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I am wondering how a helicopter manages to stay balanced and stay in one spot while it is in the air.

I understand that there is a second rotor that is used to counteract the angular momentum and also the use of swashplate to direct the helicopter forwards and backwards.

Assuming the angular momentum of the main rotor is not an issue, Is there a sensor on the helicopter that can detect if it is not upright? Is the swashplate then adjusted to tilt the copter into an upright position?

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    $\begingroup$ The sensor is the pilot and it stays upright and hovers with pilot skill. Note to hover in one spot with no wind you actually have to tilt the main rotor a bit to counter the tail sidethrust. With a horizontal main rotor it will move sideways due to the tail sidethrust. $\endgroup$
    – DKNguyen
    Aug 29, 2020 at 21:16
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    $\begingroup$ @DKNguyen Pretty sure you know by now not to answer questions in the comment section. $\endgroup$
    – pipe
    Aug 29, 2020 at 22:19
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    $\begingroup$ @pipe Not really answer. But mainly just wanted to point out a misconception in the question that if the helicopter is upright that it will hover in place. $\endgroup$
    – DKNguyen
    Aug 29, 2020 at 23:52

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Most helicopters of modern manufacture are dynamically unstable while hovering- they require constant, "hands-on" control inputs from the pilot to keep things in balance. This task is very demanding and requires a lot of practice to master.

Hiller helicopters (which have been out of production for many years) were one of the rare examples of dynamically stable helos, which could actually hover "hands-off".

Hovering can also be performed by an automatic pilot system or something called a stability augmentation system (SAS) which assists the pilot by reducing the degree of dynamic instability, rendering the helicopter more easily controllable. Both types of systems require sensors to detect tilt and accelerations.

BTW many radio-controlled helicopter models are SAS-equipped to make them less difficult for hobbyists to fly.

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  • $\begingroup$ I'd guess that being physically in the cockpit makes it easier for a pilot to be part of the feedback control loop, letting your inner ear and/or vision react to changes more quickly and accurately than looking at a hovering RC helicopter from outside. Or is it more a matter of staring at an instrument either way, at least in the worst case? $\endgroup$
    – Peter Cordes
    Aug 29, 2020 at 16:19
  • $\begingroup$ yes, in the case of a piloted helo, the pilot furnishes the "sensor inputs" and as you point out, the RC helo furnishes none of that. $\endgroup$
    – niels nielsen
    Aug 29, 2020 at 16:22
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    $\begingroup$ Are quadcopters dynamically stable, or do they use an SAS to hover? $\endgroup$
    – nick012000
    Aug 30, 2020 at 16:01
  • $\begingroup$ Quadcopters are fundamentally unstable and essentially always have 3-axis rate nulling based on MEMS gyros and software making them at least neutrally stable. Many by default also have a level-seeking behavior based on accelerometers in an outer control loop (ie, positive stability), in some models this can be disabled letting the pilot directly control the inner rate loop for aerobatics. $\endgroup$
    – Chris Stratton
    Aug 30, 2020 at 16:33
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    $\begingroup$ Hillers had an extra pair of stubby rotor "blades" containing counterweights which were mechanically coupled to the main rotor blades' cyclic pitch control. If the main rotor disc began to tilt for any reason, the extra blades would work the cyclic in such a manner as to cancel the tilt tendency automatically and so the hillers could hover hands-off. $\endgroup$
    – niels nielsen
    Aug 31, 2020 at 16:06

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