The minimal vertical rate of descent at which flight is safest is achieved when gliding at the economical speed. However, if the engine fails while hovering, the helicopter speed V = 0. In order to transition into a glide at a speed close to the economical speed, some altitude must be lost in
order for the helicopter to acquire a definite kinetic energy E =
Only part rather than all of the helicopter’s potential energy is used in acquiring the velocity (approximately two tenths of the total potential energy). The remaining energy goes to overcome parasite drag and main rotor profile drag, to turn the tail rotor and the accessories. The total potential energy of the helicopter is
Eb = GH,
where G is the helicopter weight;
H is the helicopter flight altitude.
We find the kinetic energy with loss of altitude from the formula
0.2GH = gl, 2g
Hence, we find the safe helicopter hover height
But experience shows that an additional height margin of about ten meters is required for the landing maneuver; therefore, the formula for determining the safe hover height takes the form
Example. The economical flight speed for the Mi-1 helicopter is Vgc = 90 km/hr or 24.8 m/sec. We find the safe hovering height.
If the helicopter has translational velocity in the horizontal direction prior to transition to the main rotor autorotative regime, the safe height is found from the formula 196
For example, the Mi-1 helicopter is flying horizontally at a speed of 70 km/hr or 20 m/sec. In this case, the safe height is defined as
Therefore, in case of engine failure in horizontal flight or in climb along an inclined trajectory, less altitude is required for transition into the autorotative regime than for transition into this regime from hover or when performing vertical climb or vertical descent with the engine operating. After determining the safe heights for transition into the autorotation regime for different flight speeds, we can plot the safe flight height diagram (Figure 84).
This diagram shows the danger zone, and we see that the safe hover height is up to ten meters or above 200 meters. The safe hover and flight height limitation at low speeds makes the use of helicopters at low altitudes difficult in practice. It is not advisable to fly, the helicopter in the danger zone except in extreme emergencies.