Phil: Lol, ah well, he was a special man with special tastes!
.. on the pad in eccentric circles.
US: Sorry, but I have to ask; do you mean
eccentric circles (I'm sure it must have looked that way) or did you possibly mean concentric!
.. 4 blades better than 3?
I was told (but I cannot substantiate this) that
the most susceptible systems to GR were three-bladed rotors and that the ultimate 'risk' was a three-bladed rotor with wheeled undercarriage. As I understand, it can occur on 4 or more bladed rotors also.
In December 2005 this Alouette III landed at Escalante National Monument in Utah and suffered ground resonance apparently shaking itself apart in all of four seconds
Here's a further description of ground resonance in case it helps:
Not all types of helicopters are susceptible to ground resonance. Two-bladed helicopters are exempt because their “teetering” rotors are a single rigid structure, like a see-saw. The only rotors that can produce ground resonance are those with three or more blades. Multi-blade rotors have lead-lag hinges, which allow blades to speed up and slow down at different points as they circle the mast while the helicopter is moving forward. The hinges keep the fluctuating lift and drag forces on each blade from inflicting excessive stresses on the rotor hub. Snubbers and dampers limit the motions of the blades.
Because it is massive and spinning at a high speed, the rotor of a helicopter must be properly balanced. If the lead-lag hinges allow the blades to depart from perfect symmetry, the rotor’s center of gravity shifts slightly to one side of the mast, throwing the system out of balance.
Anything that’s springy has a favorite frequency of vibration—its natural frequency—which is determined in part by its size and mass. That’s why tuning forks always produce a certain tone, and why boats of different sizes rock at different rates. When two things with the same or similar natural frequencies are in contact, or sometimes even merely close to each another, and one of them begins to vibrate, it may “excite” the other to vibrate as well. The ability of one vibrating object to create this sympathetic vibration in another is what enables the rotor blades to gain control of the entire helicopter.
The helicopter’s airframe has its own natural frequency, which can be excited by an out-of-balance rotor. Usually there is a triggering event: a bump or a landing or takeoff on sloping ground or with a little sideways or forward motion. A jolt moves the mast while the blades, because of the freedom of motion allowed by their hinges, lag a little behind. The rotor, now slightly out of balance, begins to wobble like a slowing top. If the characteristic vibration frequency of the airframe is close enough to the rate of rotation of the rotor, it joins the dance, amplifying the rotor wobble.
The destruction is wrought by the considerable energy stored in the rotor blades. The shaking rapidly grows in violence, exceeding the strength of the mast, transmission mounts, and landing gear. The cyclic control in the cockpit flails about so violently that the pilot cannot hold it, the rotor blades strike the tail boom or the cockpit, parts begin falling off, and moments later the helicopter may be a heap of scrap.
If ground resonance begins, the pilot’s best option is to get the helicopter into the air. Once the tires or skids are no longer touching the ground, the vibration fades. If the rotors do not have sufficient speed for flight the next best remedy is to eliminate lift by reducing blade pitch; shut down the engine; and hope for the best while waiting for the rotor to slow.
The wait-and-hope approach is only sometimes successful, so a better solution to ground resonance is to prevent it. Helicopters with multiple-blade rotors have shock-absorbing landing gear with powerful dampers that allow it to soak up the energy that would otherwise set the helicopter shaking. When ground resonance occurs in these craft, it is usually because tires or shock absorbers have been improperly serviced.