Consider the following –
Aircraft need Thrust, they always have, and probably always will.
Aircraft don’t need Power, they never have, and probably never will.
Jet engines DIRECTLY produce Thrust, and Fuel Flow is directly related to Thrust output.
Propeller engines DIRECTLY produce Power, and Fuel Flow is directly related to Power output. The propeller converts Power to Thrust as a function of speed (TAS).
Power = Force X Velocity, i.e. Power = Thrust X TAS, so, for the propeller aircraft –
Thrust = Power Divided by TAS, as TAS increases, Thrust decreases.
Speed Effect for propeller aircraft is that Thrust declines directly with speed, not quite linear due do propeller characteristics, but slightly kinked.
Speed Effect for Jet aircraft is that Thrust is fairly constant with speed, but declines slightly up to about M0.5, and then increases again.
If we plot Thrust available from a propeller on a Drag Vs TAS graph, a constant quite steep decrease is noted with increasing speed. If we plot Thrust available from a jet on the same Drag Vs TAS graph, it is almost ‘flat’, but dips slightly, descending slightly at typical speeds in the vicinity of VMD.
A picture is worth a thousand words –
For the Jet, we note a very shallow negative gradient for the Thrust line, leading to tangency with the Thrust Required curve very slightly below VMD. For practical purposes, it is VMD, and, in the interests of speed stability, a speed slightly above VMD will result in negligible fuel penalty. (For the purists, if the holding is at a considerably higher Altitude where VMD was above about M0.5, Best Holding Speed would be slightly above VMD).
For the Propeller however, we note a quite steep negative gradient for the Thrust line for a given Power, leading to tangency with the Thrust Required curve somewhat below VMD. Flight at VMD would require considerably more Power, and be fuel inefficient.
The relative constancy of jet thrust with increasing speed, compared to the rapid decline in thrust from the propeller with increasing speed, serves to illustrate why Sir Frank Whittle went to all the bother of inventing the jet engine.
Best Regards,
Old Smokey