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Technically, unless a helicopter's centre of gravity and centre of lift (centre of the rotor) are in perfect alignment, it would generate a pitching moment which would make it unstable.

So, how does a helicopter manage to solve this issue. (I'm assuming it's impossible to perfectly align the CG and the centre of lift due to practical loading conditions.)

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    $\begingroup$ It is unstable, the pilot hovers by constantly adjusting controls. $\endgroup$
    – Stone
    Jan 17, 2018 at 13:26
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    $\begingroup$ Helicopters can't fly, they're just so ugly the Earth repels them ;) $\endgroup$
    – Hobbes
    Jan 17, 2018 at 15:59
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    $\begingroup$ functional description of a helicopter: an oil leak, looking for a place to crash. $\endgroup$
    – niels nielsen
    Jan 17, 2018 at 21:13
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    $\begingroup$ The truth is, nobody knows. Some say it's God, others say it's the pilot's ego keeping the helicopter afloat. I personally think it's a little of both. Seriously though it's extremely unstable, you are constantly making tiny adjustments every few miliseconds to account for every tiny variable. Go try it sometime, it'll cost at most $500 for an intro flight, and that's if you're too fat for the R-22 like me :D $\endgroup$
    – Noah Wood
    Jan 22, 2018 at 10:05

2 Answers 2

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A helicopter can hover without the CoG being directly underneath the rotor hub, because the rotor hub can apply hinge moments:

  1. Fully articulated hubs with a hinge offset and fixed hubs apply direct mechanical moments to the fuselage, which line them up with the rotor angle. Own drawing The rotating blades are subject to centrifugal forces, which pull the rotor hub in the plane of the rotor deflection. As soon as the rotor plane changes as a result of cyclic stick input, the resulting rotor hinge moment will change fuselage angle accordingly, providing the desired feedback cue.

  2. Any helicopter, regardless of the type of rotor hub, has an aerodynamic coupling between fuselage attitude and rotor attitude. A teetering rotor such as on Bell 212 and R-22 does not produce above mechanical rotor hinge moment. In flight, a helicopter is suspended from its rotor, with the Centre of Gravity underneath the centre of lift, unless the rotor can apply a hinge moment to the fuselage. With a rearward CoG and no hinge offset, the following could happen: own drawing The helicopter has taken off vertically, nose tipped up first, and cyclic forward was input. If the rotor had a hinge offset the fuselage would tilt forward, but with a teetering hinge like in the question there is no centrifugal force acting on the hub, and fuselage position now depends only on CoG location. The upward nose angle has consumed some of the useful forward tilt of the cyclic stick. Visibility is impeded. Fuselage tilt provides no direct feedback of the cyclic position. enter image description here Fortunately there is an aerodynamic rotor hinge moment, created by the aerodynamics of flapping. The picture is from Helicopter Performance, Stability and Control by Raymond Prouty. It shows that at first, the rotor acts as a gyroscope and stays in its plane. But the blade feathering is referenced to the shaft, and angle of attack of the right hand blade is increased, left hand blade decreased. The rotor will then flap until it is perpendicular to the shaft. With teetering rotors, there is an indirect aerodynamic coupling between fuselage and rotor shaft: the fuselage attitude still follows the rotor attitude but at a slower rate.

The CoG cannot be placed anywhere, it must remain in the area where the moment caused by gravity pulling on the CoG is lower than the maximum hinge moment. But with all helicopters, there is a region of useful CoG.

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  • $\begingroup$ You should link to your previous answer that you took a lot of this information from since it can provide context (for example, why you talk about a teetering hinge). $\endgroup$
    – Ron Beyer
    Jan 17, 2018 at 17:01
  • $\begingroup$ @RonBeyer The teetering hinge and the articulated hinge are both mentioned in this answer: one would think that the teetering hinge behaves like OP states, but it does not. The other question was a different one (does the fuselage follow the rotor). There was a lot of static on the line with that question, a link would be more confusing than enlightening in my opinion. $\endgroup$
    – Koyovis
    Jan 18, 2018 at 1:23
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If the center of lift is above the center of mass then the craft will rotate until the center of mass will be directly below the center of thrust. After that the rotation will come to a stop. If dampened the pendulum action will stop eventually

The rotor blades of a helicopter change their lift depending on where they are in the rotation and the position of the cyclic. This allows for precise control of the center of lift by the pilot which in turn will control pitch and roll the craft.

In hover the rotor will be horizontal and produce very little to no horizontal thrust.

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    $\begingroup$ "If the center of lift is above the center of mass then the craft will rotate until the center of mass will be directly below the center of thrust." - Are you sure about that? Do you have a source? That sounds similar to the pendulum rocket fallacy. Also, do you have a definition of "center of thrust" which defines it as a specific point? $\endgroup$
    – Tanner Swett
    Jan 17, 2018 at 17:01
  • $\begingroup$ @TerranSwett Since the is a hinge between the blades (which produce lift) and the helicopter fuselage (which includes the cg), a pendulum effect is indeed possible and active in helicopters. However, cyclic control and a stiff rotor shaft mask this somewhat. $\endgroup$
    – Peter Kämpf
    Dec 6, 2021 at 12:08

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