Re: Airplane Drag

[John Denker <jsd@MONMOUTH.COM>]

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