Why does best glide speed decrease with increased form drag, specifically gear/flaps
 

Hi all,

I'm trying to figur out why the best glide speed decreases with increased form drag.

I started thinking about it when I started training on a retractable landing gear airplane with the airspeed for best glide beeing lower with the gear extended.

Correct me if I'm wrong, I got this far: best glide speed is derived from the AOA corresponding to best L/D. With increased (form) drag the polar curve (CL as function of CD) moves right. The tangent from this curve gives the AOA for max glide range (as each point on the curve represents a specific AOA) and the glide angle is the angle between the vertical axis and the tangent line. Thus the tangent will give a slightly lower AOA for the max glide range but I can't figur out how this causes the best glide speed to decrease, or increase for that matter as I don't understand how the airspeed is derived from the AOA.

Can somebody help me out?

Thanks!

To answer your AOA question first, if we deal with shallow glide angles, approximating level flght, then if AOA goes up speed goes down and vice versa

As to why best angle of glide speed goes down with extra profile drag it is because you want to fly at minimum drag speed, Vimd, which is lower at higher profile drag values. Not minimum drag! That has gone up. It's just Vimd that has gone down

Dick