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Background

I'm a model aviation enthusiast and really enjoy seeing how far people can push the limits of aerobatics. Model aircraft are able to push the limits well beyond full scale due to their small size. I understand that this is only possible because models are small* so they can withstand forces that would kill any occupants in a manned craft. Also, model aircraft are typically overpowered when compared to full scale aircraft

For example, I don't have a model plane that isn't capable of a 30 degree climb, and it's quite common for aerobatic model airplanes to have thrust to weight ratios exceeding 1.5:1 or even 2:1.

Question

When I first saw some of what model helicopters were capable of I was blown away. Having also seen some full scale helicopter aerobatics (Red Bull Demonstration) I've been wondering just what full scale helicopters are capable of.

In the Red Bull Demonstration it looked like the helicopter might use some negative (or at least neutral) collective while rolling, however it did not appear that it would have enough to sustain an inverted hover or inverted flight. This got me wondering: has there been a full scale helicopter capable of sustaining inverted flight or an inverted hover (which takes even more power)? If there hasn't, would it be possible to purpose build a helicopter that would be capable of doing so?

Note

I'm just wondering about technical possibility of creating a machine capable of this maneuver. For the scope of this question, please ignore the regulatory, legal, and economical obstacles that would need to be overcome to attempt such a feat (not to mention finding a pilot willing to try).

Also, I'm only asking about full scale inverted flight. I understand physics get in the way very fast if someone were to try to design a full scale helicopter capable of all the aerobatics models do.

*some model airplanes and helicopters can be greater than 35% full scale, but that is still small when compared to full scale aircraft.

Vikki
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JustWannaFly
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    I believe that the AH-64 can roll 360° with no forward airspeed, loosing altitude in the process. Either I've seen it or an illusion. I cannot find any evidence on Youtube, however. I would love for someone to either correct me or post video evidence, I've been wondering what I've seen for years. – dotancohen Apr 13 '16 at 06:16
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    Bear in mind that designs don't scale with the same power of their linear size. It's not just occupant killing that is the concern. Never mind acrobatic tricks, a faithfully scaled up model aircraft probably would be torn apart by the most boring of attempts to fly. – Nathan Apr 13 '16 at 09:45

1 Answers1

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There is no real helicopter capable of sustained inverted flight and certainly not of hovering inverted.

The simple reason is that there is no operational need to do so.

Is it theoretically possible? Yes, but you have a lot of engineering problems to overcome. In addition to inverted fuel and oil systems, the rotor head will be much more complex than it needs to be.

When flying normally, the fuselage effectively hangs like a pendulum underneath the rotor with a large nut (OK, it's more complex than that) holding the rotor onto the rotor shaft hub. The rotor pulls the fuselage up.

To fly inverted, the rotor must be able to push the fuselage up so the hub would have to be engineered to apply and withstand the appropriate forces in both directions.

Then, the rotating blade cuffs, and associated hinges, must be designed so that the blades can have negative pitch (as seen from the normal attitude) across the full range. In turn, this means that the collective mechanism must be designed so that it is possible to set positive and negative pitch as required.

The blades would also need to be more rigid, so that they do not cone excessively and contact the fuselage. This would add more forces to the head joints which would need to be strengthened.

Finally, you must "reverse" the cyclic controls so that the expected collective pressure, cyclic back; nose up; reduce speed; works in the opposite sense.

I've probably not considered other factors.

All of this would add significant complexity and weight which, given that there is no requirement for inverted flight, will not happen.

By the way, the power required to fly or hover inverted would be the same as for normal flight. There is no intrinsic reason why inverted flight would need more power - blades rotating at a certain speed, with a particular angle of attack will generate the same amount of lift no matter which way they are pointing.

The reason model helicopters can do this is that the ratio of the weight of the rotor to the weight of the fuselage is much lower than for a real helicopter, and therefore requires a lot less power to produce the required lift, and models have a much higher power to weight ratio.

The reason real helicopters can go inverted for a short time in aerobatic manoeuvres is because the momentum on the fuselage is pulling away from the rotor so that the lift vector pointing up, toward the fuselage, is countered. Consider an outside loop, which would be impossible, since the momentum of the fuselage and the lift vector would combine.

You should be able to demonstrate this in a model with a normally designed rigid head. Add some (secure) ballast in the fuselage until you must accelerate in ground effect to gain translational lift in order to be able to climb out. Now try to fly inverted!

Simon
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  • That's pretty much what I figured. The lack of need combined with extra engineering required mean it just won't happen. – JustWannaFly Apr 12 '16 at 17:12
  • With regard to your point on power, would I be correct in assuming the blades have a symmetrical airfoil if they produce the same amount of positive or negative 'lift' for a given angle of attack? – JustWannaFly Apr 12 '16 at 17:14
  • @jdkorv11 Not necessarily, although many blades are symmetrical. If not, all you need to do is increase the angle of attack. Remember that power is needed to overcome drag which is a function of how much lift is produced. To produce the same amount of lift from any given aerofoil needs the same amount of power, regardless of where the lift vector is pointing. Imagine a stationary blade. You can make the angle of attack (pitch) whatever you like with no power since no speed = no lift = no drag. – Simon Apr 12 '16 at 17:17
  • Thanks for clarifying. I was operating on the assumption that the rotor was at speed and being supplied with the power necessary to sustain flight. I should have stated that. Also, are helicopters typically capable of providing some negative collective? I'd guess so but that's just a guess. – JustWannaFly Apr 12 '16 at 17:30
  • @jdkorv11 Not as far as I know. All the types I've flown have slight positive pitch when the collective is fully down to control the rotor RPM in autorotation. The ability to generate negative pitch without the stronger head and stiffer blades I mention in the answer would make the chances of a rotor strike higher. – Simon Apr 12 '16 at 17:32
  • Thanks for the insight. While I'm pretty comfortable with the models, I don't know about the challenges faced by full scale helis. For example, models typically don't need to worry about rotor strikes so I've never really thought about those as problems before. Also aerobatic models will often allow as much negative collective as positive so that's why I was wondering about full scale. – JustWannaFly Apr 12 '16 at 17:40
  • @jdkorv11 You're welcome. – Simon Apr 12 '16 at 18:02
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    The pendulum argument does not work. In a pendulum the force at the hinge must be always upwards, but the lift tilts with the rotor and that tilts with the fuselage. Rotorcraft are not stable and higher or lower CoG does not change it (as it does not change roll stability in fixed wing aircraft). – Jan Hudec Apr 12 '16 at 21:54
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    I feel like the part about "Gravity will try to pull the fuselage down and sideways, the pendulum will try to swing down" might be suffering from something akin to the pendulum rocket fallacy. – user2357112 Apr 12 '16 at 22:10
  • So how did they shoot the scene near the end of Escape to Witch Mountain? This was in 1975, long before CGI effects. – JDługosz Apr 13 '16 at 02:59
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    @JDługosz I have no idea, you'd be better asking the film makers. Unless you are suggesting that because CGI didn't exist, the helicopter must have flown upside down in which case I'd ask you how they got the van to fly? Or any other number of special effects used. BTW, the first use of CGI in films was in 1973 and there were many techniques used before that such as matting and compositing. – Simon Apr 13 '16 at 05:57
  • @JanHudec Actually, it does. I started to write some fairly long comments explaining why but it just kind of clogged things up so I'll simply delete the paragraph. I think I've done a reasonably good job of explaining why helicopters don't fly inverted. Vertical CofG certainly does affect roll stability, it simply isn't relevant in a fixed wing since it is not possible to move the CofG significantly, vertically, away from the aerodynamic centre. which is roughly where the wings are. In a helicopter, the CofG is significantly below the aerodynamic centre, at least when not inverted :) – Simon Apr 13 '16 at 06:22
  • @Simon, yes, it's complicated (for fixed wing aircraft it actually does matter for slip too; it does not matter for roll alone). Anyway, all the other reasons you gave are good ones. – Jan Hudec Apr 13 '16 at 07:51
  • @user2357112, yes, it does. But then it gets more complicated, because as the aircraft actually starts flying sideways, another aerodynamic force will appear and that will now be able to create torque. So there actually is some effect, but it can't be explained with pendulums. – Jan Hudec Apr 13 '16 at 07:54
  • I don't think the cyclic needs modification: a pilot could be trained to handle inverted flight. And I guess we'll have to convince some daredevil to actually modify a helicopter - as I see it, there's only practical considerations, no fundamental limitations. – Sanchises Apr 13 '16 at 11:04
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    @user2357112: The pendulum fallacy is the idea that mounting the lift generator above the CoG will lead to passive attitude stability. This is false; no matter whether the rotor is above or below the craft, you need some active control of the direction of lift to achieve stable flight. In a helicopter that's what the cyclic provides. Still, however, you need quicker and/or more precise control in order to be able to balance your craft atop the lift point than you need for dangling from it. That's easier to provide for rockets because they're not expected to hover anyway. – hmakholm left over Monica Apr 13 '16 at 21:57
  • "Consider an outside loop, which would be impossible, since the momentum of the fuselage and the lift vector would combine." By that logic, outside loops should also be impossible for fixed-wing aircraft. – Vikki Oct 15 '18 at 02:56
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    Most full size helicopters can apply some negative collective; this is required to be able to autorotate on power failure (which all helicopters are expected to be able to do in order to be flight certified). Having enough negative collective (along with all the other factors) to fly or hover inverted is another thing entirely. – Zeiss Ikon Feb 24 '22 at 19:42
  • Back then I wrote that there is no pendulum, but that's not entirely true. The fuselage is hanging below the rotor pretty much exactly like a pendulum. The rotor is tilted by adjusting the aerodynamic forces it creates, but the fuselage can swing around below it. This still does not contribute stability for the abovementioned reasons, but it would create additional instability in inverted flight. In fact some designs have a problem, called mast bumping already at low positive loading. – Jan Hudec Feb 25 '22 at 20:48