Ground Resonance

Ground resonance occurs when the Helicopter is light on the skids or wheels during landing or taking off. Vibration from the skids (in light contact with the ground) can be amplified and reflected by that contact. This can quickly develop into violent shaking and vibration. The aircraft can be destroyed right there, shaken to pieces.

The problem is restricted to fully articulated rotor heads which go hand in hand with more than two blades. The 'teetering' head is only used by two bladed helicopter types which are not susceptible to ground resonance (as long as the POH is closely followed, especially with reference to ice or snow on the blades. It must be completely removed before any start is attempted).

Ground resonance is caused by the rotor disc weight not evenly distributed around the axis of rotation. This is easily possible with the type of head that has lead and lag hinges. (See Hookes Joint Effect)

Fully Articulated Head

The proper operation of the dampers whether on the head or in the landing gear is crucial. If one of these is faulty or has the incorrect pressure within it, that can cause ground resonance. Tyre pressures are just as important. The same problems apply with ice, snow or other objects adding their weight to the blade(s).

As the blades have the freedom to lead and lag it is possible for one or two of the blades to become unsynchronised with its partners. This can be when one skid makes a harder than normal contact with the ground, for instance.

Ground resonance can be the source of some very heavy vibration which is amplified by the light ground contact. The head will normally have some type of damper arrangement connected to the blades to stem the effect.

Dampers (shock absorber) may also be included in the landing gear. These are normally called “Oleos”. Both measures are only prevention, not a cure.

In this fully articulated head the blades are allowed to move about the axis of rotation and regain their position by lead and lag hinges. Herein lays the potential for an unbalanced disc. In a rigid system the blades simply flex to move, automatically flexing back when the tilt of the disc is complete.

Be ready for Ground Resonance

This is the reason the RRPM are maintained at optimum during any landing or take-off (especially with slopes as these involve light contact on one skid initially). RRPM are maintained so that the pilot may quickly lift back to the hover. The problem only happens with ground contact.

What if ground resonance occurs?

If full RRPM is available take off as soon as possible. The effect will subside when the aircraft is airborne. If this cannot be done then shut down and stop the rotor as quickly as possible using the rotor brake (if fitted).

Prevention is better than cure

Proper maintenance, a good pre-flight checking all the items mentioned, an adherence to the manufacturer's limits on icing, snow, water in blades, wind velocity at take-off will be the best way to ensure this does not happen.