Why do airplanes need wings?

[SU3SU2U1]

No one integrates momentum flow, because it's basically computationally impossible, even for low Reynolds number flow.

Thats certainly not true (the part about computational impossibility). While I don't have much experience with fluid flow in planes, I have lots of experience with numerical fluid flow in GR (I like to say I spent my masters dropping stars into black holes). We integrate momentum flows across boundaries all the time to verify conservation laws and validate simulations. If you have the flow, its not computationally complex at all.

Not at all. But you can't compute it with a simple momentum-based "air goes down" argument, because you cease the ability to track the momentum of a fluid element once it leaves your cube.

Why would you ever need to track the fluid element once it leaves your cube? I think you seem to have some sort of idea that you need to keep track of things away from the plane, which is wrong. The fluid is only capable of exchanging momentum with the plane when it is near the plane.

Lets back off from order of magnitude for a second, and just consider the following- if we draw a large cube around the plane, integrating just the pressure won't be enough to give us the forces on the plane? We also have to take into account fluid flow at the boundaries?

[gorcee]

No one integrates momentum flow, because it's basically computationally impossible, even for low Reynolds number flow.

Thats certainly not true (the part about computational impossibility). While I don't have much experience with fluid flow in planes, I have lots of experience with numerical fluid flow in GR (I like to say I spent my masters dropping stars into black holes). We integrate momentum flows across boundaries all the time to verify conservation laws and validate simulations. If you have the flow, its not computationally complex at all.

Not at all. But you can't compute it with a simple momentum-based "air goes down" argument, because you cease the ability to track the momentum of a fluid element once it leaves your cube.

Why would you ever need to track the fluid element once it leaves your cube? I think you seem to have some sort of idea that you need to keep track of things away from the plane, which is wrong. The fluid is only capable of exchanging momentum with the plane when it is near the plane.

Lets back off from order of magnitude for a second, and just consider the following- if we draw a large cube around the plane, integrating just the pressure won't be enough to give us the forces on the plane? We also have to take into account fluid flow at the boundaries?

1.) I had meant "computationally impossible" strictly with regards to actual aerospace applications. It's actually not even computationally impossible then, but if you're doing momentum calculations, you're also not directly computing lift. Most large-scale, numerical flow models use Reynolds-Averaged Numerical Simulation (RANS) or Large Eddy Simulation (LES), which, I believe, are not momentum conservative at all (I'd have to double check on the derivation, which I have not looked at in quite some time, but you get the idea). In fact, I am quite familiar with a lot of aeroelastic code, and you'd be surprised at how many times you'll see things like "k = 0.45; // set to 0.45 just because it works". In other words, actual FEM codes for flight mechanics have a crapton of "magic" happening underneath.

2.) Regarding the fluid element argument: Imagine you're a fluid element in stationary air, minding you own business. *WHOOOOOOOOSH* an airplane goes past you and imparts momentum onto you. If we're looking at balancing the momentum perfectly, then we need to account for all of your motion well after the airplane goes past, because you're going to be caught in vortices, etc. In practice, we don't have to do this from t=0, of course. But it is well known that if you place an object downstream from a flight vehicle that obstructs the flow in some way, you change the upstream flow over the vehicle. Or, in other words, the pressure field travels upstream. So you really need to account for the fluid element's motion for quite some distance past the airfoil, typically. Trying to compute the momentum balance would be heinously difficulty even if it wasn't practically impossible due to turbulence.

3.) You need velocity at the boundaries, from which you can obtain pressure without loss of generality. So basically yes, you can't integrate the pressure field without considering the velocity at the boundaries. Or, in other words, there is no pressure field without velocity at the boundaries.

[gorcee]

The larger the mass flow rate you are accelerating downwards, the less power you need to stay in the air.

That's really actually the opposite of what is true.

How so? I think we have all agreed that heavier than air flight ultimately requires the acceleration of air downwards for the plane to stay up. Anyways, the weight is fixed so the net force is fixed. This means on an ongoing basis, accelerating a small mass a lot or a larger mass a little. The minimum energy required increases with the velocity change. For an enormous mass flow to push against it would require miniscule power to stay aloft even before considering the added fluid drag of that velocity change.

Supersonic jets only need tiny stubby wings because they already get an enormous air flow from their velocity. Almost none of their thrust is going to lift induced drag compared to slower aircraft. In other words, the difference in drag between a supersonic jet flying with no lift (some sort of parabolic trajectory), and one with the flaps set for enough lift to keep it in the air indefinitely, will be much less than the equivalent difference in drag for a slower plane of the same mass. Of course the total drag is much higher for the faster plane, but not the lift induced drag.

There are only (basically) two sources of drag: Induced drag and parasite drag. The induced drag is typically a small component of the overall drag, and is a much smaller component on a supersonic jet than a non-supersonic jet.

Even if an airplane is "falling", it's still producing enormous amounts of lift induced drag. Lift-induced drag has nothing to do with drag coming as a result of moving the airplane in the opposite direction of gravity. It has everything to do with drag acting in the opposite direction of the oncoming flow. So, for a supersonic jet, you are practically 100% wrong with this statement: "Almost none of their thrust is going to lift induced drag compared to slower aircraft". For a supersonic jet, the majority of the thrust is countering induced drag, typically by a factor of 3:1 or more.

For instance, the F-104 had a C_D of 0.048, with a zero lift C_d0 of 0.017, which is an estimate of parasite drag. Thus, over half of the thrust is countering induced drag for fixed angle of attack, regardless of attitude or trajectory. And this was a plane designed in the 50s, before advanced flow-shaping technology and manufacturing processes could lower the parasite drag even more.

[SU3SU2U1]

Most large-scale, numerical flow models use Reynolds-Averaged Numerical Simulation (RANS) or Large Eddy Simulation (LES), which, I believe, are not momentum conservative at all

I think with RANS you mean Reynolds-Averaged Navier-Stokes, which as the name implies is a navier-stokes methods. If your process is working correctly, you need to conserve the reynolds average momentum of fluid elements (or else Navier-Stokes is broken, and you'll get run-away) in the absence of pressure gradients. The users of the code might not care, but the authors must have spent time verifying momentum conservation as part of their validation. I don't know anything about LES.

Any method based on Navier-Stokes will conserve fluid-momentum away from obstacles/boundaries (at boundaries, it depends on the boundary conditions- momentum can be exchanged) thats how you derive Navier-Stokes.

Imagine you're a fluid element in stationary air, minding you own business. *WHOOOOOOOOSH* an airplane goes past you and imparts momentum onto you. If we're looking at balancing the momentum perfectly, then we need to account for all of your motion well after the airplane goes past, because you're going to be caught in vortices, etc.

We aren't looking at the total momentum in the system. The whole point is that momentum of the fluid won't, in general, be conserved because of the plane (plane pushes on air/air pushes on plane). So if we look near the plane, it acts as a source of momentum in the fluid. The magnitude of the momentum being imparted to the fluid can be calculated locally, and its equal and opposite to the force on the plane.

A fluid element can only impart momentum to the plane (and visa versa) if it comes in contact with the plane. After that, all of the flow is fluid elements exchanging momentum with other fluid elements. This means we only have to care what the fluid is doing locally near the air craft.

Similarly with a rocket- what happens after the exhaust leaves the rocket doesn't matter at all. It could fly through the void of space, or someone could walk behind the rocket and catch all the exhaust with a bucket.

[Danny Uncanny7]

There are only (basically) two sources of drag: Induced drag and parasite drag. The induced drag is typically a small component of the overall drag, and is a much smaller component on a supersonic jet than a non-supersonic jet.

Even if an airplane is "falling", it's still producing enormous amounts of lift induced drag. Lift-induced drag has nothing to do with drag coming as a result of moving the airplane in the opposite direction of gravity. It has everything to do with drag acting in the opposite direction of the oncoming flow. So, for a supersonic jet, you are practically 100% wrong with this statement: "Almost none of their thrust is going to lift induced drag compared to slower aircraft". For a supersonic jet, the majority of the thrust is countering induced drag, typically by a factor of 3:1 or more.

For instance, the F-104 had a C_D of 0.048, with a zero lift C_d0 of 0.017, which is an estimate of parasite drag. Thus, over half of the thrust is countering induced drag for fixed angle of attack, regardless of attitude or trajectory. And this was a plane designed in the 50s, before advanced flow-shaping technology and manufacturing processes could lower the parasite drag even more.

I stand corrected on supersonic planes. But I still stand by the fact that if you are accelerating air to achieve lift, the greater the mass of air you accelerate, the less power you need.

Given a denser than air object that that can by some mechanism impart momentum to the air at its surface with 100% efficiency, what is the minimum power necessary to keep it aloft? Does it have an ideal shape?

[gorcee]

I stand corrected on supersonic planes. But I still stand by the fact that if you are accelerating air to achieve lift, the greater the mass of air you accelerate, the less power you need.

That's totally counterintuitive. That's like saying, "the heavier the wheelbarrow you have, the less force you need to push it."

[gorcee]

Most large-scale, numerical flow models use Reynolds-Averaged Numerical Simulation (RANS) or Large Eddy Simulation (LES), which, I believe, are not momentum conservative at all

I think with RANS you mean Reynolds-Averaged Navier-Stokes, which as the name implies is a navier-stokes methods. If your process is working correctly, you need to conserve the reynolds average momentum of fluid elements (or else Navier-Stokes is broken, and you'll get run-away) in the absence of pressure gradients. The users of the code might not care, but the authors must have spent time verifying momentum conservation as part of their validation. I don't know anything about LES.

Any method based on Navier-Stokes will conserve fluid-momentum away from obstacles/boundaries (at boundaries, it depends on the boundary conditions- momentum can be exchanged) thats how you derive Navier-Stokes.

Imagine you're a fluid element in stationary air, minding you own business. *WHOOOOOOOOSH* an airplane goes past you and imparts momentum onto you. If we're looking at balancing the momentum perfectly, then we need to account for all of your motion well after the airplane goes past, because you're going to be caught in vortices, etc.

We aren't looking at the total momentum in the system. The whole point is that momentum of the fluid won't, in general, be conserved because of the plane (plane pushes on air/air pushes on plane). So if we look near the plane, it acts as a source of momentum in the fluid. The magnitude of the momentum being imparted to the fluid can be calculated locally, and its equal and opposite to the force on the plane.

A fluid element can only impart momentum to the plane (and visa versa) if it comes in contact with the plane. After that, all of the flow is fluid elements exchanging momentum with other fluid elements. This means we only have to care what the fluid is doing locally near the air craft.

Similarly with a rocket- what happens after the exhaust leaves the rocket doesn't matter at all. It could fly through the void of space, or someone could walk behind the rocket and catch all the exhaust with a bucket.

Durr, yes, I meant Reynolds Averaged Navier Stokes. I just kept mis-typing average, and then I got distracted by thinking about something else along the way!

The difference with your rocket example, however, is all of that rocket exhaust was at some point inside the rocket. With an airfoil, the vertical momentum of the downwash is not the same as the vertical momentum of the aircraft!

Su, that's surely wrong? If you draw a small box, you need both pressures and flows on the boundaries of the box. As you take the boundary closer and closer, the pressure terms become dominant, until a box fitted to the shape of the aircraft has only pressure terms and no fluid momentum exchange on the boundary.

That's different from a rocket, where the exhaust overpressure is often a small contribution to net the thrust. If you draw a small box around a rocket and only look ar momentum exchange, you often get a reasonable approximation of what's going on.

Also, in a rocket, the fuel is stored onboard. So you can compute it using momentum considerations because the momentum is internal to the body. This is not the case in an airplane, where the fluid momentum is external to the body.

Just to be sure I get your main point: your main objection to momentum explanations is that while you can calculate lift if you know momentum exchange and pressures on a boundary, it doesn't actually give you anything to figure out those values, right? Unlike a rocket, where you can say "it's tossing out X kg/s at m/s, therefore it produces roughly Z force". And no matter whether you draw a boundary close or far from the rocket, you will indeed find roughly that same momentum flow if you integrate over the boundary.

If that's roughly your point, I can only agree. Downward-deflected air is part of the picture that you need in your head to understand lift, but it's not enough.

Yes, this has been my point all along,

[SU3SU2U1]

The difference with your rocket example, however, is all of that rocket exhaust was at some point inside the rocket. With an airfoil, the vertical momentum of the downwash is not the same as the vertical momentum of the aircraft!

It doesn't matter whether the exhaust was inside the rocket. Contact forces are contact forces. The change momentum of an exhaust element leaving the rocket = change in momentum of rocket. Similarly, change in momentum of the air elements contacting the wings = change in momentum of the plane.

[gorcee]

The difference with your rocket example, however, is all of that rocket exhaust was at some point inside the rocket. With an airfoil, the vertical momentum of the downwash is not the same as the vertical momentum of the aircraft!

It doesn't matter whether the exhaust was inside the rocket. Contact forces are contact forces. The change momentum of an exhaust element leaving the rocket = change in momentum of rocket. Similarly, change in momentum of the air elements contacting the wings = change in momentum of the plane.

Surely, however, there are rocket fuel elements that never actually contact the rocket. Momentum is conserved even absent contact. That's why fluid momentum is different that just computing the momentum at the boundary layer over a surface. Yes, it's because fluid elements "contact" each other, but when we're talking about fluid elements contacting each other, we're not really talking about momentum anymore. We're talking about pressure.

The point still stands that fluid momentum of the downwash is a necessary, but not sufficient, component for computing lift. (Actually, it is more or less sufficient, but not in a straightforward, intuitive manner, which is my point).

[Danny Uncanny7]

I stand corrected on supersonic planes. But I still stand by the fact that if you are accelerating air to achieve lift, the greater the mass of air you accelerate, the less power you need.

That's totally counterintuitive. That's like saying, "the heavier the wheelbarrow you have, the less force you need to push it."

No the force is constant. Given that you push a wheelbarrow with force F, the heavier it is, the less power you will need to keep that force applied as the wheelbarrow accelerates away.

[gorcee]

I stand corrected on supersonic planes. But I still stand by the fact that if you are accelerating air to achieve lift, the greater the mass of air you accelerate, the less power you need.

That's totally counterintuitive. That's like saying, "the heavier the wheelbarrow you have, the less force you need to push it."

No the force is constant. Given that you push a wheelbarrow with force F, the heavier it is, the less power you will need to keep that force applied as the wheelbarrow accelerates away.

That's also not really true.

I think I see what your'e trying to say, however.

You're basically saying, "Let's say I have a plane, and it requires say a 100 HP engine to achieve lift. Now, I double the wings, so I generate more lift! I no longer need a 100HP engine." Am I right in thinking this? If so, that's not true.

The equations for lift, L, and drag, D, are as follows:

L = \frac{1}{2}\rho U^2SC_L

D = \frac{1}{2}\rho U^2SC_D

Where U is the freestream velocity, rho is the air density, S is the planform (as viewed from top/bottom) area of the wing, and C_L is the lift coefficient.

If I double the size of the wing, and therefore double the amount of air that gets moved, then I also double the drag.

If I double my velocity, and in doing so double the mass flux over a spanwise segment (equivalent to doubling the mass flow rate over a wing), then I quadruple my lift. However, I also quadruple my drag.

In general, the downwash of an airfoil, w, is defined as w = U \tan \alpha_i, where alpha_i is the induced angle of attack. This is related to the lift by L=\rho U\cos \alpha_i, or, in other words, L = \rho U \Gamma \tan^{-1} \frac{w}{U}, where Gamma is the circulation. Likewise, D = \rho U \Gamma \sin\alpha_i

So, basically, if you want to increase the downwash, you increase the drag, and you increase the lift. Practically, you cannot do this forever. But even for small increases in downwash, either through airfoil shaping or geometric angle of attack changing, you increase both lift and drag. If you look at the equations, you basically get the thrust required as a function of the tangent of the downwash, and tan(a) < 1 for a < 90 degrees. So, really, you can't improve efficiency in this way.

[Sockmonkey]

You sort of can, in the sense that the larger your wing is, the slower you can go, (to a point) and therefore can get away with a much smaller engine since you're dealing only with mostly lift-drag and very little viscosity-drag.

[jmorgan3]

So, basically, if you want to increase the downwash, you increase the drag, and you increase the lift. Practically, you cannot do this forever. But even for small increases in downwash, either through airfoil shaping or geometric angle of attack changing, you increase both lift and drag. If you look at the equations, you basically get the thrust required as a function of the tangent of the downwash, and tan(a) < 1 for a < 90 degrees. So, really, you can't improve efficiency in this way.

But if you make the wing longer (i.e. increase the aspect ratio), then you can decrease your drag coeffiecient for a given lift coefficient, as shown by lifting-line theory. If you keep the gross weight, wing area and flight conditions the same, then you will need to keep the same C_L to maintain altitude, but your C_D will decrease because the aspect ratio (AR=span^2/area) will increase. This happens because you are moving a larger amount air at a lower change in velocity, so less energy is required. Another way to think about it is that your induced AOA is lower because your span loading is lower, and this decreases drag. That is why an airplane needs wings: the fuselage has a terrible aspect ratio and therefore will have a very low L/D ratio. Also, the very high "wing" loading will require very high takeoff and landing speeds.

[gorcee]

So, basically, if you want to increase the downwash, you increase the drag, and you increase the lift. Practically, you cannot do this forever. But even for small increases in downwash, either through airfoil shaping or geometric angle of attack changing, you increase both lift and drag. If you look at the equations, you basically get the thrust required as a function of the tangent of the downwash, and tan(a) < 1 for a < 90 degrees. So, really, you can't improve efficiency in this way.

But if you make the wing longer (i.e. increase the aspect ratio), then you can decrease your drag coeffiecient for a given lift coefficient, as shown by lifting-line theory. If you keep the gross weight, wing area and flight conditions the same, then you will need to keep the same C_L to maintain altitude, but your C_D will decrease because the aspect ratio (AR=span^2/area) will increase. This happens because you are moving a larger amount air at a lower change in velocity, so less energy is required. Another way to think about it is that your induced AOA is lower because your span loading is lower, and this decreases drag. That is why an airplane needs wings: the fuselage has a terrible aspect ratio and therefore will have a very low L/D ratio. Also, the very high "wing" loading will require very high takeoff and landing speeds.

This is a good point -- hence the reason that gliders have high aspect ratio wings -- that I had forgotten about.

If I remember correctly, there's also an eccentricity factor in there for elliptical wings.

Edit: yeah, I remember these equations.

It's worth mentioning, though, that the increase in L/D efficiency from the high aspect ratio wings is less to do with the amount of air being "moved", and more to do with the mitigation of 3-D effects, namely the reduction of the influence of tip vortices. However, one interpretation of the effect of the high aspect ratio wing is that it moves more air in the "proper" direction as a result of the high aspect ratio, because the tip vortices are less pronounced.

So "moving air efficiency" is a better description of the effect moreso than it is of the cause.

Incidentally, if you've ever noticed "winglets", the little upturned tips at the edge of the wings, these are also designed to manage tip vortices without the increase in weight and drag penalties incurred by longer wings.

[Sockmonkey]

High-aspect-ratio wings also tend to have a smaller frontal area for their size than low-aspect ones, which is another way they have less drag.

[Zamfir]

For bodies with attached flow around them, frontal area is of very little relevance too drag.

[gorcee]

For bodies with attached flow around them, frontal area is of very little relevance too drag.

Also, where it gets a little confusing, is that if you compute drag for say a car or a baseball, you use the frontal area. But for aircraft we use the planform area.

The reason why is because it's a lot easier to calculate planform area for an airplane, and it jives better with other considerations, ie wing sweep, etc. Other than that, there's no mechanical reason to use one or the other -- the difference just gets wrapped into the coefficient of drag.

[scootwhoman]

This topic relates in a way to space exploration. Because our atmosphere is dense enough to allow winged flight, escaping it is very difficult. We must move very fast to reach orbital velocity, but high speed is impossible in our atmosphere. Therefore, we must lift ourselves above most of the atmosphere before we can begin to accelerate towards orbital velocity.

The earliest aircraft were interested only in flight, not in travel. They used numerous wings to create lift, because the engines provided very little power. As engine technology improved, wings were changed to reflect higher flight speeds, and a single wing became the standard.

If we are interested in lifting weight to altitude, would not a biplane be more efficient than a monoplane?

[Carnildo]

The earliest aircraft were interested only in flight, not in travel. They used numerous wings to create lift, because the engines provided very little power.

No, early aircraft had multiple wings for stiffness: for a given amount of lift, a pair of wings braced against each other is more rigid than a single wing, at the cost of increased drag (both from the wings interfering with each other and from the inter-wing struts). As materials technology and aircraft design techniques improved, monoplane wings became stiff enough, and the biplane shape was abandoned.

[aydee]

First... I am not a aeronautical engineer or a physicist.

However, the things to take into consideration. How important are wings. Well lets look at the purpose of an aircraft. It's to get to a destination quickly, regardless the surface below (Land or sea) and SAFELY!

Now when LANDING an aircraft one of the most important things they do is 'lower the flaps'.. Translation: Increase the wing surface area.

This has 2 effects. 1: Increasing lift. 2: Increasing drag.

So.. You can slow down more, and you can come into the ground for landing SLOWER! (Fast landings are bad. Fast enough landing and even a cessna would run off the end of an international airports runway).

So.. Let's look at the next option.. Vectored thrust. Jump-jets (Harrier being the most famous of course).

Great concept. Brilliant for it's purpose. But inefficient. It's like using brute force when something more elegant is available. For just a brief moment, consider the amount of thrust and how many engines would be required to hover (not take-off which is even more power) a 747 full of cargo/people. You already have 4 jet engines putting out massive amounts of thrust, with 2 surface designed to provide lift under high velocity. It would take more than those 4 to vertically lift you off and hover.

Harriers have similar fuel capacities to many other military jets, but their range is greatly reduced in comparison. They are interceptors. If VTOL was 'efficient' then they would use it to take off and land. They don't. The use runways to do a normal runup/slow down. It's too inefficient otherwise. VTOL is TYPICALLY used to vectored thrust in a military aggressive situation (No citation available. Just personal conversations with an older friend who is an ex-RAF pilot who was one of the first people to be involved in experimenting if "harriers in a vectored thrust in combat situation is viable" test. Testing involved going up many an F-16 pilot to test the capability of the idea. This was shortly after the "War of Attrition").

Do airplanes need wings? Yup.

Helicopters need wings too. They call them "Rotary wing aircraft" for a reason.

AD

[scootwhoman]

The earliest aircraft were interested only in flight, not in travel. They used numerous wings to create lift, because the engines provided very little power.

No, early aircraft had multiple wings for stiffness: for a given amount of lift, a pair of wings braced against each other is more rigid than a single wing, at the cost of increased drag (both from the wings interfering with each other and from the inter-wing struts). As materials technology and aircraft design techniques improved, monoplane wings became stiff enough, and the biplane shape was abandoned.

Some of the earliest successful aircraft designs were monoplanes, but their weight carrying ability was very poor. There were also tri-planes, which had three wings. The whole concept of aerodynamic lift was a little foggy to some experimenters, and many different wing designs were tried, some which emulated bird wings. But birds fly by cupping their wings and pushing down and back, while airplanes plane, that is skim through the medium. Looking at the first Wright aircraft the wing shape is very pronounced, as they sought to achieve enough lift with a very puny motor. As the ability to fly became more established, range and weight carrying quickly became the most important design factors. Wings became flatter, and aircraft speeds rose rapidly.

With the advent of jet engines, speed became as important as range, and wings were swept back to reduce drag while maintaining lift. Speed, payload, and range have become the holy grail of aircraft design, so our concepts of aircraft have become biased by what we see all the time. No one has built an aircraft just to carry weight to altitude until now, with the Burt Rutan White Knight being the first. Long, straight wings provide the maximum amount of lift, but they suffer from flex and vibration when they get too long. A biplane may be a better solution for launching a space ship than a monoplane, because the only thing that is important is getting the space ship to a high enough altitude that it can use all of its power to go fast and not waste a bunch on climbing straight up.

The enormous amount of drag of a big biplane can be overcome by using 8, 10, or even 12 big turbofan engines, the best of which now weigh about 20,000 pounds, yet produce around 120,000 pounds of thrust. By using a catapult, the carrier and the spaceship can be accelerated to a high enough velocity to insure take-off in just about a mile, and have a run-off track which could be used in aborts. The track would also support the huge weight of the carrier and the space ship, which could total around 2,500,000 pounds. That way, miles and miles of very thick runway are not required.

(Is this what they call hijacking a thread?)

[gorcee]

First... I am not a aeronautical engineer or a physicist.

However, the things to take into consideration. How important are wings. Well lets look at the purpose of an aircraft. It's to get to a destination quickly, regardless the surface below (Land or sea) and SAFELY!

Now when LANDING an aircraft one of the most important things they do is 'lower the flaps'.. Translation: Increase the wing surface area.

Note: increasing effective area is only one of the roles of the flaps. More importantly, perhaps, is that the flaps "turn" the velocity field, in effect moving the trailing edge. This keeps the flow attached over the wing, and has the effect of altering the induced angle of attack, predominantly through increasing downwash without requiring a similar adjustment to the geometric angle of attack.

[scootwhoman]

There is no theoretical limit to the size of a wing, the only restraint is the amount of power needed to overcome the drag. Some things have been done to 'spoof, the air into behaving like the wing is larger than it actually is. Flaps are one of those techniques, and date back to the 1930's. Prior to that, pilots had to 'slip' an airplane just prior to landing, in order to reduce the landing speed. This is somewhat similar to a crosswind landing, as the nose of the plane is not pointing at the runway until just before touchdown.

Another method is called 'boundary layer control', which I really know nothing about, other than it is supposed to lower the stall speed of a wing by a certain percentage. However, as it generally uses bleed air from the aircraft engines, it has not proven to be highly reliable, at least, the U.S. Air Force thought so, back when the F-4 was equipped with the stuff. Wing slats are another variation of the BLC approach, I believe, but, again, I do not know anything about it. The aim of all of this technology is to allow an aircraft to fly slower than it is supposed to be able, or to allow it to take off at a lower speed.

The U.S. Navy came up against a big problem when they started flying jets, because these cast iron birds just have to go really fast before they can claw their way into the sky. So, big steam catapults were developed, which can hurl an aircraft into the air at about 140 miles per hour. Because that is still too slow for many jet fighters, the aircraft carrier must head into the wind at high speed, to create an effective wind across the flight deck.

Soon, steam catapults will be a thing of the past, as electromagnetic catapults replace them. These are capable of much higher speeds and accelerations, insuring that launching from ships will continue even as aircraft get heavier. In order to get a massively laden carrier wing into the air, a speed of over 300 miles per hour may be required. Exceeding the stall speed by at least 20 miles per hour insures that there will be no take-off stalls due to wind shear or low air density from high temperatures. By using a 'rail gun', as some people call a linear induction motor, a carrier wing carrying a 1.5 million pound spacecraft could still be able to lift off, even though it weighs another million to 1.5 million pounds.

Many people do not realize that the space shuttle had consumed nearly half of the fuel in the external tank by the time the Solid Rocket Boosters were jettisoned, even though the vehicle was only traveling about 1 mile per second at that time in the launch. The remaining 4 miles per second were achieved using just one half of the fuel capacity of the external tank. By carrying the orbital spacecraft above the majority of the atmosphere, a carrier wing would allow a much larger payload to be put in orbit when compared with what the same size vehicle could achieve launching vertically.

The technology exists, the money is lying around looking for a profitable return, all we need now is someone with vision to sell the investors on the idea. Considering that they bought into credit default swaps and collateralized debt obligations, it should not be that hard to convince investors that they will see profits from investing in space flight.

[gorcee]

Another method is called 'boundary layer control', which I really know nothing about, other than it is supposed to lower the stall speed of a wing by a certain percentage. However, as it generally uses bleed air from the aircraft engines, it has not proven to be highly reliable, at least, the U.S. Air Force thought so, back when the F-4 was equipped with the stuff. Wing slats are another variation of the BLC approach, I believe, but, again, I do not know anything about it. The aim of all of this technology is to allow an aircraft to fly slower than it is supposed to be able, or to allow it to take off at a lower speed.

As someone who has worked in flow control, and is co-author of a number of technical reports on file with NASA and the USAF on the topic, no.

[scootwhoman]

Another method is called 'boundary layer control', which I really know nothing about, other than it is supposed to lower the stall speed of a wing by a certain percentage. However, as it generally uses bleed air from the aircraft engines, it has not proven to be highly reliable, at least, the U.S. Air Force thought so, back when the F-4 was equipped with the stuff. Wing slats are another variation of the BLC approach, I believe, but, again, I do not know anything about it. The aim of all of this technology is to allow an aircraft to fly slower than it is supposed to be able, or to allow it to take off at a lower speed.

As someone who has worked in flow control, and is co-author of a number of technical reports on file with NASA and the USAF on the topic, no.

"No" what? No, the aim is not what I stated? No, that is not how it works? Please elucidate.

[wam]

Another method is called 'boundary layer control', which I really know nothing about, other than it is supposed to lower the stall speed of a wing by a certain percentage. However, as it generally uses bleed air from the aircraft engines, it has not proven to be highly reliable, at least, the U.S. Air Force thought so, back when the F-4 was equipped with the stuff. Wing slats are another variation of the BLC approach, I believe, but, again, I do not know anything about it. The aim of all of this technology is to allow an aircraft to fly slower than it is supposed to be able, or to allow it to take off at a lower speed.

As someone who has worked in flow control, and is co-author of a number of technical reports on file with NASA and the USAF on the topic, no.

having done a bit of this during my degree, my understanding is that BLC can be used for vairous purposes,

Lowering drag
Improving stall charateristics
Increasing Stall angles

scootwhoman I think you were confusing stall charaterstics with speed alone, rather than with angles invloved.

On a side note takeoff speed is not determined by stall charateristics, more by the amount of lift required. Lift is proportional to airspeed squared.

Slightly OT

Spoiler:

There are also two types of boundary layer control, passive and dynamic. Passive is simple stuff like surface textures and vortex generators (those triangles you see on wings). Dynamic normally involves pulsed jets/suction. I will say most of the dynamic stuff went way over my head, but I did pass an exam in it somehow

[gorcee]

Flow control has a number of purposes, generally pertaining to increasing flight envelope, or increasing controllability near the boundaries of the flight envelope. This includes maintaining controllability at high angles of attack, increasing redundancy in the event of actuator failure, delaying flow separation to enable better control authority through mechanical actuators, reducing drag through various mechanisms, including 3-D wing effects, manipulating flow to reduce flutter/buzz/LCO, and so on.

The ability to increase takeoff weight is a peripheral benefit of flow control technology, and if anything, the majority of this benefit would come through purely synthetic flow control actuation by eliminating the weight penalty of mechanical actuators and wing surfaces. This technology is not yet ready, and will likely never be used alone on anything other than a UAV. It is true that many structural techniques achieve flow modification: winglets are an example of this, but these rigid structural/skin elements are static, and as such don't actually achieve any dynamic control over the flow regime.

Whenever you talk about dynamic modification of the flow around an aircraft, you're immediately involving yourself in a control systems discussion. Aircraft are generally nonlinear, and most modern aircraft are unstable in at least one axis. Any sort of non-traditional (ie, non-actuated, non-vectored) flow control scheme needs to be associated with a feedback control system -- generally, a feedback control system that is somehow coupled with the rest of the flight control system. The computational and algorithmic tools to make this work haven't been around for very long, and it would probably take 10 years to V&V them even for a single platform. The only application I could possibly foresee justifying this expense purely to increase takeoff weight would be for an STOL cargo aircraft. I don't believe the C-17 uses active flow control, and it's brand new. So we won't see it in a large-scale aircraft for decades, most likely.