Disk Loading and Power Loading

A parameter used frequently in helicopter analysis that appears in the preceding equations is the disk loading, T/A, which is denoted by DL. Because for a single rotor helicopter in hover, the rotor thrust, T, is equal to the weight of the helicopter, W, the disk loading is sometimes written as W/ A or W/nR2. Disk loading is measured in pounds per square foot (lb fir2) in British (Imperial) units or newtons per square meter (N m~2) in the SI system. One may also use kilograms per square meter (kg m~2) in the SI system. The direct use of the kilogram (kg) as a unit of force is frequently found in engineering practice, particularly in the aerospace field. For the purposes of computing the disk loading for multi-rotor helicopters such as tandems and coaxials, or for tilt-rotors, the convention is to assume that each rotor carries an equal proportion of the aircraft’s weight. Values of rotor disk loading for a selection of rotating-wing aircraft are given in the appendix.

The power loading is defined as T / P, which is denoted by PL. Power loading is measured in pounds per horsepower (lb hp-1) in Imperial units, or newtons per kilowatt (N kW-1) or kilograms per kilowatt (kg kW-1) in the SI system. Remember that the induced (ideal) power required to hover is given by P — Tvn. This means that the ideal power loading will be inversely proportional to the induced velocity at the disk. To see this, recall that the inflow velocity at the disk vt can be written in terms of the disk loading as

(2.24)

According to Fig. 2.6, the ratio T/P decreases quickly with increasing disk loading (note the logarithmic scale on the abscissa). Therefore, vertical lift aircraft that have a low ef­fective disk loading will require relatively low power per unit of thrust produced (i. e., they will have high ideal power loading) and will tend to be more efficient; that is, the rotor will require less power (and consume less fuel) to generate any given amount of thrust. Calcu­lation of the actual power loading and rotor efficiency, however, requires the consideration of viscous losses.

Figure 2.6 Hovering efficiency versus disk loading for a range of vertical lift aircraft.

Helicopters operate with low disk loadings in the region of 5 to 10 lb fir2 or 24 to 48 kg m-2, thus they can provide a large amount of lift for a relatively low power with a power loading up to 5 kg kW^1 (50 N kW_1 or 10 lb hp_1). Figure 2.6 shows that the helicopter is a very efficient aircraft in hover compared to other vertical takeoff and landing (VTOL) aircraft. Tilt-rotors can be considered a hybrid helicopter/fixed-wing aircraft and have higher rotor disk loadings. Therefore, they are less efficient in hover than a conventional helicopter of the same gross weight but still are much more efficient than other VTOL aircraft without rotating wings.