Forward Flight

In forward flight there are two natural trim conditions: zero bank angle and zero sideslip. Since very few helicopters have sideslip indicators, pilots tend to fly at zero bank angle, where they are most comfortable. In these: cases, the pilots find the sideslip angle that makes the sideforces on the fuselage equal and opposite to the tail rotor thrust.

For test purposes, instrumented helicopters generally have sideslip indi­cators, and the pilot may be asked to fly at zero sideslip to minimize drag and maximize performance. In actual flight, of course, the helicopter is usually in a condition of some bank angle and some sideslip.

The relationships of bank angle versus sideslip angle are imbedded in the three equations of lateral-directional equilibrium of Table 8.11.

The solution of these equations results in relationships of Ф, and Tr as a function of the sideslip angle, p. The pilot can sense the roll angle, Ф, but not the other two variables. These show up indirectly as lateral and directional control positions. To calculate these positions, the sideslip is accounted for in the following manner:

Lateral:

Ay (Ay + bp_o by^ + By sin P

Directional:

(Use the method of Chapter 3—"Tail Rotor”—to find 0^.)

Using the rigging plots of Appendix A, the results for the example helicopter at 115 knots have been obtained and are plotted on Figure 8.31. These indicate that this helicopter has three types of positive stability: positive directional stability, positive dihedral effect, and a positive sideforce character­istic.

The amount of pedal position required to hold a sideslip angle indicates the magnitude of the directional stability and tail rotor control power. Although a

TABLE 8.11

Lateral-Directional Equilibrium Equations in Forward Flight

Y-Equilibrium Equation

Component Source

Main Rotor Y-Force

Main Rotor H-Force Tail Rotor Thrust

Tilt of Vert. Stab. Drag

Fuselage Side Force

Yf

Tilt of Fuse. Drag

Tilt of Gross Weight Equilibrium

Left Sideslip Angle, p, deg. Right FIGURE 8.31 Trim Conditions in Sideslip; Example Helicopter at 115 Knots

helicopter might resist flying with sideslip because of high directional stability, only a little pedal displacement may be needed to hold sideslip if large changes in tail rotor thrust can be obtained with small pedal movements.

The change in lateral control position to hold a sideslip angle is an indication of the dihedral effect. Positive dihedral results in a roll to the left when the helicopter inadvertantly slips to the right—just as on an airplane with both wings tilted up. Positive dihedral is desirable to insure dynamic lateral-directional stability.

The change in roll attitude to hold a given sideslip is an indication of the sideforce characteristics of the helicopter. Strong sideforce characteristics help the pilot make coordinated turns by giving him a "seat of the pants” feel whether he is skidding to the outside of the turn or sliding to the inside.

EXAMPLE HELICOPTER CALCULATIONS

page

Horizontal stabilizer characteristics 501

Vertical stabilizer characteristics 511

Longitudinal trim in hover 517

Longitudinal trim in forward flight 522

Speed stability 527

Angle of attack stability 529

Power effects on trim 530

Lateral trim in hover. 532

Directional trim in hover 534

Lateral-directional trim in forward flight 538

HOW TO’S

page

Aerodynamics of the fuselage 512

Aerodynamics of the horizontal stabilizer 488

Aerodynamics of the vertical stabilizer 502

Angle of attack stability 529

Directional trim in hover 534

Lateral-directional trim in forward flight 535

Lateral trim in hover 532

Longitudinal trim in forward flight 517

Longitudinal trim in hover 516

Power effects on trim 530

Speed stability 525