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.
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.