Boeing CH-47D Chinook helicopter

Ground Resonance

Ground Resonance is an aerodynamic phenomenon associated with fully-articulated rotor systems. "Ground" implies the airframe must be in contact with the ground. "Resonance" refers to the natural vibration frequency of the airframe resulting from the design and manufacturing process. Each part of the airframe, including the rotor system, vibrates at a certain frequency. During the development process, engineers will design the helicopter in such a way as to avoid having the fuselage and rotor system vibrate in sympathy with each other. Otherwise, the helicopter would be difficult to control. Instability can occur when the lead-lag frequency of the rotor couples with one of the natural frequencies of the fuselage, usually due to a shock to the airframe.

The CH-47D Chinook helicopter fully-articulated rotor head.

The term "fully-articulated" means that the rotor head allows the blade to move in three independent planes. A horizontal hinge pin allows the blade to move up and down. This is known as "flapping". Components within the pitch varying housing allow the blade to rotate about its span (the distance from end-to-end). This is known as "feathering". A vertical hinge pin allows the blade to swing forward and aft with respect to rotor-head rotation. This is known as "leading and lagging" (hunting). The vertical hinge pin has also been called the "drag hinge".

It is the drag hinge that allows any given pair of blades to either get closer to or farther away from each other. When the pair of blades get too close or too far away, the balance of the entire rotor disk becomes off-center.

Ground Resonance does not occur in rigid or semi-rigid rotor systems, because there is no drag hinge. In addition, helicopters with skid-type landing gear are not as prone to ground resonance as one equipped with wheel-type gear. The flexibility of an air-filled tire tends to amplify and increase the oscillation, as does the air-oil (oleo) landing gear struts normally associated with wheel-type landing gear. Improper servicing or poor condition of the tires, struts, as well as the lead-lag dampners all contribute to the tendency to experience ground resonance - an experience that ain't all that much fun!

Ground Resonance develops when the rotor blades move out of phase with each other and cause the rotor disc to become unbalanced. In a three-bladed rotor system, one can consider the in-phase condition to be one where the blades are approximately 120° from each other, plus or minus a small amount, as the blades rotate around the tip-path plane. In a four-bladed system, the blades are approximately 90° apart.

Columbia Helicopters Incorporated Chinook helicopter N234CH lifting a water bucket used to help fight forest fires.

Columbia Helicopters Incorporated (CHI) Chinook helicopter N234CH lifting a water bucket used to help fight forest fires. This photograph shows the approximate 120 degree rotor blade separation.

Chinook 65-07992 after modification as the Boeing 347 test aircraft. This photograph shows the 90 degree phase separation of the rotor blades.

Chinook 65-07992 after modification as the Boeing 347 test aircraft. This photograph shows the approximate 90 degree phase separation of the rotor blades.

When the blades become out-of-phase, the weight of the rotor system becomes concentrated on one side of the rotor disk, rather than being equally distributed. The weight does not remain in one spot - rather it moves as the rotor system revolves. Think of a tire on a car that has lost the balance weights - the vibration experienced by the driver as the vehicle speed is increased is similar to what the helicopter will under-go. In a helicopter the situation gets much worse very quickly.

Of course, aircraft are not designed to be as structurally strong as ground vehicles. The out-of-phase condition can cause a helicopter to self-destruct in a matter of seconds. For this condition to occur, the helicopter must be in contact with the ground. If the pilot sets the helicopter on the ground firmly on one corner of a skid or on one tire of a wheel equipped helicopter, a shock is transmitted through the fuselage to the main rotor system. This shock causes the blades to move out of their normal relationship with each other. Remember, this movement occurs along the drag hinge.

Corrective Action

If the Rotor RPM (N_R) is low, the corrective action to stop ground resonance is to shutdown the engines immediately and fully lower the thrust to place the blades in low pitch. Shutting down the engines reduces the N_R, hence the lift of the blades. Lowering the thrust further reduces the lift of the blades. By reducing the lift in the blades, the tendency for the individual blades to "hunt" is reduced. Reducing the hunting may allow the blades to return to their normal in-phase condition. This action may prevent the total destruction of the helicopter - if the N_R can be reduced quickly enough (Good Luck!).

If the N_R is in the normal operating range, the pilot must immediately lift the helicopter off the ground, and allow the blades to automatically realign themselves. By smoothly lifting off without hesitation at the onset of ground resonance, disaster will be avoided. A normal, gentle touchdown may then be accomplished. If the pilot lift's off and allows the helicopter to firmly re-contact the surface before the blades are realigned, a second shock could move the blades again and aggravate the already unbalanced condition. This could lead to a violent, uncontrollable oscillation.

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