Mad Dog 11
An aircraft’s best glide angle is found at max L/D, which does not vary with weight but is purely a function of the design and aerodynamic efficiency of the aircraft.
A variation in weight however does affect the speed at which the max L/D occurs, the best glide speed increasing as the aircraft weight increases.
If we consider two aircraft, identical apart from their weights, both trying to achieve the best glide angle possible in their descents, we will find that the heavier aircraft, flying at it’s best glide speed (max L/D) will be flying faster than the lighter aircraft, which can still achieve the same glide angle, but only at a slower speed.
In practice, the best glide speed for both aircraft is likely to occur at too slow a speed for schedule regularity or ATC purposes etc., and so many operators have a fixed speed in descent for their aircraft, regardless of weight, and this fixed speed is generally well above the theoretical best glide speeds of either aircraft.
Let us assume that our airline requires us to descend at 300 kts IAS, and that the best glide speed for our heavier aircraft is 270 kts and our lighter aircraft is 250 kts.
Both could achieve exactly the same glide performance if allowed to fly at their varying best glide speeds, but they have both been made to fly at the same speed.
We can see that at 300 kts the heavier aircraft is much nearer to it’s best glide speed (only 30 kts too fast) than the lighter aircraft (50 kts too fast). Being closer to it’s best glide speed will mean a better glide performance, and so our heavier aircraft will lose less height over a given ground distance than the lighter aircraft. As both are flying at the same speed, this means that the heavier aircraft has a lower rate of descent than the lighter one!
This ability to increase the speed at which max L/D occurs by increasing the aircraft weight is used to good effect by competition glider pilots, who, on good thermalling days, will often load up to 500 lbs of water into their gliders, but that’s another story!
Hope this helps!
Bellerophon