How would you design a cyclocopter?

2jujube7

Well-known member
I think you could pull off a configuration like in your flight attempt video which works for control of roll/pitch/yaw, but also have the rotors braced on both sides. Kind of having the airframe wrap around the rotors like a cinewhoop.

You'd take a weight hit on the frame structure, but the added stiffness I think would let you push the cyclos to higher RPMs for more thrust with all your planned upgrades
The problem with that design is that the roll axis did not have the RPM induced torque leveling. I was able to get the roll PIDs stable, but there wasn't a whole lot of control bandwidth as there was no impact of the RPMs on roll axis, and the moment for increased thrust impacting the roll axis was pretty small as the rotors were right next to each other. I also did not have the capability to translate left/right while remaining level.

I think this is what I'm going to go with. I have to mess around with the spacing/frame a little bit but the general layout is there. I'll have to build two additional rotors, but the overall design goal that I'm going for is maneuverability and thrust to weight. This should be the best design that would allow peak maneuverability without going up to 8 rotors? I'll also be able to extend the frame out pretty easily to wrap around. Might be good to move the two independent rotors inside the frame? I'd have to put electronics on the outside but that might be the move.
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Cool progress. Its always interesting to see.
I am trying to imagine if it would be possible to build a big single free rotor cyclocopter driven by props like Nick's giant flying ceiling fan. Driving a fuselage and the linkages in pitch instead of yaw. Maybe drive that fuselage framework from both sides and twist the blades of the rotor for roll? Just dreaming...
That would be sweet, but the moments and control coupling would probably be way too high. Might be doable though if a creative guy like Nick would try.
 

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OCPatch

New member
Jude: Thanks for the bearing explanation, and the Texas A&M paper link.

Regarding the yaw control:
You have uncontrolled gyroscopic precession forces pretty much under control with the two pairs of two spools spinning opposite each other. Great idea. Why not use gyroscopic precession forces to your advantage?

In the image of the new design you're contemplating:
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You have four spinning gyroscopes in the form of your cyclorotor spools. Look specifically at the upper left spool and the (so far) unconnected right end of its axle shaft. If you were to use a servo to raise the end of that axle slightly you would generate a counter-clockwise yawing moment due to gyroscopic precession. Lower that end and you get a clockwise yawing moment. And when the craft yaws, moments from the other precessing gyroscopes (other spools) virtually cancel out, provided you match the movement on the opposite end of the axle on the bottom right spool - and you get twice the yawing moment for a given movement of the axle end. If you only control one of a pair of spools then you will get some rolling moment produced. Further, you are actually tilting the net lift vector in the correct direction on each of the opposing spools to give additional yawing moment, though I think the gyroscopic precession moment could be significantly larger than the tilted lift depending on the angular momentum of the spools, the rate of movement, and the amount you actually tilt the spools.

You don't quite get it for free - you would need to add two servos (only one servo if you are REALLY clever with the mechanical linkages), plus linkages if you want to locate servos closer to the center of mass, maybe bellcranks to change direction, and some sort of secure pivot or hinge on the opposite end of each spool axle, but I think it would be a relatively clean method to achieve yaw rotation without hanging some other spinning part off of the vehicle. You would also introduce new forces on the blade pivot arms with this control method, but they would probably be relatively small in comparison to the centripetal force needed to keep the blades from pulling the whole thing apart as they spin.

Note that the yawing moment generated is proportional to the *rate* that you raise the axle end (plus the effect of the tilted lift vector). There will likely need to be some "wash-out" logic where, in order to stop the yawing and return the spool to "flat", the movement will likely need to be fast outward from neutral to get the yawing moment, with a slow return to neutral to be ready for your next yawing maneuver. Note this only works well when you don't need a steady state yaw rate - just quick bursts to change or stabilize heading. I guess if you raised the axle enough (trim out a yaw imbalance?) you would get a small steady-state yawing moment by the opposite tilting of the spool lift vectors.

Basically, the yawing torque generated is the cross product of the angular momentum (moment of inertia * spinning rate of the spool) and the angular rate at which you are forcing it to precess. If anyone is interested, please check me for errors, but this does track with some observations and quick measurements I have made.
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And I call dibs on being made a co-author on your paper if the method works ;-)
 

OCPatch

New member
Oh, Jude, one more gut reaction as I look at your drawing: If the rotation was for the descending blades on the inside, then the 'swirl component' of the outflow of a spool would point outward and downward and the 'lift' vector would point slightly inward. This should act kind of like dihedral on a wing to provide some natural stability.
But I'm sure there are many factors that go into these decisions and I'll bet I don't appreciate all of them.
 

2jujube7

Well-known member
Jude: Thanks for the bearing explanation, and the Texas A&M paper link.

Regarding the yaw control:
You have uncontrolled gyroscopic precession forces pretty much under control with the two pairs of two spools spinning opposite each other. Great idea. Why not use gyroscopic precession forces to your advantage?
Not a bad idea. The difficult part would be maintaining mechanical rigidity with that mechanism. Given that I don't have a center shaft for each rotor in this design, It'll be important to design the frame strong enough so that each side of the rotor has the axis of rotation properly aligned. Adding in another mechanical mechanism like this will introduce a good bit of extra weight, slop, and complexity. It does seem like a pretty decent solution though, ignoring all that. It would work much better on the previous rotor design that has the center shaft.

Oh, Jude, one more gut reaction as I look at your drawing: If the rotation was for the descending blades on the inside, then the 'swirl component' of the outflow of a spool would point outward and downward and the 'lift' vector would point slightly inward. This should act kind of like dihedral on a wing to provide some natural stability.
But I'm sure there are many factors that go into these decisions and I'll bet I don't appreciate all of them.
Yeah, the rotors rotating outwards is pretty important. It allows the counter torque from the RPMs to level the aircraft out. If they spun the other way, the increase in RPMs would lead to that side dipping more.
 

2jujube7

Well-known member
Progress update - got the transmission working flawlessly and the 2 DoF vector system working great. There's surprisingly little resistance with the transmission and barely any slop with the vectoring so I'm very happy with how these came out.

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I tried making blades out of molds using expanding insulation spray foam, but the surface finish was trash. The material should be perfect for these airfoils but my manufacturing process just isn't right. Not sure what was going wrong as I've seen people making full flying wings out of that stuff. If anyone has had success or experience with that then any input or comments would be appreciated. I'm taking on some undergrads for this project so I should just be able to assign a couple to this issue and see if they're able to do anything with it?

Currently waiting on more pink foam to come in so that I can hot wire cut it out using my previous method. I built a CNC foam wire cutter though so it should go smoother than trying to do all of them by hand again.
 

NickRehm

Member
I tried making blades out of molds using expanding insulation spray foam, but the surface finish was trash. The material should be perfect for these airfoils but my manufacturing process just isn't right. Not sure what was going wrong as I've seen people making full flying wings out of that stuff. If anyone has had success or experience with that then any input or comments would be appreciated. I'm taking on some undergrads for this project so I should just be able to assign a couple to this issue and see if they're able to do anything with it?

Maybe lining the insides of the molds with a thin film (packing tape) could help with the surface finish? Not sure how well the spray foam would adhere though

If you've got the manpower and funding, might be worth it to start looking into composite blades;)
 

2jujube7

Well-known member
Maybe lining the insides of the molds with a thin film (packing tape) could help with the surface finish? Not sure how well the spray foam would adhere though

If you've got the manpower and funding, might be worth it to start looking into composite blades;)
Yeah that was the next thing I was going to try. There were also difficulties with getting it to cure in the middle so I'll have to figure that out too.

I'm putting a carbon fiber spar through the foam now and screwing the bearings directly into that. This shouldn't add a whole lot of weight because it allows me to get rid of the plastic plates I was epoxying on the sides of the airfoil to screw bearings onto. I'm pretty sure that using just solid foam is lighter (or at least not significantly heavier) than a complex composite layup. I'm at 1g of blade mass and 1g of plastic/epoxy mass on the ends for the previous 150x50mm blades. I didn't look everywhere but I think its one of the lightest blades out of what I've seen? The big thing though is avoiding extra weight and complexity - the new 3x1.5x200mm carbon fiber spar should be sufficient for strength (I'm going from 150x150mm rotor to 200x200 at the same rpm) so I'm just going to try the single spar and the airfoil out of the same XPS foam.

Have you tried 3d printing whole rotor blades using the vase-mode technique?
Yep, it was the very first thing that I tried. Wayyy too heavy, I think the blades I did were 18g compared to the 1-2g foam ones.
 

Egil

New member
Your progress sounds solid! It’s great to hear that the transmission and vector system are working well. For the blades, lining the molds with packing tape could improve the finish, but it’s worth considering composites if you can swing it. The carbon fiber spar sounds like a smart move for keeping things light while adding strength.
 

2jujube7

Well-known member
Got some videos of it running. I'm currently going through and rebuilding all of the parts in carbon fiber nylon to improve stiffness. I also bought some load cells and I need to set them up with an aluminum extrusion frame to make a test stand to measure thrust. Things are looking great though and I'm excited to get everything put together and do a full throttle run.

For the blades I ended up setting up a CNC foam wire cutter and cutting them out of xps foam like the last one.

 

2jujube7

Well-known member
The video above didn't end up working that well. I believe there is too much slop from the increased dimensions. I'm going back to the smaller scale rotor, but there will be a few changes.

I've been scared to use gears because of noise and the potential to have bad-fitting teeth, but we have some Bambu printers that I printed a planetary gearbox out on and it's working great. Lower weight, more compact, don't have to worry about pulley teeth slipping, and probably higher efficiency.

This one is a 6:1 ratio and it's very smooth, I'll probably up it to a 7:1 or 8:1 for the final design so that I can go up in voltage and stay relatively efficient.

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CampRobber

Active member
I've been scared to use gears because of noise and the potential to have bad-fitting teeth, but we have some Bambu printers that I printed a planetary gearbox out on and it's working great.

Planetary gears are cool, but in this case, if you'd used plain spur gears, you'd be able to put the motor off to the side and run control linkages through a hollow shaft. Though I guess that's only a problem if you were trying to make a single-ended rotor.
 

2jujube7

Well-known member
Planetary gears are cool, but in this case, if you'd used plain spur gears, you'd be able to put the motor off to the side and run control linkages through a hollow shaft. Though I guess that's only a problem if you were trying to make a single-ended rotor.
True. I'm doing a hollow rotor anyway so I figured that running the linkage from the other side wouldn't matter.
 

CampRobber

Active member
The video above didn't end up working that well. I believe there is too much slop from the increased dimensions. I'm going back to the smaller scale rotor, but there will be a few changes.

I've been scared to use gears because of noise and the potential to have bad-fitting teeth, but we have some Bambu printers that I printed a planetary gearbox out on and it's working great. Lower weight, more compact, don't have to worry about pulley teeth slipping, and probably higher efficiency.

This one is a 6:1 ratio and it's very smooth, I'll probably up it to a 7:1 or 8:1 for the final design so that I can go up in voltage and stay relatively efficient.

View attachment 246636 View attachment 246637

And also as a total aside, I'm now really curious whether a print like that could be used to make a geared-prop quadcopter.

Maybe if Nick is feeling angry about vortex drones and wants to prove that disc loading is just disc loading...
 

OCPatch

New member
The design looks pretty efficient material-wise.
I did a three-planet, two-stage planetary gear box for a 1:49 reduction and it eventually worked out pretty well. The first article in PLA was deafeningly loud.
Things I learned along the way (I'll bet you are already there; posting for others):
1) Nylon is a really good material for printing gears; lubricity on the gear faces and shafts is great and noise is manageable (but not with added carbon fiber if you want long wear),
2) Tooth profile can make a difference, but shape precision can be a challenge with the 3D printers - hopefully you find a shape that knocks off the high spots after a little running and then settles on something that has some operational life. Possibly allow for a little extra space between spur tooth top and the engaging ring valley so shed debris does not become a binding problem, and
3) Tolerances can be a bit fiddley for optimal performance. Expect multiple iterations before you settle on something. Determine design, material, print process, and then iterate for performance.

Best of luck on the latest incarnation!
 

2jujube7

Well-known member
Hah, yeah a 1/16" is probably a little small to handle any sort of loading.
That does seem like a good Nick problem to solve.

I considered a two stage, but I figure that the extra weight and bearings needed for the multiple components won't be worth it. If I go up in size any more though, it would be perfect. I'm using nylon and a gear generator that I found, everything surprisingly fit very well the first time. Bambu printers have flow compensation and lidar so I don't have to fiddle a whole lot with horizontal expansion.

What as the 1:49 gearbox for?
 

OCPatch

New member
The 1:49 gearbox is for a fun project: a contra-rotating, large, slow rotor, super lightweight helicopter project. One rotor mounts with the BLDC motor and the other mounts on the output shaft of the reduction drive. It is shelved right now in favor of more pressing things. I do hope to get back to it in 2025.