High hover height

If the pilot suffers a total power failure when hovering some distance from the ground his subsequent actions are somewhat different than those described above. He cannot simply bleed energy from the rotor in an attempt to arrest the rate of descent since the hover height is too high and he would achieve minimum sustainable rotor speed long before reaching the surface. Instead the pilot will initially dive the aircraft, increasing the kinetic energy of the vehicle at the expense of its potential energy, whilst attempting to maintain rotor speed within the power-off limits. At some altitude much lower than the original hover height he will conduct a cyclic flare EOL at a speed close to VMP. Since VCR is dependent on both aircraft and rotor performance (VMP and CL/s) it is possible to relate the high hover height (hHI) to this critical speed [2.28 and 2.29]. For the example light helicopter, hHI = 276 ft.

1.15.1 Scaled avoid curve

Using the four key data values: hLO, hCR, VCR and hHI, a scaled avoid curve can be constructed. Figure 2.40 shows such a curve for the light helicopter example and compares it with published data for an actual helicopter of broadly equivalent size and mass. Note that the avoid area expands somewhat if the AUM is raised from 1800 kg to 1900 kg (the limit of applicability of the published data) and a minimum power speed of 65 KTAS is used. Alternatively if the original VMP is used with the higher AUM and a longer delay time is assumed then although modified charts are required to determine VCR and hHI [2.6] a closer approximation can be achieved, see also Fig. 2.40. CR HI

image43

Fig. 2.40 Scaled avoid curves.

 

Chapter 3

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