Best Glide Speed

"Raymond Swartz" wrote in message
...
Yes, weight does have an impact on best glide speed. That is why glider
pilots in competition (or whenever they need a high speed, like long
flights limited by daylight) carry water ballast in the wing tanks. It
doesn't affect the glide angle too much

"Peter Duniho" wrote in message
...
It doesn't affect the glide angle at all, oddly enough. The lift/drag
ratio
remains the same, which results in the same glide ratio. You just have
more
lift and more drag.

It does affect the glide angle a little.

The model that infers that glide angle is independent of weight assumes a
parasitic drag coefficient that is independent of speed. In reality it's
not. Skin friction drag coefficient reduces as speed to the power 0.2 for
turbulent boundary layers and speed to the 0.5 for laminar boundary layers.
Thus the faster (heavier) glider actually has a higher L/D.

Thus at 20% higher weight, 10% higher glide speed, the skin friction
(approximately a quarter to a half of the drag) is between 2 and 5% lower.
So the glide ratio will be something like 0.5 to 2.5% better.

Is it significant? Well, maybe. Bear in mind that we're always taught to
nail that best glide speed. Getting the airspeed wrong by 5% costs you
just 0.5% of your glide angle, and getting it wrong by 10% costs you just 2%
of your glide angle. So the numbers are the same sorts of magnitude.

Julian Scarfe