Re: Airplane Drag

[brian whatcott <inet@INTELLISYS.NET>]

At 02:04 9/13/99 -0600, Jim Green wrote:
> > ... I was asking about flaps and ailerons.
> I have been told most assuredly that extending flaps at high speed produces
> less drag than extending them at lower speed....
> I find the physics of this a little hard to fathom -- if true (:-)

> Jim


Well, the answer to Jim's Son's conundrum finally dawned, as these
things will.
 But first, I deduce that Jim's Son flies a commuter jet or an airliner,
but probably not for Delta or United.

The issue is one of pragmatic engineering, rather than physics.
 At least, that is a viable position.

     Flaps provide vital lift gains at low speed which allow smaller
 wings to be used which provide less drag etc., etc.

   Engineers specify these devices for the flight regime at which they
will be useful, namely lowish speeds.
   On some small to medium size airplanes, if a pilot were so foolish
as to extend flaps at a speed greatly exceeding the max flap operating
speed, he would find that the flaps and their high accompanying drag,
would rapidly depart the scene.
    This scenario is circumvented in more modern and larger aircraft
by various means of locking out or blowing down or modulating control
extensions that would over stress the airframe.

One can easily see why one would want to use ailerons even at high speed:
this is after all the legacy of the Wright Brothers - controlability!
    And it's not hard to accept that a controls designer would moderate
control extension so as to hold resultant stresses within the allowable
load factor range ( which tends to be lower in big planes than in
smaller ones).

But now you ask: why would you want to lower flaps at high speed?
If other devises like  speed brakes etc are not fitted, the flap
can provide useful drag increments - and these days, rapid descents
into terminal airspace are not unknown.

 So, on rather scanty evidence, I postulate that J's S is looking at
the plots of control drag at various speeds, and puzzling over the
decrease in drag at high speeds (or perhaps, expecting us to puzzle
over an engineering feature he is already privy to.)

Sincerely

brian whatcott <inet@intellisys.net>
Altus OK