OPTIMUM TAKEOFF PROCEDURE AT HIGH GROSS WEIGHTS

A study of the takeoff maneuver of helicopters too heavily loaded to hover out of ground effect is reported in reference 5.17. The conclusions of that study are that the shortest—and safest—takeoffs are achieved by accelerating into forward flight with as much ground effect as possible until reaching a rotation speed and then climbing out at that speed. An equation for the distance required to accelerate from hover to a given forward speed can be derived by considering that the acceleration capability is linear with speed. That this is a satisfactory assumption is shown by Figure 5.14. If x is the distance, then x is speed and x is acceleration. Between hover and VmiX (or xmix), the equation for acceleration is:

dx.

X = *HIGE + X

where dx/dx is a negative number.

The solution to this differential equation is:

Combining these last two equations gives the equation for the distance, x, required to accelerate to the speed, x:

Once the helicopter is accelerated to the rotation speed, x rot, a climb is started. The additional distance to climb over an obstacle with height h is:

where an approximation to the rate of climb from momentum considerations an be used:

where

Ah. p. h. p.avai| h. p. flt@irot

The total distance required for the maneuver is thus:

The optimum rotation speed depends on the height of the obstacle to be cleared. For a low obstacle the rotation speed will be low, but for a high obstacle (such as a mountain) the speed will be that for maximum climb angle, which could be found from a plot of rate of climb versus forward speed as in Figure 4.48 as the speed at which a ray from the origin is tangent to the curve. Calculations have been done for the example helicopter at a gross weight of 28,000 lb—a weight just above that at which hover out of ground effect is possible on a sea-level standard day according to Figure 4.35. Figure 5.16 shows the results of the calculations: First the distance required to clear a 50-ft obstacle as a function of the rotation speed, and then the optimum rotation speed and minimum distance as a function of obstacle height.