# How would an airfoil react if it was flown backwards?

Out of curiosity what would happen?

Would pressure on both top and bottom flip? or stay the same or act as a normal wing with less lift?

I'd assume it would still take longer to travel over the top than the bottom. Then conservation of energy takes effect and produces lift due to pressure, just wondering what negative effects happen?

• Sounds like you want to subscribe to the "equal transit theory" of lift, which has been disproven. Dec 4 '16 at 19:24
• Dec 4 '16 at 19:59
• Wow, your picture shows the airfoil at at an extreme negative (backwards) angle of attack. Obviously it would "dive for the dirt" and crash violently into the ground. End of story, enough said. Oct 9 '21 at 0:08

What would happen? Flow separation on the suction side, but it would still produce lift like a regular airfoil. The L/D ratio would be lousy, however.

Only at a small angle of attack range will the wing show attached flow on both sides: When the stagnation point is right at the tip of the trailing edge. This behaviour is similar to that of a flat plate and produces a rather limited range of useable lift coefficients and substantially more drag than when used properly. But still you get more suction on one and more pressure on the other side. The blunt rear edge will cause flow separation at all angles of attack and substantially increase pressure (or form) drag.

An airfoil with a blunt trailing edge has an advantage over a flat plate because it will perform acceptably over a slightly larger angle of attack range, but still this will be no comparison to the behaviour with the blunt side facing forward. Indeed, the front of an airfoil needs to be blunt to allow its use over a larger angle of attack range, while its rear end needs to be pointed to reduce the area over which the flow separates.

Now I feel I should drop a line or two about the cause of lift. Essentially, a wing creates lift by accelerating the air that flows around it downwards. The inclination of the airfoil will already be enough to cause this acceleration, regardless which side faces forward. The plot below shows how several airfoils perform over the first 180° angle of attack.

Lift coefficient over the first 180° angle of attack (picture source)

• Misunderstanding of cause and effect. The acceleration of the air is not the cause of the lift. How can it be? the air is accelerated after it hits the airframe. The acceleration is simply a consequence, (that means an effect, not a cause), of the Principle of Conservation of Momentum. This principle is of course very, very important, and all aviators need to understand it, but it is not the cause of lift. Lift is caused, by the result of the impact of the atmosphere molecules on the surface of the airframe. Oct 8 '21 at 15:04
• @CharlesBretana "impact of the atmosphere molecules on the surface of the airframe"? Do you really mean Newton's impact theory? Maybe read this instead Oct 9 '21 at 5:43
• No I do not mean Newtons Impact Theory. Your answer to that other question is excellent. It is in more detail, exactly what I mean. Perhaps you misunderstand what I am saying above. when I say impact of the atmosphere molecules on the surface of the airframe I am talking about the same thing you are talking about when you say The molecules will bounce against the wing skin more at the lower side than at the upper side, and the difference is lift. The difference you refer to is the difference between the forces exerted by all the molecular collisions on the bottom and the ones on the top. Oct 10 '21 at 0:04
• @Charles: There are many papers that try to correct this notion of impetus. "It is not necessary to regard the equation of motion as a cause-effect relationship." 1 Take a tanker that drops its payload of water: does the reduction in the force $W$ (aka $F$; $mg$) precede the change in $m$? No, as both are simultaneous (in Newtonian mechanics); "Some textbooks use Newton's second law as a definition of force,[5][6][7] but this has been disparaged in other textbooks.[8]: 12–1 [9]: 59" 2
– ymb1
Oct 10 '21 at 21:32
• @CharlesBretana: No QM or GR here (but speaking of which, in GR, an accelerating object feels no force). Back to Newtonian land: using the Kleppner & Kolenkow ref.: "It is the interaction [between systems] which is physically significant and which is responsible for the force." You've used F=ma to make a point, and my counter example was the tanker scenario. In other words regarding wings: the net acceleration of the air and the net lift vector, are one and the same.
– ymb1
Oct 10 '21 at 23:51

If you take the pictured wing, the same wing blunt side forward, and something in between you start to see what was explored in the 1940s by North American aviation as "laminar flow", attempting to reduce drag by delaying flow separation as much as possible.

Moving the thickest part of the wing back to around 30% and greatest camber to around 40% reduces drag by not only reducing turbulent flow on the top rear of the wing but also adds thrust by tilting the lift vector forward. The resulting wing has an improved lift to drag ratio but suffers the same issue as the reverse wing: a very sharp, unpredictable stall due to lack of warning buffet and a lower stall AOA from the sharp leading edge.

Rounding the leading edge greatly improves stalling characteristics, leading to the use of slats to have the best of both worlds for cruising or slow flight.

The Kline-Fogelman design even tried removing the rear top portion of the wing, and was studied by NASA (inconclusively), but the time honored way of minimizing drag, as seen in gliders, is as high an aspect ratio as possible, in other words, removing the entire back of the wing. This is also seen in the more modern 787 airliner wing when compared with the older 707.

• Re "the time honored way of minimizing drag, as seen in gliders, is as high an aspect ratio as possible, in other words, removing the entire back of the wing." -- I think you need to elaborate a bit more on that, not following what you mean by "removing the entire back of the wing"! Oct 7 '21 at 19:09
• The next line illustrates well. At a given speed, flow is smoother near the front of the wing. That, and proportionally reducing the wingtip vortex helps reduce drag. Oct 7 '21 at 19:16