Tandem heli CAD project, help needed

[widell]

I'm a mechanical engineer and recently I've been interested in RC helicopters. My project right now is designing my own CP tandem helicopter. It's not so much about actually building and flying it, more about designing and solving the technical challenges that come up.
I'm trying to use as many standard parts as possible.

I might pick up the actual flying part of this hobby later, so far I've only tried simple quads. If so, this forum will prove very helpful, I'm sure.

I need some basic information about dimensions. Lots of stupid questions ahead. Also, English is not my first language and I'm not too familiar with the helicopter lingo, so bear with me.

I'm imagining this helicopter about 450 size. I assume two rotors with about the same area as one 450 would be able to generate about the same lift?
That's about 2 x 320. What blade length would you recommend? What's the main shaft diameter for a rotor of that size? Is there a suitable donor?

What is the distance between frames? I have found a bunch of motor mounts and main shaft bearings online but none with dimensions.

Can swashplates be separated, and is it possible to make custom lower halves for a reasonable price? I don't need elevator since that is achieved by different pitch on the forward and aft rotor on a tandem. Only two servos per rotor is needed for aileron and pitch, but that requires some sort of custom swashplate. I have seen tandems with two regular three-servo setups, but I'd like to avoid that if possible.

I'm thinking of using something like an Arduino micro computer to handle the custom mixing needed for a tandem. Has anyone here done anything like that before?

Any help is much appreciated!

[peteski]

Originally Posted by widell

I'm imagining this helicopter about 450 size. I assume two rotors with about the same area as one 450 would be able to generate about the same lift?
That's about 2 x 320. What blade length would you recommend? What's the main shaft diameter for a rotor of that size? Is there a suitable donor?

First things first, a 450 size heli actually has blades that are around 325-360 mm long, not 450 mm. So you might want to think about what size heli you actually want to end up with. A pair of 325 mm bladed rotors would actually lend itself quite well as it's a pretty common size, although most newer "450" helis are now on 350-360 mm blades.

Yes swashplates can be separated although custom halves are not going to be cheap as one off items. But even if you don't need elevator control, you will still need some way of fixing the elevator axis, maybe with a fixed link from the frame to the elevator swash position? So do you really need a custom swashplate? I've only given this about 10 sec thought btw, so could be talking rubbish!

[Waxxingibbous]

Nice stuff here, I'm learning cad so I can design some stuff too.
325 mm blades are nice on an underpowered system
350-360 offer more stability but usually need more power


I'd look at the blade 450x/blade 360cfx/ trio 360cfx/ for basic build design dimensions


I'd look at what the oxy 3 by Lynx addressed in their frames and servo placement

And from there you should see major differences in the frame integrity and rotor head system as well as balance/coG.

The good thing is a lot of these models share the same exact parts almost 60% with subtle changes.

Main focus will be rotor head, main shaft, bearing blocks, tail assembly, and battery type for flight ready weight and center of gravity

[widell]

Originally Posted by peteski

First things first, a 450 size heli actually has blades that are around 325-360 mm long, not 450 mm. So you might want to think about what size heli you actually want to end up with. A pair of 325 mm bladed rotors would actually lend itself quite well as it's a pretty common size, although most newer "450" helis are now on 350-360 mm blades.

That's right! I've read about the size nomenclature, but I forgot about that when asking. I'll have a look at what sizes are available.
I need blades that can be used both clockwise and counterclockwise. I saw 3D blades with "symmetrical airfoil". Are all 3D blades symmetrical? I assume regular blades are not?

Originally Posted by peteski

Yes swashplates can be separated although custom halves are not going to be cheap as one off items. But even if you don't need elevator control, you will still need some way of fixing the elevator axis, maybe with a fixed link from the frame to the elevator swash position? So do you really need a custom swashplate? I've only given this about 10 sec thought btw, so could be talking rubbish!

I'm afraid you might be talking rubbish.
The three links on a regular swashplate do not individually control pitch, elevator or aileron. They need to be used in conjunction. In my case, the third one can not be fixed, because then I would not be able to use collective without inducing elevator on that rotor. You are right that I need some sort of elevator fixing, I have a couple of ideas.

[peteski]

Originally Posted by widell

I'm afraid you might be talking rubbish.
The three links on a regular swashplate do not individually control pitch, elevator or aileron. They need to be used in conjunction. In my case, the third one can not be fixed, because then I would not be able to use collective without inducing elevator on that rotor. You are right that I need some sort of elevator fixing, I have a couple of ideas.

Well I did warn you and yes you are obviously correct! But my point still holds about not necessarily needing a custom swash. Your best bet might be to use a conventional 3 servo setup with a custom mix on the FBL to keep a constant elevator pitch throughout the collective range regardless of Tx elevator commands.

[extrapilot]

There are other options on the swash. One would be to re-drill the linkage points, so that you have 3 at 90deg offset per. Then, use a simple fork to link the opposing 2 as collective. The problem is that you have to resolve mixing the aileron here- since you have to bias the aileron servo offset by the collective offset to not induce aileron with collective.

Because of the rotor dynamics- there is a 90 deg lag between control input and rotor response in cyclic. Normally, this is handled by offsetting the linkages between the head and the swash by 90deg, so that a swash that leans right cause the rotor to lean right. But that is not a requirement- you can rig it how you want- where the swash may pitch forward/aft to induce rotor roll, etc.

There are extremely complex caveats with this stuff, and so you may be better off not swimming upstream on this- and just running a 3-servo swash at each end. The parts are readily available, and you don’t need expensive/massive/power hungry servos for an application like this.

Yea, 3D blades are symmetrical- and you don’t lose a ton of efficiency as a result. The bigger loss is no twist along the blade- but that generally doesn’t exist for retail customers of conventional RC heli blades.

On the rotor size, you have to be a little careful on this idea. Where the rotor overlaps, you have a change in efficiency- generally reduced. And, often there is additional loss due to download- which is just a technical term for the effect the rotor wash has on the fuselage. That is much more a problem with tandems- since there is much more fuselage area in that wash. Normally, you lose about 5% lift from that, and then depending on rotor overlap- maybe no loss, maybe 10%+

Kind regards-

[R_Lefebvre]

Originally Posted by widell

I'm thinking of using something like an Arduino micro computer to handle the custom mixing needed for a tandem. Has anyone here done anything like that before?

I suggest not starting from scratch.

Use Ardupilot. It might become a bit more of an adventure instead of just making something really simple from scratch, but you could learn a lot about programming and UAV's.

I'm a mech eng, and after being involved in the project for 5 years, I can now put C++ and Embedded Systems on my resume.

[stmpngrnd]

Yes Adrupilot is good...however...

...here is another option as well by Skookum http://www.skookumrobotics.com/products/sk-720-black/
setup thread: https://www.helifreak.com/showthread.php?t=662056
This controller is plug and play and has a setup wizard for Tandem Helis. Have flown SK controllers since they first came out at the dawn of FBL. I personally believe they are the smoothest most natural flying FBL controller and I have flown pretty much all of them and owned most.

While it does not have the high ceiling of autonomus/nav features of an Adrupilot or it's open source DIY heart..the unit is rock solid and I would venture to say it would make at least part of your project rather simple and relatively trouble free.

[The_crack_fox]

Have a look at raptor 30/50 swash plates. They have 4 equally placed pivot balls on both the inner and outer ring. Might be usable without modification.

[Waxxingibbous]

Sk720 is a great piece of electronics
Tandem rotor supported very easy setup