Re: Power Loading

Paul Butler (
Wed, 17 Jan 1996 12:17:45 +1100

Hi Guys:

I feel I must chip in on this discussion about surface finish.

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.

Increased wing loading will have a significant effect on drag 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).

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

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.

Paul Butler