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:
- The form and function of an ordinary car in driving mode
- A robust VTOL mode
- An energy-efficient cruising flight mode
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?
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?