Glide distance vs. weight

On Sun, 27 Feb 2005 21:14:25 GMT, "Julian Scarfe"
wrote in ::

"Larry Dighera" wrote in message
news
While your statement above is generally accurate, it's not absolutely
true (as was pointed out to me by a glider pilot in e-mail). Here's
some empirical evidence of L/D changing with a change in weight (note
the right hand polar graph under 'Technical data'):
http://www.dianasailplanes.com/szd55.html

The data there indicates an L/D of 51 at higher weights, 49 at lower (about
50%). That seems consistent with the idea that at higher Reynolds numbers
(in effect, higher speeds) the skin friction drag coefficient reduces a
little.

Reynolds number: http://aerodyn.org/Frames/1flight.html

Given the "clean" design of the glider, the increase in parasitic
drag at higher speeds is probably insignificant compared to the "skin
friction drag" reduction.

"Larry Dighera" wrote in message
...

Given the "clean" design of the glider, the increase in parasitic
drag at higher speeds is probably insignificant compared to the "skin
friction drag" reduction.

I think so. Words for drag vary, and I've always used parasite drag to
include skin friction, but I think we mean the same thing.

For a laminar boundary layer, skin friction is proportional to Re^-0.5, and
for a turbulent boundary layer to Re^-0.2. If skin friction drag is about
2/3 of the total parasite drag (by which I mean skin friction + form drag),
which in turn is 1/2 the total drag at best glide, that would suggest that
the L/D should improve by between 1/15 and 1/6 of the increase in speed.
The data you quoted, with a 50% speed difference for a 2-3% difference in
L/D suggest something like the 1/15 expected of a turbulent boundary layer.

Julian

Larry Dighera wrote:

Given the "clean" design of the glider, the increase in parasitic
drag at higher speeds is probably insignificant compared to the "skin
friction drag" reduction.

But given that the original poster was most probably talking of
airplanes with noisemakers, I suspect that for him, best glide gets
dramatically worse at higher speeds. As I always say: Airplanes don't
*need* airbrakes because the whole plane *is* just one huge airbrake.

Stefan