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Re: Airplane Drag



At 07:33 PM 9/11/99 -0600, Jim Green wrote:
Can someone please write an expression for the frictional force on a smooth
aircraft as a function of airspeed.

I think it is something like f=kv^n where n=n(v), but it might be a series
of terms.

To a very useful approximation the *skin friction* term goes like V^2. To
a good approximation, n=2 for all V, unless you're talking about Mach
effects or other highly technical things that don't belong on this list.

Thus the frictional drag on the plane increases with airspeed.

Sure.

BUT

I am told by my pilot son that the frictional drag due to the extension of
the airplane's ailerons or flaps _decreases_ with increasing airspeed -- at
least at the normal extensions during landing.

Is this true?

No.

a) There are drag contributions that *do* decrease as a function of
airspeed, but the skin-friction contributions are not among them.

b) There's nothing about flaps or ailerons that changes the previous
sentence. The coefficient of frictional drag -- the "k" in the equation
above -- is increased when you hang out the flaps. The V^2 dependence is
unchanged.


For a pilot-oriented discussion of all this, check out
http://www.monmouth.com/~jsd/how/htm/4forces.html