The
axis of the power V TAS curve can be broken down as follows:
Power is Thrust x TAS so we can show our power available lines for the jet and
prop.
Power required is drag x TAS so we can show the power required curve. (Note
that it is derived from the 'typical' drag curve (for simplicity) which is EAS
squared. As the speed is increased, TAS in this case, then the power required
is proportional to EAS cubed.)
The tangent to the power required curve gives the best ratio of drag x TAS to
TAS. Cancel out the TAS (mathematically) and we're left with drag, and as this
was the best ratio of drag x TAS to TAS then it must be the point of least
drag, Vmd.
A jet flies at Vmd for max endurance.