TonyR,
Each aircraft has a "minimum drag speed", This is the slowest speed that the aircraft will mantain level flight at a given power setting, (just before the drag curve beats you)
I don't understand why you say this. Vimd (min drag) is the best glide, best range and best rate of climb speed for a prop a/c! It is the point on the drag curve where power available it at its most over power required.
If you know this speed for your aircraft then it can also become your minimum approach speed.
Mind drag speed (Vimd) has nothing to do with approach speeds! Vimd on the AA5 is 80kts, approach is 70kts with 20 flap. 75kts with no flap. Vimd for a C152 is 65kts, approach is 65kts with 20 flap and 70kts clean –c oincidence.
Also what do you mean by beats you? Vimp (min power) is the best angle of climb speed for a prop and it is slower than Vimd. I assume you mean that aircraft speed becomes more unstable when slower than Vimd?
The CAA recommends that the minimum approach speed is 1.3 x stall speed for your configuration. For PT flights that figure is the mandatory minimum! But that may not be the best approach speed when matched with the correct speed over the threshold (Vat). These figures are in the POH and surely should be understood and followed??
The point of increase by the 1.3 factor is to increase safety margins. This accounts somewhat for static port/pitot port errors due to the a/c attitude and configuration as you only see IAS and not corrected airspeed as well as unexpected small variations in wind direction/speed. Also, if the wind is a significantly variable or wind shear is possible I think you would be bonkers not to add a bit extra on.