Compound Helicopter Design

Starting a new discussion topic dedicated to the design of compound helicopters. It would allow users to share ideas.
For those that aren’t familiar with the compound helicopter configuration, it has a single main rotor but the tail rotor is replaced by two forward facing propellers that provide both antitorque snd directional control like a tailrotor but also provide thrust to pull the heli through the air like an airplane.
I have already built and flown one so I plan on sharing the details of that design and also my plans for the new and improved design. I will post those soon but wanted to kick off the topic.


What are the benefits of the compound design?

Sorry I didn’t didn’t talk about that. The big benefit is fast forward flight speeds. So instead of the main rotor system providing the propulsive force, the propellers do and In most cases a compound design also has stub wings that help offload the main rotor system in forward flight. The combination of those two things allows the heli to go faster compared to a traditional heli at the same rotor speed.

I plan on using the winglets on mine.

I’ve been looking up as much engineering information as I can find on the EuroCopter x3. The machine is now sitting in a museum and not being flown anymore, and I don’t think AirBus intends on doing anything with it. Sikorsky went forward with their x2 concept and it evolved into the S-97 Raider. It is a coaxial compound heli.

Yes, as much as 30-40% greater than conventional cruise speed. I don’t know about efficiency yet as compared to conventional. There is no data on it I can find. In theory the efficiency should be better due to reduced drag on the main rotor system in forward flight.

@bnsgeyer take a look at the dev thread on how to incorporate the mixer for the thrusters. Wondered what you thought of my idea to use the dynamic flight flag to make it fully automatic in all flight modes.

It’s evolved to the Airbus Racer. See this link

I did see that and meant to reply. In short I think it can be treated like we discussed with heli trimming and have maybe a cruise pitch attitude parameter that would be set based on the dynamic flight flag. That way it wouldn’t affect your hover pitch attitude.

Jakob, Bill’s first design uses gear and shaft driven helicopter tail units for the thrusters. DDVP drives can be used as well. I don’t know yet if DDFP drives could be used. I think it is possible with reversing ESC’s and use the SERVO outputs for the variable pitch servos to drive the ESC’s, similar to a DDFP tail rotor. So 1500 pwm signal would be motor off. 1000 - 1499 is motor reversed. 1501 - 2000 is motor forward.

DDFP thrusters would reduce the weight and complexity of the drives considerably. And I don’t think it would take too much re-write of the code to use DDFP, other than having to change it to map two DDFP drives to two SERVO outputs for it to work.

I believe I captured your DDFP changes in my proposed compound Heli motors library. So this should be doable once accepted into master.

And I think it should work too. I’ve been looking at my logs where I tested a vertical stabilizer on a conventional heli. By adjusting the vert stab to counter main rotor torque I was able to reduce the power by ~50 watts to the tail rotor. And the SERVO output to the tail servo shows it was running at zero pitch in tranlational lift (starting at ~5-7 m/s flight speed) except for making minor yaw corrections. Which means that DDFP drives, with a suitably tuned vert stab, would both be running in forward direction that point with only minor changes in assymetric thrust for yaw correction.

I do not see an engineering reason that it won’t work.

I have nowhere near the amount of experience that you guys have, but is there a big need to fly at more than 12-14m/s? Or do you expect a big increase in efficiency even at those speeds?

I’ve been flying Section surveys the last 6 weeks at 27 m/s doing 1/16 Section (40 acre) shots for NASS. Takes 4 minutes to fly a Section (640 acres) plus turning time.

The biggest market for high-speed VTOL aircraft is agriculture, mining, foresty, and powerline/pipeline. The local power cooperative here flies powerlines now with a Vario XLC Carrier turbine helicopter flown with HeliCommand (now Bavarian Demon) with a Profi GPS. Those helicopters are built in Europe and are quite expensive to operate. I think a compound could do what they do in electric configuration in a smaller size.

Right. Here in Canada, BVLOS is all but impossible, so that type of surveys are not yet feasible.

Interesting project nevertheless. I would think that a direct drive would be the simplest solution.

Once I get my new build on the way (Except UPS has actually lost my helicopter!), I would been interested in joining in, as I would then have a spare airframe for it.

It’s not in the US either under Part 107. But I got a Section 333 with restrictions - can’t fly over state roads or Interstate highways, stay 500 feet from occupied buildings, ADSB-OUT required, commercial pilot certificate for full-size (with remote pilot rating) requiried.

I certainly hope that doesn’t turn into a nightmare.

It kinda already has, but ultimately it’s insured. The level of incompetence of UPS Canada is very high.

How well I know. We live just a hop, skip and jump from Canada. We have friends in Ontario and if we send them something on UPS we send it to Ryden’s Border Store on the US side of the border, and they drive down to get it. Otherwise the chances of them actually getting it are about 50-50, even with the VAT and declaration already taken care of for Customs.

I’l post some details of my x3 build. It may be awhile as I’m still in the designing stage.

I have some experience with doing composite airfoils, etc. Let me know if there’s anything I can help with.

Hah. My new Soxos 600 did turn up!. (Took a lot of shouting at useless UPS people for that to happen)

So I had a presentation that I developed to document my development of the X3. I modified it to bring it up to date.
Compound Helicopter Design
I briefly describe my plans for the new design. I will provide more detail on my plans for the new design in another post.

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So, I was looking up reversible ESCs. Most modern min-quad blheli compatible ESC’s should be able to do the job.
However, it also made me think of how to control the amount of thrust in forward flight?. Do you tie it into the collective or cyclic?. You could put it on a separate channel, but then you’ll need to map it to an easy-to-access control, like a slider.

I used a slider to control the thrust of the propellers in stabilize and althold modes. I was not sure how to implement the boost control in controlling the aircraft in velocity command modes and auto modes. I started a discussion on the developer team section to get feedback and ideas.

The trick with DDFP is to select the right props. Normal quadrotor props only have a one-way airfoil. They produce some thrust in reverse, but only about 40-50% of what they produce running the right direction.

There are some props for quads with reversible motors that may work fairly well, but I don’t think those have a very efficient airfoil.

I theory, I think it could be done with DDFP without reversing props too. it would use just one thruster for yaw control in hover like a DDFP tail does. And the heli would hover with a nose-up attitude. But the size of the heli would probably be limited to 500 class or smaller. I have never been able to get DDFP to work all that will with a 600-class heli. It has no problem as far as available thrust. It has a problem with how fast the tail rotor can be throttled and respond for accurate yaw control. A friend of mine flew a Trex 550 with a DDFP on it for awhile and it worked reasonably well. But not as good as a conventional tail. He eventually went to DDVP with a y-cable on the main rotor ESC/tail ESC. And is now flying our dev code where we have DDVP working again.

In the end, I think DDVP will give better results. But I would not hesitate to try DDFP on a 500 class compound.