Tandem Heli

I've had a dream for a while of building a tandem rotor heli.  And I think I finally figured out how to do it.  The reason for wanting a tandem is for a number of potential benefits.  Tandems are more efficient than single rotor helis, because they don't need to waste up to 30% of their power simply for anti-torque control.  They also pack a LOT of payload capacity into a compact frame size.  A "600" size machine would be a meter long, and could theoretically lift about 50lbs of payload.  That would be pushing the limits of the power system, but 10-20lbs would be easy.  And it would fit in a golf-bag sized package. A "500" sized machine would be only about 70cm long, and should be able to lift 5lbs or maybe even 10. Really attractive specs for a workhorse UAV. A tandem rotor helis is also more efficient flying forward than a single rotor heli, with a higher theoretical top speed.

This has been done in the past with things like the TwinRexx.  But I wasn't satisfied with the way that machine was done, it had a lot of problems.  A big problem it had is that it used gear drive between the two rotors.  The gears used were coarse pitch, and probably create a lot of vibrations.  Also, gear drive requires a very rigid structure or else you get geartrain instability which leads to premature wear and failures of the gears, which I think was the main problem with the TwinRexx. 

One of my requirements for the project is that it would be all belt drive.  I like belts for a lot of reasons.  They are efficient, smooth and quiet.  When sized correctly, they are very durable.  They have particular advantages in a tandem heli application because any flex in the frame between the two shafts is easily accommodated by the belt. 

Another big desire for me was twin motors.  I'd like to have twin motors for the exact same reason full-size aircraft do.  That is obviously the ability to continue flying if you lose one motor.  Helis can auto-rotate, but that means you have to land *right here, right now*.  In a workhorse UAV application, that just isn't acceptable.  Adding a second motor in the most basic way isn't very difficult.  But if you want the system to keep running with one of the motors stopped, you need a one-way on the motor output shaft, NOT the main shaft as most single rotor helis employ.  Otherwise the dead motor would suck a lot of power from the good motor.

I've hummed and hawed about this, but I'd basically be undertaking a ground-up design of a completely new heli and drivetrain from scratch, which is daunting.  I'm certainly capable of it, but it's a lot of work.

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I've finally found a solution.  The MSH Protos line of helicopters almost seem tailor made for tandem conversion.  They use a single stage belt drive for the entire drivetrain.  They don't have a one-way on the main shaft, instead they have it on the motor shaft.  This would make it very easy to basically strap two of these things back to back with a single long belt. 

Hopefully I will start this project this winter.  

I should say that I actually really like the Protos line of helis in single rotor applications as well.  They have a lot of really desirable features.  A smooth, quiet and efficient, single stage belt-drive.  Nice, simple and clean servo installation.  3 mainshaft bearing blocks.  And a really nice space for mounting an APM or PX4. I'm strongly considering switching over to these from the Align machines I've been using.  The 500 size Protos is at a very attractive price.  They also have a 450 size machine.  And a 700-800 is just coming out soon.

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  • Talk to Frederick Hedberg, he has some experimental code.



    ChrisgA1A said:

    Hi Rob,

    About a year ago I put a tandem together using two FBL 450 kits just to see the results. I ended up with a fairly robust air frame with one motor and ESC on each head using the torque tube to keep them synchronized. It's not ideal but very functional. The Skookum SK720 (blue) was supposed to fly a tandem so I installed and meticulously set it up. Well...it will fly, but not well at all.. I was told by the guys at Skookum that the 720 black "may" work better but I don't have $450.00 to spend on a chance. I heard that the Bavarian Demon works well for a tandem but it uses two servos on each swash so that is another fabrication issue that's just not worth it. Because at the end of the day, it would still only be a RC model..

    I understand that you're no longer a supported developer and think that SUCKS! With that, I don't see Tandem support for ArduCopter in the near future.. I thought the OpenPilot guys were going to do it but that whole camp seems to have fallen apart..Do you know of any affordable FC that will fly this thing properly?

    Or, maybe know someone that may have some experimental compiled code and a GCS for APM or Pixhawk :)

    If so, I'd be happy risk my hardware on some tests.

    Thanks,

    Chris

  • Hi Rob,

    About a year ago I put a tandem together using two FBL 450 kits just to see the results. I ended up with a fairly robust air frame with one motor and ESC on each head using the torque tube to keep them synchronized. It's not ideal but very functional. The Skookum SK720 (blue) was supposed to fly a tandem so I installed and meticulously set it up. Well...it will fly, but not well at all.. I was told by the guys at Skookum that the 720 black "may" work better but I don't have $450.00 to spend on a chance. I heard that the Bavarian Demon works well for a tandem but it uses two servos on each swash so that is another fabrication issue that's just not worth it. Because at the end of the day, it would still only be a RC model..

    I understand that you're no longer a supported developer and think that SUCKS! With that, I don't see Tandem support for ArduCopter in the near future.. I thought the OpenPilot guys were going to do it but that whole camp seems to have fallen apart..Do you know of any affordable FC that will fly this thing properly?

    Or, maybe know someone that may have some experimental compiled code and a GCS for APM or Pixhawk :)

    If so, I'd be happy risk my hardware on some tests.

    Thanks,

    Chris

  • hello bob . I care a lot about your idea and I would be willing to help in the design and construction of mechanics. I am a mechanical engineer and I have already made ​​three mechanical for trex 700 gasser the only problem really is my bad English .

    I hope you both willing and able to proceed with the project .

    Enzo

  • Hello Rob,

    I am very_interested in this tandem config heli. For all the reasons you mentioned in your first post. 

    How far did you get?

    • Never really got very far with it actually.  Still just a concept at this stage.  I think it would be interesting to do, but I don't have resources to experiment with stuff like this on my own.

      • Rob, 

        I was flying a 450 heli with APM this morning. I've built tons of multirotors in the past and have just started messing around with collective pitch heli's recently. I still have yet to tune the heli right, but I already like it.

        During flight, I also realized it would be exciting to design a tandem heli from 2 traditional heli kits. Not for additional lift or just to be exciting, but I found a legitimate use for a tandem that neither a traditional heli or multirotor can perform perfectly. 

        As a result, after I perfect tuning this heli, I plan to double up and start a tandem heli project. I can't say I'm super familiar with Arducopter's code, but I wrote my own flight control code in the past. Adapting APM or Pixhawk for a tandem heli would be the biggest challenge for me, but I sincerely appreciate all the work you put into Arducopter for heli's. I have to say, your work has inspired me to push the limits. 

        I definitely need to keep any results posted. This should be interesting!

  • Hi Rob,

    you can get 'S-Schlag' = reflexed airfoils for 'links'=left and 'rechts'=right in very good quality from this German company: http://derblattschmied.com/

  • Well I am in the process of making some 350mm asymetrical CW and CCW blades if you are interested.  40mm chord.  I can send you a PM when they are ready if you like.

  • Looks really interesting. Have progressed this at all?

  • A tandem UAV does makes sense and sounds awesome! I happen to see a Hirobo Chinook scale heli recently and thought mechanics would be great for a uav conversion, unfortunately its rather expensive... Worth having a look anyway even just for inspiration. Another advantage of tandems for uav use is (if flown side-ways) that a camera can be mounted in-between the rotors for a clearer front view. Maybe tandem tiltrotor is worth looking into as well (for some reason I want one) much bigger flight envelope and could go a long way for redundancy (glide or even auto-gyro emergency prop hehe) but would be more complex.

    Tandem shouldn't be too much work in APM code, right ? You'll need to setup some rudder>swash mixing and obviously add the second swash but it should be pretty close to trad heli, correct ?

    Good luck and keep us posted.

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