How would an airfoil react if it was flown backwards?

Question

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. — Ron Beyer, Dec 04 '16 at 19:24
Related : Is there an aerofoil that gives reasonably good lift for both air flow directions: forward and backward? — Peter Kämpf, Dec 04 '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. — quiet flyer, Oct 09 '21 at 00:08

score 14 · Accepted Answer · edited Apr 13 '17 at 12:59

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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)}

edited Apr 13 '17 at 12:59

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answered Dec 04 '16 at 20:16

Peter Kämpf

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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. – Charles Bretana Oct 08 '21 at 15:04
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@CharlesBretana "impact of the atmosphere molecules on the surface of the airframe"? Do you really mean Newton's impact theory? Maybe read this instead – Peter Kämpf Oct 09 '21 at 05:43
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. – Charles Bretana Oct 10 '21 at 00:04
Actually, as I would state it, Lift is just the normal (normal to the airspeed vector) component of the vectorial sum of all the forces exerted by ALL the molecular collisions of air molecules hitting the airframe, just as Drag is the Parallel component of that same vector sum. – Charles Bretana Oct 10 '21 at 00:06
@CharlesBretana Then we are on the same page. To use Newton again, force is mass times acceleration. The air gets accelerated at the same time it is pushing against the wing. No need to confuse cause and effect: It is all happening at the same time. – Peter Kämpf Oct 10 '21 at 07:20
Yes, we are on the same page, almost... But I believe basic understanding is dealt a severe blow when we confuse cause and effect. They do indeed occur close to the same time, well, very close to the same time, but F=ma has two variables. It would be absurd to say that the Acceleration causes the Force. – Charles Bretana Oct 10 '21 at 14:57
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@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 – Oct 10 '21 at 21:32
@ymb1, The only process that attempts to disparage the notion of cause and effect is the "spooky" action at a distance, in Quantum Mechanics, that purports to explain Quantum Entanglement. Are you saying that F=Ma is an example of Quantum Entanglement? It may not be necessary to regard the equation of motion as a cause-effect relationship, but that doesn't mean it is not a cause & effect. It is still cause and effect, regardless of whether you regard it that way or not – Charles Bretana Oct 10 '21 at 22:24
@ymb1, And they are not simultaneous! Heck, the entire notion of simultaneity, by Einsteins general Relativity, is proven to be a perceptual fiction. Newtonian Mechanics is just wrong. Any two events in Space-Time, can be defined as either A Before B, A after B, or are not even in each other's time cone (they cannot be aware of one another) see https://en.wikipedia.org/wiki/Relativity_of_simultaneity – Charles Bretana Oct 10 '21 at 22:28
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@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. – Oct 10 '21 at 23:51
@ymb1, In your tanker example, before the tanker releases the water. The Force is the Lift on the wings. The mass is the mass of the tanker and water. The resultant acceleration is 32 ft/sec2 (if in Level flight). After the release, the force is the same (until the pilot eases off on the back stick to lower the AOA, lift does not change), the mass is the mass of the tanker without the water, and the acceleration is greater than 32 ft/sec2 by some amount. – Charles Bretana Oct 11 '21 at 00:34
@ymb1, Also, in GR, an accelerating object will ALWAYS feel the force, at least if you measure the acceleration properly, in a non-accelerated (free-fall) frame of reference. – Charles Bretana Oct 14 '21 at 14:42

Robert DiGiovanni · Answer 2 · 2021-10-07T16:33:54.990

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"! — quiet flyer, Oct 07 '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. — Robert DiGiovanni, Oct 07 '21 at 19:16

How would an airfoil react if it was flown backwards?

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