Glide distance vs. weight

[news.mcgraw-hill.com]

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?

[Marc J. Zeitlin]

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

[CryptWolf]

"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.

[Larry Dighera]

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.html#sec-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.

[Don Hammer]

>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.

[Stefan]

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

[Montblack]

("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

[Dan Luke]

"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

[Ken Kochanski (KK)]

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.html#sec-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.

[Larry Dighera]

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.

[Darrell S]

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?
>
>

[Julian Scarfe]

"Larry Dighera" > wrote in message
...

> 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

[Larry Dighera]

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

>"Larry Dighera" > wrote in message
...
>
>> 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]

"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

[Stefan]

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

[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]

> 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

[Stefan]

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