Carbon 60 - a flying Bucky ball.

My first RC heli design.

Last November (2004).
I thought it would be neat to take some aerial photos of a home site I am interested in. Kites, balloons or paying for professional aerial photos were all options but I thought it would be fun to try to do it from a model air-craft of some kind. I had never flown models before and the saga of learning to fly a model is elsewhere on this site along with some videos. I fly full sized gliders and it wasn't too hard to fly fixed wings after the initial prangs. I completely ruled out regular helicopters as being too expensive and too hard to fly. I was impressed with the quad rotor dragonfly but it was too small and the professional version too expensive.

April 2005.
I bought a dragonfly micro heli. It took a month to arrive and had serious drift problems in the gryo so it took quite a while to install “separates”, get them working and then learn to hover.

July 2005.
Bought a Blade runner. This is an amazing kids toy. Co-axial rotors with no cyclic. If floats like a balloon. At 50 grams and limited to totally still air make it is useless for camera work but steered me away from quad rotor towards co-axial.

August 2004,
I started my co-axial project.
I wanted to use off the shelf parts as much as possible. I was fairly familiar with dragonfly parts and they are relatively cheap. Other types are probably better for this but this was an experiment.
Early on I was thinking of using two dragonfly (sans tails) mounted right way up. I was also thinking of modifying the rotors so the shaft extended through the rotor head so it could also be held at the top as well.
One problem with this arrangement is a lot of wiring has to be run between the two helis. I was thinking of controlling both cyclics at that point so there would have been a lot of wire to route and some of them passing high currents.

All the co-axial designs I've seen have both rotors on one shaft, this means major modifications and probably requires a fairly decent workshop plus a lot of hours.

Turning the problem upside down.
By turning the bottom heli over all the interconnection problems go away. The down side - so to speak - is the C.G. is very high. The heli would not be inherently self-righting. I thought (wrongly) that is would be stable though. By stable I mean it stays in the attitude you put it in.

The frame.
The frame is a bucky ball made from 2mm carbon fibre rods - 90 rods 13cm long - 60 vertexes.
The idea is in part to provide a undercarriage but mainly it is to partly protect the rotors from bumps when flying through clutter (trees).
There are a lot of possible shapes that might be better, this was a starting point that I thought would be fun.
The ball is about 650mm across so it will fit through a standard doorway.

The rotors.
Making the reverse pitch blades for the bottom was problematic. You can't buy them as far as I know so I bought symmetrical profile blades (like-90) and tried to modify then to add pitch.
The first set I removed material at the root to add 10 deg pitch but they were destroyed on the first test run due to a stupidity error.
The next set I used PCB material to make wedges. I later damaged one blade in a heat gun accident.
I had serious vibration problems which I blamed on the damage but later found other factors.
The blades were a little long anyway, then I used some "CP twister" blades fitted using 15 deg balsa wedges.
These wedges didn't last long and I replace them with spruce.

The top rotor is powered by a brushless motor - bottom is brushed for now. It remains to be seen if a brushless motor will respond fast enough for yaw control.
With a 3 cell poly battery fitted the top rotor has enough power to lift off without the lower assembly is place.
Ideally the blades should be the same top and bottom.

Yaw control.
Yaw control is through differential speed control of the two rotors. I don't have a co-axial mixer so I've fudged it for the tests by using a heading hold gryo to control the lower rotor speed. This is not ideal but works well enough for basic hover testing. Ultimately I'll probably have to build a custom mixer.

Only the top rotor has cyclic control. Servos are fitted to both rotors but the bottom ones are electrically disconnected.
Initially I had fly bar stabilization on the lower rotor but this doesn't work well, particularly if there is a wind shadow from the innards.

Sept - First test.
First tests were hopeless due to extreme vibration. I expected the frame to vibrate but not this much. The thing was literally shaking apart and I had some rotor strikes – not clear which happened first, the strike or the frame failure. I've only broken one rod so far due to rotor impact all the other failures were glued joins. The first tests were with just the top assembly fitted. I tamed the shake a little by adding a cross brace and balancing the rotor better.
With a three cells lipo fitted the top rotor has enough grunt to lift off (I was holding it to stop it spinning).
With the bottom assembly fitted (and a few mistakes and accidents) I got the gyro to lock properly and it held a (fairly) stable heading.
I have more vibration problems but eventually got it good enough to increase power to what I think was the point of lift off – it just fell over.
No matter what I did with the controls it would always fall over to the left.
After much head scratching I figured the problem was that the lower fly-bar paddles were passing through the wind shadow of the battery at the back. If I understand fly-bar theory correctly this would cause the symptoms I was seeing.

Off with the bottom fly-bar paddles.
I removed the lower paddles and found a way to re-arrange the linkages to lock the pitch neutral. I didn't get it quite correct because the swash plate position is moved in the process. So now it always fell over backwards.
I re-adjusted and still had problems. Again bad vibration which I then but down the a warped swash plate, it is more like I have a bent shaft. I was able to compensate for the wobble but still had it falling over.
I added “training wheels” (as it the top photo) to give me a better chance of keeping it upright and finally managed to briefly lift off.

Lift off and let down.
I could lift off but rather than float like a helium balloon as I'd hoped it flew more like a balloon being deflated..
This was one of the many times I was about to give up and try something else – maybe a blade-runner on steroids.

The ball in flight.

Click the image for a larger version.
Blue is the front, yellow is the “tail” with the antenna dangling from it.

I see a problem.
I have a theory (or two) about why it is so unstable. It goes some thing like this.
The lower rotor is in the wash of the upper one. It is has to work hard to generated lift in the down flow – like trying to swim up a waterfall.
The upper rotor has a similar problem with the air being suck away from under it but I think this is a lesser effect.
If you now bank or pitch (or get hit by a side gust) the lower rotor hits cleaner still air on one side.
Let's say you bank right, the lower rotor goes left relative to the upper one and momentarily hits still air one the left – this would cause it to bank right even harder, in other words we have positive feed back. The wind shadow from the innards is also hitting a different part of the rotor but this would be a stabilizing negative feedback.
This problem is worse with a large rotor separation like I have. One possible improvement might be to add a duct but this would need about half a square meter of material and significantly add to the weight.
My dragonfly has generally been more unstable that I think it should be and I suspect the flybar would work much better on top away from the rotor wake. Other people seem to manage OK but there have been reports similar to mine in some of the forums.

More gyros.
I borrow an extra gryo and added gryo stabilization to the cyclic. Initially it made no difference at all. I think the vibration was just too bad for the gyros to cope. The next day I tamed the vibration and the thing suddenly became flyable.
The wind had briefly died down and I took it out-side. It seem to fly quite well for a while but then the wind returned and I lost control and had it land on the roof of the shed.
One downside of the ball shape is it rolls, it rolled off the roof, fell to earth and I had to re-glue about 10 of the joins.
I've never got it to fly as well since.

Some of the vibration problem seem to be due to damage or a defect in the lower rotor head. It is likely to be a bent shaft but I haven't dug into it yet.

I've had a couple of brief attempts at flying without the top fly-bar (paddles). I noticed a shift in the effects of the controls. I would expect the controls to keep working in their normal sense because of the way I coupled the linkages. I expected a 90 deg phase lag from when the rotor angle of attack was changed to when the rotor tilts but I observed something like a 65 to 70 deg shift instead. I guess the rotors are too light and slow to act like perfect gyroscopes. In any case I couldn't fly it but it will be worth revisiting the idea at some point.
The little co-axials hitting the shops (twister bell-47 or alpine mountain rescue) don't use fly-bars but do have some spinning masses on top.

The bottom line is that even on a good day it probably flies a little worse than my dragonfly.
I had to add lots of angle to the flybar paddles to be able to fly it at all.
Part of the problem is visual, it is easy to loose track of what is front and what is back. Even top and bottom can be mixed up.

I don't know how far I will take this. There are lot of things to do to improve it. Some of the things will be re-usable on other models so the effort isn't totally wasted if I can't sort it out.

The frame will most likely be replaced by something simpler and stiffer. The whole lower rotor assembly could be replaced by a simple hub. Since the lower rotor doesn't seem to be making the thing stable there might be little point in trying the match it to the top rotor – it is possible that a four blade lower rotor might work better.

A mixer needs to be made and there is scope for improved control electronics for the cyclic.

If you read this far you might be interested in seeing a little video clip, it is not particularly stable but you can see the yaw control is working. Also bear in mind this thing has only been flying for a few days so I haven't got the hang of it yet. It does go better out of ground effect as well.

This project is unlikely to be continued because I bought a flying brick.

Cheers Eddie,

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