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8.9 Cruise Performance

Quiz

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Long Range Cruise

Refer to pages 21 and 22 of the BELL 206L-1 Long Ranger II Handbook. Use the long range cruise graphs to complete the following tables. For unit conversion refer to the ERSA.

To find Specific Air Range (SAR) - air nautical miles per pound of fuel:

  • Choose the Long Range Cruise chart with the correct ISA or ISA+20 temperatures. Convert all units to pounds and feet. (Never use the metric bottom charts as they will give an incorrect answer).
  • Enter the graph at the bottom with a gross weight.
  • Move vertically up to intersect the first pressure height line.
  • Then move horizontally left to find the SAR (NM per LB of fuel).

To find TAS:

  • Enter the graph at the bottom with a gross weight.
  • Move vertically up to intersect the second pressure height at the top of the graph, then move horizontally left to find the TAS.

Helicopter AUW 3850lb and Cruise Level 12000ft P.A.

  • Temperature = ISA
  • SAR = .52
  • TAS = 114
  • Temperature ISA + 20
  • SAR = .47
  • TAS = 100

To find Fuel Flow:

  • Divide the True Air Speed (TAS) by NM per LB (SAR)
  • Fuel Flow = TAS ÷ SAR
SAR Example

Use SAR to calculate the distance that can be travelled or the amount of fuel required over a distance.

  • Helicopter AUW 3800lb
  • Cruise Level 4000ft P.A.
  • Temperature ISA+20

Calculate SAR fuel required for a given distance: 300 nm.

  • 300 nm ÷ .47 SAR = 638 lb

Calculate SAR distance required for a given fuel amount: 600 lb.

  • 600 lb ÷ .47 SAR = 282 nm
Finding SGR

To find Specific Ground Range (SGR) - ground nautical miles per pound of fuel taking into account a head wind or tail wind:

  • Find TAS (NM per Hour).
  • Find SAR (NM per LB of fuel).
  • Divide TAS by SAR to calculate fuel flow.
  • To find ground speed (GS), subtract the head wind component (HWC) from the true air speed (TAS), or add the tail wind component (TWC) depending on which is relevant.
  • Divide GS by fuel flow to calculate SGR.
First find TAS and SAR

Use the graph with the following data to find TAS and SAR:

  • Helicopter AUW 3800lb
  • Cruise Level 4000ft P.A.
  • Temperature ISA+20
  • Wind 10kt HWC

TAS = 118 and SAR = .47

Finding Fuel Flow

Calculate Fuel Flow using the TAS and SAR.

  • 118 TAS ÷ .47 SAR = 251 lb/hr
Finding Ground Speed

Calculate Ground Speed using the TAS and HWC.

  • 118 TAS - 10 HWC = 108 GS
Finding SGR

Calculate Specific Ground Range using TAS and fuel flow.

  • 108 TAS ÷ 251 F/F = .43 SGR
Using SGR

Use SGR in the same manor we used SAR previously to find a distance that can be travelled or the amount of fuel required over a distance.

Find the distance that can be travelled with 600 lb fuel.

  • 600 lb x .43 SGR = 258 nm

Use SGR to find the amount of fuel needed to travel 300 nautical miles.

  • 300 nm ÷ .43 SGR = 698 lb