Collective and Cyclic Pitch for Trim

A fallout of the calculations for power required is the collective and cyclic pitch associated with the trim points. Figure 4.41 shows these for level flight at sea level at the example helicopter’s design gross weight.

Cruising Flight

There is no single accepted definition of cruise speed. Depending on the situation, it may mean the speed for maximum range, the speed for 99% of maximum range,

the speed at maximum continuous power, or any other continuous speed required to do a specific mission.

The speed for maximum range may be determined using a plot of fuel flow vs. forward speed such as the one for the example helicopter on Figure 4.42. The speed for maximum range is the speed at which a ray through the origin is tangent to the fuel flow curve for this case, 114 knots. Note that this is slightly faster than the speed for maximum equivalent rotor lift-to-drag ratio in Figure 4.40. The reason is that a turbine engine is more efficient at high power than at low power because of the fuel flow that must be used just to keep the gas generator spinning, regardless of the power output. The effects of head – and tailwinds are also shown. A headwind requires a higher cruise speed than no wind, and a tailwind a lower speed. The specific range, S. R., is the distance flown whil? burning one unit of fuel. It is generally expressed in nautical miles (N. M.) per pound of fuel and is determined as:

s R Ground speed RM./hr N. M.

Fuel flow Tb/h^ ’ lb

If one knows the engine power required as in Figure 4.38 and the engine fuel consumption characteristics as in Figure 4.3, the specific range can be determined

as a function of gross weight, altitude, and forward speed. (Note: For multiengine helicopters, divide the power required by the number of engines before going to the fuel flow curves.) The calculation has been done for the example helicopter, and the results are plotted on Figure 4.43. At low gross weight, maximum range is obtained at high altitude whereas at high gross weight it is obtained at sea level. At low gross weight and low altitude, the average blade element angle of attack is below the condition for the maximum lift-to-drag ratio. For this case, a higher value of CT/o obtained by flying at a higher altitude is beneficial. This is analogous to the dependency of hover Figure of Merit on CT/o, which was discussed in Chapter 1. The better specific range at low gross weight and high altitude can be important during a long flight in which the helicopter is allowed to drift up as fuel is used. One word of caution: if the cruise altitude is expected to be over 15,000 ft, oxygen should be provided for the crew.

The speed for maximum range corresponds to the maximum specific range, but it is common to use the speed to the right of the peak where the specific range is 99% of maximum, the rationale being that the mission time can be shortened with little sacrifice of economy by flying at this speed.

Figure 4.44 is a cross-plot of Figure 4.43 showing the effects of gross weight and altitude on the 99% maximum specific range value. The cruise performance is better with one engine than with two. This is due to the characteristics of a turbine engine, which is more efficient near full power than at partial power since a large part of its energy is consumed internally in driving the compressor. (This effect is not so pronounced on reciprocating engines.) The single engine advantages are generally not used because of the risk of not being able to restart the dead engine in an emergency. It does, however, give the pilot an option—for example, in stretching the range at the end of a long over-water flight.

The distance traveled at the speed for 99% maximum specific range while burning a given amount of fuel can be found by integrating the area under the specific range curves of Figure 4.44, since: