In level flight, assume L = W
At this weight, for minimum drag, it is necessary to fly at the maximum lift/drag ratio:
But L/D = CL / CD
The CL vs. α and CD vs. α curves for the aircraft are unique to that aircraft and independent of weight.
This means that, to fly at minimum drag speed, the speed must be that which maintains level flight for the weight of the aircraft at the time at the (CL / CD) max value of Angle of Attack. Thus if the weight changes, so will minimum drag speed.
For jet aircraft, best endurance speed is at (CL / CD)max, whereas best range speed is at (CL**-2 / CD)max. Best turn rate is at CL max . Hence in the F4 we flew by AoA for each requirement - no worrying about the exact speed, just decelerate to fly level at the appropriate AoA for either range or endurance. For best turn, full A/B and pull to the best turn AoA at the speed corresponding to max permitted +G!
Last edited by BEagle; 30th January 2008 at 15:53.