How would you design a cyclocopter?

[NickRehm]

This might be a little too dense for you to fully follow, but it's something for you to chew on if you are interested. One of my final dynamics projects was on modeling the dual cyclo. It might help explain the nature of the forces/moment acting on the system. You can see in the early time history data, there are some oscillations: this is from the cross-coupling of roll/pitch/yaw.

 

[NickRehm]

As for your pitch oscillation problem...I'm thinking that you should not be assigning pitch_pid to the rear rotors, just the nose rotor. Reason being is that if you spool the rear motors in response to pitch (lets say it is nose up but should be level, so the rear motors spool up to counter), the torque from the rotor on your config from spooling up will actually create a nose-up moment, where you intended on creating a nose-down moment with the thrust.

Keeping the rear motors fixed in response to pitch, whose torque is inversely related to the desired pitch, will eliminate this weird canceling between rotor torque and rotor thrust when responding to pitch disturbances. This will mean that only the front rotor stabilizes pitch which will couple to roll as it changes its speed. So your roll axis needs to be able to handle the disturbances from the nose rotor as it responds to pitch (you can see what I mean by roll-pitch coupling with this example). Not a problem if you have roll tuned well.

 

[2jujube7]

As for your pitch oscillation problem...I'm thinking that you should not be assigning pitch_pid to the rear rotors, just the nose rotor. Reason being is that if you spool the rear motors in response to pitch (lets say it is nose up but should be level, so the rear motors spool up to counter), the torque from the rotor on your config from spooling up will actually create a nose-up moment, where you intended on creating a nose-down moment with the thrust.

Keeping the rear motors fixed in response to pitch, whose torque is inversely related to the desired pitch, will eliminate this weird canceling between rotor torque and rotor thrust when responding to pitch disturbances. This will mean that only the front rotor stabilizes pitch which will couple to roll as it changes its speed. So your roll axis needs to be able to handle the disturbances from the nose rotor as it responds to pitch (you can see what I mean by roll-pitch coupling with this example). Not a problem if you have roll tuned well.

Alright so I switched the directions of the two rear rotors, which seems to greatly increase pitch stability. To be honest, I designed the cyclocopter to go together like this in order to make the pitching more stable, but it appears that my logic was backwards for some reason and I did the opposite of what I was trying to do.

The thought is that I can then take pitch control out of the nose rotor, which should in turn eliminate the roll/pitch coupling?

Haha I thought that the stabilization/flight controller would come together easy once I got the rotors generating enough thrust.

 

[NickRehm]

The thought is that I can then take pitch control out of the nose rotor, which should in turn eliminate the roll/pitch coupling?

That'll definitely help. You will still run into the issue at higher throttle where the torque from the nose rotor will require one of the rear rotors to spool up much higher than the other side. So might be worth doubling the i_limit on the controller so the roll integral can spool up high enough to compensate.

 

[2jujube7]

Doing better. I'll try increasing the i_limit to see what that'll do.

I feel like it could use a little more fine tuning, but overall it flies significantly better. It's getting good enough for possibly a flight outside where I'll have room to let off the string completely.

 

[NickRehm]

How does it do if you give the tether a little slack so it is effectively hovering on its own?

 

[2jujube7]

I haven't tried it yet. I'm not super confident on the cyclocopter's abilities to not destructively oscillate, so I'm going to do a little more tethered testing. There's not quite enough room for me to completely give it slack with my current setup, but that last video was pretty close to a free flight.

 

[NickRehm]

I'd say go for a little hop in ground effect! Just keep your finger on the disarm switch lol

 

[2jujube7]

I decided that I'm definitely still having problems with oscillations, and although I may be able to fly it, it would be a bit of a stretch to call it a "stable hover". I'm going to restart the tuning and try an actual PID tuning strategy, instead of the "attempt to (somewhat randomly) change values in a way that would make it more stable" that I've been doing.

I'm going to attempt a free flight tomorrow afternoon though no matter where it's at.

 

[2jujube7]

It's been a while but here's a quick summary of whats been going on.

- I was unable to get stable hovering, so i'm going back and rebuilding the cyclocopter to have a larger moment of inertia in order for it to hopefully be more controllable. While doing that, I'm reworking some things on the rotor.
- I'm going to focus on thrust to weight ratio instead of efficiency. Part of this is reducing weight, so I was able to get the rotor from 94g to 70g. This includes a 6s motor that I calculated to be roughly equivalent to the motor that I've been using when running both on 3s, while somehow weighing 1/2 as much? Not sure whats going on there, but I think that the motor I was using was just low quality and had a lot of extra structure on it. Plus I'll be 4s+ capable in case I want to try that later on.
- Re-gearing the motor to increase thrust at the cost of a little efficiency

All of this brings the rotor from a 2.65:1 thrust to weight ratio to an estimated 4:1 (estimated range is from 3.75:1 to 4.32:1) including motor and transmission weights, excluding ESC. I'm finishing on getting the new motor worked into the rotor right now so I'll have the data soon. I'm not sure what the historical values are for a cyclorotor, but I was able to find several, as new as 2017, with a <2.5:1 thrust/weight ratio.

 

[2jujube7]

And... It is official. Last night I reached 348g thrust with a 74g cyclorotor weight. (98 Watts, so 3.56 g/w) This gives me a 4.70:1 T/W ratio, which a) i'm happy about and b) surprised about because my calculations are correct, and even a little bit of an underestimate. The thrust should actually be able to be higher, as my max thrust was limited by the pulley slipping instead of the actual max throttle. Not sure what to do about that as I already have the belt tensioned and wrapped around the smallest pulley properly, so I might just have to add extra teeth? It would mess up the already probably low gearing ratio so I don't really want to do that, but worth a shot I suppose.

 

[NickRehm]

And... It is official. Last night I reached 348g thrust with a 74g cyclorotor weight. (98 Watts, so 3.56 g/w) This gives me a 4.70:1 T/W ratio, which a) i'm happy about and b) surprised about because my calculations are correct, and even a little bit of an underestimate. The thrust should actually be able to be higher, as my max thrust was limited by the pulley slipping instead of the actual max throttle. Not sure what to do about that as I already have the belt tensioned and wrapped around the smallest pulley properly, so I might just have to add extra teeth? It would mess up the already probably low gearing ratio so I don't really want to do that, but worth a shot I suppose.

Awesome!! I think I'm giving up on pulleys because of the slipping... Noise reduction was great, but gears are just so much easier
Flight when?

 

[2jujube7]

Awesome!! I think I'm giving up on pulleys because of the slipping... Noise reduction was great, but gears are just so much easier
Flight when?

Does this mean that you've returned to it?

I've found that ~18-20 teeth is the lowest I can go with my GT2 belt properly tensioned. I dropped down to 16 recently in order to try and fit a higher kv motor, but I'm having trouble so I'm going to need to go back up.

The motor is running a little hot, and I have to go up a belt size in order to gear it down a little more. I'm probably going to be messing around with gearing ratios for a little bit, so the flight won't be super soon. Like I said a couple posts ago, I'm going to be rebuilding the cyclocopter with the rotors spread out a little more in order to hopefully make it more stable. All that said, I'm not sure when I'll be going for another flight. It will happen though.

 

[Ondelori]

Thank you NickRehm for a helpful thread on this forum

 

[2jujube7]

To preface this, I had discovered that the thrust stand that I was using was faulty (2nd time I've had this problem?) I made a direct mount to the scale, so it now reads exactly the thrust that the rotor is producing. Retesting, I got to 307g thrust (so *only* a 4.3ish:1 thrust to weight ratio). But now, I know that the scale is right, because there are no other possible factors influencing it.

Well... On a whim I decided to try a higher voltage on the rotor. I hit 370g thrust with a depleted 5s battery (at 18.8v, so if it was full there would be ~20% more power available). Not a great increase in thrust for nearly double the power consumed, but I'll take it. But this means that the thrust to weight is now measured at 5.3:1, and it is quite possibly even higher if I have a full 5s.

To be honest, I'm extremely surprised that the rotor hasn't flown itself apart at these speeds.

Next steps are that I'm going to play around with some of the pitching (I'm going to try to reduce it by 5 degrees because that might decrease the motor hotness by a good bit while having a small effect on the thrust) and gear ratios to try to find a good mix between motor hotness and thrust (It's rated for 6s but Its getting a little hot so I'm going to have to mess around with it).

 

[Scotto]

To preface this, I had discovered that the thrust stand that I was using was faulty (2nd time I've had this problem?) I made a direct mount to the scale, so it now reads exactly the thrust that the rotor is producing. Retesting, I got to 307g thrust (so *only* a 4.3ish:1 thrust to weight ratio). But now, I know that the scale is right, because there are no other possible factors influencing it.

Well... On a whim I decided to try a higher voltage on the rotor. I hit 370g thrust with a depleted 5s battery (at 18.8v, so if it was full there would be ~20% more power available). Not a great increase in thrust for nearly double the power consumed, but I'll take it. But this means that the thrust to weight is now measured at 5.3:1, and it is quite possibly even higher if I have a full 5s.

To be honest, I'm extremely surprised that the rotor hasn't flown itself apart at these speeds.

Next steps are that I'm going to play around with some of the pitching (I'm going to try to reduce it by 5 degrees because that might decrease the motor hotness by a good bit while having a small effect on the thrust) and gear ratios to try to find a good mix between motor hotness and thrust (It's rated for 6s but Its getting a little hot so I'm going to have to mess around with it).

Wow great job! So what is your drive ratio now? And what motor? Got any new pictures?

 

[2jujube7]

Wow great job! So what is your drive ratio now? And what motor? Got any new pictures?

I'm still figuring the drive ratio out, but I've been going back and forth between 6:1 and 7:1 trying to figure out which has the best balance of thrust and efficiency. I think I'll stick with 6:1 when I'm on 3s. I've been using a racing motor (1700kv) that is actually 6s capable, but I've been using it on 3s and it is working quite well.

Here's a picture. Not a great angle to see the other side, but it shows the new thrust stand pretty well. Again, 70g rotor weight with 307g max thrust at full throttle at 3.91g/W on 3s.

 

[2jujube7]

 

[CampRobber]

Wow I'm super impressed with the performance you've achieved. This needs to fly ASAP!

I haven't worked on mine much since I last posted ... this is what the full vehicle would have looked like, plus the single rotor I actually built and tested. Unfortunately neither the power-to-weight, efficiency, nor reliability were what I wanted before actually trying to fly it.

Theories: (1) flat foamboard airfoil section is bad; (2) too high a "fill factor" i.e. total chord / circumference; (3) too much surface area in general for the 1606 motors; (4) 1606 motors just bad; (5) timing belt efficiency due to badly printed pullies.

I eventually came around to your "3d print everything" plan, but unfortunately, my printer is a potato and was frustrating my development efforts.

 

[2jujube7]

Theories: (1) flat foamboard airfoil section is bad; (2) too high a "fill factor" i.e. total chord / circumference; (3) too much surface area in general for the 1606 motors; (4) 1606 motors just bad; (5) timing belt efficiency due to badly printed pullies.

Looking good! A few thoughts from things I've read or learned:

(1) Not sure what to say. If you wanted to try something thicker like a naca 0020 you probably should, but that adds time and complexity to manufacturing so that might not be the best idea.
(2) Aim for around a 0.66:1 to 0.5:1 chord to radius ratio. Maybe lower because you have 5 blades instead of the conventional 4. If you want to look up Adam Kellen's master thesis it examines solidarity.
(3 and 4) I discovered that racing drone motors work the best. Not sure what you're using, but racing drones tend to have very powerful and weight-optimized motors.
(5) Not much you can do here besides getting your 3d printer settings better. I too was experiencing a lot of trouble with printing, but I was lucky enough to end up with my school's old commercial grade 3D printer. It made a bit of a difference lol.

And then I'd recommend taking out that center piece that connects all the pitching linkages. It's back a few pages, but I tried that a year or so ago and it completely messed up the pitching and reduced thrust.

Looks great!

Wow I'm super impressed with the performance you've achieved. This needs to fly ASAP!

I bought some more electronics and 3 other motors and I'm waiting for them to come in, so I'm back to trying to get it to fly. I've gone from a 2.6:1 to a 4.6:1 thrust to weight ratio since the last cyclocopter, so it'll work a lot better. The copter will pretty much fly on idle.