Lift on a plane is created by fast moving air flowing over the sloped top of a wing creating a low pressure area which the high pressure below the wing moves towards thereby creating lift.That much I know but how do some planes fly upside down for long distances like daredevil and acrobatic planes?? Are the wings shaped the same on the top and bottom?? How is it that they do not stall?
I think by changing the angle of attack you can create lift on any wing-like surface.
Oh, man, I sense a mega-thread coming on…
Actually, it’s either an oversimplification or wrong to say that lift is created by a ‘low pressure area’ on top of the wing. It’s more accurate to say that lift is created by taking a mass of air and accelerating it downwards. Newton’s third law, and all that.
As for flying upside down, How easy it will be really depends on the airfoil of the wing. A wing must have a positive angle of attack to create lift. With a symmetrical airfoil (curved equally on the top and bottom), a plane will fly just as well upside down as rightside up, although the fuselage will be at different angles because the wing is set at a certain angle of incidence to the fuselage.
With high-lift airfoils with no camber or undercamber on the bottom, flying upside down can be difficult because you often have to maintain a pretty high angle of attack. So airplanes that are used for aerobatics typically have symmetrical airfoils.
There are discussions questioning the whole ‘creates low pressure on the top’ theory.
If you think about it, just because the air needs to go faster over the longer distance to meet up with the air traveling the shorter distance on the bottom, why would it? Who says it has to travel the same distance in the same amount of time?
As Sam Stone correctly said, almost all the lift a wing produces comes from the mass of air deflected downwards; a handy demostration of that is a helicopter, the blader are nothing else than wings, rotating wings, it´s blindingly obvious to note that a hovering helicopter blows a lot of air downward, the mass of that air, multiplied by it´s speed equals the weight of the machine. A fixed wing aircraft does the same but, since it doesn´t (usually) stay at a same location, the downwash is not so evident; a plane flying very low over water leaves a wake just beneath it for example.
heh 
I mean, the blades
The increase-speed-reduce-pressure thing is usually called Bernoulli’s principle. It likely has some part in keeping the plane up, but it appears the bulk comes from downwash, which is the effect other posters have described (the wing takes air and sends it flowing down relative to the plane, and the plane is pushed up as a result).
Now, you can make downwash with things that aren’t shaped laike wings are ‘supposed’ to be. You can move a piece of plywood thorugh the air at an angle and create downwash. Try sticking your hand out the window of a car, parallel to the ground. Then rotate your hand a bit in one direction or another, and see what the wind tries to do with your hand.
If you take an airplane, and turn it upside down, the wings will, indeed, generate Bernoulli-style lift downwards. And the ‘downwash’ will now be ‘upwash’, which contributes to downward ‘lift’. However, if you point the nose of the plane up (away from the ground) a bit, the wings can hit the air in such a way as to create a downwash effect. You need enough power to overcome the drag and the downward lift of the wings.
And you need good seat straps to keep from hitting your head on the canopy.
I expect there are other methods, as well.
All of this is conjecture and observation from a private pilot and student of fluid and classical mechanics. I haven’t flown inverted, but some of my coworkers have described it to me.
When the wing deflects the air downwards, it creates 100% of the lift.
But if we have a pressure difference above/below the wing, this also creates 100% of the lift.
In other words, the low pressure above the wing is inextritably linked with the downwards deflection of the air. It’s not seventy percent of one and thirty percent of the other. Instead we have two different ways of explaining the same thing. Each of them creates 100% of the lift.
Here’s a general rule: regardless of the shape of a wing, if it doesn’t deflect air downwards, then there will be no pressure-difference and no lift.
Also, many textbook diagrams contain a flaw involving the wing’s tilt (the angle of attack.) Since the rear half of the wing determines whether the air is deflected downwards, we must look at the rear half of the wing to determine whether the whole wing is tilted or not. Here’s a diagram I drew to illustrate this problem (see section 4):
http://amasci.com/wing/airgif2.html
More stuff:
http://amasci.com/wing/airfoil.html
http://amasci.com/wing/rotbal.html
Plotting lift coefficient as a function of angle-of-attack shows you that the zero-AOA lift is pretty negligible for most real wings (lift is usually zero at a small, negative AOA). This means that lift caused purely by the shape of the wing can be relatively insignificant. Telemark’s cite has a depiction of this - another example of lift vs AOA is here.
An aerobatic Super Decathlon has pretty close to a symmetric airfoil, so to sustain inverted flight just point your nose sufficiently skyward to push enough air down to keep the plane up (as mentioned before).
If it’s mainly AOA, why have the airfoil shape? I’m not an engineer but as I read it, the curved upper wing surface throws more air down than a flat wing would, and the shape also delays airflow separation as the wing slows hence delaying the onset of drag associated with a stalling wing.
BTW, flying upside down sucks. It’s like hanging upside down.
Aaaagh, I get sloppy when I’m lazy and tired. Realized I wasn’t clear above.
Correction:
Plotting lift coefficient as a function of angle-of-attack shows you that the zero-AOA lift coefficient is relatively small for most real wings (lift coefficient is usually zero at a small, negative AOA). This means that lift caused purely by the shape of the wing can be relatively insignificant.
and . . .
I’m not an engineer (no kidding) but as I read it, the curved upper wing surface throws more air down than a flat wing would, and the shape also delays airflow separation as the wing AOA increases hence delaying the onset of drag associated with a stalling wing.
Sorry for the confusion.
That first link of yours gives a very good explanation, bbeatty. It was very well written. Good job.
I knew someone who flew a plane upside-down for a half hour. He said the engine had to be specially adapted with pumps to circulate the oil (which is normally pumped up but drips down) and a couple other adaptations.
Excellent link bbeaty. Just excellent. Pulsed smoke… brilliantly simple.
Until I learned otherwise from these very boards, I had always believed the bernoulli explanation. Heck, it was what I was taught. Now I fight the ignorance fight. The real explanation is so intuitive, that I don’t understand the persistence of the bernoulli low pressure explanation.
I see Hari Seldon beat me to it, but I have an investment in this post so here it is.
The aerodynamic thing is the easy part. Making the engine run upside down is the main problem except, maybe, for turbojets.
Carburation is a problem so fuel injection is needed. Lubrication is also a problem. For an in-line engine with a sump, the oil in the sump would fall down onto the pistons. Flat, opposed layouts can have two sumps, top and bottom and with fuel injection are probably best.
Radial engines use the dry sump method. There is a small collecting sump at the bottom and from there the oil is immediately remove by a scavenging pump that puts it in a storage tank. However, when inverted the oil doesn’t fall into the sump. Two such sumps could be used for inverted operation.
Continuous, inverted flight isn’t normally done so if the engine cuts out while upside down that’s OK except for those idiot stunters who fly inverted over a field and pick up a handerchief with a wing-tip hook.
Heh. I’ve had about fifteen years to polish it. I started ranting about this misconception around 1988 while working at a science museum. It’s part of this collection of grade-school textbook errors:
PHYSICS ERRORS IN K-6 GRADE TEXTBOOKS
http://amasci.com/miscon/miscon4.html
Apparently some of the experts in aerodynamics learned the wrong explanation in fourth grade, and it stuck with them until adulthood. They can’t correct the error because they won’t admit that such an error could even exist.
It’s religion! It’s much like trying to convince a fundamentalist that the bible is full of mistakes. If you succeed, then you’ve pulled the rug out from under their entire world.
Some people (pilots, esp. teachers and authors) have a huge emotional investment in the “Bernoulli” explanation. They can’t change, since this would involve their admitting that their fundamental understanding of their own subject is wrong in a major way. When you get right down to it, the problem is that they’re more interested in “being right” than in learning the truth. Make that “more interested in BEING SEEN as being right.” When you attempt to lead them slowly through the correct explanation, they tend to (1) suddenly change the subject, (2) come up with increasingly desparate and irrational counterarguments (3) explode in rage, then hate you for years afterwards. Number (3) is stunning when it’s pulled by presumably rational professionals with advanced degrees and decades of experience.
Arguing about lifting force has taught me that some of the rationality of science is lip service only, and insanity lurks below the calm surface of many people. You’d never know this unless you find a major flaw in someone’s expertise and watch it send them right off the deep end.
There’s a well-known solution: scientists must remain humble, be like little kids; remain students always and never consider themselves to be experts. Einstein is a good example. So’s Feynman. I think they’d find this “infectious misconception” stuff fascinating rather than threatening.
Okay, bbeaty, I’ve read through your links, and they make sense to me. In fact, I reached some of the general physical conclusions just from seeing the pictures of over- and under-flows not lining up at the trailing edge. This explanation is much more internally consistent than the one based on flowpath length.
I’ll begin using this. I’m a flight instructor (and also studying fluid mechanics in school), so I’ll adjust my classes accordingly.
I do have a question, though. Since a wing can generate lift without a cambered upper surface, why do so many wings have the same shape (strong upper camber in the leading 20% or so)?
How come? What makes this better than just a flat plate angled at the same slope as the upper surface of the existing wing? In this case, the lower surface would have a steeper slope, which suggests an increase in downwash… or does it?
Related to that is the question of which path the downwash takes. Is it parallel to the upper surface, or parallel to the bisector of the angle between the upper and lower surfaces at the trailing edge?
I’m all excited about this now.
As I understand it, there’s no purpose for camber as long as there are no turbulence effects; particularly the airflow-detachment effect or “stall.”
If turbulence physics did not exist and “stall” was impossible, then a barn door would make an excellent wing. Diagram:
J. Denker’s SEE HOW IT FLIES: barn door cfd sim
from 3 Airfoils and Airflow
“Stall” is when the air flowing above the wing peels loose from the upper surface. During stall, the air above the wing is no longer deflected downwards, a large region of turbulent appears above the wing, and the lift becomes small and the drag large. Flat unstreamlined airfoils such as barn doors will go into stall-mode if the angle of attack is made larger than a very small value. Streamlining improves things (make an airfoil thicker, with a blunt nose and a sharp trailing edge.) Even better, cambered airfoils don’t stall until their angle of attack is made fairly large. In other words, when using a variety of attack angles, a cambered airfoil gives a very large MAXIMUM lifting force, while a symmetrical airfoil or an unstreamlined airfoil cannot be tilted nearly as much without triggering the stall mode and losing the lifting force…
Since “stall” and the need for camber is an advanced topic, maybe introductory textbooks would be improved if they focused entirely on symmetrical wings. Only add the camber back in when attempting to teach the complicated non-ideal effects (parasitic drag, boundary layer, stall.)
I think its parallel to the bisector, at least in an idealized situation. In flow calculations they usually remove the streamlined shape and replace it with an infinitely thin wing (use the camber line that’s drawn through the center of the thick-wing shape.)
I think that, since the air is accelerated when flowing over the wing, the upper mass of air, upon reaching the trailing edge, has a larger momentum than the mass of air moving bellow the wing, thus the resulting downwash should have a vector slightly offset downward from the bisector.
I definitely appreciate wanting to teach private pilot students the straight dope. Like bbeaty mentions, your trouble comes from the fundamentalist adherence/emotional investment people have with the Bernoulli explanation - and that includes some flight examiners. I went back to AC 61-23C and although it gives an acceptable explanation for the relationship of AOA, wing shape and lift, the FAA also retains the Bernoulli text:
I believe this is the AC that some of the PPL oral prep guides cite. I’m definitely not suggesting to teach your students what some check pilots might want to hear, but unfortunately student pilots who cannot explain how Bernoulli’s principle creates wing lift will be seen as ignorant by some unenlightened examiners. That kind of forces CFIs to teach Bernoulli in addition to “real life.”
In “real life” I find an understanding of AOA (along the lines of Langewiesche’s “fly the wing” mindset) much more valuable than imagining my plane being sucked up by hurried air molecules. For a CFI (or any pilot, I guess) I recommend Langewiesche’s Stick and Rudder - gives a decent “pilot-level” primer on flight dynamics and lift.
The shape of the wing allows for lift while minimizing drag. A flat plate would haver increased drag cause by violent turbulence as the air came over the sharp edge of the wing.