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According to Bernoulli's principle and for a given angle of attack would that not lower the lift force of the airplane and increase its drag and therefore increasing its demand in thrust and fuel consumption?

This is anti-economic and puts the airplane under unnecessary mechanical stress.

I am not referring to the angle of attack adjusted by the pilot during flight or to any preinstalled by the manufacturer angle of incidence on the wings to support lift but solely to the shape of the wings. For example aerobatic airplanes (i.e. stunt planes) and most modern fighter jets, they don't rely at all on wing shape for lift. Their wings are evenly shaped flat upside and downside (i.e. instead of being usually in other airplanes, curved upside and flat downside).

Therefore, their flat evenly shaped wings do not support additionally to the angle of attack and incidence angle the lift of the plane.

Why not this additional feature of upside curved wing shape is not present in this kind of airplanes? Is there any particular reason(s)?

What is the trade-off here? A curved wing creates lower resistance on the upside and therefore faster air speed and lower air pressure (Bernoulli's principle) upside. Thus, increases lift in addition to the main lift generated by the angle of attack and incidence angle.

It would be logical to make use of this feature in fighter jets and aerobatic airplanes to generate larger lift with less thrust and less fuel consumption.

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    $\begingroup$ Related The Aerfoil Myth $\endgroup$
    – Farcher
    Oct 22 '21 at 7:34
  • $\begingroup$ Also Related question: physics.stackexchange.com/q/13030 $\endgroup$
    – Markoul11
    Oct 22 '21 at 10:30
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    $\begingroup$ I think you've got a major misconception here. The wings of fighters & areobatic planes are NOT flat. A bit of searching will find images - cross sections &c - that show this. They're thin, but definitely curved. Of course with sufficient power, you could get a flat wing to fly - ask anyone who's tried to move sheets of plywood in a strong wind - but it's not efficient. $\endgroup$
    – jamesqf
    Oct 22 '21 at 17:01
  • $\begingroup$ @Farcher Unfortunately, this article is still wrong (cited from it's second effect paragraph: "Because of the Bernoulli effect this causes a low-pressure pocket on the upper surface of the wing"). The truth is, that it still applies Bernoulli the wrong way round. Correct is: The air follows a curved path along the wing's surface, hence is accelerated downwards, hence forms an area of low pressure above the wing, and because of Bernoulli, we have a faster air flow within this low pressure area. The air is basically sucked into the low pressure region at the leading edge. $\endgroup$ Oct 24 '21 at 11:08
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The "equal-time fallacy" is alive and kicking, as shown in your question and the other answers. Look, here is the best explanation I've seen about how wings work and airplanes fly: https://www.av8n.com/how/

The "equal-time fallacy" says when two molecules of air get separated at the leading edge of a wing, they must come together at the trailing edge. So in order to get faster flow over the top (for lift), there must be a longer curve over the top. WRONG. The molecules do NOT meet up. The upper one gets there first. The result is a vortex in which air is pushed DOWN, and that makes the lift.

Wings lift because they push the air down, not because they are shaped a particular way, but because they fly at an angle-of-attack (AOA). The shape is just an optimization for typical flight. Wings on aerobatic airplanes are symmetrical because they typically fly inverted just as well as upright.

Is Bernoulli wrong? No. Bernoulli is absolutely true. What is wrong is the typical explanation, which is still being taught to kids. Again, read https://www.av8n.com/how/

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    $\begingroup$ "Wings on aerobatic airplanes are symmetrical because they typically fly inverted just as well as upright." Bingo! :) $\endgroup$
    – Markoul11
    Oct 22 '21 at 12:56
  • $\begingroup$ IMO the curved wing upside, reduces even more air resistance and friction therefore also air pressure on the upper side of the the wing when the plain flights upright straight, Surely the largest piece of lift is generated by angle of attack+i incidence angle but the curved wing shape helps also and creates additional lift and reduces drag. Of course it is clear to me now that this curve would mess up the lift on an inverted flight and would be an instability factor and maybe is the only reason why this kind of airplanes have flat symmetrical wings. Thank you. $\endgroup$
    – Markoul11
    Oct 22 '21 at 13:11
  • $\begingroup$ "The shape is just an optimization for typical flight." I take that to mean that the curvature does help to produce lift, in which case, I agree. There's a different fallacy going around that it doesn't matter at all. I was told recently by a friend that watching a B52 take off will disabuse you of that notion. $\endgroup$
    – JimmyJames
    Oct 22 '21 at 20:14
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    $\begingroup$ I must have been about 10 years old when I first saw the "explanation" that "the air over the upper surface has to speed up". I thought, "Why does it?". As if the air molecules care that they've been separated from ones they were near to a second ago! Ridiculous! And it seems, to this day, people keep trotting out this sentence like it has some meaning and actually explains something. $\endgroup$
    – Lefty
    Oct 22 '21 at 21:16
  • $\begingroup$ That link does not go to an explanation of how planes fly. It goes to an index of other links, which maybe explain how planes fly, but aren't labeled clearly enough for me to know. $\endgroup$ Oct 23 '21 at 16:46
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Is there any particular reason(s)?

There are actually two reasons.

Fighter jets are designed to fly at supersonic speed. Airfoil camber (that is the proper name you were looking for) helps at subsonic speed to make lift creation more efficient, but is a source of drag (wave drag) at supersonic speed. Fighter wings have very little relative thickness and little camber in order to reduce wave drag.

When those fighter wings need to produce lots of lift (which happens either at take-off and landing, or in maneuvering around Mach 0.7), they rely on vortex lift which is caused by flow separation at the leading edge of a swept wing. With separated flow, the details of the wing's contour don't count anymore, so camber is again not needed in fighter wings when high lift is required.

Aerobatic planes do away with camber completely, and for a different reason. It is much easier to fly rolls with an uncambered wing. The main advantage is less in inverted flight per se but in the transition between upright and inverted flight. This answer gives you the details.

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For example aerobatic airplanes (i.e. stunt planes) and most modern fighter jets, they don't rely at all on wing shape for lift. Their wings are evenly shaped flat upside and downside (i.e. instead of being usually in other airplanes, curved upside and flat downside).

Symmetrical wings still rely on their curved shape for lift, but need a positive angle of attack to produce it. A symmetrical shape has the advantage that the wing produces the same lift whether it is right way up or inverted, which is important for good aerobatic handling.

The lift of any wing increases with airspeed, but so does drag. A thicker wing produces more lift, but also more drag. An aircraft designed for high speed needs a thin wing to keep drag down, and doesn't need camber because it can produce enough lift without it. However when taking off and landing it may need camber to produce enough lift at low speed, thus the reason for flaps.

A truly flat wing is inefficient because without a curved shape to follow the air 'unsticks' from the top surface and turbulates as it goes over it, increasing drag and reducing lift. But the bottom surface still produces lift so it still functions as a wing, just poorly. Model aircraft are sometimes made out of thin flat sheets of balsa or polystyrene foam for easy construction. They have poor efficiency, but this doesn't matter because fuel economy is not an issue.

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  • $\begingroup$ Good all around answer. Thanks. $\endgroup$
    – Markoul11
    Oct 23 '21 at 10:18
  • $\begingroup$ Asymmetrical wings also need a positive AoA to produce lift. In fact, much of the reason for having camber is to get a the flow right when the AoA is too high for air to flow smoothly over a flat wing. $\endgroup$ Oct 24 '21 at 13:51
  • $\begingroup$ @leftaroundabout depends on how you define AoA. Usually geometric AoA is implied, and then cambered wings do have lift at 0 degrees. aviationchief.com/angle-of-attack.html en.wikipedia.org/wiki/Airfoil $\endgroup$ Oct 24 '21 at 18:52
  • $\begingroup$ "Symmetrical wings still rely on their curved shape for lift" - no, no, and NO. They don't rely on it. It helps, sure, but you could fly even if you had absolutely no curve in your wings whatsoever! It would be less efficient, but it would still be possible to fly! (as you correctly stated in the last paragraph) $\endgroup$
    – vsz
    Oct 25 '21 at 9:15
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Fighter jets should be fast (> Mach 1), fast rolling and small flight curve for fast turning (good manoeuvrability). For > Mach 1 it is of advantage, in regard to good manoeuvrability (M < 1) as well. Nobody cares about the economical aspects and yes higher stress, but if better manoeuvrability than the other fighter is given, this is taken into account. Utmost object for those fighters is manoeuvrability and speed, so they have to design it on the instability limit of a flight process (in flighter jets a pilot couldn't handle it, the basic stability is controlled by the electronic system) and material strength.

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