Glide distance vs. weight

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.

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.

"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

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?