Ideal Power

The minimum—or ideal—power required to produce rotor thrust may be determined from momentum considerations as the energy per second dissipated at the rotor:

Er/scc = (Force) (Velocity) = T vx ft lb/sec

Since 550 ft lb/sec is the equivalent of one horsepower, the ideal power is:

or

h. p

At sea level, the ideal power is:

Ty/51.

38

From this equation, it may be seen that for a given rotor thrust, the higher the disc loading, the higher the power required. During the early days of helicopter development, when power plants were relatively heavy, designers minimized the power required by using low disc loadings of 2 to 3 lb/ft2. The introduction of lightweight turbine engines has made it possible to put less emphasis on low power requirements and more emphasis on designing compact helicopters with minimum structural weight. For this reason, over the years disc loadings have been increasing steadily until values of 10 to 12 lb/ft2 are considered feasible for helicopters that do not have to operate close to loose surfaces.