Best Glide Speed

[john smith]

Does Best Glide Speed reduce with weight the same as other airspeeds?

[kontiki]

Best glide speed is a determined by the interection of the parasitic drag
curve and the induced drag curve. Parasitic drag is not going to be affected
by weight but induced drag is somewhat affected by weight as well as the
location of the CG. So, yes, it *could* reduce or increase slightly depending
on the CG.

john smith wrote:
> Does Best Glide Speed reduce with weight the same as other airspeeds?
>

[Raymond Swartz]

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, but it does have an impact at
which airspeed that best glide angle occurs.

Go check out a glider contest sometime.

I recall crewing for one once, and fondly remember the view, as the
afternoon wore on, of the gliders returning to the airport from the
direction of the setting sun. We could first see them at about 15 miles
distance as bright points where the sun was reflecting off the white gel
coat. As they got closer we could make out the glider shape. Then about 5
miles out the sun started refracting through the "contrails" of water
ballast being dumped as they approached the finish line.

That particular contest had 6 gliders crossing the finish line at > 100
knots within 2 minutes of each other.

Ray Swartz


john smith wrote:

> Does Best Glide Speed reduce with weight the same as other airspeeds?
>

[G.R. Patterson III]

john smith wrote:
>
> Does Best Glide Speed reduce with weight the same as other airspeeds?

Yes.

George Patterson
If you want to know God's opinion of money, just look at the people
he gives it to.

[Peter Duniho]

"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

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.

[Julian Scarfe]

> "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