S.E. Best Glide

[FEW+SCT]

Hi all...
I was always told that best glide speed (dependant on weight) would always give you the best range, regardless of wind.

The other day, I was told that if you have a, say 20 knot headwind, you could get further by increasing that speed by about 10 kts. (due penetration). Seems very doubtfull.

Any comments????

[Intruder]

Best glide speed is max endurance, not max range.

Once you know your no-wind max range airspeed (somewhat higher than max glide), the rule of thumb is to add 40-50% of the headwind component, or subtract half the tailwind component. Max speed in a significant headwind will depend on drag characteristics of the airplane and engine limitations. Never fly slower than max glide speed in the tailwind.

[Genghis the Engineer]

I Disagree with intruder, quoted best glide speed is usually best glide angle (=range) speed. Best endurance is the min sink speed which will be slightly slower and may be quoted separately.

This speed will give you best range in still air, for a headwind you'll technically want a slightly faster speed, for a tailwind you'll want a slightly slower speed. But, without knowing the shape of the glide-angle .v. airspeed curve it's not possible to put an accurate figure on what speed should be used at what wind conditions.

With that in mind, I'd be very cautious about such rules of thumb which were probably worked out for one type only, and possibly by somebody without full possession of the facts for that. Although the basic principal is correct, I'd recommend sticking to the placarded best glide speed but if you need to stretch it pick a field downwind rather than into wind (assuming that you've enough height to have a choice).

G

[ROB-x38]

A good way to visualise it is to take it to the extreme:

- best glide speed 65kts
- headwind 65kts
-> range = zero

Therefore with a headwind, increase your speed by some amount to get some range (and vice versa).

Rob.

[bookworm]

With that in mind, I'd be very cautious about such rules of thumb which were probably worked out for one type only, and possibly by somebody without full possession of the facts for that.

As a general principle of aviation, Genghis, I heartily agree. However in the case of performance calculations like this one, you can get a long way on nothing more than an assumption that the drag curve looks something like the norm. The change in the best-range glide speed due to wind should be expressable in terms of wind component as a fraction of best-range glide speed.

[fudgy2000]

the best glide speed is derived from the speed that gives you the best L/D ratio. The best L/D ratio is gained by flying the aircraft at its optimum AoA. A heavy aircraft will have to fly faster to maintain its best AOA, compared to a light aircraft. In a headwind situation, if you fly slightly faster than the best glide speed you will be in the head wind for less time, hence you will travel further over the ground!

Endurace is a different thing. Maximum time in the air. Gained by flying at the minimum power on the power required curve.