Capt P.B.,
Sorry for the poor discription with regard to energy "conversion"!!!
If you don't have an engine to provide that nice arrow forward labled "thrust" then you are in a glider and you need to use gravity insted.
I have always tried to use "thrust" since that avoids efficiency issues.
--------
C. G. B.
Thank you for the diagram.
As shown in your diagram, total drag varies with airspeed. In order to maintain a constant speed, the thrust force along the flight path must be equal to the drag force which acts opposite along the flight path.
In your diagram, the drag at the point you marked "Minimum Power Speed" is shown to be greather than the drag at the point marked "Minimum Drag Speed".
If the drag is greather then in order for the speed to be constant, the opposing force must be equal. Thrust opposes drag.
Perhaps you prefer to lok at the diagram as follows;
An aircraft is flying in steady flight at the minimum drag speed.
If a gust disturbs the aircraft and causes the speed to increase by say 10 Knots, then since the drag has increased while thrust has remained the same, the speed will tend to return to the minimum drag speed (stable).
If however, a gust causes the speed to decrease by say 5 knots then drag has increased and again thrust has remained the same, speed will reduce further (unstable).
The above speed stability cases (front and back of drag curve) show that the thrust required for a constant speed is higher at speeds both above and below the minimum drag speed.
-----------
The bottom of the drag curve is minimum drag and hence the best endurance speed for a jet aeroplane (B747 in my examples above) and speed for the flattest glide (glider).
If you want to talk about minimum power for a piston engine then you need to plot a power required curve which will be different from the drag curve and that is a different debate I think!
Regards,
DFC