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(Aside from crashing, injury, and possibly death.)

Airplanes which are out of balance will have either poor stall recovery or noses which are too heavy to lift.

What are the consequences of a helicopter which where the CG is outside the envelope?

  • Is it simply that the rotor will reach its stops and thus there is insufficient command range in all directions?
  • Or are there subtle effects such as when an aft-CG in an airplane prevents recovery from a stall?
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  • $\begingroup$ When you say “out of balance” you just mean a CG which is outside the envelope? On helicopters rotors can also be out of balance. Also note that being prevented from receiving from a stall IS a problem of insufficient control authority. $\endgroup$
    – Jim
    Commented Jul 19, 2022 at 22:25
  • $\begingroup$ Yes, the helicopter and not the blades. I'll update the question. I'll disagree that the aft-CG is insufficient control authority in the sense that the question means, although you are, of course, technically correct. $\endgroup$
    – Kenn Sebesta
    Commented Jul 20, 2022 at 3:22

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A C of G out of range on a helicopter will cause it to hover with an appropriate slant, whatever it takes to get the C of G under the rotor head, and if the C of G is displaced far enough, yes you would hit the blade flapping limit stops as you have full cyclic input to try to get the thing to hover in one place, and from there it would just keep drifting sideways.

If you have an articulated rotor that can tolerate blade flapping to the physical limits that is.

If it's a teetering rotor system like on a Bell or Robinson product, the same effect will occur, but the stops that are contacted are the teetering stops inside the rotor head that rest against the mast, and the result is "mast bumping", and this will cause the mast to snap off and the machine becomes a falling object. Hopefully while you're still just a couple feet in the air.

So, say you had a huge lead weight on the tail of the machine and you tried to lift off into a hover.

With an articulated rotor, say a Hughes 269/300, the thing would start to lift into the hover with the nose pointing up, and you would eventually have full forward cyclic in, and that wouldn't be enough and the machine would immediately start to fly backwards because even with the rotor tilted as far forward as it will go with the blades running at the swashplate flapping limit and possibly hitting the flapping stops, the Cof G is still aft of the rotor axis so the rotor disc can't get level, so back you go and at some point you probably crash.

With a teetering rotor, say a Bell 47, the thing would start to lift off into the hover with the nose pointing up, and the moment you applied enough forward cyclic, in trying to stop any backward drift, to make the bump stops in the rotor head contact the mast, there'd be a vibration, a big bang, the machine would drop to the ground, and the entire rotor assembly would fly off for a second or two on its own flight powered by its rotating inertia, like a big boomerang that hopefully doesn't come back, and land nearby.

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    $\begingroup$ "like a big boomerang that hopefully doesn't come back" <-- that was good for a sensible chuckle! $\endgroup$
    – Kenn Sebesta
    Commented Jul 20, 2022 at 3:24

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