Power Loading

Dr. Larry J. Crabb (docc@aristotle.net)
Tue, 16 Jan 1996 20:29:24 -0600


You wrote:
>I feel I must chip in on this discussion about surface finish.

Join the crowd, Paul.

>I accept that a rougher surface finish MAY produce more drag and probably
>ONLY if it pokes through the boundary layer adhering to the surface. If
>the boundary layer is thick then the quality of the surface finish will
>have little effect on drag.

I beg to differ. We're not necessarily talking about wings or stabilizers
here. All surfaces of an airplane are exposed to the ravages of air
passing over them, around them, in between them. Look, picture a molecule
of air (78% nitrogen, 20% oxygen and 2% other) racing over a surface with a
molecular configuration of 800 microns. That's like a Cadillac running
into a 12' high brick wall and that's what's happening to every molecule of
air passing over a rough surface. In fact, electronmicroscopy eddy
currents are formed by the millions and are only dissapated by the rush of
new air which does the same thing. It's called parasite drag and it exists
to a greater degree on the 747 jets, than on the surfaces of an R/C scale
models. But, it does exist on the later.

>Increased wing loading will have a significant effect on drag

And, visa versa because both are an element of lift.

>because drag is a byproduct of lift. If the airplane is heavier it must
>generate more lift and to do that it must fly faster or at a higher angle
>of attack. Thus there is more drag in both cases. To fly at the same
>speed at a higher angle of attack or at a higher speed at the same angle
>of attack requires more power (this is
where power loading becomes important assuming equal propellor efficiency).

I couldn't have said it better!

>So it is better to be light than heavy for the same wing area.

Hooray! Someone else understands this! I'm glad you said that!

>Counter to that argument is the one where a higher wing loading is less
>affected by gusting conditions but wing profile camber is also important
>in this case. Light pattern ships with zero camber are much less affected
>than trainers or WW1 models with significant camber.

Right on, Paul.

>I would expect an SE5 or SPAD to be less affected by gusts than an
>Albatros because the wing profiles of the first two have much less camber.
>In fact the wing section used by many British types were little more than
>modified flat plates. Sections with little camber generally have a
>smaller Cd than those with more. That is a significant reason why the
>boxy SE5 had a slightly better performance than the streamlined Albatros
>D.Va. Less profile drag from the wings and therefore better performance
>with similar amounts of power.

You are so right! My Spad does have less camber than my Albatros and it
performs only somewhat better. However, consider this: if the Albatros
had a boxy shape and the S.E. 5 had a streamlined shape, the difference in
performace would have been even greater. All the S.E. 5a needed was a
streamlined fuselage along with their less cambered wings. NOTE: now we're
discussing "Form" drag, and that's another topic altogether.

By the way, Paul, did you get my msg. re: the Albatros thing?