Glide distance vs. weight

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?

news.mcgraw-hill.com wrote:

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?

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?

--
Marc J. Zeitlin
http://marc.zeitlin.home.comcast.net/
http://www.cozybuilders.org/
Copyright (c) 2005

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
so with no lift, the glide range is the same but you'll get there
faster. Since glider records are all speed over distance, that's what
you want.

BTW the Boeing 707 glide ratio is 19 to 1, about the same as an old
Switzer glider. The best 25 meter glass ships are around 53 to 1.

Don Hammer wrote:

BTW the Boeing 707 glide ratio is 19 to 1, about the same as an old
Switzer glider. The best 25 meter glass ships are around 53 to 1.

Make that 60 (Nimbus 4, ASH 25). The new ETA project scratches even 70.
Most impressive.

Stefan

("Stefan" wrote)
Make that 60 (Nimbus 4, ASH 25). The new ETA project scratches even 70.
Most impressive.


http://www.leichtwerk.de/eta/en/gallery/photos.html
Photos of the ETA sailplane and its (31 m) 101 ft wingspan.

Montblack

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.

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

"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?

Lift/Drag = Velocity/sink rate = Glide distance/Altitude = glide ratio

The angle of descent doesn't change. As the weight increases,
the speed also increases to keep the L/Dmax the same.

Looking at it another way, more weight means a higher sink rate.
To maintain the glide ratio of the lower weight, velocity must
increase.

I just went over this working on my commercial. It doesn't feel right
unless you remember that velocity is allowed to change and the
glide ratio remains constant.