It is certainly the case that if DP is proportional to EAS squared and DI is proportional to 1/EAS squared, throughout the entire speed range, and we ignore all other possibilities, then the best range will be at 1.316 VMD. Unfortunately this is rarely the case.
For DP to be strictly proportional to EAS squared, it is necessary for CDP to be constant. And for DI to be strictly proportional to 1/EAS squared it is necessary for CDI to be proportional to 1/EAS to the fourth power.
But above MCDR we get an increase in drag that is not described
by these equations. If this increase is very large then the right side of the drag curve will be very steep. It is entirely possible that this will cause best range to occur at a speed lower than 1.316 VMD.
If however the aircraft is designed for high speed flight, with for example area rule, supercritical wings, or swing wings, then the increase in drag above MCDR is unlikely to be so severe and MCDR will itself be greater. This will make the right side of the drag curve less steep, and more like the theoretical shape.
The use of things such as winglets will also affect both the DP and DI, thereby changing the shape of the drag curve. The exact effects in this case will depend upon the speed for which the winglets have been optimised.
Because of all of these possible factors, it is far too simplistic to simply state "the tangent to the drag curve is always at 1.32vmd, it's calculus".
As I said in my original contribution to this string "the 1.32 VMD figure is just conventional wisdom". It is reasonable starting point when teaching the subject or making initial guestimates. But to get really accurate results we must take all of the relevant factors into account.
Last edited by Keith.Williams.; 6th March 2005 at 15:53.