CALCULATION OF TRIM CONDITIONS

Level Flight

Since the trim conditions are dependent on each other, the calculations are done as an iteration of the angle of attack of the tip path plane and the rotor thrust, using the equations:

Df + Нм + Ht G. W. – Lf

T = ^(GM. – LPy + (Df + HM + HTy

The fuselage lift and drag characteristics as a function of angle of attack of the fuselage may be estimated from previous experience or measured in a wind tunnel. Typical fuselage aerodynamics that will be assumed to apply to the example helicopter are shown in Figures A2 through A4 of Appendix A. The lift and drag forces are:

Lp = q(Lp/q)

Df = qf

where the fuselage angle of attack is:

Ctf = CtTPp — is ais ~ A&d. w.

where AaD W is the downwash angle produced at the fuselage by the rotor. A wind tunnel investigation reported in reference 3.31 shows that the rotor downwash at the fuselage is approximately equal to the value corresponding to the momentum – induced velocity at the plane of the rotor:

Thus

aF = — — іs — a! radians Iі

Trim conditions for a helicopter can now be evaluated from the equations just developed. A typical case for the example helicopter has been prepared. The assumptions that were used in the iterative process were:

First Iteration Subsequent Iterations

« = H = (H0 + H()prcv iter

= ® Clp — (Qp)prcv. iter.

The results of the calculations are shown in Table 3.2.

Figure 3.48 shows the distribution of the local angle of attack along the advancing and retreating blades for level flight and the two other flight conditions to be discussed next. For the example helicopter with 10° of twist, the highest local angle of attack is not at the retreating tip but at the 70% station.