How does an airplane fly?


by ProgressNation
Tags: airplane
boneh3ad
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#127
Jul2-13, 02:26 PM
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Quote Quote by rcgldr View Post
I had the impression that an adverse pressure gradient usually (but not always) triggers a transition to turbulent flow (assuming turbulent flow hasn't already begun due to other factors).
Like I said, it is a related pair of phenomena but not one in the same. At the risk of digging too deep into this topic, I will try and clarify fairly succinctly here what I mean. As an example, the boundary layer on a flat plate will transition to turbulence without the action of an adverse pressure gradient. Boundary layers are essentially very complicated, nonlinear dynamical systems, in many ways like a mass-spring-damper system, only more complicated, and instead of being governed by Hooke's Law and some damping, they are governed by the Navier-Stokes equations.

Much like their simpler counterparts, they have various instability modes that can grow and eventually get large enough to transition to turbulence. It turns out on a flat plate, the instability mode that leads to turbulence is a streamwise wave called a Tollmien-Schlichting wave. On a flat plate, these will eventually grow large enough to transition to turbulence. As it also happens, they are dominant on a 2-D wing. They are also remarkably unstable to an adverse pressure gradient, so when you get farther downstream on a wing and they encounter the adverse pressure gradient, they grow much faster than usual and it leads to early transition. They don't need the adverse pressure gradient in order to cause transition, but the adverse pressure gradient does speed the process along. In this case, your understanding is correct.

For a swept wing, Tollmien-Schlichting waves exist but are not dominant. In those situations, you have what is called the crossflow instability that dominates. That instability, it turns out, is actually made more stable by an adverse pressure gradient. However, on most practical swept wings in service, it transitions well before the adverse pressure gradient occurs anyway. In other words, most commonly-used swept wings actually transition independently of the adverse pressure gradient, and the pressure gradient itself would actually delay the transition somewhat.

Quote Quote by rcgldr View Post
I thought that the streamlines ending and beginning were an issue for profile drag (the "partly" part), like a bus traveling down a highway, where the stagnation zones front and rear do not have the smae pressure, and most of the profile drag is usually due to the lower pressure aft of an object (depending on the shape).
That's true enough, but that only occurs on an airfoil when you have separation. Otherwise there is no issue of the streamlines not meeting up neatly. On a bus, you have that massive separation.
sophiecentaur
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#128
Jul3-13, 12:23 PM
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Quote Quote by Traz 0 View Post
Silly flight school. They wanted us to understand aerodynamics. Lol

Also, that wing cross-section diagram posted a few entries ago does show "normal force" as acting perpendicular to the chord line, which IS as I was taught. And, so, again my initial question: Is this still considered the proper model for diagramming wing function? Or is it a rough approximation that's good enough for government work?
I think so. They can hardly expect a specialist flier to be a specialist Physicist at the same time (or vice versa ).
The meaning of the word 'understanding' is very wooly. I am sure that the flight school course didn't require you to do more than be 'comfortable' with the techinical stuff at a reasonable level. What they told you would probably not have been sufficient for you to have designed a wing, for example.


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