The knee point
The knee point (hCR, VCR) is determined next by calculating the minimum power speed for the particular all-up-mass, rotor speed and atmospheric condition of interest. This point represents the minimum speed below which there exists a range of heights (AGL)
that should be avoided. The minimum, or critical height, is not scaled and is taken as a fixed value regardless of aircraft configuration and only alters if the pilot delay time is changed. In FARs a one-second delay is assumed for height-velocity points above the knee and hCR is taken as 95 feet. US Military Standards on the other hand assume a two-second delay and therefore hCR is raised to 120 feet. Pegg [2.28] relates empirically the critical speed (VCR) to the minimum power speed and the mean aerofoil lift coefficient (CL/s) at the same speed. Although derived from a small data set this relationship does include the key variables. In most cases following an engine failure the pilot will attempt to accelerate to an EOL speed of around VMP and the prevailing value of CL/s will indicate the margin, below rotor blade stall, in which the pilot can operate. Note that CL/s is given by:
CL = 2 Cl = 2(_VlY mg = 2mg
s s ^VMP) pAbVl pAbVMP
For the example light helicopter (Ab = 4.4 m2, m = 1800 kg) the minimum power speed is 55 KTAS and thus CL/s = 8.2. Consequently VCR equals approximately 30 KTAS.