Range and Endurance Lesson

Specific Air Range (SAR): Specific air range is defined as the number of air miles travelled per unit of fuel.

Specific Fuel Consumption (SFC): Specific fuel consumption is used to define an engine’s fuel efficiency. SFC is defined as the fuel flow per unit of power produced by the engine.

Range is all about getting the highest number of miles for the amount of fuel you have. In order to fly an aircraft for maximum range, the aircraft must abide by certain airframe and engine conditions. Maximum range is achieved at the TAS that allows either:

  • A maximum air distance for a given fuel burn-off; or
  • A minimum fuel burn-off for a given air distance (i.e., the lowest fuel burn-off / air distance ratio)

By converting fuel burn-off and air distances to rates, this ratio becomes fuel burn-off per unit / air distance per unit time, i.e., fuel flow / TAS.

Since fuel flow depends on power, the ratio becomes power/TAS, and maximum range will be achieved at the TAS for which this ratio is least. For this reason, the TAS gained at altitude could be advantageous. As we have seen previously, best range can be found on an aircraft’s power required curve. If you were to draw a tangent line to the power required curve, you would find the best lift–drag ratio and, therefore, range.

Airframe Factors:

  • Altitude: For a given weight, the aircraft will have a best lift–drag ratio. As altitude increases, both power and TAS need to increase for the best lift–drag ratio, but they increase in the same proportions, meaning that the lift–drag ratio remains unchanged. Therefore, altitude has no effect on range (airframe factors).
  • Weight: As weight increases, the angle of attack for a particular power setting increases, meaning drag increases. Power will need to increase to maintain TAS. But this increase in power is out of proportion to the increase in TAS required, meaning range decreases with an increase in weight.
  • Wind Velocity: Wind will affect range. With a tailwind, range will increase. With a headwind, range will decrease.

Engine Factors:

Engine factors revolve around trying to obtain the greatest specific fuel consumption.

  • RPM and Manifold: The best SFC will be obtained with a low RPM and a high manifold pressure.
  • Mixture: A lean mixture will obtain the best SFC.
  • Temperature: A cold temperature improves SFC with power being able to be produced at lower RPMs.
  • Altitude: A low altitude will obtain a better SFC than at altitude. Full throttle height (FTH) should be used where possible to obtain the best SFC. At a certain altitude, FTH will no longer be able to be maintained, meaning SFC will progressively get worse with increasing altitude.
  • Carburettor: Application of carb heat will increase the temperature of the air, decreasing SFC.

Have a look at this table from the C172R flight manual. This details the conditions to obtain best range. You should have a good working knowledge of how to obtain range from this graph.

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Endurance is all about time in the air. Maximum endurance means either:

  • The maximum time in flight for the given amount of fuel; or
  • A given time in flight for the minimum amount of fuel

It is appropriate to fly at maximum-endurance speed when the speed over the ground is not significant, for instance, when:

  • Holding overhead or near an aerodrome waiting to land; or
  • Carrying out a search in a specific area

Since fuel flow for an engine–propeller combination depends on the power set, minimum fuel flow (and therefore maximum endurance) will occur when minimum power to maintain steady straight and level flight is required.

Factors Effecting Endurance:

  • Altitude: Minimum power setting for straight and level flight will allow the greatest endurance. An increase in altitude will require a greater power setting for level flight, which reduces endurance. Therefore, the greatest endurance will occur at the lowest safe altitude.
  • Weight: Increasing weight will increase the required power setting for straight and level flight, reducing endurance. Therefore, the greatest endurance will occur at the lowest permissible weight.
  • Engine: Increasing RPM will increase fuel flow, reducing endurance. The greatest endurance will occur and the lowest permissible RPM at the leanest permissible mixture.

Practically speaking, in order to obtain best endurance, fly at the safest lowest altitude, at the lowest permissible RPM at the correctly leaned mixture.

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