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On a helicopter, the collective is used to adjust the attack angle of all the main-rotor blades in the same direction by the same amount, causing the amount of lift generated to increase or decrease symmetrically across the whole main rotor, which, in turn, causes the helicopter to climb or descend. If the collective is pulled all the way down to zero, the main-rotor blades move to zero attack, generating no lift (this is useful when on the ground).

Negative collective, if possible, would cause the main-rotor blades to move to a negative pitch, causing them to generate negative lift and accelerate the helicopter downwards. This would enable a very rapid descent, which could be useful in certain circumstances (for instance, in combat). Can any helicopters actually do this?

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    $\begingroup$ If this was possible, it would be a potential failure mode of the control system. That would be bad. $\endgroup$
    – MD88Fan
    Jul 9 at 0:02
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    $\begingroup$ How often do you think you might need or want to fall faster than 32ft/sec squared? Dumping power and pitching full forward is aggressive enough, I can't imagine how much stress it might put on the rotors to suddenly go to negative pitch... $\endgroup$ Jul 9 at 23:56
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enter image description herePic source

Negative thrust, as in enabling the helicopter to fly upside down...but no, the aerobatic Red Bull helicopter does not perform sustained inverted flight, it does barrel rolls and loops which maintain positive n.

Providing negative collective is not a design feature in helicopters. Ray Prouty briefly discusses sustained inverted flight in Helicopter Performance, Stability & Control, commenting on the extra collective travel required for negative collective pitch:

No actual helicopters are rigged in this manner, for two reasons. First, it would require a collective control system with twice the normal travel...Second, it would lose the important safety feature of having the down collective stop approximately corresponding to the right position for autorotation.

Upon a power failure, the collective needs to be lowered into the autorotation position within a couple of seconds, to prevent the rotor to lose RPM. With the collective down stop at correct autorotation position, the blade profiles must retain positive angle-of-attack, like in the pic below from the pretty good description on this site.

enter image description here

It seems that the autorotation setting at the collective down stop is a way more important design feature than negative thrust. Which would also bend the blades towards the tail boom, with possible catastrophic results.

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  • $\begingroup$ What you describe for autorotation (the rotor blades being sucked forwards by their own lift) requires an airfoil with negative induced drag, which is physically impossible. $\endgroup$
    – Vikki
    Jul 9 at 21:13
  • $\begingroup$ @Vikki It’s a matter of torque balance of the blade. Hard to visualise, reasonably described in the linked article. $\endgroup$
    – Koyovis
    Jul 9 at 21:56
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During an autorotation the collective is lowered to its lowest position and this decreases the angle of attack (aoa) of the rotor blades. Once the helicopter begins its descent the resulting downward and forward profile creates a positive aoa for the blades. Maintaing the correct rotor RPM ( within a very narrow rage) is crucial in arresting the descent and making a successful touchdown (primary purpose of an autorotation). The pilot uses the cyclic control to adjust the descent angle to maintain the correct autorotation airspeed and rotor RPM. At the proper point just above the surface the pilot will lift the collective up thereby increasing the rotor blades' aoa. This creates more lift and quickly decreases the descent rate (and rotor RPM) for a minimal forward movement and soft touchdown.

So, specific to your question, a negative rotor blade pitch (and aoa) would likely cause (in the case of an autorotation) the helicopter to descend perhaps vertically and at a high descent rate out of control.

I can't visualize this type of rotor blade system and and have never seen it.

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A helicopter applying downforce through its rotors would fall faster than a rock dropped next to it. Pulling out of that manoeuvre would demand excessive lift forces. Helicopters are not stressed for high manoeuvrability in the same way that some aircraft are, as the various forces imposed on the spinning rotor blades and driveshaft bearings would demand excessive strengthening and added weight. Only a few can even pull low-gee party tricks such as barrel rolls or loops.

Also, being in an effectively upside-down state, the helicopter would no longer have pendulum stability and would become dynamically unstable.

In autorotation the air flows up through the rotor disk. I don't know if any designs can set the blade incidence negative with respect to the disk, but even if they can the blade will still have a positive angle of attack to the oncoming airflow.

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  • $\begingroup$ Pendulum stability is a fallacy $\endgroup$
    – Koyovis
    Jul 11 at 14:12
  • $\begingroup$ @Koyovis Only partly. It does affect the flight dynamics, though not in the simplistic manner often assumed. For example accelerating forwards will tend to push the nose up instead of down. $\endgroup$ Jul 11 at 14:51
  • $\begingroup$ It affects flight dynamics. You make a statement about pendulum stability, which is a fallacy. $\endgroup$
    – Koyovis
    Jul 11 at 21:11
  • $\begingroup$ @Koyovis The fallacy is in its assumed effect. Were pendulum stability a fallacy per se, fixed-wing aircraft with high wings would not be given anhedral in order to reduce its excessive effect. This is not the place for a fuller discussion. You have offered your comment and I have replied. That is enough. $\endgroup$ Jul 12 at 8:59
  • $\begingroup$ The comment is meant for insights that may have been missed, for amending the answer. I won't discuss it any further, since the speed stability of an underslung rotor is clearly explained. $\endgroup$
    – Koyovis
    Jul 12 at 9:47

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