looking for unwanted wobble

by Pillip Flop | October 8, 2012 | (7) Posted in Just Fun

Recently I watched the flight test video “Laser balancing props” and loved the idea how you could visualize hidden vibrations.

So I copied the idea and built a rig to look for hidden vibrations caused by turbulence on different shaped quad arms

I built a rig out of bits of old greenhouse,( which as it happens make good quad frames) and rubber bands to try and simulate a quad arm as it self corrects.

I then hung it over a table to try and cancel out as much ground affect as possible and used the laser trick at a different angle to try and catch any up and down movement . 

After looking at images of aerodynamic shapes I was surprised to see exactly how much drag and turbulence there is even on round tubing, so I went for the teardrop shaped arm which seemed to be the most practical to copy and compare it to a square shaped arm.( the most streamlined shape is called the symmetrical aerofoil… I think).

I used nylon kite string round a plastic strip held in place with hot glue to get the shape. Then finished it off with ca glue and tape.(by the way that nylon kite string and ca glue combo sets like steel  I`m Sure there`s a chemical reaction going on) .


Anyway I made the square shape out of card held in place with rubber bands. All I had to do was cut away the rubber bands to compare the two. On the first video you can hardly see any difference.

But add the laser and you can clearly see turbulence and bouncing in-between the different rates of throttle. I wonder if this would be useful in FPV getting rid of unwanted wobble.

I can’t help notice, but even in the most expensive multicopter frames there is little or no consideration for aerodynamics directly under the props. The only place where it`s really needed.

I’d love to do a before and after using FPV to see if there is an improvement. That`s a little beyond my reach at moment. But I will get there. Please let me know if I`m way of the mark here or if there`s anything in it.










COMMENTS

colorex on October 8, 2012
You must be a scientist! Good point on the aerodynamic qualities of commercial frames!
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ukarmy04 on October 10, 2012
Nice analysis, Philip. You're right about the drag caused by a circular tube - it can be much higher than you would expect. I'm not sure if you know about the early multirotor prototypes that started off with a downward faced motor. They still generated thrust downwards, but the inflow came from the side with the arm. Just an interesting comparison because the inflow field of the propellor can be distanced from the arm of the multirotor fairly easily enough so that the inflow isn't affected by the arm. Also, another thing that might need to be taken into consideration is the effect of the shape of the arm in terms of it's vibrational properties. I think there was a multirotor user who did an analysis of the difference in resonance between an aluminum square tube and an aluminum U-channel (with the aluminum U-channel being worse for vibration). These properties would also change with material (carbon fiber, glass fiber, plastics, wood, other metals, etc). And one final note...that's an interesting idea you had for controlling the yaw on a tricopter. I'm pretty sure it would work (unless I'm forgetting some basic physics here), but I think the stress on that component might be more than moving the motor itself. Considering you have to move a small "wing" against the airflow and get it to deflect in each direction. I think it would work, but it might end up being less inefficient that the current design and it wouldn't be as responsive.
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Pillip Flop on October 11, 2012
Hi ukarmy04, Thanks for your comments I`ve been looking at some early quadcopter prototypes, I see what you mean about the motors, 2 upside down and 2 the right way up. I noticed they were brushed motors, all going clock wise if you turn them the right way up. It`s hard to believe that was only 6 years ago. If you had a quad and dangled each motor, with the arms curving up to the centre then down toward the c/g sort of spider like that would get round the problem completely wouldn`t it?. Perhaps a bit too much extra frame weight but it would make a wild looking quad. The snag would be all the props being over exposed. I bet there is a way round it though. About the trycopter rudder control idea, Shaun_Mileson commented and said it`s been tried but wasn`t very good. So you might be right there. I`m defiantly going to make a rig to test some streamlining and yaw ideas. If I make arm that`s got aerodynamic qualities and resembles a rudder maybe the load could be split between 3 arms by adding control surfaces to each one for better response. I`m getting some scales next week so I can accurately measure the yaw. Thanks for your comments ukarmy what you say is really helpful.
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Shaun_Mileson on October 10, 2012
It would also be interesting to see how much (if any) power could be saved if the aerodynamics under the props were better, maybe resulting in 30 seconds to a minute extra flying time? I too have pondered this same subject so well done for taking the time to investigate it. I am more than sure it would make a difference in the reduction of "jello" in video.

As for commercial frames I'm sure they don't do it to keep production costs down. Also alignment of each "wing" must be exact otherwise a weird yaw or pitch behavior could arise...
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Pillip Flop on October 10, 2012
Hi Shaun, it`s interesting you mentioned the power thing. There is a bit of extra thrust as it happens, only marginal, but it`s definitely there. If you take in account the extra stability and thrust is times 4 in the case of a quad, there would definitely be extra flight time. I don`t know about a minute but I`d love to see it tested properly to see if there were a difference. The reduction of “jello” and extra battery life would be win win if you fly FPV or any multi.
I think I understand what you meant about “wing” alignment causing weird yaw or pitch behaviour. A wing sort of implies it has aerodynamic responsibilities like a wing generates lift or an elevator changes pitch. In our case the quad arm is just sort of” in the way”. I suppose if the quad arm had any responsibilities they would be. Be the right length, the right weight the right strength, carry 3 wires and separate the down wash as little as possible so any turbulence and negative air pressure is controlled and in one place. Instead of being all over the place, random and causing “jello”. By being teardrop shaped, small or just thin. Any weird yaw behaviour could be controlled or trimmed out …maybe. Or in the case of a trycopter you could even have a small symmetrical aerofoil “wing” with a 9 gram servo controlled rudder instead of a full sized servo mounted motor, saving weight. I don`t know if that`s been done or would even work. Thanks for your comments Shaun what you say is really interesting and has got me thinking. Please understand I am no expert on this subject, but I do find it endlessly fascinating. I t would be nice to give something back. Cheers.

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Shaun_Mileson on October 11, 2012
I think for a person on a quest to make the most efficient multirotor he can this is worth a serious look, for us average joes the accepted norm of the frames is "good enough" I am going to do my own testing and see with my rpm/kv meter as well as my power meter to see how much difference it can make. You're onto a good thing here...As for the "rudder on a tricopter, I have seen it done with limited success on rcgroups, but the guy trying it did not have a shroud around the prop, which I think would make a difference.
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Pillip Flop on October 11, 2012
Hi Shaun, that`s fantastic news an rpm/kv meter, Sounds like a good bit of kit. Better than my rubber bands and bits of green house . I bet you could even work out the amount possible of extra flight time . Even if it were only an extra 20 seconds. That and the extra stability would be good news. Thanks Shaun and good look with that.
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Pillip Flop on October 9, 2012
I don’t know about the scientist bit, I wish! Bits of old green house and rubber bands .A lot of messing around to prove something we already knew. Glad you agree about the commercial frames. Cheers.
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tramsgar on October 15, 2012
Anyone doing investigations according scientific principles is a scientist. There, go update your CV =).
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looking for unwanted wobble