Retreating Blade Stall Limitations

The preceding section showed that retreating blade stall results in a severe increase in the required power. An equally important effect of the onset of retreating blade stall is the accompanying increase in the vibration level and blade stresses. It is therefore important in the design of a helicopter that the rotor be selected to avoid the occurrence of retreating blade stall throughout the intended operating regime.

It has been found that the inception of retreating blade stall can be pre­dicted relatively simply by assuming that the loading distribution is similar for all rotors. It follows then that the section lift coefficient near the tip of
the retreating blade is proportional to the blade loading and the resultant tip velocity.

c kWb

,m“ (p/2)(cdR – V)2’

where к = constant of proportionality, wb = blade loading.

With some algebraic manipulation this can be written as

The constant of proportionality varies with the blade twist, for varying the twist shifts the loading in – or outboard on the blade. From a series of power-required calculations, as previously described, the constant к is given

by

к = 3.17 – 2.79T,

where вт = total twist in radians, usually negative.

Consider the example helicopter just studied in terms of Eq. (5-51):

— 6Cr CL = —T – = 0.555, a

C, = 1.25,

(max 7

к = 3.5.

Thus

С, к

C, 3

•max

detail = 1 — 0-72
= 0.28.

Indeed, it was found that the rotor suffered a power loss at p = 0.31, which is in excess of this value.