Glide distance vs. weight

Take several aircraft at the same altitude. Make it a B-52 H since it can
carry over 300,000# of fuel so it can vary considerably in weight. Make one
weigh 300,000#, another weigh 400,000#, and a third one weight 500,000#.
The heavier the aircraft, the more energy it took to get it to that same
altitude but once it's there and the engines are shut down the heaviest one
has more potential energy (due to its greater weight affected by gravity).
The heaviest aircraft would have a much higher driftdown speed than the
lightest one and would reach the ground sooner than the others. It's higher
speed would induce more drag so that would counter the greater energy
available due to the greater weight. In no wind conditions all 3 should
glide the same distance. The lightest one would stay aloft longest but at a
slower driftdown speed producing the same glide distance. The driftdown
vertical speed of the heaviest would be the highest. It would travel across
the ground faster than the others... but for a shorter period of time.

Throw in a wind and you dramatically change things. Gliding into a headwind
the heaviest aircraft will glide the furtherest since it's effected by the
headwind a shorter time. Gliding with a tailwind the lightest aircraft will
glide the furtherest since it glides for a longer time thereby using the
most tailwind assist.

Darrell R. Schmidt
B-58 Hustler History: http://members.cox.net/dschmidt1/
-

"news.mcgraw-hill.com" wrote in message
...
During a review of the V-speeds for an airplane I've never flown before,
my
instructor asked me about glide speed vs. weight, and total glide
distance.

I got the glide speed vs. weight part right, but the distance part seemed
counterintuitive - that the total distance covered (by flying at the
correct
best glide speed for the weight) would be the same, regardless of the
weight.

Can anyone explain this so that it makes sense?

"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