Power Required and Drag vs Airspeed
Range and Endurance Flying, III
Maximum range flying is done so we can fly the maximum distance for the amount of fuel onboard. This can be set against distance covered through air or over ground. It is of interest to a pilot when he wants to fly with the least amount of fuel flow for a distance traveled to save fuel. Maximum endurance flying is done when the pilot wants to remain airborne for the maximum time possible given an amount of fuel.
The engine needs to run at its maximum efficiency to use the least amount of fuel for the distance flown. We will see what influences this.
Engine SFC Considerations
The get the best range or endurance we need to run the engine at its peak efficiency 1/SFC. There are a number of factors which have a pronounced effect on efficiency, they are: MAP and RPM, mixture, ambient temperature, altitude and carburettor temperature and air filter.
MAP and RPM
To achieve a certain amount of power there are a number of MAP and RPM combinations available to the pilot that will have the same result. The lowest SFC, however, is reached with the highest MAP and lowest RPM for that power setting. Low RPM reduces friction in the engine and improves the air intake (volumetric efficiency) and a high MAP also contributes to a good air intake (less restriction by the throttle valve). Limits are placed on MAP/RPM combinations by the engine manufacturer regarding the prevention of detonation. Just go by the book numbers and no harm is done.
Correct leaning of the engine during cruise is also very important to reduce gross fuel consumption, select best economy or the lowest fuel flow obtaining the maximum EGT/CHT permissible for the power setting.
Ambient temperature (OAT)
Cold air is more dense and improves the volumetric efficiency of the engine. The required amount of power can thus be obtained at a lower RPM if OAT is low and this will also reduce friction losses in the engine.
By the same token carburetor heat should not be used during range or endurance flying as this will increase intake temperature and reduce the efficiency of the engine. Any obstruction in the air intake, like an air filter or partially closed throttle valve, reduces the MAP. So if ram air is available, use it, because it can increase the full throttle height and TAS, again improving volumetric efficiency by pressurizing the intake.
The engine runs at maximum volumetric efficiency at that altitude where full throttle can be used for the power required, also known as full throttle height. The engine breaths better and power losses in the induction and exhaust system are much reduced. Higher altitudes also gives lower ambient temperatures. For those with a turbocharged engine, check the power performance graphs to see which altitude should be used for peak SFC as that will be much higher than without a turbo or supercharger.
To achieve the best possible SFC for range or endurance flying it is important that the aircraft has the least amount of drag power = drag x TAS so that the least amount of power and thus fuel flow is required.
To fly at maximum range speed we need to fly the aircraft at the minimum drag speed, it will require more power than endurance but it will provide the maximum TAS/power ratio and the highest distance for the fuel used.
Maximum endurance flying is done at the minimum power speed, there will be more drag but the least amount of power is used to fly level altitude. Fuel consumption is at its lowest so we can remain airborne for the maximum amount of time.