Glide distance vs. weight

On Sat, 26 Feb 2005 17:11:46 -0600, Don Hammer wrote
in ::


Glide distance is determined by the glide ratio (L/D ratio for the
airplane as a whole) and the altitude. If you're 1 mile up, and your
L/D is 10, you can glide 10 miles. Since the best L/D ratio for an
aircraft doesn't change with weight (although the SPEED to fly at best
L/D goes up with increasing weight), the distance you can fly isn't
dependent upon weight.

The heavier you are, the faster you'll get there, but where you get TO
is the same :-).

Howzzat?

That's what they do in gliders. Put on 400 pounds or more of water
when conditions are strong and dump it when it gets weak or before
landing.

Glide ratio is a function of the design and doesn't change with weight

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

According to the e-mail I received, this weight induced change in L/D
is apparently more significant on aircraft with wing aspect ratios
(length/chord) from 22-26, so the OP may not find it of too much help
in correcting his instructor's assertion depending on the particular
aircraft they were discussing.

"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.

Julian Scarfe

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

Julian Scarfe wrote:

The data there indicates an L/D of 51 at higher weights, 49 at lower (about
50%).

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
*have* airbrakes because the whole plane *is* just one huge airbrake.

Stefan

Julian Scarfe wrote:

The data there indicates an L/D of 51 at higher weights, 49 at lower
(about 50%).

"Stefan" wrote in message
...

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.

What leads you to that conclusion? I don't think there's any basis for it.
Just because the L/D for airplanes is much less doesn't mean that the
variation of L/D with speed is different.

Julian

Julian Scarfe wrote:

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.

What leads you to that conclusion?

Ok, I don't know the math, so I might be wrong. But my understanding is
that the better L/D at higher weight (hence higher speed) depends on
aerodynamically "clean" aircraft. Airbrakes will change the equation
dramatically. The common light singles (Cessna, Piper and the like) have
lots of airbrakes attached (or, as I said, are just huge airbrakes
themselves). (If you're talking of Cirri or the like, things may be
different.)

Stefan