Heres my Question...
> I understand that Endurance is greatest when FuelFlow is least and
FF = SFC x Thrust I dont understand the part where SFC increases with temperature .. SFC = FF/THRUST .. wont an increase in temperature (decrease in Density) effect both THRUST and FF(fuel flow) equally ?
If temperature increases this will cause the air to expand. So if the engine RPM remains constant, the mass flow rate of air passing through it will decrease. This will decrease the thrust. In order to restore the thrust to balance the constant drag, we must increase the fuel flow. This will reduce endurance directly.
Another way of looking at the situation is to consider what is happening to the SFC.
SFC is the mass of fuel that is used per hour to produce each unit of thrust (for a jet engine) ot power (for a piston/prop or turboprop engine). So if SFC increases (at constant thrust or constant power output), the fuel flow will increase and the endurance will decrease.
SFC is affected by a number of factors including the following:
ENGINE RPM
For jet engines SFC is typically lowest between about 90% to 95% RPM. For pistons it is lowest when the throttle if fully open to minimise inlet restrictions.
AIR PRESSURE AND DENSITY
SFC is lowest when operating in high pressure, high density air. (You will probably find that your performance notes are wrong on this matter.)
AIR TEMPERATURE
SFC is lowest when operating in very cold air. The colder the air the lower the SFC.
So if air temperature increases the SFC increases. This means that although our required thrust (which is equal to drag) remains constant, the mass of fuel that we must burn to produce that thrust has increased. So endurance decreases as air temperature increases.