Figure 5-7

CHAPTER 5

HELICOPTER AERODYNAMICS WORKBOOK

point where their combined center of gravity is displaced from the center. In most flight

conditions, this situation will rapidly right itself as the individual blades sort themselves out. In

this process, each blade leads and lags in such a way as to spiral the CG toward the mast where it

belongs.

The problem exists if the aircraft is not airborne. A gust of wind, sudden control movement

or hard landing can displace the blades. The resulting motion due to the offset centrifugal force

may be just at the right frequency to rock the airframe on its landing gear.

Figure 5-7 illustrates this situation. Once this occurs, these two motions get in step, causing

the CG to spiral outward violently, producing a rotating force at the rotor hub, which can shake

the aircraft to pieces almost immediately.

Despite this dire possibility, ground resonance does not happen every time it has an

opportunity; just often enough to scare everyone concerned. The first recorded instance was in

the 1930's, when a Kellett autogyro apparently hit a rock while taxiing. This accident attracted

the attention of scientists, who eventually produced a mathematical and physical understanding

of the phenomenon. They found ground resonance can be prevented with damping, but the

damping must be used in the rotor around the lead/lag hinges and the landing gear.

As far as the pilot is concerned, prevention consists of making sure all dampers are

operational during the preflight inspection. If an oscillation is detected and the aircraft is up to

flying rpm, the primary recovery method is to lift off. An alternate method is to land, secure the

engine, and apply the rotor brake. These actions should bring the rotor system back into balance.

Figure 5-7

5-8 FLIGHT PHENOMENA