Range And Endurance

Question 1

Define specific air range and specific fuel consumption

Answer 1

Specific air range = air distance flown / fuel used

Specific fuel consumption = gross fuel consumption / power used

Question 2

State the general conditions for achieving maximum SAR. (6)

Answer 2

• Highest MAP/lowest RPM combination within the recommended range
• Use lean mixture
• If possible, avoid continued use of carburettor heat and hace ‘ram air’ selected
• Minimise drag
• Fly at the lowest gross weight.
• Fly at the power setting/altitude/TAS at which indicates the best gnm/USG that is obtainable. (Full throttle height)

Question 3

Explain airframe considerations of flying for range (effect of altitude,weight and wind velocity) and what is the best range speed?

Answer 3

The maximum airframe efficiency and ​best range speed​ ​occurs at the speed for minimum drag/maximum L/D ratio.

Effect of altitude​: Altitude has no effect on the best range IAS but the ‘best range’ TAS increases with altitude.

Effect of weight​: Increase of weight means that the AoA for the best L/D ratio is reached at a higher IAS; the speed for best range has thus increased.

This increase in IAS means an increase in TAS, drag has also increased and as PR=drag x TAS the power required has increased thus. Range is therefore reduced if weight is increased.

Effect of wind velocity​:
A headwind component will decrease the range achieved while a tailwind component will increase it.

Best Range: (maximum TAS/power ratio)

Question 4

Engine considerations for flying for range

Answer 4

to achieve maximum range the engine must be operated so that the power required is produced in the most efficient way with the minimum specific fuel capacity (SFC)

Question 5

Factors affecting the specific fuel capacity. (RPM & MAP, mixture, altitude, temperature, carburettor air intake)

Answer 5

RPM and manifold pressure (MAP):
​The lowest SFC is obtained by using the lowest rpm with the highest MAP within the allowable engine limits.
- Low RPM: Use of low rpm reduces friction losses and improves volumetric efficiency.

• High MAP: Maximum MAP for the rpm being used is limited by the cylinder pressures above which a rich mixture must be used for cooling for the prevention of detonation.

Mixture strength:​
Lean mixtures and the power settings that permit are essential to achieving the lowest SFC

Altitude:
​Higher altitude the engine breathes better and the power losses due to friction in the induction and exhaust system are reduced.

Temperature:
​Cold air at a given altitude improves the SFC, since the power available can be achieved at a lower rpm and the power required by the airframe reduces as TAS is reduced.

Carburettor air intake: ​application of carburettor heat is used the SFC will deteriorate.

Question 6

Define flying for endurance and where it would be found on the PR curve

Answer 6

remaining in the air for the greatest amount of time for the least amount of fuel used.

The speed for maximum endurance is the TAS that coincides with the bottom of the power required curve.

This is the speed at which the minimum amount of power (and the lowest fuel consumption) is required to maintain level flight.

Question 7

Differentiate between range and endurance flying

Answer 7

• Range flying is to get Maximum range in level flight is achieved when the greatest distance is covered for the amount of fuel used.
• Endurance is the most amount of time in the air for the least amount of fuel used.

Question 8

State the factors affecting endurance (altitude, weight and engine considerations)

Answer 8

Effect of altitude: ​
The minimum power speed will coincide with a given IAS for a particular weight. At a higher altitude, drag will be the same at that IAS but the TAS will be increased. As power = drag x TAS, power required and gross fuel consumption to maintain the endurance speed increases with altitude. Thus best endurance is achieved at the lowest safe altitude.

Effect of weight:
​Minimum power speed increases with increased weight, the drag at higher speed also increases. Thus power required and gross fuel consumption both increase because of the higher drag and TAS, endurance thus decreases with increased weight.

Engine Considerations:
​For maximum endurance, the engine must be operated with minimum gross fuel consumption (minimum fuel flow). Acheived by using lowest permitted rpm in the lean range, with MAP adjusted to maintain the ‘minimum power speed’.

Question 9

Explain practical endurance flying techniques (5)

Answer 9

• Fly at the recommended gliding speed, with small adjustments to power, determine the lowest setting that will comfortably hold the aircraft in level flight.
• Use the lowest permitted rpm for the lean range which will give smooth running, and enable the generator/alternator to charge. Adjust MAP to maintain the selected
  speed.
• In turbulent conditions fly at slightly higher speed
• Ensure the mixture is correctly leaned
• Fly at the lowest practical altitude.

Question 10

What are the best endurance and best range speeds?

Answer 10

Best endurance is found at the lowest point for PR/TAS curve (bottom of the curve)

Best range is found at a tangent to the PR/TAS curve, (maximum TAS/PR) or the best L/D ratio point.