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Does a Rotorcraft hoisting CG chart show what the CG will be as the load is hoisted from top to bottom or vice versa?

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  • $\begingroup$ Can you give an example of a "Rotorcraft hoisting CG chart", or a link to one? Is this just a hypothetical thing that you are imaging might exist, or something that you've actually seen? (See last para of my answer as to why such a thing might exist.) $\endgroup$ Sep 4, 2023 at 11:43

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Presuming that the winch is in a fixed location, (not on a track that allows it to move fore and aft or sideways) the longitudinal and lateral center of gravity of the helicopter would not change as the load is raised or lowered.

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The dynamics of an aircraft with something dangling from a single long line have little in common with an aircraft with the same object attached below the aircraft by a completely rigid (long) rod or pylon.

So we won't find any chart that treats raising or lowering an object with a hoist, as being equivalent to a vertical shift in a helicopter's CG.

Incidentally, that's why hang gliders, where the pilot suspension system can be modeled as a single flexible line, at least when the pilot is not using his or her muscles to make a control input, don't have the same hands-free pitch and roll stability as do paragliders, where the multiple suspension lines keep the pilot essentially rigidly fixed in position relative to (and well below) the rest of the aircraft.

In the hang glider case, when the pilot exerts no muscle force with his arms, or releases the control bar completely, he is free to swing fore and aft and side to side and so his weight acts as if it is located at the point where the flexible tether (called a "hang strap") connects to the rigid structure of the glider. Not as if he is rigidly fixed in position below the glider.

The same will be true during helicopter hoisting operations, at least as long as the hoisting rate is constant, and assuming that the object is not "penduluming" or swinging wildly, which can change the object's apparent weight due to centrifugal loading. If the object is simply dangling at the end of a line which is fixed or being raised at lowered at a constant rate, for stability purposes the object can be viewed as being located at the upper end of the hoisting line, i.e. at the point where the hoist line meets the pulley of the winch.

For a given hoist location, one might well imagine that only a certain part of the total allowable helicopter CG envelope might actually be compatible with hoisting operations, and that this might vary according to the weight of the object being hoisted. Is this perhaps what you've seen a chart relating to? If so, the length of the hoist line would not matter, so nothing changes as the object is raised or lowered, due to the dynamics explained above.

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