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new design

My friend and I recently prototyped this radical new design for a helicopter. In this design, instead of using the main rotor to control the pitch of the helicopter, there are two other auxiliary motors at the tail rotor. This means that the main rotor doesn't tilt in any way to control the helicopter, it only controls the altitude. For the yaw, there would be another motor at the tail rotor area. Is this even feasible? Even at a model scale?

Aadiraj Anil
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  • Why two vertical tail rotors? One would surely be sufficient? – Sanchises Dec 26 '22 at 12:30
  • This is technically a tandom rotor https://en.wikipedia.org/wiki/Tandem_rotors with the second rotor on a long lever arm and scaled down. It can probably work in scale model but as drawn is will lack roll/lateral control. – GremlinWranger Dec 26 '22 at 12:53
  • You'll be relying entirely on the "pendulum effect" to keep it upright, it appears (much like this -- https://www.youtube.com/watch?v=f4cAylbWvmU ) – quiet flyer Dec 26 '22 at 13:36
  • Roll control is missing. There is no pendulum effect with fixed rotors. – U_flow Dec 26 '22 at 13:37
  • @quietflyer pendulum effect can only happen if the connection between the lifting element and body is flexible. If the axis is rigid, it doesn't matter if the rotor is above at or below the body. If the connection is flexible, you now have to manage the tilt of the rotor disc directly, and presto, you have Bell's teetering rotor system. – John K Dec 26 '22 at 13:43
  • Perhaps one of these might interest you. Who says you could not put an electric motor in the rotor for vertical take-off, then recover energy from the rotor once forward flight was achieved. – Robert DiGiovanni Dec 26 '22 at 14:40
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    Not as drawn. There is a fundamental problem that would make directional flight impossible. It looks to me like one of your design goals is to simplify the main rotor hub by eliminating the complexity of the cyclic mechanism by having a fixed hub, and you modulate only rotor RPM. This helicopter could theoretically hover, but once in directional flight (presumably forward) an entirely fixed main rotor hub without any cyclic mechanism would have no means to compensate for the difference in lift produced by the advancing and retreating blades. It will roll into the retreating blade. – Max R Dec 26 '22 at 14:52
  • @U_flow -- see link in my comment -- I've been threatening to ask an ASE question about this topic for years, maybe it's time-- – quiet flyer Dec 26 '22 at 15:42
  • @quietflyer a common misconception. If you draw the situation up, you will quickly realize that there is no pendulum effect for fixed rotors – U_flow Dec 26 '22 at 16:20
  • Do some research into cyclic pitch and why its needed. This was the mistake made by many of the early would-be inventors of the helicopter. – Jim Dec 27 '22 at 01:31

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It should be feasible (with some modifications). And unfortunately is not radical nor new: Any RC helicopter toy without swashplate is controlled in a similar way, like the one in this picture (source):

enter image description here

Here the two contra-rotating main rotors produce the thrust to lift the helicopter and they balance their own torques.

To make the thrust pointing forward and give the helicopter a forward speed, the small rotor on the tail is activated so that it generates an upward thrust: this produces a nose-down pitch which makes the helicopter and the main thrust tilt forward, producing therefore also a forward force. If the rotor on the tail is activated to generate an downward thrust, it makes the helicopter and the main thrust tilt backward producing this time a backward force. This part is similar to your idea.

Yaw is controlled making the two main rotors rotate with a slightly different speed. In your case the yaw is controlled via a third rotor, much like a conventional helicopter.

Obviously there's no means to tilt the helicopter with its thrust to the right or left, so lateral movements are not possible.

As also pointed out in some comments, stability might be an issue. Thanks to the rotors on the tail, you might be able not only to manoeuvre but also to keep it stabilised in yaw and pitch but only if its own proper movements around those axes are not faster than your reflexes. Anyway, as already said, in roll there's no way you can control and stabilise it. A mechanical stabilising bar like the one in the picture is therefore necessary.

Another problem is the dissymmetry of lift that would make the helicopter tip over as soon as it moves somewhere horizontally. To avoid this, the blade shouldn't be rigidly connected to the rotor head but free to flap either via an hinge or making the blade's root soft.

So, taking into account these modifications, at the end you obtain an RC helicopter just like the one in the picture I posted :)

sophit
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  • Does this question, and associated answers, suggest that the stability dynamics of the helicopter in your picture are considerably different than the one in the question? https://aviation.stackexchange.com/questions/64969/how-does-this-rc-helicopter-keep-itself-upright?rq=1 – quiet flyer Dec 26 '22 at 13:29
  • @quietflyer: for sure not, the same stabilising bar might be needed, but the general idea isn't something new and should be feasible. – sophit Dec 26 '22 at 13:36
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    I’d recommend being more specific in your answer. The design you show is feasible because of the unique characteristics of counter-rotation, which is absent in the concept. As I mentioned in another comment, the counter-rotating blades don’t just simplify the design by eliminating a tail-rotor gearbox; They simplify the design by countering asymmetric lift. Without counter-rotation, you have two problems to solve, not just torque. – Max R Dec 26 '22 at 17:22
  • @MaxR: you're right, I actually wanted to expand that part of my answer but it's always a bit difficult to understand when it gets too long, technical and boring... Maybe I'll link some other answers dealing with those topics. – sophit Dec 26 '22 at 19:08
  • @sophit just to be clear. The design the question poses is not feasible. It possesses no roll stability, an no means to counteract. I am sure that you are aware, that this is due to the fixed propeller. The design which you link has fly bars and flexible linking of the blades which is part of its stability concept. The proposed design lacks these features. I propose that you modify your answer accordingly. Because your first statement is wrong. The proposed design is not feasible. – U_flow Dec 26 '22 at 21:00
  • @U_flow: thanks for your suggestions, now the answer should be clearer – sophit Dec 26 '22 at 21:47
  • @MaxR: thanks for your suggestions, now the answer should be clearer – sophit Dec 26 '22 at 21:47
  • The Wikipedia article about the "dissymmetry of lift" seems to have several issues, to the extent that I would not cite it here. The flapping and lead-lag hinges on a rotor head are for mechanical reasons (avoid overload and failure) and certainly not to compensate for asymmetric lift. Flapping / lead lag motions are anyway too small and hence too slow to have a significant influence. – Apfelsaft Dec 27 '22 at 03:05
  • @CarlBerger: I couldn't find anything better... and at least the pictures are correct ;) – sophit Dec 27 '22 at 15:39