. The tip path axis

The axis at right angles to the tip path plane is generally called the tip path axis or disc axis. The resultant thrust of the rotor aligns within about a degree of the tip path axis, and for most purposes they can be considered to be coincident. The tip path axis is also called the axis-of-no-flapping in some texts, although this term is not strictly correct because the existence of harmonics in the blade motion cause the blades to flap slightly with respect to the tip path axis. By the same token the tip path plane is not strictly a plane.

. The tip path axis

Control Rotor thrust

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Fig. 4.3 (a) A helicopter in steady forward flight requires a combination of forward and lateral cyclic inputs to neutralize flapback and inflow/coning roll. Note the tip path axis that is very nearly aligned with the rotor thrust, the control axis, and the shaft axis that are generally not coincident. When observed with respect to the control axis, the blades do not feather, but instead appear to flap. Note that even a hingeless rotor with rigid blades can flap with respect to the control axis. (b) If the pitch control rods are parallel to the shaft and 90° from the feathering axes, then the axis of the swashplate is identical to that of the control axis. (c) The angular difference between the tip path and control axes is equal to the amount of flapping observed with respect to the control axis and to the amount of feathering observed with respect to the tip path axis.

 

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Designers often try to trim the machine so that in cruise the action of the tail plane opposes couples due to drag on the hull in order to align the shaft axis with the tip path axis because this minimizes the amount of flapping and hence the amount of wear and vibration. However, this will often be compromised because of variations in the longitudinal position of the CM. In tandem helicopters it is easier to align the tip path axis with the shaft axis. During a roll manoeuvre the tip path axis and the shaft axis must diverge in order to roll the hull.

An observer turning with the tip path axis would see the feathering action changing the pitch of a given blade sinusoidally about the collective pitch setting at one cycle per revolution but as he is turning in the tip path plane he would not see any flapping (except for harmonics). In the steady hover no dragging would be observed, but in forward flight dragging would be seen due to the increased drag on the advancing blade compared with that on the retreating blade.

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