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

"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.

On Sat, 26 Feb 2005 14:48:17 -0500, "news.mcgraw-hill.com"
wrote in
::

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?


Here's a clue:
http://www.av8n.com/how/htm/power.ht...weight-effects

The short answer is that the L/D is virtually unaffected by weight.

For a more technical explanation, I've crossposted to
rec.aviation.soaring.

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

"news.mcgraw-hill.com" wrote:

I got the glide speed vs. weight part right, but the distance part
seemed
counterintuitive

It's not counterintuitive when you remember that extra weight at
altitude is extra potential energy. The energy that was used to lift
the weight to altitude can be recovered in the descent.
--
Dan
C172RG at BFM

When you add weight to an aircraft ... all points on the polar shift to
the right. i.e. minimum stall increases ... the speed for minimum sink
increases ... the speed for best L/D increases ... all almost the same
magnitude at the speeds we typically fly at.

So, if your L/D was 10:1 at 60 MPH when empty ... when loaded to gross
it could be 10:1 .... but at 75 mph. So, if you were gliding from a
mile up in a no wind situation ... you would go 10 miles distance in
both cases ... but, of course, at a higher speed and in less time if
you were heavy.



Larry Dighera wrote:
On Sat, 26 Feb 2005 14:48:17 -0500, "news.mcgraw-hill.com"
wrote in
::

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?


Here's a clue:
http://www.av8n.com/how/htm/power.ht...weight-effects

The short answer is that the L/D is virtually unaffected by weight.

For a more technical explanation, I've crossposted to
rec.aviation.soaring.

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.