Re: Airplane Drag

["Prof. John P. Ertel (wizard)" <jpe@NADN.NAVY.MIL>]

Probably the best expression to describe the drag (related to the
frictional force) on any A/C(AirCraft) would be series.  But, pilots are
substantially less mathematically oreiented than most physicist (speaking
as both a physicist and a former active duty USMC pilot).  Pilots look at
a plot of the Drag vs. A/S(AirSpeed) or a plot of a family of curves
for Lift vs. Drag for the particular A/C that they are about to fly.  These
curves usually have separate curves for "dirty" configurations (trailing
flaps, leading edge slats, landing gear, speed break, canopy, etc.).

The Lift vs. Drag curves are generally the most informative as they yield
the "best" speed or angle of attack for landing, max glide distance.  The
speed for L/Dmax (Lift over Drag, maximum) is a good target speed for
landing.  Note that faster A/C with what we call "flat" wings (minimal
camber) generally have such plots done in terms of angle of attack instead
of speed.  AoA (Angle of Attack) is generally a better indicator for such
A/C than speed.  Such A/C are equipped with AoT meters (as well as ASI, Air
Speed Indicators) to help the pilot fly at the optimum AoT (max glide
range) for landing.  Flying the approach at L/Dmax gives the pilot the
maximum time to recover should anything unfortunate happen while in the
landing pattern and/or on final approach.

Sorry we don't have a simple function (or even a consistently reliable
more complicated function) for the drag.  Perhaps those in AeroEngineering
may have something.  I'll pass the question over to our Aero Dept. here at
Canoe U.

On Sat, 11 Sep 1999, 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.
> >
> > Thus the frictional drag on the plane increases with airspeed.
> >
> > 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?
> >
> > Why?
> >
> > TX
> >
> > Jim Green
> > mailto:JMGreen@sisna.com
> > http://users.sisna.com/jmgreen

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