how airplanes fly.

I read your recent article about how airplanes fly. I want to add my thought. Should we start as follows: A plane is on the runway. Air pressure at 35 pounds per square inch is pushing both up and down on the wings. As the plane accelerates down the runway the configuration and position of the wings (and perhaps the horizontal tail as well) causes the air pressure on top of the wings to gradually diminish until the difference between the air pressure on top of the wings falls ( in pounds per square inch) to a level where the pressure on the bottom of the wings minus the pressure above the wing X the total area of the wings in square inches surpasses the weight of the plane, then the plane lifts off.
If this is correct, then the question is what causes the air pressure above the wing to diminish. Maybe one factor is the inertia of the air above the wing which would slow down the air filling the partial vacuum developing above the wing.

           John Price

Referring to this column, I’m guessing:

How do airplanes fly, really? by SDSAB member aerodave, July 12, 2005.


Not exactly recent, though. These Straight Dope articles are all reprints.

Anyhow, welcome to the Straight Dope Message Board, John Price.

Be a good boy, don’t piss off the mods :slight_smile: and enjoy the ride!

Some more ETA: Hang tight for a bit … I’m going to hunt up some cites that I’ve seen before … There are all kinds of theories about how airplanes fly, and it seems that it’s even less certain than we though back 45 years ago when I was first learning this stuff…

Oops – Looks like the cite I was trying to find is, in fact, the same one that aerodave gives at the end of his article. If you click through the pages, it has several pages showing wrong theories and some pages showing some of the better current theories.

Back in the day, we all learned the Bernoulli theory. But I don’t recall ever hearing about the “equal transit time” idea. As far as I can figure, that was somebody’s idea of what the Bernoulli theory was all about. But I can still believe the Bernoulli theory, at least partly, and I can’t see any logical reason why one needs to imagine an “equal transit time” theory to go along with it.

We know in detail what an airfoil does but not so much about how it works. Modelers have found that a wing works about the same if you turn it around and fly with the trailing edge first. You do not need an airfoil envelope. Flat plates and curved plates work well as do wings with a curved airfoil for the first third of the wing and a flat plate from there to the trailing edge.

And, check out Magnus effect or Flettner aircraft. They have virtual airfoils. Some great stuff on You Tube.

Slight nitpick, the air pressure would be 15 pounds per square inch, not 35

Airplanes fly by pushing down air. It’s an F=ma type of thing.

I believe the Bernoulli explanation is at last (mercifully) falling out of favor, and the Newtonian one is (deservedly) becoming more common.

To summarize: planes fly because their wings deflect air downward, which (in accordance with Newton’s laws of motion) produces an upward force (equal to the weight of the plane when it is in unaccelerated flight).

The best way to grasp this is to observe a helicopter hovering over water: The rotor blades (which clearly are wings) keep the machine airborne by pushing a huge amount of air downward, made obvious by the way the water surface is disturbed.

The Bernoulli explanation is the Newtonian one. Bernoulli’s equation is just F = ma as applied to fluids (as opposed to discrete objects). What’s thankfully falling out of favor is the explanation which is frequently taught as “the Bernoulli effect” but which has nothing to do with it, nor with any other aspect of reality.


A helicopter stays aloft because it pushes air down. An airplane stays aloft using the same principle: it pushes air down.

Re: Shape of wing.

People have been studying wing shape for over a century now. And your standard airliner wing cross section is still basically the same as it was in the 1920s.

This shape provides more lift at cruising speeds than a flat wing and therefore saves fuel.

Sure you can fly a plane with flat or inverted wings, but you’re going to burn more fuel.

It’s absurd to deny this reality of wing shape. The standard shape increases lift!

IANA aerospace engineer. But it’s my understanding that a “barn door” shaped wing can (and will) provide lift by deflecting air down, but there are problems when the wing has a simply flat geometry (i.e. no airfoil shape):

  • The bottom surface of a flat wing will deflect air down, thus providing lift. But the air won’t follow the profile of the top surface, and thus the top surface won’t deflect air down and provide lift.

  • The air above a flat wing is very turbulent, thus producing drag (and inefficiency).

An airfoil is shaped so that, in addition to the bottom surface, the top surface of the wing also provides lift. The top surface is shaped such that the air wants to follow the profile. (I am not sure how this occurs, exactly, but it does.) And if the air follows the top surface of the wing, it will be deflected down. In addition, the air is trying to “pull away” from the top of the hump on the top surface of the airfoil, but is unsuccessful in doing so. So it creates a slight vacuum between the air and the top of the hump, thereby providing additional lift.

One more thing: in addition to being inefficient, a barn door shaped wing would have a terrible stall angle compared to an airfoil.

I think the right way to say this is that airfoil shapes are solutions to the problem of making wings efficient. If you don’t care about efficiency, you can produce large amounts of lift with a crude approach, up to and including “barn door” wings.

And note that there is no standard shape - airfoils vary a lot in their details, according to the job(s) the wing is expected to do. Airfoil research is ongoing - future wings will not be the same as today’s, which in many cases are quite different from those of the past.

Most of the time, pretty well.

It is the speed of air flow that produces the lift. Bernoulli’s principle, which says that if air speeds up the pressure is lowered. Thus a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift.

Lift is a function of area and angle of attack, not airfoil shape. The airfoil shape determines drag.

The air that is deflected down at the trailing edge of the wing does not produce lift. The angle of deflection is inversely proportional to the aspect ratio of the wing. A long, thin wing produces the same lift as a short wing of the same area, but has little or no downwash.

This is pretty much the standard “Bernoulli-based” explanation. It’s not wrong, but it’s a bad explanation for two reasons:
It encourages the listener to overlook the important fact that to create that region of low pressure, an enormous amount of air must be continually deflected downward.

It amounts to explaining the unknown (how a plane flies) in terms of the unfamiliar (Bernoulli theory of fluid flow). By contrast, Newtonian action-reaction is familiar - something every toddler encounters every day: “When I push on the chair, it moves away from me, and I move away from it.”

Regardless of aspect ratio, no downwash = no lift.

The short wing acts energetically on a relatively small volume of air, thus embedding a relatively large amount of energy in the wingtip vortices that are an inevitable consequence of lift. The long thin wing acts more gently on a relatively large volume of air, with consequently lower induced drag. But there ain’t no such thing as (unaccelerated) winged flight without downwash.

I don’t have a dog on this catwalk, but I have been reading you guys’ discussion with interest. So, of course, the internet gods provided this article on my home page this morn. From Scientific American, it discusses that “No One Can Explain Why Planes Stay in the Air”.

If the wing was entirely flat and perpendicular to the ground and pushing air like a bulldozer out of the way but generating no lift would it be surprising that there would be a pressure difference in the fluid on the two sides of the wing? On one side you are compressing the fluid on the other side you are pulling away from the fluid.

Put the same flat wing at an angle to the ground other than parallel, while in motion, and you still are pushing fluid forward and down while pulling away from a fluid on the opposite side of the wing.

If downwash produced lift then long thin wings would produce less lift than short broad ones. They don’t. Lift is a function of area.

If lift was created by pushing down on the air then parking at the end of a runway to watch 747s take off would result in you car being flattened by the supporting column of air. I’ve done it and my ears did not even pop.