Maximum Ferry Range

The calculation of the maximum ferry range uses the same general procedures as for the payload-range curves. Typical special considerations that might be used, however, are:

Conditions

No wind

Sea-level, standard day

Cruise at speed for 99% max. range

2-rnin warmup and takeoff (WUTO)

at max. cont. power

30-min reserve for cruise speed

at landing weight

Min. op. wt. = 10,430 lb

Std. fuel tank capacity = 3,000 lb

Auxiliary fuel tank

weight = 0.1 x fuel wt.

400 800 1,200 1,600

Range, N. M.

FIGURE 4.45 Payload-Range. Example Helicopter

• 20-knot headwind.

• Cruise at speed and altitude for best range.

• Warm up and takeoff: 2 min at maximum continuous power.

• Reserve no. 1: 45 minutes at speed and altitude for best range at minimum operating weight.

• Reserve no. 2: 10% of fuel used for cruise over 3 hours.

• External tanks may be dropped when empty.

• Noncritical items of equipment may be left behind (weapons, passenger seats, etc.).

• Special items of equipment may be required (life rafts, oxygen, etc.).

For illustration, the maximum ferry range of the example helicopter will be determined using these conditions. The assumed takeoff gross weight is 28,000 lb, which is almost the maximum that an be flown in level flight on maximum continuous power.

The Group Weight Statement for the ferry mission in Appendix A can be used to find the minimum operating weight as:

Empty weight 10,000 lb

Crew 360

Unusable fuel 30

Engine oil 40

Auxiliary cabin tank 481

Survival kits 100

Lift rafts 50

Oxygen equipment 200

Minimum operating weight 11,261 lb

The total usuable fuel for the ferry mission is contained in five tanks:

Forward internal 1,485 lb

Aft internal 1,485

Cabin auxiliary 4,809

First external 4,091

Second external 4,091

Total 15,961 lb

Each external tank weighs 409 pounds and is assumed to have an equivalent flat plate area of 2 square feet.

Figure 4.46 shows the specific range plot based on these conditions. The maximum range is calculated by integrating under the curve from the mission start weight, which is the takeoff gross weight minus fuel for warmup and takeoff, and the landing weight, which is the minimum operating weight plus reserves. The reserves consist of two amounts:

Reserve no. 1 = Fuel for 45 minutes at minimum operating weight.

Reserve no. 2 = .1/1.1 [Total fuel — (Fuel for first 3 hr + WUTO + reserve no. 1)]

For this example, reserve no. 1 is 420 lb when calculated at 11,261 lb, 20,000 ft altitude, and 120 knots air speed. The fuel required for the first 3 hours was found by calculating the distance and time represented by burning 1,000-pound increments of fuel from takeoff. In the first 3 hours, 4,400 lb of fuel will be used. Using a warm-up and takeoff (WUTO) fuel allowance of 56 lb, reserve no. 2 becomes 1,008 lb. Thus the landing weight is 12,689 lb, and the takeoff weight is 27,944 lb. Integrating the envelope on Figure 4.46 between these two weights and accounting for dropping the external tanks when empty gives a maximum ferry range of 1,289 nautical miles.

In this example, the optimum altitude is sea level until more than half the fuel is burned off. At that time a slow climb is initiated, which reaches 15,000 feet at the end of the mission. The extra fuel required for this climb can be calculated and subtracted from the fuel available for cruise; but unless it is a significant quantity it can be ignored by assuming that the same amount of fuel is saved during the descent at the end of the flight.

Gross Weight, lb

FIGURE 4.46 Ferry Mission Conditions, Example Helicopter