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4m rotor diameter and Teetering rotor

Hi every one

I am new to ardupilot, we are currently building a large UAV helicopter with 4m rotor diameter with a teetering type two bladed rotor head. And the total take off weight is about 150kg. I am wondering if I can use Pix hawk and Ardupilot for autonomos flights of this heli. Can any one explain me if it is possible to go with Ardupilot for such a large heli.

Thanks

Possible, yes, it is. But it will need some special tuning. Are you doing simulations? How proficient are you in Control theory?

I have some experience in gyro tuning and RC helicopter flying. I have some theoritical knowledge on helicopter stability and control. And nothing much, can you suggest me something here.

Post photos, and more vehicle details.

@mohanakannan
As @amilcarlucas said, it is possible to control a heli that size with a pixhawk and the arducopter firmware. The software is pretty flexible providing the ability to tune most vehicles. Are you designing this aircraft from scratch or is this a proven helicopter? You will have to figure out what servos you will need and how to power them through the pixhawk. I am not familiar with what is needed for that size heli. Is the rotor system a freely teetering system or does it have stiffness like a spring or rubber dampers that force it back to the plane of rotation? If you have picture or details you can provide, that would be great.

This Helicopter is being designed from scratch. We are using tonegawa seiko servos. These servos take external power supply from a 24v battery and signal from the receiver. The rotor system is a free teetering and does not have any stiffeness similar to the rotor system of a Jetranger. Sure, I can share the pictures of my heli under development. Kindly tell what do you think.

Just a proposal: try ardupilot on a cheap but well known helicopter first. Play a lot with different settings, get a feeling of control gains, and install it on your large heli only after you are already familiar with it.

In other case you would have to deal with two unknowns at the same time: mechanics and flight control. And even the best professionals could have some mishaps, crashes before the success.

I dont know any RC helicopter with teetering rotor. After a short search found this one:

@mohanakannan Looks great! Have you ever designed a helicopter before? I agree with @perwollf that it is probably a good idea to use a less expensive helicopter to learn about how to tune the flight controller and get some experience with the system you’ll be using. Hopefully you will have a good sense of the structural dynamics of the helicopter you are designing. Vibration management is one of the biggest challenges to helicopters. Along with that is dynamic stability. In tuning the flight controller, you will have to know what gains could drive the rotor unstable. I am developing an automated tuning feature which should be able to provide that information without having to actually adjust the gains to that point. being that it is a true teetering rotor head, You will be on the far end of the tuning spectrum from most heli’s that I have helped users tune. The RC helicopters have very stiff rotor heads.

What is this helicopter designed to do? Surveillance? package delivery? …

@perwollf. Thanks for the advice, I am any way planning to do that. I will have to use the pixhawk with a smaller kit rc helicopter to know some stuffs before using it on to the big one. I too havn’t seen teetering rotors on RC helicopters, thanks for sharing the video. Teetering rotor on such a small RC heli won’t work because of the light weight blades and high head RPM. We need to have heavy blades and low RPM or use a bell-hiller flybar to have some lag otherwise it will follow the rotor too quik. Sorry for the over explanation, I believe you knew it already.

@bnsgeyer. Thanks for the advice to use pixhawk first with a smaller heli. And good to know that you’re developing an automated tuning feature. It’s interesting to know that some gains could drive the rotor unstable. I understand that I may need to use the far end of the tuning spectrum for my helicopter. We are planning for long range surveillance and also package delivery.

Legal Disclaimer:
ArduPilot cannot be used to fly aircraft carrying human passengers. It is not certified software for such a purpose. The following test information from was from a flight test conducted under strict safety mitigation requirements for data gathering purposes and obtaining a grant of waiver to develop, test and fly an unmanned helicopter over 55 lbs gross takeoff weight in the U.S. The aviation authority is the FAA Flight Standards District Office, Minneapolis, MN. The grant of waiver was denied based on the test data.

We did successfully hover a Mosquito XET helicopter with a modified version of ArduPilot. But I was flying the helicopter (in the seat) while my assistant tried to hover it manually via RC control. We used S-tec autopilot servos with clutches that I could over-ride with the cyclic. We only had cyclic control hooked up to the RC, I controlled collective, torque and engine throttle from the seat.

That being said, some warnings:

  • The Mosquito has a teetering rotor system similar to a 206B.
  • We only used acro, did not let the autopilot actually fly it with any sort of stabilization.
  • I had to make significant corrections from the seat to hold it in an acceptable hover.
  • We did it primarily to get some logging information on what would happen if the autopilot tried to fly it.
  • My conclusion was, that while my assistant (who is an expert RC pilot) could hold it in a pseudo hover using visual reference, the rotor system was too disconnected from frame attitude for the autopilot to work. It would not be able to hover it.
  • In hover, the logs showed the frame attitude (that the autopilot measures) having significant delay from actual control inputs - the rotor system does all the work and the frame stays pretty much static.
  • If we would’ve tried to use a mode like Stabilize, I concluded the autopilot would start “chasing it” and result in a dynamic rollover because the autopilot does know what the rotor system is doing, as it “sees” no response in frame attitude from a control input, so it would tend to over-control.

In flight, where the rotor system is a little less disconnected I think it would be able fly it and perform basic roll leveling or stabilization. But not in the pitch attitude. However, if it can’t pick it up into hover, it’s a non-starter to get it into forward flight. The control inputs to hover a teetering or hinged rotor system are very minute - your arm is resting on your leg and the movements on the cyclic are usually no more than 5 or 6 mm in any direction.

The Mosquito is quite a bit heavier and larger than your model, obviously. But if you can, I would suggest using a rigid rotor system, then it will work fine if the input shaping is adjusted properly for the response of a larger size helicopter like you are building.

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Hello @ChrisOlson

Thanks for your valuable inputs. Your efforts are highly appreciable. My helicopter under development is very similar to that of the Mosquito XET with a similar rotor system. The feedback is off course slow because of the fact that no moments will be transferred from the rotor system to the frame on a centrally hinged rotor. The only way we get a moment is by the thrust line offset from the CG due to rotor tilt, which will have a large delay compared to small rc helis. In the case of offset hinge rotor moment transfer is possible directly from the rotor to the frame. Is it possible for you to mount the controller in faraway places like the bottom so that we get large magnitude of displacements? It would be great if you could suggest any other way to handle this in our case.

Best Regards
Mohan

this is extremely dangerous. arudpilot was not made for use in manned aircraft. this post needs to be removed

Based on testing I have done, I do not believe it is possible at the present time. I would suggest using a rigid rotor design, or a flybar design on your model, although it can be damped with rubber dampers like most RC models are. Bill and I have discussed articulated rotor systems in the past, and the difficulties of being able to fly them. But at the present time the design of the attitude controller is such that I can definitely state that it will not work with a FBL rotor.

Even with a rigid rubber-damped rotor system, blade flex alone will provide for a significant challenge in tuning the attitude controller with a 4m rotor. If you designed your rotor system to use a flybar, aka the Yamaha RMax/FAZER-R, so the autopilot doesn’t have to perform basic stabilization of the rotor system with the rate controller, I believe you will have better results vs a FBL rotor with a larger sized machine. Flybar-stabilized helicopters, even in RC, have a teetering rotor system, although with very soft dampers on RC models and not usually an actual teetering hinge. But ArduPilot is able to fly flybarred helicopters because the rate controller is basically turned off.

There is nothing wrong with the Bell-Hiller flybar-stabilized rotor system design. It was used successfully for many decades, reduces the loading on your cycle servos, and greatly simplifies tuning of the attitude controller for a large machine that is going to be difficult and dangerous to tune. Based on the testing I did with the Mosquito it was the solution I arrived at that would be necessary to fly it as an unmanned platform.

Based on my work over the past year developing an autotune algorithm, I wouldn’t go so far as to say it couldn’t be done. I believe it could be done safely but I wouldn’t expect it to fly like a sport helicopter. We would probably need to dial the gains back quite a bit from the typical RC heli gains so as to not overdrive the system thus making it chase attitude. And we would have to dial back the command model to be more in line with the rates and accelerations achievable by this vehicle.
It would be worth developing a simulation model with the same characteristics as your heli to see what would be achievable. I remember seeing a paper on the simulation of a Bell 206. We could use the state space model of that and see how well the aircraft can be controlled. The paper was presented by Zivan and Tischler at the American Helicopter Society forum in 2007 titled Development of a Full Flight Envelope Helicopter Simulation Using System Identification. It has the derivatives that could be used for the simulation experiment.

Good point. I would have to concede that it might be possible. Just based on what I saw doing some data gathering, I rejected it as too dangerous to try to come up with a “tune” that would work.

The biggest problem was hover. With the autopilot mounted under the seat the frame attitude changes and accelerations were basically undetectable in static hover.

I wasn’t willing to proceed with it unless I had a much smaller scale model of the same machine to experiment with. Even flybar RC heads are quite a bit stiffer than a true semi-rigid rotor system. ArduPilot flies flybar helicopters with supreme stability and stable flight. That’s what I would select for a large unmanned model - I think there is a reason Yamaha went with flybar on their large umanned helicopter with their YACS system. Like you say, it doesn’t fly like a sport helicopter. But it is extremely stable and easy to fly for a remote pilot. And minimal load on the cyclic servos vs a FBL head. Our test heli has a mixer box in it so it is H1 with only two servos. Most are going to use servo direct to swash, and that is very high load on the servos with a large rotor. Especially during runup when the helicopter rocks around and physically tries to jerk the cyclic out of your hand until the rotor system “flies out” and becomes stable. The 206B is hydraulic assist. These smaller machines are direct linkage.

Hi, I hope you don’t mind me asking asking a couple of questions, but I’m genuinely curious, why are you developing such a large unmanned helicopter? And why did you pick the teetering hinge style of rotor system?

Hi @Jay-Bruce

  1. We wanted a large model mainly to carry large payloads internally and also to carry more fuel which improves range and endurance.
  2. I chose Teetring rotor system mainly because I come from 1-seat & 2-seat microlight helicopter back ground. So I have some expeience with them which makes me confident in the system. If I had started from RC helicopters I’m sure I would have used rigid RC styled rotor heads.

Sorry @bnsgeyer & @ChrisOlson

I had some personal commitments and had no time to respond. Thanks again for all your valuable inputs.

Cheers
Mohan

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