Flying cars and jet-powered batwing

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stoppedcaring
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Flying cars and jet-powered batwing

Postby stoppedcaring » Wed Oct 15, 2014 4:40 pm UTC

NOTE: JUMP TO DISCUSSION OF MILITARY BATWING HERE

Came across this news bit the other day about the promise of yet another flying car. Looks neat, and the road-worthy version is far nicer-looking than most I've seen, but it's still just a plane-you-can-drive. Being able to drive to the airport, fly my car to another airport, and then drive away is nice if I'm filthy rich and need to commute from town to town by myself, but it's not going to change transportation.

A flying car that will actually change our systems of transportation needs three things:

  1. The form and function of an ordinary car in driving mode
  2. A robust VTOL mode
  3. An energy-efficient cruising flight mode
Without (1), you might as well put an axle on an ultralight. Without (2), you're stuck going to and from the airport. Without (3), you're burning pounds of fuel with every minute you stay aloft and you have no meaningful range.

It seems that the designs have to get bigger and bigger to accommodate the different components, which requires more powerful components, which requires more size, and eventually it's just too large to fit in an ordinary parking space. The biggest problem is that most designs either involve separate drivetrains for all three elements (axle for drive, something for vertical thrust, and a pusher prop) or a hybrid system for two that doesn't work well (tiltrotors, for example).

So I thought...how can I just have a single drivetrain?

Idea:

Two-seater (front to back) motorcycle chassis with quadcycle wheelbase. 45-degree camber on front and rear wheels; 90+ degree wheel turn radius. Independently chain-driven. Oversized wheels: 1.4m (so with the sharp camber they fit in a 2m wide, 1m high, 1.4m deep volume). Cowlings over each wheel that can fold out.

Because of the sharp camber, the tires are mounted asymmetrically on the inside of the wheel. The spokes are blades, making each wheel function like a ducted fan (most ducted fans have a stationary shroud rather than a corotating one, but it's close enough). In ground mode, the cowlings cover the fan intake and prevent air from being sucked through.

To take off, the four cowlings are folded out into short wings and legs are extended from the chassis to raise the wheels off the ground. Power is delivered to all four wheels at once, and the four combined fans produce enough thrust to lift the vehicle into the air. Once airborne, the steering mechanism is used to rotate all four wheels to point backward, propelling the vehicle forward. Once enough speed is built up that the wings can provide lift, the power can be reduced to save fuel and maintain cruise.

Because the wheels serve to propel the vehicle on the ground and also provide airborne thrust, while the steering mechanism acts as a makeshift tiltrotor, the need for separate components is avoided, meaning weight and complexity are dramatically reduced.

As far as the physics are concerned: each of the four wheels has an area of 1.6 m2. The static thrust of a ducted fan depends primarily on fan area, air density (1.3 kg/m3), and power, according to the following equation:

F3 = 2AρP2

If we assume that the total weight of the vehicle with two passengers, cargo, and fuel will be less than one tonne (a reasonable goal, I think), then we should plan on needing 10 kiloNewtons of thrust to get off the ground with relative haste. Each fan is then responsible for 2.5 kN. Plugging in the numbers and solving for P, we find that each fan will need to draw 61.3 kilowatts of mechanical power, meaning our engine needs to push out roughly 245 kW.

I don't think that's unreasonable. A stock Suzuki Hayabusa motorcycle engine delivers a whopping 130 kW, and it's not uncommon for track car enthusiasts to power roadsters with a pair of Hayabusa engines or other high-performance motorcycle engines; according to Wikipedia, John Hartley's Caterham Seven is powered by a pair of Hayabusa engines weighing a total of 91 kg and delivering 300 kW.

If we were to power it with a turboshaft engine, we might be able to increase the power-to-weight ratio even further. Depending on the configuration, it might be a good idea to combine a large microturbine (or a pair of microturbines) with an alternator, a battery, and a motor (thought that might drive weight up too far).

I don't see anything too difficult to overcome here. Thoughts?
Last edited by stoppedcaring on Tue Oct 28, 2014 7:12 pm UTC, edited 2 times in total.

Hypnosifl
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Re: Flying car (engineering heavy)

Postby Hypnosifl » Wed Oct 15, 2014 5:59 pm UTC

I'm pretty ignorant of this subject, but could there be any difficulty in delivering the engine's power to the fans? Looking online, I see that small turbofans like this one can deliver thrust in the range you suggest, but as I understand it these fans are driven directly by superheated gases in a jet engine--what system would be used in your proposal? If electrical cables, for example, would existing cables be able to take the necessary current? I know there have been car-sized vehicles that can fly and hover using ducted fans, like the AirMule (video of a test flight here) and this hoverbike, but they use much larger fans, which makes me wonder if there isn't some engineering difficult with using smaller ones in the absence of jet engines.

Another question: what would be the weight of the fuel needed to run one of the high-powered engines you mention at maximum power for, say, 1 hour?
Last edited by Hypnosifl on Wed Oct 15, 2014 6:46 pm UTC, edited 1 time in total.

stoppedcaring
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Re: Flying car (engineering heavy)

Postby stoppedcaring » Wed Oct 15, 2014 6:40 pm UTC

Hypnosifl wrote:I'm pretty ignorant of this subject, but could there be any difficulty in delivering the engine's power to the fans? Looking online, I see that small turbofans like this one can deliver thrust in the range you suggest, but as I understand it these fans are driven directly by superheated gases in a jet engine--what system would be used in your proposal? If electrical cables, for example, would existing cables be able to take the necessary current? I know there have been car-like vehicles that can fly and hover using ducted fans, like the AirMule (video of a test flight here), but they use much larger fans, which makes me wonder if there isn't some engineering difficult with using smaller ones in the absence of jet engines.

Another question: what would be the weight of the fuel needed to run one of the high-powered engines you mention at maximum power for, say, 1 hour?

I was thinking independently chain-driven off of a transmission, not unlike a motorcycle (but obviously requiring a larger transmission). The chain approach should be able to flex with the movement of the wheels easily.

The difficulty with smaller fans is simply that they require much more power for the same amount of thrust. (Note: this isn't pure RPM; a ducted fan is optimized for a specific thrust/RPM configuration.) Something like the AirMule is kept aloft exclusively by the downward thrust of its main fans, so the fans must be fairly large to moderate power consumption and conserve fuel. But high power consumption is fine if you only need it for takeoff and landing and switch to a forward-thrusting, aerodynamic-lift-based configuration with lower power requirements once you're airborne.

You certainly wouldn't ever run the engines at maximum power for an hour, because takeoff is only going to be a few moments. Instead, you take off at maximum power and then begin moving forward, reducing your throttle as you build up speed. Estimating the fuel consumption of a motorcycle engine is tricky because it's always given in MPG, not kg/hr, but I can take a stab at it. The Hayabusa gets 33 mpg, which at its top speed of roughly 190 mph would equate to 5.8 gallons per hour. Probably an underestimate because I doubt that its top speed has the same fuel economy, but it'll get us in the ballpark. If we assume fuel consumption rate is proportional to horse power, then the 250 kW engine is going to drink 11.2 gallons of fuel in one hour, or about 70 pounds. Probably closer to 100 pounds once you factor in the lower fuel efficiency of maximum RPM.

Of course, not only would it be impractical to hover in place for an hour, but it would also overheat your engine long before you hit that mark. Which, again, is why maximum RPM is limited to VTOL.

Hypnosifl
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Re: Flying car (engineering heavy)

Postby Hypnosifl » Wed Oct 15, 2014 7:31 pm UTC

Ah, I was thinking of something that could resemble a Back to the Future hovercar, that just folds down its wheels and flies around, but I see you're just talking about having all the thrust generated by the fans during takeoff and landing, and using extendible wings the rest of the time. That might well be workable, although I doubt they'd ever catch on for widespread use because really lightweight planes bounce around so much that people tend to get nauseous fast, which isn't as much of a problem for helicopter-like small crafts.

I've read about proposals to power turbofan-like ducted fans using lightweight superconducting engines for all-electric planes--the paper here says at the end that
Detailed design studies for HTS propulsion motors supported by experimental validation have convinced us that superconducting rotating machines today can achieve power densities comparable with that of turbine engines (3-8 kW/kg). This remarkable achievement, however, is still not enough for deployment into commercial aircraft. Electrically-propelled airliner aircraft would become feasible when power densities approach 25 kW/kg for motors and 50 kW/kg for generators, which appears to be achievable with fully superconducting machines (both inductor and armature).

So, maybe if we get the kind of advances in superconducting engines the paper projects for 2030 or so, they could be used for a car-like vehicle that could fly continuously like a helicopter with a few small high-speed fans sticking out the side and back, without the issue of engine overheating that you mentioned would prevent this with a motorcycle-like engine.

stoppedcaring
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Re: Flying car (engineering heavy)

Postby stoppedcaring » Wed Oct 15, 2014 7:47 pm UTC

Hypnosifl wrote:Ah, I was thinking of something that could resemble a Back to the Future hovercar, that just folds down its wheels and flies around, but I see you're just talking about having all the thrust generated by the fans during takeoff and landing, and using extendible wings the rest of the time. That might well be workable, although I doubt they'd ever catch on for widespread use because really lightweight planes bounce around so much that people tend to get nauseous fast, which isn't as much of a problem for helicopter-like small crafts.

In cruising flight, lift would be split between the thrusting fans and the wings. The 45-degree fixed camber means the fans won't ever thrust straight down or straight back (though on liftoff, the mirrored design will allow the fans to push off of each other and the flow will be mostly straight down). The faster you go, the less downward thrust you need to maintain altitude.

I don't think it would bounce around so much because the quadrotor design is extremely stable. Like I said, there is no need for vertical stabilization because the rotor-wheels are all independently steered/driven and would be designed to maintain stable, smooth flight.

I've read about proposals to power turbofan-like ducted fans using lightweight superconducting engines for all-electric planes--the paper here says at the end that
Detailed design studies for HTS propulsion motors supported by experimental validation have convinced us that superconducting rotating machines today can achieve power densities comparable with that of turbine engines (3-8 kW/kg). This remarkable achievement, however, is still not enough for deployment into commercial aircraft. Electrically-propelled airliner aircraft would become feasible when power densities approach 25 kW/kg for motors and 50 kW/kg for generators, which appears to be achievable with fully superconducting machines (both inductor and armature).

So, maybe if we get the kind of advances in superconducting engines the paper projects for 2030 or so, they could be used for a car-like vehicle that could fly continuously like a helicopter with a few small high-speed fans sticking out the side and back, without the issue of engine overheating that you mentioned would prevent this with a motorcycle-like engine.

It would still be a tremendous waste of fuel. Aerodynamic lift is what makes powered flight sustainable.

Hypnosifl
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Re: Flying car (engineering heavy)

Postby Hypnosifl » Wed Oct 15, 2014 8:26 pm UTC

stoppedcaring wrote:In cruising flight, lift would be split between the thrusting fans and the wings. The 45-degree fixed camber means the fans won't ever thrust straight down or straight back (though on liftoff, the mirrored design will allow the fans to push off of each other and the flow will be mostly straight down). The faster you go, the less downward thrust you need to maintain altitude.

I don't think it would bounce around so much because the quadrotor design is extremely stable. Like I said, there is no need for vertical stabilization because the rotor-wheels are all independently steered/driven and would be designed to maintain stable, smooth flight.

Got it, so the fans are being used for stability in flight as well as for thrust and vertical takeoff/landing.
stoppedcaring wrote:It would still be a tremendous waste of fuel. Aerodynamic lift is what makes powered flight sustainable.

Good point, as long as you can make the flight stable it would make much more sense to have extendible wings for this reason. Just to check I'm understanding this correctly, the reason this argument doesn't apply to helicopters is just that the blades are so much larger that the aerodynamic lift from angled blades is much larger than it would be for a design with small powerful fans, right? If so, a possible alternative to extendible wings might be something like the foldable-helicopter-blade design imagined by DARPA here.

stoppedcaring
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Re: Flying car (engineering heavy)

Postby stoppedcaring » Wed Oct 15, 2014 10:03 pm UTC

Hypnosifl wrote:
stoppedcaring wrote:In cruising flight, lift would be split between the thrusting fans and the wings. The 45-degree fixed camber means the fans won't ever thrust straight down or straight back (though on liftoff, the mirrored design will allow the fans to push off of each other and the flow will be mostly straight down). The faster you go, the less downward thrust you need to maintain altitude.

I don't think it would bounce around so much because the quadrotor design is extremely stable. Like I said, there is no need for vertical stabilization because the rotor-wheels are all independently steered/driven and would be designed to maintain stable, smooth flight.

Got it, so the fans are being used for stability in flight as well as for thrust and vertical takeoff/landing.

Right. Quadrotors are super popular for small-scale hobbyists because they are so easy to control and so stable. A lot of UAVs are quadrotor-based. It's just that battery capacity starts to go south as you scale up, meaning that you need to go to a gasoline engine well before reaching manned-vehicle size, and then you've basically got an oversized helicopter. A heli can generate lift in forward flight, which makes it both faster and more energy-efficient than a quadrotor, which would burn fuel far too fast.

stoppedcaring wrote:It would still be a tremendous waste of fuel. Aerodynamic lift is what makes powered flight sustainable.

Good point, as long as you can make the flight stable it would make much more sense to have extendible wings for this reason. Just to check I'm understanding this correctly, the reason this argument doesn't apply to helicopters is just that the blades are so much larger that the aerodynamic lift from angled blades is much larger than it would be for a design with small powerful fans, right? If so, a possible alternative to extendible wings might be something like the foldable-helicopter-blade design imagined by DARPA here.

Well, helicopters can generate lift in forward flight using an exceedingly complex pitch control system. They're also sweeping out a HUGE area; the common Bell 407 helicopter has a rotor diameter of 35 feet and sweeps out almost 90 square meters, allowing it to "push against" dramatically more air. This means it can generate more thrust for the same power compared to a smaller rotor.

Incidentally, when we're talking about hover flight, there's a similarity here between helis and rockets. The rocket equation says that the more velocity your exhaust has, the less of it you'll need to produce the same amount of thrust. So you need to carry a LOT of fuel if you have low-energy exhaust, but less fuel if you have high-energy exhaust. In the same way, a lifting fan/prop will need more power if it's pushing against a small volume of air than if it is pushing against a larger volume of air. But in both cases, more power (energy consumption per unit time) during static hover means lower overall energy efficiency.

Aerodynamic lift is upward force generated by air flowing over an airfoil, which basically converts drag into lift. IIRC, a helicopter's blades do double function as airfoils during forward flight, which makes them slightly more efficient. But of course they aren't nearly as efficient as a plane. We have helicopters for VTOL/hover ability; that's about it.

The DARP flying car is really just a helicopter with wheels and folding blades, which is nice but not terribly useful outside of military applications. It's not going to fit into a parking spot, and its takeoff radius is going to be just as big as an ordinary heli, so it can't really do much to change day-to-day transportation.

I wish I was a better artist, 'cause I'd draw a concept. But the idea I've got here is basically an extra-long, slightly wider-than-it-ought-to-be two-seater four-wheeler with cowlings over the wheels...one smaller than an SUV, but that's capable of taking off vertically and transitioning to horizontal flight anywhere, and that runs on ordinary gas.

Tyndmyr
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Re: Flying car (engineering heavy)

Postby Tyndmyr » Thu Oct 16, 2014 7:57 pm UTC

stoppedcaring wrote:Came across this news bit the other day about the promise of yet another flying car. Looks neat, and the road-worthy version is far nicer-looking than most I've seen, but it's still just a plane-you-can-drive. Being able to drive to the airport, fly my car to another airport, and then drive away is nice if I'm filthy rich and need to commute from town to town by myself, but it's not going to change transportation.


The aerodynamics of a plane matter far more than the aerodynamics of a car, so I suppose the plane side will always be more dominant in that respect.

A surprising amount of requirements of both are antithetical to the other. I mean, even at the basic suggestion you started with of "add things to an ultralight", most ultralights don't have a ton of spare lifting capacity, and so, adding another axle, wheels, the various street legal requirements pretty directly conflict with it's capacity to be a functional aircraft.

Perhaps we should go back to the basics, and consider what problem we are trying to solve with a flying car. Is it the difficulty of transiting from private plane to one's car? Some sort of modular pod cockpit might help there...

But honestly, most of us don't have private planes to begin with.

stoppedcaring
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Re: Flying car (engineering heavy)

Postby stoppedcaring » Thu Oct 16, 2014 8:38 pm UTC

Tyndmyr wrote:
stoppedcaring wrote:Came across this news bit the other day about the promise of yet another flying car. Looks neat, and the road-worthy version is far nicer-looking than most I've seen, but it's still just a plane-you-can-drive. Being able to drive to the airport, fly my car to another airport, and then drive away is nice if I'm filthy rich and need to commute from town to town by myself, but it's not going to change transportation.


The aerodynamics of a plane matter far more than the aerodynamics of a car, so I suppose the plane side will always be more dominant in that respect.

A surprising amount of requirements of both are antithetical to the other. I mean, even at the basic suggestion you started with of "add things to an ultralight", most ultralights don't have a ton of spare lifting capacity, and so, adding another axle, wheels, the various street legal requirements pretty directly conflict with it's capacity to be a functional aircraft.

Well, an ultralight isn't airport-worthy, so the driven form would be more like a gocart conversion. In which case just run a chain from the motor down to the rear axle and add a switch to disengage the prop, then figure out some way to steer on the ground.

Honestly I meant more of a powered hang glider than a true ultralight. Though the line gets really blurry:
Spoiler:
Image


Perhaps we should go back to the basics, and consider what problem we are trying to solve with a flying car. Is it the difficulty of transiting from private plane to one's car? Some sort of modular pod cockpit might help there...

But honestly, most of us don't have private planes to begin with.

That is the problem we're trying to solve. Why can't I choose whether to fly or drive to work with the same vehicle? Obviously it would be limited to heavily licensed restrictions at first...probably just law enforcement to begin with. But having ANY functional, usable flying machine with the footprint of an ordinary vehicle is the first step.

We don't need a several-hundred-mile range if we can just land at the nearest gas station any time we want. That's what I'm envisioning -- a lightweight, two-person, road-licensed 4-wheeler with the capacity to take off vertically and dart a dozen miles or so if need be. Once police and rescue workers and military have been using them for a while, they could be opened up to privately licensed pilots, and so forth.

The vision for a flying car wasn't a vision of a car that transformed into a plane when you drove it onto an airport, but a world where cars take off from your driveway. The only way we can get closer to realizing that kind of world is to make a vehicle capable of doing exactly that.

tbburg
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Re: Flying car (engineering heavy)

Postby tbburg » Fri Oct 17, 2014 6:17 am UTC

A big issue you'd have to overcome with your design is is power loss from drag and friction in the power train. Trying to chain-drive 4 fan-wheel structures that can hinge would be next to impossible. There's also the safety-redundancy problem. break a chain and loose 2 lift fans on the same end of the vehicle. Automatic crash and burn. Gears and shafts would be just as bad. A more efficient design would be to have the motors power a alternator/generator, and have the fan/wheels individually powered with electric motors built into the hub.

The flying car idea is kind of obsolete anyway. It's centered on having to use a runway to take off or land. If the vehicle has true VTOL capability you don't need a runway, so you don't need to drive to the airport. What would be the point of having a car/plane transition if the vehicle were capable of VTOL? Why not just fly everywhere, and land next to or in your chosen parking space?
There are few laws about where you can land a helicopter, which is why they end up parked in some funny places. (I once saw a Hughes 500 in a Bob Evans parking lot, and there was a really entertaining incident in the '90s involving a Columbus, OH police helicopter doing a doughnut run.)

Another issue rarely considered is crash worthiness. If you have a "roadable plane" it would have to meet impact standards. A current production C-172 weighs under 1700lb. Hit that with a Toyota Prius and it'll crumple up like a beer can. Add enough structure to pass a crash test, and it now weighs more then the plane is rated to carry.

Tyndmyr
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Re: Flying car (engineering heavy)

Postby Tyndmyr » Fri Oct 17, 2014 1:46 pm UTC

stoppedcaring wrote:
Perhaps we should go back to the basics, and consider what problem we are trying to solve with a flying car. Is it the difficulty of transiting from private plane to one's car? Some sort of modular pod cockpit might help there...

But honestly, most of us don't have private planes to begin with.

That is the problem we're trying to solve. Why can't I choose whether to fly or drive to work with the same vehicle? Obviously it would be limited to heavily licensed restrictions at first...probably just law enforcement to begin with. But having ANY functional, usable flying machine with the footprint of an ordinary vehicle is the first step.

We don't need a several-hundred-mile range if we can just land at the nearest gas station any time we want. That's what I'm envisioning -- a lightweight, two-person, road-licensed 4-wheeler with the capacity to take off vertically and dart a dozen miles or so if need be. Once police and rescue workers and military have been using them for a while, they could be opened up to privately licensed pilots, and so forth.

The vision for a flying car wasn't a vision of a car that transformed into a plane when you drove it onto an airport, but a world where cars take off from your driveway. The only way we can get closer to realizing that kind of world is to make a vehicle capable of doing exactly that.


Why is "the same vehicle" important, though?

And take-offs/landing at a gas station seem to have, even with VTOL, significant issues integrating with current infrastructure. Gas stations almost invariably have a roof over much of the pad, and aren't really designed with approach lates and skyward visibility in mind.

Also, there's a reason why VTOL isn't used a ton commercially. It's bloody complicated/expensive. If you want widespread adoption, these things are your greatest enemy.

f5r5e5d
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Re: Flying car (engineering heavy)

Postby f5r5e5d » Fri Oct 17, 2014 4:31 pm UTC

https://www.google.com/#q=cartercopter does VTOL, but can't hover

Hypnosifl
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Re: Flying car (engineering heavy)

Postby Hypnosifl » Fri Oct 17, 2014 5:11 pm UTC

tbburg wrote:The flying car idea is kind of obsolete anyway. It's centered on having to use a runway to take off or land. If the vehicle has true VTOL capability you don't need a runway, so you don't need to drive to the airport. What would be the point of having a car/plane transition if the vehicle were capable of VTOL? Why not just fly everywhere, and land next to or in your chosen parking space?

That probably wouldn't be practical in cities if large numbers of people owned such vehicles, even if the vehicles had computer-controlled flight rather than being piloted manually--a Fifth Element type situation with flying cars clogging the spaces between buildings might be too dangerous, even with computer controls I wonder if there could still be dangers from unpredictable gusts of wind, and people might not feel comfortable with all those heavy vehicles over their heads even if the statistical risk was small. Plus, unless there's some radical improvement in making high-speed fans much quieter, lots of flying vehicles in the same area would create way too much noise pollution. So if you wanted to go somewhere in a distant city, it would be handy to be able to fly to a spot just outside, then drive the rest of the way (likewise if you wanted to leave a city you lived in, you'd presumably want to first drive out and only then take off in a more secluded area).
tbburg wrote:Another issue rarely considered is crash worthiness. If you have a "roadable plane" it would have to meet impact standards. A current production C-172 weighs under 1700lb. Hit that with a Toyota Prius and it'll crumple up like a beer can. Add enough structure to pass a crash test, and it now weighs more then the plane is rated to carry.

For now that would be a big problem, in the future if there's widespread adoption of self-driving cars, perhaps all cars can become lighter without leading to more deaths (it would certainly help with fuel economy).

stoppedcaring
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Re: Flying car (engineering heavy)

Postby stoppedcaring » Fri Oct 17, 2014 6:58 pm UTC

tbburg wrote:A big issue you'd have to overcome with your design is is power loss from drag and friction in the power train. Trying to chain-drive 4 fan-wheel structures that can hinge would be next to impossible. There's also the safety-redundancy problem. break a chain and loose 2 lift fans on the same end of the vehicle. Automatic crash and burn. Gears and shafts would be just as bad. A more efficient design would be to have the motors power a alternator/generator, and have the fan/wheels individually powered with electric motors built into the hub.

Or built into a wrap-around fender. Having the wheels so sharply cambered might pose some engineering challenges when it comes to making sure weight is supported and distributed, but I'm sure it can be overcome. Using four induction motors wouldn't add too much weight and would have the added benefit of allowing regenerative braking. A battery pack would be needed to start the thing, anyway, so might as well take the next step and make it a hybrid (though obviously the battery isn't going to provide enough juice for flight, so the hybrid benefit would only be in rolling mode).

Another issue rarely considered is crash worthiness. If you have a "roadable plane" it would have to meet impact standards. A current production C-172 weighs under 1700lb. Hit that with a Toyota Prius and it'll crumple up like a beer can. Add enough structure to pass a crash test, and it now weighs more then the plane is rated to carry.

Rolling mode would be more like a motorcycle than a car. I'm not sure what crash test requirements a motorcycle or motortrike or quadbike has, but I'm sure they're different than an ordinary car.

As far as weight is concerned: I know I was suggesting the use of a double Suzuki Hayabusa engine, but a souped-up single Hayabusa might actually do the trick. Turbocharging can get them up to 190 kW, which, when spread out over four 1.6 m2 ducted fans, produces a solid 8,440 Newtons of thrust. Take 25% off for efficiency considerations and that's still enough to lift 645 kg. If we assume the engine, alternator, and battery system weigh a combined 100 kg and estimate a cargo/passenger payload of 245 kg, that leaves 300 kg for the wheels, frame, cowlings, cockpit, fuel, and a reasonable acceleration margin. I think that's doable.

This thing would also be ungodly fast on the ground.

The flying car idea is kind of obsolete anyway. It's centered on having to use a runway to take off or land. If the vehicle has true VTOL capability you don't need a runway, so you don't need to drive to the airport. What would be the point of having a car/plane transition if the vehicle were capable of VTOL? Why not just fly everywhere, and land next to or in your chosen parking space?

Well, I think this question is notably answered by the consideration Tyndmyr posed:

Tyndmyr wrote:Take-offs/landing at a gas station seem to have, even with VTOL, significant issues integrating with current infrastructure. Gas stations almost invariably have a roof over much of the pad, and aren't really designed with approach lanes and skyward visibility in mind.

That's exactly why the same vehicle needs to be used for both rolling and flying. If you're limited to ttburg's notion of flying everywhere, then you are still essentially limited to helipads with fuel handy nearby. An actual flying car, on the other hand, can go anywhere a car can go, just as easily as a car can. Gas stations aren't the only advantage: just think of how many neighborhoods and shopping centers and downtowns have trees everywhere. It's hard enough to find parking in a city; you really don't want to be limited to parking in spaces with direct sky access. But if you can land in an open area and then drive a couple blocks to the underground parking garage, you're set.

Besides, flexibility. If I want to go pick up burgers from the Golden Arches four blocks away, I don't want to go through a preflight checklist, take off, dodge light poles and trees, and hope they have a nice open space to land. Not when I can drive there on the normal road in less time.

Also, there's a reason why VTOL isn't used a ton commercially. It's bloody complicated/expensive. If you want widespread adoption, these things are your greatest enemy.

And yet VTOL is the only way for widespread flight to become feasible.

f5r5e5d wrote:https://www.google.com/#q=cartercopter does VTOL, but can't hover

That's awesome. Too bad it's far too large to fit in a parking space or road lane.

Something else that hits the same VTOL-with-efficient-flight niche (items 2 and 3 of my original checklist) is the Ray VTOL Aircraft. Takes off and lands vertically, flies efficiently. Just way too big to fit anywhere other than a helipad, so it's still basically a helicopter for all the good it does to transportation.

The challenge of VTOL is the static thrust equation I provided in my first post: F3 = 2AρP2. For a given fan area, your power requirement is proportional to the 3/2 power of weight, but the power-to-weight ratio of engines is usually linear. That's why small quadrocopters are way more feasible than large ones. You're forced to make your fan area as great as possible if you want to make something large enough to lift a human being, because increasing engine power will only increase engine weight, which drives your power requirements further up.

Helicopters generate a lot of lift using lower power with very long blades, but the blades are dangerous, inefficient for cruising flight, and all but impossible to fit into the volume of an ordinary car. Having four cambered fans which serve triple duty as wheels and pusher props is the best way I can think of to pack maximum lifting area into the volume of an ordinary car. Maybe there's a better way; I just can't think of it.

Hypnosifl wrote:If you wanted to go somewhere in a distant city, it would be handy to be able to fly to a spot just outside, then drive the rest of the way (likewise if you wanted to leave a city you lived in, you'd presumably want to first drive out and only then take off in a more secluded area).

Exactly. The difference between VTOL-only and VTOL+rolling is night and day. Even if you were going to fly from one part of the city to another, you'd still want to drive your vehicle out of its garage to an open space, then take off and fly to your destination, then land in an open space and drive to the exact spot you wanted.

I imagine that widespread adoption would eventually result in landing/takeoff zones -- mini-helipads which opened onto the road and would be synced to transponders, so that you would only have one vehicle landing or taking off at a time. Otherwise takeoffs and landings would be limited to private property.

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Fri Oct 17, 2014 9:39 pm UTC

The closest thing I've seen to this previously is the Terrafugia VTOL flying car concept:

https://www.youtube.com/watch?v=bp2TWNpTA7s

Seems great in theory, but the logistics seem a little more challenging. VTOL and initial forward movement uses tiltrotor prop blades that fold away in cruise mode and in rolling mode. They claim a megawatt of electrical power is used to lift the vehicle off the ground. The blades appear to be about a meter long, meaning each prop set is 3.14 m2; by the equation I cited before and an assumed 80% prop efficiency, this would provide a lift capacity of two tonnes. Definitely enough to get you off the ground...only, I'm curious as to how they could ever manage a megawatt-class battery. That 300 hp engine prop sure isn't going to do the trick. Top-of-the-line lithium-ion batteries have a power-to-weight ratio of 2-4 kW/kg, which would require a battery of over 200 kg at the very least. Not insurmountable, but challenging.

But as far as the driving functionality is concerned, it's very very similar to what I'm talking about with my concept. Three separate modes, etc.

Another possibility would be fold-away wings with in-wing fans, like a much much smaller version of the old Ryan XV-5 Vertifan. But managing to get fold-away wings with enough area to work while keeping weight down and maintaining a carlike profile is really challenging.

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Re: Flying car (engineering heavy)

Postby tbburg » Sat Oct 18, 2014 4:21 am UTC

My personal negativity aside, here are some more thoughts.

Problem: A fan-wheel combination would be very heavy, and hard to keep in balance. Spinning the fan up to speed with the weight of the tire would take a lot of power. An out of balance tire is a minor problem on the highway, but when you spin the assembly up to flight speed, it could rip itself to shreds.

Possible solution: Instead of sharply cambering wheels with fans incorporated in the hub, separate the fan and wheel. Mount the wheels conventionally with a gear reduction in the hub(like the old 3-speed planetary hubs they used to make for bicycles.) Have no dedicated drive train for the wheels.
Have the fan mounted on a hinged joint above and outboard of the wheel, In "drive mode" it is hinged down (vertically)and the hub of the fan engages the hub of the wheel. Power to the wheel is supplied by the fan motor. The fan blades would probably have to have a "feather" mode to keep from pushing air into the wheel and under the car. In "flight mode" the fan assembly hinges up(horizontal) When the fan moves into the "up" position, it disengages from the wheel. If you're moving, you would start coasting. Now the fans are free to spin up for flight, unencumbered by the weight of the wheel/suspension gear, and unaffected by tire wear or damage. The design still has the problem of a tire that has to be bigger then the lift fan diameter, if that is in fact a problem.

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Mon Oct 20, 2014 6:48 pm UTC

tbburg wrote:My personal negativity aside, here are some more thoughts.

Problem: A fan-wheel combination would be very heavy, and hard to keep in balance. Spinning the fan up to speed with the weight of the tire would take a lot of power. An out of balance tire is a minor problem on the highway, but when you spin the assembly up to flight speed, it could rip itself to shreds.

Possible solution: Instead of sharply cambering wheels with fans incorporated in the hub, separate the fan and wheel. Mount the wheels conventionally with a gear reduction in the hub(like the old 3-speed planetary hubs they used to make for bicycles.) Have no dedicated drive train for the wheels.
Have the fan mounted on a hinged joint above and outboard of the wheel, In "drive mode" it is hinged down (vertically)and the hub of the fan engages the hub of the wheel. Power to the wheel is supplied by the fan motor. The fan blades would probably have to have a "feather" mode to keep from pushing air into the wheel and under the car. In "flight mode" the fan assembly hinges up(horizontal) When the fan moves into the "up" position, it disengages from the wheel. If you're moving, you would start coasting. Now the fans are free to spin up for flight, unencumbered by the weight of the wheel/suspension gear, and unaffected by tire wear or damage. The design still has the problem of a tire that has to be bigger then the lift fan diameter, if that is in fact a problem.

Good ideas. Not sure if it fixes more problems than it causes, though.

The reason for the extremely high camber is to fit larger-area fans into a smaller volume. The wheels I'm proposing have a diameter of 1.4 m, larger than most 18-wheeler tires. If these wheels were mounted vertically, they'd stand 1.4m off the ground on either side of the vehicle, which would be extremely ungainly. By cambering them into each other, they can be mounted in front and back of the chassis and take up a volume that's only 2 m wide, 1 m high, and 1.4 m deep.

Another disadvantage of having fans mounted over the wheels is added complexity.

Here are the various views of the wheel/fan orientation I was previously thinking of:

Image

The axles, drivetrains, and cowlings/fenders are obviously not pictured. But the advantage of this design is that you end up with a very large fan area packed into a reasonably small set of dimensions. The other advantage is that the same mechanism you use for steering on the ground is also used for producing a tiltrotor effect. Yeah, this does mean balance issues are going to be SUPER critical, but maybe that's just the price you pay. With fans mounted over the wheels, you need an entirely separate mechanism for lifting and tilting them.

Here's a mocked-up example (obviously minus fenders and cowlings) of what it would look like, both in rolling mode (making turns on the ground) and in flight mode (both takeoff/landing and forward flight):

Image

The entire vehicle would have a footprint approximately the same as a Toyota Camry, while of course maintaining a much lower profile and dramatically lower weight. With the cowlings unfolded into wings, the footprint would expand to approximately double the vehicle's width. The angle of the wheels can be automatically adjusted via the steering mechanism, both to stabilize flight and to enable in-flight turns. In rolling mode, the rear wheels would probably angle slightly during turns, depending on speed.

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Re: Flying car (engineering heavy)

Postby tbburg » Tue Oct 21, 2014 2:30 am UTC

Yeah,.. I was thinking about the wheel diameter/height issue after my last posting. I tried some "back of napkin" type sketches. They weren't pretty. I didn't draw out a 45deg, sketch, but I could see the upright 1.4m tire probably wasn't going to work, especially in front where they would interfere with driver visibility.
Another problem occurred: wheel-tire weight. Not just balance, the overall weight of a 1.4m tire tough enough for roadway travel would be pretty heavy. Also, probably no way to lighten the tires, they're going to have to be speed-rated to handle the rotational forces of the fans in flight mode.

There may also be an insurmountable problem with the 45deg. tire camber: turn in. When you lay a tire over at an angle it turns toward the lean. Canted tires would be scrubbing/sliding, and I'm not sure that any amount of toe-out would prevent this.

It seems like your design is aimed mainly at power-train simplicity. Do you think the advantage of the multipurpose power-train would outweigh the drawbacks of the suspension geometry and tire weight?

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Tue Oct 21, 2014 4:52 pm UTC

tbburg wrote:Yeah,.. I was thinking about the wheel diameter/height issue after my last posting. I tried some "back of napkin" type sketches. They weren't pretty. I didn't draw out a 45deg, sketch, but I could see the upright 1.4m tire probably wasn't going to work, especially in front where they would interfere with driver visibility.
Another problem occurred: wheel-tire weight. Not just balance, the overall weight of a 1.4m tire tough enough for roadway travel would be pretty heavy. Also, probably no way to lighten the tires, they're going to have to be speed-rated to handle the rotational forces of the fans in flight mode.

One advantage in this department: the vehicle is already going to be as light as it can possibly be, which will reduce the stress on the tires somewhat. They will probably have to be some sort of shock-absorbent composite anyway; I don't foresee a typical tubeless tire working out so well here. Using a fan that detaches from the wheel wouldn't accomplish anything in terms of overall weight reduction, obviously, because the wheel is still going to be there.

There may also be an insurmountable problem with the 45deg. tire camber: turn in. When you lay a tire over at an angle it turns toward the lean. Canted tires would be scrubbing/sliding, and I'm not sure that any amount of toe-out would prevent this.

I don't know much about racing or suspension or front-end alignment, but the tires themselves would be very asymmetric (as shown in the mockups above) so that might help in itself. At speed, the cowlings will act as spoilers, so that will help increase the normal force and add to grip.

It seems like your design is aimed mainly at power-train simplicity. Do you think the advantage of the multipurpose power-train would outweigh the drawbacks of the suspension geometry and tire weight?

Weight is definitely an issue, but space is a bigger issue. Unless you go with folding or telescoping helicopter blades of some kind (which is viable, but necessitates an entirely different frame and design), it's simply VERY difficult to stow any reasonable amount of fan-area in the dimensions of a vehicle. There's no way to get around the power-to-weight relationship in the static thrust equation, and engines with higher power-to-weight ratios have very high fuel requirements, so the only way to get enough thrust to lift a manned vehicle efficiently is to have large fan area.

The only way I can think of to embed significant fan area in a small enough volume is to use the wheels in a cambered configuration. Fans can't go on the sides of the cabin without axeing visibility, and the necessity for balance (since you're lifting, after all) limits a top or bottom mount to a symmetric configuration, further limiting the radius of your fans and requiring additional weight-bearing systems to extend and retract them. The tremendous advantage of having the a single location, suspension system, steering system, and drivetrain for the wheels and the fans would tend to justify the greater complexity, I think.

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Re: Flying car (engineering heavy)

Postby Tyndmyr » Wed Oct 22, 2014 6:43 pm UTC

stoppedcaring wrote:Well, I think this question is notably answered by the consideration Tyndmyr posed:

Tyndmyr wrote:Take-offs/landing at a gas station seem to have, even with VTOL, significant issues integrating with current infrastructure. Gas stations almost invariably have a roof over much of the pad, and aren't really designed with approach lanes and skyward visibility in mind.

That's exactly why the same vehicle needs to be used for both rolling and flying. If you're limited to ttburg's notion of flying everywhere, then you are still essentially limited to helipads with fuel handy nearby. An actual flying car, on the other hand, can go anywhere a car can go, just as easily as a car can. Gas stations aren't the only advantage: just think of how many neighborhoods and shopping centers and downtowns have trees everywhere. It's hard enough to find parking in a city; you really don't want to be limited to parking in spaces with direct sky access. But if you can land in an open area and then drive a couple blocks to the underground parking garage, you're set.

Besides, flexibility. If I want to go pick up burgers from the Golden Arches four blocks away, I don't want to go through a preflight checklist, take off, dodge light poles and trees, and hope they have a nice open space to land. Not when I can drive there on the normal road in less time.


But why does this transportation have to be simply one vehicle? Why isn't it sufficient to say, utilize Uber or an equivalent short-hop drive solution in conjunction with flying?

Also, there's a reason why VTOL isn't used a ton commercially. It's bloody complicated/expensive. If you want widespread adoption, these things are your greatest enemy.

And yet VTOL is the only way for widespread flight to become feasible.


Why? VTOL brings in a number of problems, what does it solve?

I'll put it this way...most drivers can't seem to manage using a turn signal, I have little faith that they understand how to deal with a vortex ring state.

tbburg wrote:Yeah,.. I was thinking about the wheel diameter/height issue after my last posting. I tried some "back of napkin" type sketches. They weren't pretty. I didn't draw out a 45deg, sketch, but I could see the upright 1.4m tire probably wasn't going to work, especially in front where they would interfere with driver visibility.
Another problem occurred: wheel-tire weight. Not just balance, the overall weight of a 1.4m tire tough enough for roadway travel would be pretty heavy. Also, probably no way to lighten the tires, they're going to have to be speed-rated to handle the rotational forces of the fans in flight mode.


Yeah, minimization of unsprung weight is a major factor in auto design. Huge tires are going to impact that negatively for sure...and even minor road wear could significantly impact performance in the air. Rims tend to pick up lots of minor scrapes, dirt, etc. Generally not ideal for props.

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Wed Oct 22, 2014 8:56 pm UTC

Tyndmyr wrote:
stoppedcaring wrote:It's hard enough to find parking in a city; you really don't want to be limited to parking in spaces with direct sky access. But if you can land in an open area and then drive a couple blocks to the underground parking garage, you're set.

Besides, flexibility. If I want to go pick up burgers from the Golden Arches four blocks away, I don't want to go through a preflight checklist, take off, dodge light poles and trees, and hope they have a nice open space to land. Not when I can drive there on the normal road in less time.

But why does this transportation have to be simply one vehicle? Why isn't it sufficient to say, utilize Uber or an equivalent short-hop drive solution in conjunction with flying?

Like I said, you may need to move your vehicle (to park, for example) without having to takeoff and land. You were the one who pointed out the implausibility of flying into a gas station to refuel; it's much safer to land nearby and then drive up. You might not always be in a place where you have easy access to a rental car. And flexibility is key. If you're driving to work and you run into traffic, you'd be able to pull off into a vacant lot, unfold your wings, and just take off immediately. Isn't that what everyone dreams about -- being able to simply "hop over" an unexpected accident or traffic jam or construction zone?

VTOL is the only way for widespread flight to become feasible.

Why? VTOL brings in a number of problems, what does it solve?

I'll put it this way...most drivers can't seem to manage using a turn signal, I have little faith that they understand how to deal with a vortex ring state.

VTOL solves everything. Right now, if you want to fly, you have to drive to an airport first. The whole point is not needing an airport. Unless you have a personal vehicle with VTOL, you've accomplished next to nothing.

Obviously, people would need some sort of pilot's license to fly this. Probably only a hobbyist license, but still. Not going to be as easy as getting a driver's license.

tbburg wrote:Yeah, minimization of unsprung weight is a major factor in auto design. Huge tires are going to impact that negatively for sure...and even minor road wear could significantly impact performance in the air. Rims tend to pick up lots of minor scrapes, dirt, etc. Generally not ideal for props.

We could always start out with a flight-only model to test the whole thing. Just use ordinary ducted fans instead of trying to do the whole fan-wheel affair. Once you had that operational, working out a way to make it drive would be more feasible.

ttburg's previous idea about separating the fan from the wheel would actually work if we maintained the same configuration, but had smaller wheels set underneath the fans on the ground at a lower camber with a jointed driveshaft. The ducted fans would lock into the jointed driveshaft to operate the wheels in rolling mode, but separate and lift away in flight mode.

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Re: Flying car (engineering heavy)

Postby Tyndmyr » Thu Oct 23, 2014 12:58 pm UTC

VTOL most emphatically does not solve everything. For instance, most VTOL craft tend to put out a significant amount of heat. The Osprey, for instance, puts out so much that carrier decks need to be redesigned. You can't just take off from the McDonalds parking lot.

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Re: Flying car (engineering heavy)

Postby Neil_Boekend » Thu Oct 23, 2014 1:35 pm UTC

People don't manage to use their turn signals because there is effectively no penalty on not using them. They don't care and forget. After a few years they don't use them at all.

Having said that, vortex ring states are something a computer should prevent. Most people won't be able to pronounce it, let alone manage it.

However, if I understand it correctly with 10 seconds of wikipedia reading it is quite unlikely with 2 angled rotors. Intuitively I'd expect the downwash of each rotor to damage the vortex of the other rotor.
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Re: Flying car (engineering heavy)

Postby Hypnosifl » Thu Oct 23, 2014 1:51 pm UTC

Tyndmyr wrote:VTOL most emphatically does not solve everything. For instance, most VTOL craft tend to put out a significant amount of heat. The Osprey, for instance, puts out so much that carrier decks need to be redesigned. You can't just take off from the McDonalds parking lot.

A lightweight craft whose VTOL was based on fans that weren't powered by jet engines probably wouldn't put out too much heat, see the Airmule (with video here) that I mentioned earlier in the thread.

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Thu Oct 23, 2014 6:07 pm UTC

Tyndmyr wrote:VTOL most emphatically does not solve everything. For instance, most VTOL craft tend to put out a significant amount of heat. The Osprey, for instance, puts out so much that carrier decks need to be redesigned. You can't just take off from the McDonalds parking lot.

My point is that VTOL is required to solve the problem of widely distributed flight, the flying car of science fiction. Otherwise you need a runway.

Problems with VTOL don't mean VTOL isn't necessary. And the problem with the Osprey is that it uses a turboshaft jet engine to run the props, producing massive heat from the jet exhaust. Ducted fans themselves do not significantly heat the air, at least not enough to cause any problems, and the engine is just an ordinary motorcycle engine.

Neil_Boekend wrote:People don't manage to use their turn signals because there is effectively no penalty on not using them. They don't care and forget. After a few years they don't use them at all.

Having said that, vortex ring states are something a computer should prevent. Most people won't be able to pronounce it, let alone manage it.

However, if I understand it correctly with 10 seconds of wikipedia reading it is quite unlikely with 2 angled rotors. Intuitively I'd expect the downwash of each rotor to damage the vortex of the other rotor.

Using four inward-angled rotors makes a vortex ring state all but impossible. Another related advantage of the inward-angling is that it will produce a high-pressure region immediately under the vehicle. Not only does this cool the engine during hover, but it generates a ground effect during takeoff to help reduce overall load.

Hypnosifl wrote:A lightweight craft whose VTOL was based on fans that weren't powered by jet engines probably wouldn't put out too much heat, see the Airmule (with video here) that I mentioned earlier in the thread.

And you can even use a jet engine as long as the exhaust isn't pointed at the ground like it is with the Osprey. Jet engines operate at highest efficiency when their intake and axial direction are lined up with the direction of travel, meaning their exhaust also has to be lined up with the direction of travel. But the Osprey's entire engines tilt with the rotor, meaning that the only way to have axial alignment during forward flight is to have the exhaust point opposite the rotor plane, forcing it to point at the ground during VTOL.

There's some neat stuff we could see in the near future (military batwing)...

A next-generation military ground-support aircraft would ideally be able to hover with the same fuel consumption as it has in sustained cruising flight. Attack helicopters are great for ground support but are complex/vulnerable and have very limited range with low forward airspeed. Turboprops and jets like the Osprey, the Harriers, and the F-35B have much better range but their extreme fuel consumption in hover flight makes them ineffective for ground support.

What would be super cool to see? A working jet-powered version of The Dark Knight Rises's Bat/Batwing/Batcopter. Primary role as a replacement for the A-10 Thunderbolt II, with VTOL/STOL capacity and a secondary role of light/personnel lift and near-ground infantry support a la the Apache/Comanche family.

It would have to be wider than the obviously un-airworthy Bat, but not too much wider. Tailless. The same center-mounted body as the Bat, but with very broad, deep wings enclosing a pair of inward-angled fans. The upper and lower surfaces of the wings would need variable-angle louvers to vector airflow during fan-assisted flight or close completely for full-forward aerodynamic flight.

I'm thinking it could be powered by two modestly-sized medium-bypass turbofans, mounted high on the back in a semi-embedded configuration reminiscent of the B-2. It would be nice if an adjustable bleed-air system from the turbofans could be used to operate the lift fans rather than bothering with a gearbox and shaft assemblies, but I'm not sure that's possible. At the very least, bleed-air could be used for vectored flight control to reduce moving parts. If a pure bleed-air drive isn't possible, the turboshafts could potentially pressurize a "fueldraulic" drive as in the F-35B.

In order to achieve equal fuel consumption in hover as in cruise, the turbofan thrust requirements to meet L/D ratio in forward flight would need to closely match the power requirements of static thrust for the given fan area. An advantage of downward-angled wings would be that the overall aerodynamics could be designed to allow for a nice controllable ground effect, reducing fuel consumption in ground-support situations and enabling an almost tank-like role.

I don't know how much ground effect could be exploited, but it might even be possible for ground effect characteristics to enable extended "skim" flight over reasonably level ground, similar in function to a Star Wars-type speeder, using only one engine. Helpful for longer-range sorties that need to conserve fuel, and also a lifesaver if one engine fails or is damaged by gunfire. If necessary, the fueldraulic or bleed-air system could be used to extend oversized flaps to enhance ground effect lift.

Four manual-option autotargeting miniguns distributed as necessary over the body could command the entire area and automatically neutralize rocket attacks. It would also carry typical antitank/antivehicle armament like an Apache-class helicopter. The suspended single-seat pilot pod would be heavily armored; a compact bay behind the pilot could carry up to two passengers or a small amount of cargo. Light grapple-based lift would be possible but only over short ranges; grapples could also be useful in lifting/moving obstacles as additional support for infantry.
Last edited by stoppedcaring on Tue Oct 28, 2014 7:13 pm UTC, edited 1 time in total.

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Re: Flying car (engineering heavy)

Postby Tyndmyr » Thu Oct 23, 2014 6:29 pm UTC

Not enough time to adequately crunch that whole thing, but usable ground effect is related to wingspan and of course, altitude. This is why the old soviet ground effect birds were so frigging huge...let them utilize ground effect at a useful altitude. Obviously, with a small craft such as this, being in ground effect means being within a few feet of the ground. In other words, if it's not a runway/road already, it's not useful.

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Thu Oct 23, 2014 7:29 pm UTC

Tyndmyr wrote:Not enough time to adequately crunch that whole thing, but usable ground effect is related to wingspan and of course, altitude. This is why the old soviet ground effect birds were so frigging huge...let them utilize ground effect at a useful altitude. Obviously, with a small craft such as this, being in ground effect means being within a few feet of the ground. In other words, if it's not a runway/road already, it's not useful.

That's the idea. If you have a sufficiently flat area or a long desert roadway, you can hover or fly in ground effect on a single engine. If not, you fire up both engines for fully-powered sustained hover or forward flight.

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Re: Flying car (engineering heavy)

Postby Tyndmyr » Thu Oct 23, 2014 7:35 pm UTC

Why not just drive, then?

I mean, flying car is hard enough. Why complicate it by going for flying car/ground effect vehicle?

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Re: Flying car (engineering heavy)

Postby Qaanol » Thu Oct 23, 2014 7:54 pm UTC

What if we tackle the VTOL from another direction entirely?

Instead of trying to make a system capable of sustained thrust outputs sufficient to lift off the ground, let's make the launch system use temporarily-stored resources.

For example, instead of fans that have to blow with the full weight-force of the vehicle, an alternative would be compressed-air tanks that shoot jets of air downward until they are empty. It might take several minutes of running a compressor to build up enough pressure for launch, and then the launch only lasts a few seconds. Once you're aloft and moving forward then some sort of wing, perhaps an autogyro-style propeller, provides lift as an airfoil. The compressor can slowly refill the tanks as needed.
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Re: Flying car (engineering heavy)

Postby stoppedcaring » Thu Oct 23, 2014 8:06 pm UTC

Tyndmyr wrote:Why not just drive, then?

I mean, flying car is hard enough. Why complicate it by going for flying car/ground effect vehicle?

Oh, sorry; I was talking about a possible A-10 and Apache/Cobra replacement: "There's some neat stuff we could see in the near future. A next-generation military ground-support aircraft would ideally be able to hover with the same fuel consumption as it has in sustained cruising flight. What would be super cool to see? A working jet-powered version of The Dark Knight Rises's Bat/Batwing/Batcopter."

So this is separate from the flying car concept; this is a suggested military ground-support aircraft using a fan-in-wing design powered by double turbofan engines for forward aerodynamic flight, using a body similar to the Bat from The Dark Knight Rises but with broader downward-angled wings. I was just pointing out that such a design would enable extended ground-effect operation, giving it a much larger range and versatility than it would otherwise have.

Qaanol wrote:What if we tackle the VTOL from another direction entirely?

Instead of trying to make a system capable of sustained thrust outputs sufficient to lift off the ground, let's make the launch system use temporarily-stored resources.

For example, instead of fans that have to blow with the full weight-force of the vehicle, an alternative would be compressed-air tanks that shoot jets of air downward until they are empty. It might take several minutes of running a compressor to build up enough pressure for launch, and then the launch only lasts a few seconds. Once you're aloft and moving forward then some sort of wing, perhaps an autogyro-style propeller, provides lift as an airfoil. The compressor can slowly refill the tanks as needed.

Well, what would be used for forward propulsion in flight? Some kind of ducted fan, I would assume. In which case, why not allow it to be angled downward? In which case you use it for lift anyway.

I suppose a combination is possible -- limited compressed air to give the extra kick to help the fan get you into the sky. Kind of a complex and low-safety-factor launch system, unfortunately. It also puts a greater load on the airfoil; the angled fans of my original design help to provide downward thrust even during forward flight, allowing a shorter wing length. It's not too hard to calculate the thrust requirements but I'm thinking the added weight of the compression tank might be prohibitive....

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Re: Flying car (engineering heavy)

Postby stoppedcaring » Thu Oct 23, 2014 10:19 pm UTC

By the way, this is basically what I had in mind with the military ground support aircraft:

Image

As you can see, the turbofan bypass can be open to produce jet thrust, or it can be closed off with "shutters" to direct bleed air into a pneumatic system that runs the lift fans and any other auxiliary power stuff. The louvers on the top of the lift fans run front to back to reduce drag during transitional flight but the ones on the bottom run horizontally to allow for thrust vectoring.

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Re: Flying car (engineering heavy)

Postby Zamfir » Fri Oct 24, 2014 2:31 pm UTC

A wing is also a structural element... It's a beam, from which the aircraft hangs. It's also a torsion-resistant closed box that keeps the wing from twisting.

If you cut a big hole in that, you need some other way to fulfill those functions. Those other ways will be inefficient and heavy.

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Re: Flying cars and other craft (engineering heavy)

Postby Tyndmyr » Fri Oct 24, 2014 4:54 pm UTC

I think they already had that concept on Avengers.

But yes, wings are structure/support/hardpoints....this ends up looking like a very inefficient replacement for a heli. If you're going to have rotary blades for lift and stubby wings...we already have helicopters that do that without introducing those significant concerns. Additionally, since the heli can sweep out much more area, it's generally going to produce much more lift.

stoppedcaring
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Re: Flying cars and other craft (engineering heavy)

Postby stoppedcaring » Fri Oct 24, 2014 6:19 pm UTC

Zamfir wrote:A wing is also a structural element... It's a beam, from which the aircraft hangs. It's also a torsion-resistant closed box that keeps the wing from twisting.

If you cut a big hole in that, you need some other way to fulfill those functions. Those other ways will be inefficient and heavy.

Huh.

I have to say, of all the engineering challenges involved with something like this, I hadn't thought this would be a particularly significant one. Surely there would be some sort of grating over the fan opening that, in combination with titanium struts, would do just fine for weight-bearing purposes.

Tyndmyr wrote:This ends up looking like a very inefficient replacement for a heli. If you're going to have rotary blades for lift and stubby wings...we already have helicopters that do that without introducing those significant concerns. Additionally, since the heli can sweep out much more area, it's generally going to produce much more lift.

But with the previously-noted vulnerabilities and complexities due to exposed rotors, large footprint, limited range, and low forward speed. Compared to an Apache, an A-10 Thunderbolt II has nearly three times the range, more than twice the rate of climb, twice the service ceiling, and 2.4 times the closing speed.

Military helicopters are already running on jet engines; they're just turboshaft ones. Why not come up with a way to reduce total weight and use combination turboshaft/turbofan engines for something a lot faster and more fuel-efficient?

Apaches are currently powered by a GE T700-class turboshaft engine; these are expected to be replaced in a few years with the GE3000. After a little digging, I came up with the following probable specifications for a combination turboshaft/turbofan engine of this size:

Mass: 200 kg
Max shaft power: 2237 kW
Max thrust: 11 kN
Fuel consumption @ 100%: 0.112 kg/sec (EDIT: corrected error; originally quoted as 0.284 kg/sec)


As far as performance is concerned...it needs to be able to compete with the takeoff acceleration of an Apache, which is roughly 1.61 gees (0.61 gees more than hover). A target performance of 1.75 gees at 95% engine capacity is ideal. At the same time, it needs to have forward acceleration comparable to the Warthog. The Warthog has a typical aircraft thrust-to-weight ratio of 0.40; if we aim for 0.50, our loaded weight (not max takeoff weight) will be 4.48 tonnes using our maximum thrust and that P/W ratio.

Lifting 4.48 tonnes with an acceleration of 1.75 gees requires 38.4 kN of static thrust per lift fan. Assuming 90% fan efficiency means effective required thrust is 42.7 kN. By the static thrust equation with each engine running a single fan at 95% capacity (2125 kw), each fan will need a disc lifting area of 6.63 m3, a diameter of 2.9 meters. Accounting for the downward-tilting wings, this gives us a wingspan of roughly 8.5 meters or 28 feet:

Image

Pretty much an exact fit to the dimensions I had in mind (though my original model didn't really factor in the angle on the lift fans).

More specs wouldn't be hard to come up with...
Last edited by stoppedcaring on Mon Oct 27, 2014 7:24 pm UTC, edited 1 time in total.

Tyndmyr
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Re: Flying cars and other craft (engineering heavy)

Postby Tyndmyr » Fri Oct 24, 2014 7:44 pm UTC

stoppedcaring wrote:
Zamfir wrote:A wing is also a structural element... It's a beam, from which the aircraft hangs. It's also a torsion-resistant closed box that keeps the wing from twisting.

If you cut a big hole in that, you need some other way to fulfill those functions. Those other ways will be inefficient and heavy.

Huh.

I have to say, of all the engineering challenges involved with something like this, I hadn't thought this would be a particularly significant one. Surely there would be some sort of grating over the fan opening that, in combination with titanium struts, would do just fine for weight-bearing purposes.


Gratings are not known for being particularly strong structural elements. In addition, gratings over your fans are going to reduce efficiency, and you'll be subjecting them to significant load while the fans are in operation. Titanium and other weight saving structural elements are already in frequent use in airframes. More exotic alloys are going to kill you in price, too.

Tyndmyr wrote:This ends up looking like a very inefficient replacement for a heli. If you're going to have rotary blades for lift and stubby wings...we already have helicopters that do that without introducing those significant concerns. Additionally, since the heli can sweep out much more area, it's generally going to produce much more lift.

But with the previously-noted vulnerabilities and complexities due to exposed rotors, large footprint, limited range, and low forward speed. Compared to an Apache, an A-10 Thunderbolt II has nearly three times the range, more than twice the rate of climb, twice the service ceiling, and 2.4 times the closing speed.


Certainly. However, your design is more of a crippled counterpart to an Apache than it is anything like a Thunderbolt. What you are trying to do, an Apache does better.

Think of it this way, if you want VTOL, you need to shove air downward to create lift. The larger the area swept out, the less force you need to put on that air to create a given amount of lift. Thus, if you're sweeping out, oh, a quarter as much area as the Apache, to match it's performance, your fans have to be spinning four times as fast. This is precisely why the Osprey and similar rely on jet engines pointed downward. You want that much thrust from a small area, speed is the way to do it.

Using wings, such as the Thunderbolt does, is a very efficient way to get lift, but that's not a way that solves VTOL issues, and thus, makes a very poor example of what you're trying to do here.

Military helicopters are already running on jet engines; they're just turboshaft ones. Why not come up with a way to reduce total weight and use combination turboshaft/turbofan engines for something a lot faster and more fuel-efficient?


If it was as trivial as "come up with a way to make it lighter, faster AND more fuel efficient", odds are very good that someone who has far more knowledge in the topic than either of us has probably explored it. If, as an outsider to an industry, you think there is an overtly obvious way to make everything way better, your first assumption should probably be "I'm missing something, and need to research further", not "God, they're all idiots but me" or similar.

I'm not an engine expert, but I do know that helicopters tend to be limited in top speed by blade rotation speed. In short, if you're moving forward fast enough that your retreating blade ceases providing significant lift, your chopper stalls. flips over and then you probably die. So, that's likely one tradeoff being considered in helicopter designs. No doubt there are many more I am unaware of.

stoppedcaring
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Re: Flying cars and other craft (engineering heavy)

Postby stoppedcaring » Fri Oct 24, 2014 10:15 pm UTC

Tyndmyr wrote:Gratings are not known for being particularly strong structural elements. In addition, gratings over your fans are going to reduce efficiency, and you'll be subjecting them to significant load while the fans are in operation.

Well, nothing is saying that you can't have narrow structural elements over the fans. And again, the shorter, stubbier wings help out here. The longer wings of most fixed-wing aircraft have much greater beam loading from torque, but these wings are nearly as deep as they are long and thus won't be as problematic.

Think of it this way, if you want VTOL, you need to shove air downward to create lift. The larger the area swept out, the less force you need to put on that air to create a given amount of lift. Thus, if you're sweeping out, oh, a quarter as much area as the Apache, to match it's performance, your fans have to be spinning four times as fast. This is precisely why the Osprey and similar rely on jet engines pointed downward. You want that much thrust from a small area, speed is the way to do it.

Actually, the Osprey is a pure turboshaft engine just like a typical helicopter engine. It happens to have its very-hot-exhaust pointed at the ground during takeoff, but the exhaust does not contribute at all to lift. Turbojet/turbofan engines actually produce less static thrust for their size and fuel consumption than a prop or rotor; they are used simply because they are able to operate at higher airspeeds and are more fuel-efficient at those speeds.

The relationship between area and power consumption for a given amount of thrust is actually a little different; it's actually inverse-squared; if you cut your area by a factor of four, you only have to double your power consumption. So moving from a single 15-meter prop to a pair of 3-meter props is actually not nearly as bad as you might think.

Of course, the key here is not just higher power, but weight reduction. For comparison, if a plane is an SUV and a helicopter is an ATV, this craft would be comparable to a dirtbike.

The weight reduction over (or would it be under?) a helicopter is achieved in a few ways. First, a helicopter requires either a tail rotor (which wastes energy and requires a much larger footprint with associated weight cost) or a pair of contra-rotating rotors (which also waste energy and require greater gearing complexity and weight). Additionally, it's a single-pilot craft (again, think dirtbike over ATV) so it has a reduced cockpit cross-section and weight requirements. Finally, its armament would be more projectile-based than ordnance-based, further reducing weight.

A few additional specs so you can get a better idea of the range of abilities we're talking about...

As I said before, the holy grail for a VTOL ground-support craft is to have equal fuel consumption in sustained hover as it has in full-forward cruise flight; this allows the engines to be tuned to a single maximum efficiency and dictates the same loiter/persistence regardless of combat mode. Using the same parameters as before, sustained hover (1 gee) would require each engine to operate at 784 kW, or just 35%; the engines would probably be set to peak efficiency at 45% to allow for maneuvering. At this level, fuel consumption is 0.26 kg/sec.

This also means that the craft can hover or cruise on a single engine running at 90%, albeit without the higher combat performance abilities.

Cruise speed is difficult to estimate, but we can use the A-10 as a ballpark comparison. The A-10's maximum level speed is 196 m/s at full engine thrust of 80.64 kN. Its frontal area is roughly 20 m2, allowing us to estimate its standard drag coefficient at 0.161 (note, this is different from the planform drag coefficient typically quoted for an aircraft). Assuming that our craft has a stubbier, deeper frontal area, we can project approximately 10 m2. Using the same equation in reverse and assuming a slightly worse drag coefficient of 0.17, our craft's maximum speed will be 141 m/s, about 315 mph. Almost as fast as the cruising speed of the A-10 (and remember that its horizontal acceleration is a full 25% greater than the Warthog). Cruising speed would be 94 m/s or 210 mph, 15% greater than the maximum speed of an Apache.

I don't know what flaps or extensions would be required to optimize ground effect, but in theory they could permit ground-effect cruise and ground-effect hover on a single engine at 45% for longer persistence.

This is using the same engine type as an Apache but at slightly lower average power consumption, meaning combat persistence will probably be 1.5 to 2x longer than a helicopter. This, plus the dramatically greater speed, gives it a range that far outstrips the Apache.

Your design is more of a crippled counterpart to an Apache than it is anything like a Thunderbolt. What you are trying to do, an Apache does better.

The only thing an Apache does better than this is carry more ordnance. Which this craft is entirely capable of, if it sacrifices a bit of vertical acceleration. But ordnance is going to be laser-guided anyway, so you can afford to sacrifice vertical takeoff speed if that's your mission because you'll just be flying over to launch. Maximum takeoff weight under full engine load is actually nine tonnes, enabling it to carry just as much ordnance as an Apache in a tank-killer configuration.

If it was as trivial as "come up with a way to make it lighter, faster AND more fuel efficient", odds are very good that someone who has far more knowledge in the topic than either of us has probably explored it. If, as an outsider to an industry, you think there is an overtly obvious way to make everything way better, your first assumption should probably be "I'm missing something, and need to research further", not "God, they're all idiots but me" or similar.

Well, it's not an overtly obvious way. It's actually a pretty complex, specific way of doing it with some pretty narrow specifications. And it's been done before (see the XV-5 Vertifan), just not in a ground-support role.

Anyway, it's just an idea.

stoppedcaring
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Re: Flying cars and other craft (engineering heavy)

Postby stoppedcaring » Tue Oct 28, 2014 6:57 pm UTC

Did a little more digging to come up with additional specifications for this concept air support craft.

The efficiency of the rotors on an Apache helicopter is actually a lot less than I had originally estimated, due to the inner-blade losses as well as the useless tail rotor. The liftoff thrust of an Apache is about 1.4 gees, meaning our craft would have twice the vertical acceleration of an Apache. This gives us a larger margin for adding ordnance while still maintaining next-generation superior speed and maneuverability. The drag equation for vertical lift implies a peak vertical climb rate nearly four times that of the Apache.

Designing the craft to take advantage of ground effect permits aerodynamic takeoff on a normal runway; even loaded down with more weight than the craft could ordinarily carry, it could still sustain a ground-effect hover and use a runway or flat open area to accelerate to aerodynamic takeoff speed. What we've done is take massively overpowered turbofan engines with respectably-sized lift fans and connect them in the smallest possible footprint/airframe. The limiting factor for payload will be space, not weight; the theoretical maximum payload weight using ground effect aerodynamic takeoff is equivalent to the A-10 Warthog.

The West has long cornered air superiority with in the fighter and heavy/stealth bomber arena, but modern warfare -- particularly urban and asymmetric warfare -- continues to challenge in the areas of close air support and counter-insurgency. These roles are currently filled almost exclusively by rotorcraft (primarily Apache and Apache Longbow) and the A-10, which were designed nearly half a century ago. The Air Force and Army have been actively looking for new aircraft with low loitering speed, long endurance, and VTOL/STOL capacity to fill these roles in a way the current fleet cannot. However, attempts at developing new craft around current platforms have stagnated.

Re-evaluating our craft's cruise and maximum speed using a planform area comparison rather than frontal area comparison generates slightly higher values, but still within a few percent of the original projection. So that speaks to the accuracy of the inital estimates and overall design.

Fuel consumption is actually lower than I had initially estimated due to a math error. Sustained hover or cruise is a fuel consumption of 0.101 kg/sec and ground-effect hover or cruise is 0.05 kg/sec. These values prompted a weight breakdown to come up with fuel and ordnance capacity.

Once you subtract the weight of engines, armor, and armament, the airframes of the Apache and the A-10 both come to almost exactly 45% of the typical loaded weight, so I used 40% to account for the more compact design of our craft. Extrapolating conservatively for all other parameters, I derived the following estimates:

Mass Estimates
  • Frame, rotors, and cockpit: 1792 kg
  • Engines: 400 kg
  • Integrated armor: 240 kg
  • Onboard guns (two 20mm rotary cannons; one minigun): 150 kg
  • Onboard fuel tanks: 1100 kg
  • Onboard ammunition (2,500 rounds): 500 kg

This leaves 200 kg for ordnance in a light attack or counter-insurgency configuration; reducing onboard ammunition can increase this up to 700 kg. However, maximum takeoff weight is nine tonnes, so the maximum theoretical payload (extra ammo, fuel, and/or ordnance over typical configuration) is 5220 kg. Compare to the Apache's maximum armament/extra fuel of 3668 kg and the A-10's hardpoint capacity of 7260 kg. Again, the limiting factor for our craft will be hardpoint space, not weight.

At cruise or sustained hover in the typical configuration, the onboard fuel tanks permit 3.5 hours of persistence, an hour longer than an Apache.

The capacity for ground effect maneuvers also allows a "gliding tank" configuration optimized for counter-insurgency support, with a much greater armament capacity and the same endurance. Other combat configurations would include typical close-air support and a ground-attack configuration with maximum takeoff payload and short-duration bombing/attack missions.

Here's the full specification listing for comparison to other craft.

General characteristics
  • Crew: 1
  • Length: 5.7 m
  • Wingspan: 8.5 m
  • Height: 3.5 m
  • Wing area: 20.2 m2
  • Disc area: 13.26 m2
  • Powerplant: 2 x GE3000 combination turbofan-pneumatic turboshafts, 2237 kW/11 kN each, 0.112 kg/sec fuel consumption at 100%.
  • Internal fuel capacity: 1100 kg
  • Empty weight: 2,582 kg

Performance
  • Typical loaded weight: 4,480 kg
  • Gliding-tank weight: 7,560 kg
  • Maximum takeoff weight: 7,840 kg
  • Cruising speed: 100 m/s (224 mph)
  • Maximum speed: 141 m/s (315 mph)
  • Stall speed: 0 m/s (lift fans allow complete maneuvering control, stall recovery)
  • Minimum airspeed for 100% aerodynamic lift: 41 m/s (91 mph)
  • Rate of vertical climb (fans): 55 m/s
  • Wing loading: 221.8 kg/m2
  • Disc loading: 337.8 kg/m2
  • Thrust/weight: 0.5
  • Power/mass: 1.00 kW/kg
  • Ferry range (cruise): 2400 km
  • Ferry range (ground effect): 4800 km
  • Nominal combat range: 1260 km
  • Combat radius (gliding tank, 1 hour combat): 538 km
  • Combat radius (close air support, 30 min combat): 570 km
  • Combat radius (ground attack, maximum takeoff weight, 15 min combat): 319 km
  • Maximum loiter (airborne): 3.5 hours
  • Maximum loiter (ground effect): 7 hours

I think that's about the whole of it. For reference, here's complete Apache specifications and A-10 specifications. Our craft outperforms the Apache in almost every category and is comparable to the A-10 in most categories.

The craft does have extraordinarily high disc loading compared to typical exposed-blade rotorcraft (2.6x greater than the Osprey, which is already very high), but that is to be expected for a ducted fan configuration; the previously-mentioned Ryan XV-5 Vertifan had an ungodly disc loading of 1213 kg/m2. High disc loading means autorotation won't be useful, but the low wing loading compared to the A-10 would enable safe landings after a power loss. The A-10 of course has faster absolute speed and a greater absolute payload, but our craft's VTOL capacity, greater acceleration and maneuverability, and greater loiter time make it the more effective craft for most mission profiles.

Integrated armor would ideally be modular and embedded in the airframe, allowing components and panels to be "swapped out" to increase protection during heavy-fire engagements as well as quickly repair damage in the field. This wasn't feasible when the Apache and A-10 were designed 40+ years ago, but it's realizeable now in a ground-up design.

Tyndmyr
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Re: Flying cars and jet-powered batwing

Postby Tyndmyr » Tue Oct 28, 2014 9:01 pm UTC

You are comparing back of the envelope numbers to numbers based on field tests and with margins of safety, etc based on extensive testing included. This is not anything like a fair comparison.

stoppedcaring
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Re: Flying cars and jet-powered batwing

Postby stoppedcaring » Tue Oct 28, 2014 9:25 pm UTC

Tyndmyr wrote:You are comparing back of the envelope numbers to numbers based on field tests and with margins of safety, etc based on extensive testing included. This is not anything like a fair comparison.

Most of the numbers I'm using to derive performance specifications are actual aircraft numbers. Things like drag, cruising speed, takeoff speed, airframe weight, fuel consumption, armor weight, and so forth...all taken, with added conservatism, from actual specifications which already have those safety margins built in. Are there particular figures you think are unrealistic?

Tyndmyr
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Re: Flying cars and jet-powered batwing

Postby Tyndmyr » Tue Oct 28, 2014 10:32 pm UTC

stoppedcaring wrote:
Tyndmyr wrote:You are comparing back of the envelope numbers to numbers based on field tests and with margins of safety, etc based on extensive testing included. This is not anything like a fair comparison.

Most of the numbers I'm using to derive performance specifications are actual aircraft numbers. Things like drag, cruising speed, takeoff speed, airframe weight, fuel consumption, armor weight, and so forth...all taken, with added conservatism, from actual specifications which already have those safety margins built in. Are there particular figures you think are unrealistic?


Sure. The following is a TERRIBLE way to estimate carrying capacity.

stoppedcaring wrote:Once you subtract the weight of engines, armor, and armament, the airframes of the Apache and the A-10 both come to almost exactly 45% of the typical loaded weight, so I used 40% to account for the more compact design of our craft.


Carrying capacity is not really a flat percentage across all designs, nor is it strongly related to compactness. In fact, longer, less compact wings, are usually far more efficient, which is why cargo aircraft tend to have them. Sure, a heavier load tends to have tougher frame requirements, but this is kind of a wild oversimplification.

Additionally, armor has been added and removed to aircraft after construction for far more than 40 years. Damaged armor plate is usually not the biggest issue for field repair. The plate of armor is usually the bit you LEAST need to worry about being hit.


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