Autorotation in Forward Flight

The basic mechanism of autorotation in vertical flight was described in Chapter 2. Autorotation in forward flight is based on the same concept—that over the entire rotor, the integrated effect of the tilt of the lift vector at each blade element is sufficient to overcome the integrated effect of the drag at all of the blade elements.

The closed-form equations can be used to calculate the autorotative rate of descent and the trim conditions in forward flight. Two calculating methods are available; the first consists of using the previous method for climb and descents by assuming several rates of descent and plotting main rotor power versus rate of descent. The rate of descent for which the main rotor power is zero—or a small negative value required to overcome losses—is the autorotative rate of descent.

The fuselage lift and drag are a function of the angle of attack of the fuselage where—assuming that the rotor downwash at the fuselage is equal to the momentum value at the rotor:

The calculation has been made for the example helicopter assuming that transmission and accessory losses amounted to 15 h. p. The results of the calculations are listed in Table 3.3.

The rate of descent in autorotation could have been roughly estimated from the power required in level flight and the rate of change of potential energy required to produce this power:

33,000 h. p.
G. W.

The calculated power required for level flight (including 15 h. p. for transmission and accessory losses) is 1,137 h. p. The corresponding estimated rate of descent is

1,880 ft/min. Comparing this with the calculated rate of descent of 1,790 ft/min indicates an "efficiency” of 105%. The apparent gain is primarily due to the reduction in the negative lift on the fuselage and a reduction in the tail rotor Id – force. This value can be used for rough estimates of the autorotative rates of descent of the example helicopter at other weights and forward speeds in the neighborhood of 115 knots. The angle of attack distribution in Figure 3.48 shows that the angle of attack is decreased on the retreating tip but increased inboard compared to level flight. For this reason, some helicopter aerodynamicists consider that the critical position on the retreating blade in autorotation is the station at which the tangential velocity is 40% of the tip speed, or:

r/Ra«. = 0.4 + Ц

if the local angle of attack exceeds the stall angle of the airfoil at this station, high drag can be expected to compromise the ability of the rotor to sustain autorotation.

Up to this point, the closed-form equations have been kept simple in order to illustrate the derivation and to produce results that can be readily used for quick calculations. The primary assumptions that have been used are:

• There is no tip loss or root cutout.

• The reverse flow region is ignored.

• A constant blade element drag coefficient is used.

When one or all of these assumptions are eliminated, the accuracy of the method is improved but the work in deriving the equations becomes greater, and the equations themselves become more unwieldly to use. In order to evaluate the effects of the assumptions, they will be examined one at a time.