Range performance

As will be seen, the ability of a helicopter to cover distance efficiently depends on the forward airspeed flown. This dependence is overcome by expressing the range perfor­mance of an aircraft in terms of its specific air range (SAR). Specific air range is defined as the distance covered (nautical miles) per unit of fuel and typical units are nm/kg or nm/lb. Now the distance covered at constant TAS (V) is given by Vt, where t is the time spent in the cruise condition. Likewise, the total fuel burn will equal Gt, where G is the fuel flow rate. So the definition of specific air range becomes:

„ distance covered Vt V

SAR =_______________ —__ —__

mass of fuel used Gt G

Introducing the specific fuel consumption (SFC), s, which is the fuel flow rate per unit power (kg/kW h), gives:

V — V_ G — sP

The power required for level flight can be divided into four components: the induced power (Pi); the parasite power (Ppar); the rotor profile power (Ppr), and the power required to drive the tail rotor, accessories and overcome transmission losses (Plosses). If the lost power is assumed to always equal a small and fixed percentage of the total power required, then:

SAR B s(P + Ppar + Ppr)

An important factor in the analysis of range performance is the variation in SFC. However, if it is assumed to be constant over the range of interest then SAR can be related directly to the variation of power required with TAS. Specific air range will be a maximum when VIP is a maximum or when P/V is a minimum. Consequently, in the absence of any variation in SFC the speed for maximum range may be obtained
from the power curve. It has already been shown that for a simple helicopter the power required by the main rotor can be expressed in the following form (Equation (2.17)):

Cp — Cpj + Cppar + Cppr + Cpm + Cps

Using typical empirical data, assuming small disk tilts and neglecting the effects of reversed flow, compressibility and blade stall:

1 f 1

CP — 1.2C + 2 ц3 A + 2 5Cd(1 + 4.3ц.2)

or:

Thus, the range performance will depend in some manner on the weight of the helicopter, the rotor speed, the external configuration and the true airspeed.