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What would it take to build helicopter blades that can fold in several parts?

This is the continuation of a question on Aviation. Unfortunately, there have been no use case for this for existing helicopters, so no-one has spent millions trying to work out if it was possible, let alone how to do it. As far as real life is concerned, folding helicopter blades at the rotor hub is good enough. As such, the question seem impossible to answer on a more reality-grounded place like Aviation, hence why it is continued here.

Assume that I have developed a magical helipack, or some other sort of ultracompact helicopter with a magical black-box power source. The helicopter itself is either contra-rotating or tip-jet to avoid the need of a tail rotor.

The problem is that while the helicopter itself is extremely compact, there is this enormous rotor and its long, long blades that get in the way when not flying.

Folding them at the rotor hub, as is done with conventional helicopters, helps a bit. However, blades of several metres long don't fit into the nice little backpack like everything else. For that, I need rotor blades that fold in several parts as well.

How would a rotor blade that can fold in several parts be designed? How good would it be as rotor blades? How much more would it mass?

Less-than-ideal blade efficiency is an acceptable tradeoff, as is (obviously) increased complexity. As the outer edge contributes more than the center, it is also acceptable to have the blade not generate lift from the inner parts, as long as the loss of efficiency is not too severe. However, the thing must be reasonably safe, so we want to avoid it falling apart as soon as one pin breaks.

If the design would be different for a conventional (contra-rotating) system or a tip-jet, please point out the differences (or their absence).

Bonus points if the blade can unfold by spinning it or in some other automatic way, so the user can press a button, deploy the rotor and have it open by spinning - allowing them to instantly fly off while screaming "I'll get you next time" at the heroes who just foiled their plans. Having to stop and unfold them by hand would simply not be as theatrical.

Extra-bonus points if it can also fold back automatically after stopping it. Again, not having it to fold it yourself right after landing, wasting no time to Save the Day, just looks better.

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  • $\begingroup$ The problem here is not "how can it be designed" but "can you imagine realistic materials that can be relaibly used". The forces involved are very high and they perform critical functions. As it stands, however, this might be too broad - it's an open ended set of list questions at the moment. $\endgroup$ – StephenG Jul 16 at 11:55
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    $\begingroup$ This isn't answer but I think your protagonist should just get one of these pal-v.com/en/explore-pal-v $\endgroup$ – Trevor Jul 16 at 12:41
  • $\begingroup$ Presumably you feel that helicopters are already too safe and too reliable. Would you consider helicopter-like alternatives as valid answers? $\endgroup$ – Starfish Prime Jul 16 at 12:58
  • $\begingroup$ @StarfishPrime That could be interesting, though what do you mean by "helicopter-like"? Autogyres cannot take off vertically as far as I am aware, and turbines would not have blades to fold (and are less energetically efficient as far as I can tell)... $\endgroup$ – Eth Jul 16 at 14:13
  • $\begingroup$ @Eth your question does not state that vertical takeoff or landing are an absolute requirement, nor does it say that the system couldn't be combined with some other takeoff assisting mechanism ;-) $\endgroup$ – Starfish Prime Jul 16 at 14:16
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Different choppers with folding blades fold them in different distances from the hub. Look at the AH-1Z Viper:

Bell AH-1Z

As Ash said, where and how wings are folded has to do mostly with material strength. You may wish to have joints like those of the V-22 Osprey:

MV-22 Osprey

Its blades are short compared to its body, and the joint seems pretty sturdy compared to regular civilian choppers with folding blades.

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  • $\begingroup$ This is interesting, though those designs still have the folding point close to the rotor hub - probably because they have no reason to do otherwise. What can we deduce from it about folding points further along the blade? $\endgroup$ – Eth Jul 16 at 14:47
  • $\begingroup$ @Eth we can deduce that it takes stronger materials and different storage setups to justify that. Most folding blade designs IRL are meant to allow helicopters to be stored in long, narrow places. $\endgroup$ – Renan Jul 16 at 15:11
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The rotor design on a toy helicopter I saw recently suggests its a matter of material strength rather than any technical difficulty. Those blades were in three sections that folded away for storage and folded out under tension when spun up. The problem is that those wings had to be made of cast aluminium with high tensile steel pins and they were on the order of only 100-120mm long when extended; helipacks, like the ones in the linked article, have 6m+ wingspans so the forces acting on the wing blades are much greater. Unfortunately I don't have the expertise to work out what would be required, nor the time, or patience, to work it out from first principles today, hopefully someone else can help you with that.

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