PPL Video Lectures covering Aerodynamics, General Knowledge, Performance, Meteorology And Navigation are now available through our website see front page for details.
Welcome to the CPL Aerodynamics question and answer forum. Please feel free to post your questions but more importantly also suggest answers for your forum colleagues. Bob himself or one of the other tutors will get to your question as soon as we can.
I will assume that you know that the "best glide for range" occurs at an angle of attack for best L/D ratio. It is the L/D ratio, and nothing else, that determined the glide angle and hence the glide range in still air.
For most training aircraft the L/D ratio will be a max at an AoA of around 4 degrees; it will depend on the wing profile but the best L/D will occur at a fixed AoA.
Now, consider two identical aircraft at different weights on a glide at an AoA of 4 degrees. Both aircraft will follow the same glide path as both aircraft will have the same L/D ratio.
The heaver aircraft will need to generate more lift than the lighter aircraft. If the AoA is fixed at 4 degrees, the only way to generate that extra lift is for the heavier aircraft to fly at a higher speed. We now have two aircraft on the same glide slope flying at two different speed. Which aircraft will reach the ground first?
The answer is, the aircraft with the greatest airspeed - the heavy aircraft. Given that they both start off at the same altitude, the heavy aircraft will have the higher rate of descent.
what if I glide on the speed higher than the best lift/drag ratio speed?
Then you are not flying at the L/Dmax speed and range will be reduced
what if I glide on the speed below the best gliding speed?
Then you are not flying at the best L/Dmax speed and range will be reduced
Here is some background theory that might help.
On a glide descent, the forward component of weight (FCW), is equal the aircraft drag. But the FCW is a function of the glide angle. The steep the glide angle the greater the FCW and the lower the glide range.
So if you want max range on the glide, you need to fly at the speed for min drag - ie the speed at which the L/D ratio is at a max.
If Drag is at a min; the FCW is at a min; the glide angle is at a min; and range is at a max.