The Topic That Wouldn't Die: Bernoulli's Principle and airfoils

DaveC426913

Recently, I was discussing airfoils with a friend. I thought I was being clever and said "No, Bernoulli's Law isn't what's keeping it in the air - that's a fallacy. If it were Bernoulli's Law keeping it in the air, then airplanes would not be able to fly upside down. What keeps a plane in the air when flying upside down is the angle of attack."

He came back with "No, what keeps a plane in the air when flying upside down is the fact that planes that fly upside down are so overpowered that simply pointing the thrust in the desired direction (say, up) will cause the plane to go whereever the engine takes it, wings or no - and despite Bernoulli's Principle."

I had to concede right then and there.


So: can a plane fly upside down using airfoil-generated lift, as opposed to thrust-lift? Evidence?

Mech_Engineer

A symmetric air foil would be a good example of one that can fly upside down or right side up without a degradation of it's properties. For non-symmetric air foil, all it takes is a small adjustment of the ailerons to give it lift in the upside down direction.



If the plane's lift were all thrust generated, it would be pointing straight up! Angle of attack, ailerons etc. can all explain it however.

There's an old thread about this as well: http://www.physicsforums.com/showthread.php?t=130414

russ_watters

....or just some angle of attack!

russ_watters

Bernoulli's law is an important part of how airplanes fly - it isn't a fallacy. The fallacy is the 'equal transit time' argument, which in a double-fallacy is often fallaciously connected to Bernoulli's principle. The two are not connected to each other.

RandomGuy88

A cambered airfoil that is upside down can still produce lift as long as it is at a positive angle of attack. It will likely need a higher angle of attack and will produce more drag but it will still generate lift.

JaredJames

Well firstly, are all aircraft that can fly upside down "overpowered"?

Secondly, whether it's through thrust or aerofoil, AoA is the key. Level flight, with all the thrust possible and no aerofoil just isn't possible - there's a b*tch called gravity that kicks in - think bullet.

DaveC426913

That is the question I'm asking, yes.

Um. Thrust can oppose gravity. Think rocket.

'tis. One of the tricks they do at airshows is having an F-15 stand on its tail.


Actually, a more on-point question is this: on an airplane flying upsidedown, does the lift from angle of attack significantly* exceed the downward force generated by the camber of the wing?

If so, AoA is a dominant lifting factor.
If not, then overthrusting is the dominant factor that keeps the plane aloft.


* I'm sure that, at a high enough AoA, the low pressure above the wing will likely exceed the pressure under the wing despite any camber. The question is: is there enough to lift the plane, or is it only enough to help?

JaredJames

Um. If all thrust is directly horizontal, there is nothing providing lift so no, it isn't possible. (Well I suppose if we start talking orbital velocity and rockets... but I don't know if aircraft count in that domain.)
Is that considered level flight? I'd think the attitude indicator and artificial horizon would disagree and say you've got one hell of an angle of attack. But that's up to you.

Perhaps I should have been clearer. When I said "level flight" I was referring to a trajectory with 0 AoA (just to make sure I'm perfectly clear here, AoA in relation to the horizontal).

JaredJames

This is easily answered by checking if there's an aircraft that can fly upside down that isn't overpowered (I'm having a look).

russ_watters

Well what does that even mean? What does one thing have to do with the other? The F-16 has a ridiculously high thrust to weight ratio - does that make it "overpowered"? That ridiculously thrust to weight ratio has nothing at all to do with what makes it good at inverted flight: what makes it good at inverted flight is its symmetrical airfoil.

An airplane with a cambered airfoil will perform worse upside-down than right-side-up. How much worse depends on the plane. How big of an impact the engine thrust has, both on the ability to overcome the higher drag of the higher angle of attack and its direct contribution to the lift depends on the plane. But only fighter jets, stunt planes and models are capable of getting the vast majority of their lift from their engines and standing on their tails.

russ_watters

That's a nonsensical question: a wing either produces positive lift or negative lift, not both at the same time. Trying to separate what the top and bottom surface of the wing does is pointless because even in normal level flight, the bottom surface typically doesn't make a positive contribution to the lift unless the angle of attack is very high and/or the airfoil very cambered.

What actually matters is lift to drag ratio. An inverted cambered airfoil needs a higher angle of attack than a symmetrical airfoil (or an upright cambered airfoil) in order to produce lift and with that higher angle of attack comes higher drag. Higher thrust is required to counter that higher drag and keep the plane moving forward.

But again, that's still different than saying that the engine itself is supporting the plane. I'd say that in neither case (even in the "overpowered" case) is the engine itself providing a substantial fraction of the lift. An F-16 can stand on its tail with a 90 degree angle of attack, but at a 20 degree angle of attack (inverted), the engine isn't providing anywhere close to enough lift to keep the plane in the air (roughly 1/3 of what is required), but it flies just fine. The wings provide the lift. Your friend is still wrong.

For most planes flying upside-down at moderate angle of attack, the wings provide the vast majority of the lift to make them fly. I'd even say that a plane standing on its tail strains the definition of "fly" for an airplane - as you said, a rocket stands on its tail. Right: not an airplane.

JaredJames

I've actually been considering that. My first thought was when supplying something with 'excessive thrust'. But then, what is excessive? Those aircraft are designed with it so it's not excessive, it's perfectly normal.

russ_watters

A possibly even bigger and again separate issue is other design features that may prevent inverted flight such as fuel flow control and structural integrity. I remember from flying WWII flight simulators that some of the airplanes would fly just fine inverted....until the engine cut out after a few seconds due to failure of the fuel supply!

russ_watters

Dave's friend seems to define "excessive" as the engine providing amost all of the thrust required for inverted flight. That's what's required for an engine to take an object where-ever it wants, wings or not.

Q_Goest

That doesn't make sense. Draw a freebody diagram of the plane. In order for this to be true, the weight of the aircraft (assuming wings are not contributing to upward force) has to be balanced by the verticle component of thrust produced by the engine. When I see aircraft flying upside down, the angle of attack is relatively slight, say much less than 45^o from horizontal, probably more like 10 to 25 degrees. So now you can calculate the thrust the engine must produce by assuming the verticle thrust is equal to the weight.

T_e = F_v / sin (A)
Where
T_e = engine thrust or force produced by the engine in the direction the aircraft is heading
F_v = Verticle force produced by the engine's thrust and is equal to the aircraft weight assuming the wings don't produce any upward force
sin(A) is the angle from horizontal that the aircraft's axis of thrust is pointed, say on the order of 10 to 45 degrees.

If we have a 5000 pound aircraft flying inverted at an angle of 15 degrees to the horizontal, the thrust produced by the engine has to be a whopping 19,300 pounds! This aircraft could easily accelerate straight up like a rocket. In that case, you'd be pinned to your seat with nearly 4 G's of force flying straight up. So no, there MUST be some upward force produced by the wings in any aircraft flying inverted.

minger

Seen this topic at lunch and didn't get a chance to respond. . . this is what I was planning on typing though.

DaveC426913

I didn't say anything about thrust being horizontal. All of this is based on your addition of that factor.

It's prefectly sensical. The contribution of the components can be separated for the purpose of comparing what is doing the contributing, even if it's abstract. If I have a single wing where 3/4ths of it has a +ive AoA, and 1/4 of the wing has a -ive AoA, I can certainly break its components out to see how they contribute to the overall lift.

Thus, overpowered.
OK, so you're saying that

1] if we were to remove the wings from the plane, in order for the plane to remain in the air at a 15 degree angle from horizontal, it would have to have so much thrust that, should it turn vertical, that much thrust could accelerate vertically at 4gs.

2] Since no plane can do 4gs straight up, that means no plane could keep itself in horizontal motion simply with thrust angled up at 15 degees.

3] Since real planes do maintain horizontal motion while angled only 15 degrees up, the only way this is possible is if the (inverted) wings are providing the bulk of the lift.

Hm.