To understand my question better, let's consider the priciple behind the lift generated by the wing - The upper surface of the air-foil is curved to deflect the air downwards using the Coanda effect. According to Newton's 3rd Law, an equal reaction is produced by the downwards deflection of air in the form of lift.

Borrowed from *simhq.com*

So here's my question - Why aren't some planes designed to look like a massive cross-section of a wing from the side? e.g. A Cessna C172 looks like an inverted wing (with the curved side on the bottom, which might create 'inverse lift' by pulling it down. From that aspect, is the Cirrus SR22 and the Cessna TTx more lift-friendly? Their upper part of the fuselage is relatively more curved than their lower part when viewed from the side. Or am I making a mistake - is it due to high-wing low-wing design philosophies?

Borrowed from *flyawaysimulation.com* Borrowed from *aircraftbluebooks.com*

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    $\begingroup$ 1. The airfoil is generally not constant over the span, 2. "The upper surface of the air-foil is curved to increase the distance of air traveling above the wing", not to increase distance, but to deflect air downward without adding too much drag (there are airfoils with both sides curved or symetrical). $\endgroup$ – mins Nov 7 '17 at 6:48
  • $\begingroup$ @mins: Thank you for clarifying - I apologise for my flawed understanding of lift generated by an air-foil - I have edited the question to better define lift, but could you tell me why there are so many varied theories as to how a wing / air-foil creates lift? Thanks! : ) $\endgroup$ – user18035 Nov 7 '17 at 7:02
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    $\begingroup$ There is only one real theory. The Navier-Stokes equations. And then many more or less simplified or phenomenological descriptions. Many of them oversimpkified or incorrect. $\endgroup$ – Vladimir F Nov 7 '17 at 7:44
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    $\begingroup$ Bernoulli's equestion is directly derived from the N.-S. equations under certain simplification assumptions. The others are phenomenological descriptions. $\endgroup$ – Vladimir F Nov 7 '17 at 8:20
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    $\begingroup$ The Bernoulli theory of lift is also incorrect as it relies on a false assumption. Basically the Bernoulli theory says that two adjacent particles that separate (one to travel over the top and one to travel along the bottom) will meet up at the trailing edge and so the one which traveled along the curved path must go faster. This is false the particles do not meet up at the trailing edge in fact the upper surface particles reach the trailing edge long before the lower surface particles. There are some very nice videos of wind tunnel tests showing this. $\endgroup$ – DJ319 Nov 7 '17 at 10:40

Your intuitive feeling that the fuselage could be used to create additional lift is correct. Indeed, some aircraft are specifically shaped such that fuselage could provide substantial lift (and that's excluding any 'flying wing' design where fuselage is completely blended with the wing).

However, these are generally supersonic fighter aircraft. At supersonic speeds, even the 'normal' tubular fuselage will create significant lift, given a decent angle of attack. Moreover, the engine air intakes, if properly designed, will create even more lift.

In GA aircraft, on the other hand, the effect must be negligible, and structural considerations (yes, including high/low wing) are much more important.

The primary aerodynamic consideration for the fuselage is, generally, minimising its drag and reducing interference with other parts, primarily the wing. In this regard, by the way, C172 (or any high-wing aircraft) is more 'lift-friendly'. High-wing design has less negative wing/fuselage interference; in fact, it can be even positive. In simple terms, the part of the wing above the fuselage still creates lift, and probably more of it than the whole fuselage (although I have no numbers at hand).

I also must duly note that the principle of lift generation has little to do with the geometric length of the upper and lower surfaces. It is a popular misconception. The air indeed moves faster over the upper surface, but this is due to entirely different reasons. There are several good explanations of lift on this site.

  • $\begingroup$ Very thorough and simple at the same time - great answer! So the difference in the fuselage shape of a C172 and the SR22 is due to low-wing, high-wing design factors only? $\endgroup$ – user18035 Nov 7 '17 at 5:00
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    $\begingroup$ @AnandS, it's hard to prove the 'only' clause without getting into the heads of the designers. But yes, I'd say primarily that. For SR22 at least, even just the aesthetic appeal could be a factor. $\endgroup$ – Zeus Nov 7 '17 at 5:23
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    $\begingroup$ @AnandS The main reason the bottom of the plane angles upward towards the tail is to avoid tailstrikes on takeoff and landing. The greater the angle between the ground and the bottom of the tail the more the plane can rotate with the wheels still on the ground. $\endgroup$ – ratchet freak Nov 7 '17 at 10:53
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    $\begingroup$ A flying wing has no fuselage at all. An aircraft where the fuselage blends smoothly with the wing, like the F-15, is a blended wing-body aircraft. $\endgroup$ – Vikki - formerly Sean May 20 '18 at 17:46

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