A Physical Description of Flight, Revisited (2009) [pdf] | Hacker News Reader

A Physical Description of Flight, Revisited (2009) [pdf]

allstar.fiu.edu

60 pointsTomte17 comments

Many ask the simple question "what makes an airplane fly?" The answer one frequently gets is misleading and often just plain wrong.

The exact same problem is found in sailing:

"Popular concepts as to how sails generate lift, and how two sails interact with each other are discussed in light of modern aerodynamic research. Much of the old sail theory in the sailing references is shown to be wrong." [1]

[1] http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...

I remember some video, probably a TED talk, about the mechanics of bikes and how hard they are to describe formally.

> Many ask the simple question "what makes an airplane fly?"

I'm reminded of perhaps the most excellent 30 seconds in all of ground school instruction:

https://www.youtube.com/watch?v=CK1UGWw15lc

His joke aside, it illustrates an important point: there are a variety of ways to produce lift. For example, when one observes a jet overfly the runway during an airshow, holding its altitude while on its side, the engine is the source of the majority of its lift in that scenario.

(Edited for clarity.)

Excellent resource and augment to the conversation here, thank you for posting! Totally love seeing the Harrier in that video - I saw one live at Oshkosh and WOW that thing was LOUD.

I think the easiest explanation to a lay person involves the observation that all surfaces in a moving fluid (e.g. air) produce lift--whether a flat plate, rotating cylinder, or extending your hand outside a car window. Airfoils are merely highly optimized shapes to reduce the (often significant) drag the lift-producing bodies have.

In an airplane, by reducing L/D enough you decrease the necessary power (which indirectly produces its lift) needed to overcome the combined weight of the aircraft (wings/body/powerplant) generated by gravity.

You mean increasing L/D?

Yes, you're correct. Increase* L/D by decreasing drag.

Wow, Great paper. The most interesting (and counter-intuitive) part for me was that the lift is generated by the top of the wing bending air downwards. In my head I always visualized lift as the bottom of the wing being pushed up, but this paper claims lift is really more like the top of the wing pulling upwards.

"[T]he shape of the wing affects the efficiency and stall characteristics of the wing but not the lift. That is left to the angle of attack and speed."

So no, the top of the wing is not pulling the wing upwards. The air beneath is "blowing" the wing upwards.

It's really both. Figure 8 in the paper depicts this circulation theory of lift. The top and bottom surfaces interact via this phenomenon, though the top is more critical as the paper explains.

Also terms like "push" and "pull" really confuse the mechanics here. What matters is that a net upward force is being generated, with positive pressure acting on all sides of the airfoil. There is more pressure acting upon the lower surfaces than on the upper surfaces, but at every point the pressure is positive because it is surrounded by fluid (not in a vacuum). If you integrated the pressure acting on a lifting airfoil and computed the average direction, it would be pointed up.

Good point, if the top of the wing diverts air downward via reduced air pressure above the wing, then it would be the higher pressure air underneath (pushing) that creates the lift. Yeah, I can see how terms like "push" and "pull" get pretty confusing.

The lift is the sum of all the pressure vectors around every point on the wing. Since the pressure vectors are all normal to the wing surface, in order to have a net upwards vector, the pressure on the bottom surface has to be higher than that on the top.

I.e. air cannot "pull" anything anywhere.

If I remember well: in normal condition the lift come 2/3 from the top of the wing and 1/3 from the bottom of wing.

So the depression above the wing is indeed "pulling up" the wing more than the surpression below the wing is pushing up the wing.

No, a vacuum never "pulls". The pressure on the other side of the object pushes it.

"Parasite drag of a Boeing 747 wing is only equivalent to that of a 1/2-inch cable of the same length."

That's. . . impressive.

Via our favorite search engine, I found this http://www.aviation-history.com/theory/index-theory.html

OP PDF is link 2 ("Airfoils and Lift - Newton's Law"), but as text/html. The other links look very informative, if they're anywhere near the quality of the OP text, it will make for fascinating reading.

This is really helpful for my ParaWing invention. I still need a lot of work / help on doing the math for the exact wing dimensions though; very intimidating. However, using some sound research and an innovative design technique I do know my path is sound versus conventional designs - more lift, less aggressive stall, and smaller dimensions are all pretty necessary for human-scale personal flight device.

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