Introduction to Rotor Trim
The trim solution involves the calculation of the blade pitch control settings, rotor disk orientation (blade flapping) and overall helicopter orientation for the prescribed flight conditions. Controlling the position of the helicopter in free-flight requires the adjustment of the forces and moments about all three axes. For a conventional helicopter there are three independent controls used for this purpose:
vector, producing both a side force and a rolling moment about the center of gravity of the helicopter. Longitudinal cyclic (0^) imparts a once-per-revolution cyclic pitch change to the blades such that the rotor disk can be tilted fore and aft. Like the lateral cyclic, this changes the orientation of the rotor thrust vector, in this case producing both a longitudinal force and pitching moment. Both lateral and longitudinal cyclic are controlled by the pilot using a cyclic stick (similar to the conventional stick on a fixed-wing aircraft), which is held in the pilot’s right hand.
3. Yaw: This is controlled by using the tail rotor thrust. The pilot has a set of floor mounted pedals, which are operated by the pilot s feet, jusi іікс & ruuuci on u fixed-wing aircraft. By pushing the pedals in the required direction, the collective pitch, 0TR, on the tail rotor is changed, producing a change in tail rotor thrust and, therefore, causing the nose to yaw right or left.
As will be appreciated, there is a considerable amount of crosscoupling of the forces and moments on the helicopter when the pilot applies the controls. The relevant equations describing the behavior of the helicopter are complicated and interdependent, and there is no simple solution to this problem. For example, a change in rotor thrust produced by pulling up on the collective will require a higher power and will create a larger torque reaction on the fuselage. This, in turn, will require a yaw correction to be made to keep the helicopter tracking in a straight line. These and other so-called cross-coupling effects are unavoidable on a helicopter, but much can be done to minimize their effects and reduce the workload for the pilot by appropriate mixing of the control inputs. On early helicopters this was done mechanically, but now these effects can be handled electronically by an on-board flight
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There are two basic types of trim solution that are of interest to helicopter engineers: “propulsive” or “free-flight” trim and “wind-tunnel” trim. Wind-tunnel trim is used when testing model rotors in the wind tunnel and is somewhat different from free-flight trim because only force equations are used. For propulsive trim, the solution simulates the free – flight conditions of the helicopter, and moment equations must be included. For a specified helicopter gross weight, altitude, center of gravity location, forward speed and flight path angle, the trim solution must numerically evaluate the rotor controls, namely the collective pitch angle в o’, the cyclic pitch angles Gc and 9s the rotor disk orientation, which is described by j80, fic and fi]S and the vehicle orientation, which is described by the inertial angles (pitch angle, 6p, roll angle, фр) and the aerodynamic angles (angles of attack, a, and sideslip, fi) and the tail rotor collective pitch, Otr.
There are many possible forms of trim solution, and several levels of approximations and assumptions are used. In all cases, the free-flight trim solution is obtained from a set of vehicle equilibrium equations. These are usually simplified by using small-angle assumptions, although, for some flight conditions, such as turns where the angles are large, such assumptions become increasingly questionable – see Chen & Jeske (1981). Some trim solutions neglect the lateral equilibrium equation and this is justified because a lateral tilt of the rotor disk does not substantially change the rotor or fuselage aerodynamics. The trim solution can be approached from two perspectives. One approach is to use the blade element theory with certain assumptions for the wake inflow to calculate analytic results for the blade flapping and control angles. This gives a good first estimate of the rotor trim state. Another approach is to use a less restrictive solution for the blade aerodynamics (which may include nonuniform inflow and nonlinear aerodynamics) and to calculate the rotor trim state numerically – see Section 10.7.6. In either case, an iterative approach is required.