Hi there,

I've been going through the ebook RPL/PPL Volume 1 and have run into an explanation regarding weight & glide distance I can't quite figure out.

To begin with it states that "there is one particular airspeed which produces the least drag for the lift being made-the best lift/drag ratio" (page 90). On page 91 it says "It is the angle of attack that decides the lift/drag ratio". This makes sense in a condition of no wind.

However, it then goes on to say that "the heavy aircraft glides faster than the light one" (page 91). This implies it would have a higher airspeed, and thus more drag. How can it then follow the same glide path?

Could anyone help clear this up for me?

Cheers
Rob

There is only one angle of attack that produces the best lift/drag ratio. However the lift required varies with the weight of the aircraft.

If the aircraft is heavy it requires more lift in both level flight and in a steady descent.

There are only two factors under the pilot's control that produce lift (and drag). They are angle of attack and indicated air speed.

It is the angle of attack that must be kept constant if the best lift/drag ratio is required. Therefore, more lift will require more indicated air speed and less lift will require less indicated air speed.

A heavy aircraft will have to glide faster if angle of attack is to be kept constant. It is true that will result in more drag, but since both lift and drag will increase by the same proportion, the lift/drag ratio will remain the same.

The statement that there is only one speed required to achieve a particular angle of attack is true only for a given weight.

Hi Bob,

Thanks for taking the time to answer this! The critical part I was missing was:


I didn't realize that the best glide speed is not constant but is also influenced by weight. Thanks again!

Cheers
Rob

Of course in light aircraft the different speeds for different weights is ignored because the weight range of the aircraft is quite small so one average speed can be used for simplicity.