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Ground Resonance

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When in contact with the ground (at touchdown), a helicopter with an articulated rotorhead (more precisely, a rotorhead with drag hinges) can get into resonance mode, which has the potential to destroy the aircraft. In this section, we look at what ground resonance is, how it is generated and how to prevent it.

In a rotorsystem with drag hinges, the rotorblades can lead and lag in the plane of rotation. It is then possible to achieve a situation whereby the blades are not evenly spaced around the circle of rotation, for example, due to a touchdown or another external disturbance. In such circumstances, the centre of mass (CM) isn't aligned with the mast (see figure), and will be rocked in all directions. Because a helicopter’s moment of inertia is greater around the pitching axis than the rolling axis, the pilot will probably experience lateral rocking.

In order to understand the exact nature of ground resonance, we will have to analyse it in more detail. This starts with the observation that each blade resonates at a certain frequency in the plane of rotation (see also the last section about Dragging Dynamics). We look at a blade in a rotating frame of reference, and assume that it resonates at a particular frequency 'x'. Next, we will look at all of the blades together (in example 3), as well as their phase relationship (figure below). Depending upon the phase, there are a number of effects. The first is no effect on CM; the CM is either whirling (=circular motion of the rotorhub) at frequency '+x', whirling at '-x' (that is, the CM travels the whirling circle in the opposite direction), or there is some lateral movement. The circumstances in which the whirling adds to the rotor frequency (RRPM), is known as progressive whirling; when it is negative, we call it regressive whirling.

We will now take the interaction between the hull and the rotorhub into account, and will, therefore, return to a stationary frame of reference. The hull experiences the rotor frequency and the whirling of the rotorhub. When the rotor is turning with angular frequency 'X' (directly related to RRPM), we have to include the whirling frequency in order to get at the frequency experienced by the hull. So, depending upon the type of whirling (progressive or regressive), this frequency is 'X'+'x' or 'X'-'x'.

The hull also possesses some natural resonance frequency. When this meets the rotor frequency, X, it will resonate. The movement of this resonant body is in an anti-phase with the rotor frequency (by the physical nature of resonant systems). This is still true at the frequencies 'X'+'x' or 'X'-'x' (because X >> x). Note that the hull will only resonate when in contact with the ground. Otherwise, there is no effective spring-mass system that can get into resonance.   One should also realize that the phenomenon we are studying, is an interaction between a hull in resonance and the whirling rotorhub.

In this interaction, there is an important difference between the 2 situations, with the whirling at either plus or minus 'x' (figure X). When whirling at +x, the hull's resonance force is in anti-phase with the whirling frequency. However, when the whirling happens at minus x, the hull’s resonant force is in phase with the whirling frequency. Now, the hull’s resonance around rotor frequency X reinforces the hub's whirling. This system isn't stable and will suffer from severe resonance. Indeed, there is enough energy to completely disintegrate the hull, mast and rotors. Clearly, this must be prevented.

The first and most common way to solve this problem is to use mechanical dampers, which simply suppress any blade resonance in the plane of rotation. Dampers are also used to dissipate kinethic energy on touchdown, preventing sudden disturbances in the rotordisc, which could initiate ground resonance. These measures will effectively prevent ground resonance. If it occurs despite these steps, (e.g. broken damper) then the most effective way to stop it is to take off immediately. This is because the hull will cease to resonate as soon as there is no ground contact. If this is not possible, the only change to make is to reduce rotor speed (thereby changing the excitation frequency of the system).

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HeliStart is authored by Peter Goossens.

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