The helicopter must follow the basic laws of physics, so a good place to start is with the physics. The rotor takes a stagnant mass of air from above the rotor and accelerated it to a given velocity below the rotor. in this sense it’s a force-mass-acceleration, that is one of Newton’s laws (i.e. Force = Mass x acceleration).
Therefore, we can work with either the former or latter part of that equation in order to obtain the force i.e. thrust/lift that we need. In the former, we can lower the disc load by increasing the rotor diameter, thereby working on more air (larger air mass) per second or the latter by working to increase the velocity through a given rotor diameter.
Rotor thrust = (Mass flow per second) x (total change in flow velocity)
Low Disk Loads:
Low induced velocities
Low autorotative rate of descent
Low power required to hover
Higher power loading
High Disk Load:
Compact rotor size
Low Empty weight
Low hub drag
It’s a series of trade-offs. Rotor systems optimized for high speed forward flight may be far less efficient in hover performance. Moreover, a given rotor with excellent hover performance may be very inefficient at high forward flight speeds. Hence, there is a balance between being efficient and effective. Most helicopters are designed with the scales tilted more toward effectiveness for a given task. Compared with other aircraft, like the airplane, helicopters in general are not as efficient.
The relationship between Disk Load (DL) and power required is given in the following equation. Also note the Figure of Merit (F.M.) and its importance in the power required. Figure of Merit (F.M.) – measure of efficiency of a hovering rotor. The geometry of the rotor basically determines the figure of merit. Things like, tip speed (function of rotor rpm and rotor radius), blade area, number of blades, taper, tip shape, twist, and airfoil section. In a perfect world, the F.M. would be 1.0; however, 0.75 - 0.80 is the general average range.
T = thrust or helicopter’s weight, since in any steady state thrust/lift must equal weight.
p = air density in slugs per cubic foot (@ sea-level p = 0.002377).
Edited by iChris, 26 August 2017 - 12:21.