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How would you design a cyclocopter?

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.
 

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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

Well-known member
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.
 
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

Well-known member

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.
 

2jujube7

Well-known member
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.
 

2jujube7

Well-known member
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

Well-known member
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.
 
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2jujube7

Well-known member
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. :D 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.
 
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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. :D 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

Well-known member
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.
 
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).
 
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?