Self-excited Vibrations

Self-excited vibrations are those which arise under certain conditions

when constantly acting forces are transformed into periodic forces and a steady motion becomes oscillatory. To these conditions we must also add coin­cidence of the periodic force frequency with the natural vibration frequency.

There are three characteristic forms of self-excited vibrations in heli­copters: "ground resonance," "helicopter auto-oscillations in flight," and

vibrations of the flutter type.

The combination of main rotor blade oscillations relative to the vertical hinges with oscillations of the entire helicopter as it moves over the ground can be termed ground resonance. The amplitude of these oscillations increases very rapidly.

Vibrations of the ground resonance type are not observed on helicopters having main rotors without vertical hinges. In this case the blades are positioned symmetrically, and the center of gravity of the entire main rotor is located on the hub axis. As the main rotor turns the circumferential velocity of the center of gravity equals zero; consequently the main rotor centrifugal force also equals zero.

When vertical hinges are used, the blades perform oscillatory motions /182

as a result of change of the moments of the rotational drag force and the Coriolis force. Such oscillations lead to shift of the main rotor center of gravity away from the hub axis. The center of gravity begins to travel along a sinuous curvilinear trajectory (Figure 112a). The main rotor centrifugal force N appears. The appearance of this force can be explained in a different way. If the main rotor blades are positioned symmetrieally relative to the hub, the resultant of the centrifugal forces of the blades taken individually equals zero. If the blades are positioned asymmetrically, the resultant of the blade centrifugal forces will be the centrifugal force of the entire main rotor.

Under the action of this force a moment is created relative to the landing gear wheel support point, which causes alternate deflection of the gear shock struts and tires (Figure 112b). Rocking of the helicopter on the gear develops. If the frequency of these (natural) oscillations coincides with the main rotor rpm, resonance occurs. The amplitude will increase

г

7??/Z^7777777//?77/7.

I

Figure 112. Occurrence of "ground resonance."

rapidly. As a result of tipping of the helicopter, moments of the blade weight forces about the vertical hinges develop. These moments amplify the oscillatory motions of the blades and lead to increase of the centrifugal force of the entire main rotor. The increase of the oscillation amplitude can lead to overturning and destruction of the helicopter. Moreover, the vibrations are amplified by the action of the gyroscopic moment of the tail rotor. The tail rotor turns at high speed and has a large gyroscopic moment.

It tends to retain its axis of rotation in a fixed position. During oscilla­tions of the helicopter the position of the tail rotor axis of rotation changes, and therefore large torsional moments develop in the fuselage tail boom and vertical fin. The actual picture of ground resonance is more complex and depends on many factors other than those examined here.

Ground resonance develops most frequently when taxiing the helicopter over rough ground, during takeoff roll and landing runout when making takeoffs /183 and landings of airplane type. But ground resonance can also occur when the helicopter is parked with the main rotor running. The basic operational causes are the following:

Low or different tension of the vertical hinge friction dampers;

Incorrect charging of the landing gear shock struts and tires.

The first factor leads to increase of the oscillatory motions of the blades and the second leads to change of the gear stiffness. As a result of the stiffness change, there is a change of the helicopter natural vibration frequency, and conditions for resonance are created.

When resonance occurs, the main rotor rpm must be decreased and the engine shut down.

Helicopter auto-oscillations in flight. Helicopter auto-oscillations are similar in nature to ground resonance. These vibrations combine oscilla­tions of the main rotor blades relative to the vertical hinges and oscillations of the elastic elements of the helicopter fuselage. During the blade oscilla­tions a centrifugal force of the main rotor arises, and this leads to whipping of the shaft and deformation of the members of the frame supporting the gear­box and the structural elements of the fuselage.

Auto-oscillations occur very rarely on single-rotor helicopters; only in case of failure of individual structural elements of the fuselage, which results in reduction of the fuselage stiffness, or when the vertical hinge dampers are out of adjustment. These vibrations are observed more frequently in the dual-rotor helicopters with tandem arrangement of the lifting rotors.

The bending stiffness of the fuselage of this helicopter in the horizontal plane is comparatively low. If there are large oscillations of the blades of both rotors, large bending moments are created, which as they change their direction cause marked bending vibrations of the fuselage.

Rotor blade flutter. Vibrations of the flutter type are the most hazardous. They are encountered on the main rotor blades and are theoretically possible on the tail rotor blades, but in view of the high stiffness of the latter they are not encountered in practice.

Main rotor blade flutter may be of two types: bending-torsion and flap­

ping. Bending-torsion flutter in the pure form is observed most frequently for blades with rigid restraint at the hub. Blades with hinged support usually show the combined type of flutter.