Power Required and Drag vs Airspeed
Range and Endurance Flying, I
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.
Flying for range or endurance can take the aircraft and maybe the pilot to their limits, do this safely.
Range flying
For maximum range flying you must operate the aircraft and engine in such a way that maximum efficiency is obtained over the distance to be flown. Do that by keeping the following factors in mind:
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Specific Range
We have two ways of expressing specific range: as Specific Air Range, which is air nautical miles per fuel unit (SAR) or as Specific Ground Range, which is ground nautical miles per fuel unit (SGR). SGR is SAR influenced by wind velocity.
If we include time (hour) in these definitions then we get the following: nautical miles / time = TAS (kts) and fuel / time = fuel flow (USG).
Specific Air Range (TAS / fuel flow)
To achieve the best possible SAR we need to get the best TAS / fuel flow ratio and as fuel flow relates to power we can also say TAS / power ratio.
Engine gross fuel consumption or fuel flow (GFC) depends on how much power is requested related to its specific fuel consumption (SFC), thus GFC = SFC x power. Combining these formula results in: SAR = TAS/power x 1/SFC.
Thus, we need to fly the aircraft to its maximum airframe efficiency TAS / power and engine efficiency 1 / SFC. Maximum airframe efficiency is at the speed for minimum drag/maximum L/D ratio. Usually equal to VY. For aircraft handling the best range speed is about 10% higher, sometimes known as the 'long range cruise speed'.
Altitude
As the minimum drag/maximum L/D ratio doesn't change with altitude (TAS and power required increase by the same amount) so therefore there is no effect on SAR. Even the IAS remains the same. Only tailwinds at altitude could mean an advantage.
Weight
Weight does have its toll on SAR. As weight increases, the angle of attack for the best L/D ratio is at a higher IAS (thus TAS is higher), more speed means more drag and more power is required and it is out of proportion because power = drag x TAS. SAR is lower with an increase of aircraft weight.
Specific Ground Range (GS / fuel flow)
To take into account the distance covered over ground we need to apply the headwind component to the TAS to arrive at groundspeed (GS). It is easy to see that a tailwind favors specific range resulting in a higher SGR. GS is used to find the best SGR, thus GS / fuel flow or GS / power.
Endurance flying
Endurance is obtained when using the least amount of fuel to maintain a given altitude, so in this case time is important and not distance as with range. Which happens when holding for weather or traffic to clear.
The speed used for flying endurance is at the bottom of the power required curve, that's almost VX. This is the speed where the aircraft has the minimum sink speed during glide also known as the best glide speed, Vglide. Using minimum power to maintain altitude translates in minimum fuel flow and therefore the maximum time aloft.
Altitude
Maximum endurance is flown at the lowest practical altitude, because endurance is flown at a IAS for a given weight. And as TAS increases (with the same IAS) with height and power = drag x TAS you will need more power (fuel) at a higher altitude thus reducing endurance. Stay low for endurance flying.
Weight
More weight means more power required. Minimum speed increases with a weight increase so endurance suffers. So the message is simple: loose the weight or loose the endurance.
