Hi Bob,

I am a little stumped by this question.
Refer to the diagram at left. The speed marked S would represent.
A) Maximum endurance in level fligh
Minumum litres per hour in level flight
C) Maximum nauticle miles per litre
D) Minimum speed in level

C is correct but to me the first 3 are all the same.

Am i missing something obvious here?

Kind regards,
Jason

Yes "C" is the correct answer.

Point "S" on the curve is V_md (V _{min drag}) and will give the best "Specific Air Range" (SAR) - ie best range per unit of fuel burnt in still air. Note that in level flight T=D.

Answers A & B are the same and relate to max endurance. Answer B is just another way of saying max endurance.

Max endurance occurs when the "Specific Fuel Consumption" (SFC) is at a minimum - ie when the fuel burnt per unit time (ie FF in lt/hr) is at a minimum. For a piston engine aircraft this typically occurs at V_mp - ie V _{min power}.

Note that V_md is not the same as V_mp.

The above only considers the airframe. When the engine-airframe is considered as a whole the story changes a little.

I hope this helps!

The curve shown is the THRUST curve. So the speed indicated is minimum THRUST, that's the same as minimum DRAG which is maximum RANGE in still air. Maximum range is the same as maximum nautical miles per litre. (a) and (b) both refer to maximum ENDURANCE which is minimum POWER not minimum THRUST.

(a) and (b) would both be correct if the curve shown had been the POWER curve.

Thank you Bob! That clears things up for me a bit.