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Old Oct 14, 2013, 04:49 PM
ustary is offline
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Help!
Yet another spherical vtol(singlecopter)

Hi, i recently built my first rc flying machine (a quad), and after that was a success, i decided to venture into more dangerous territory... So im building a singlecopter with a spherical shape (much like the japanese one). I have read all the other posts out there about other endeavours of this kind, but im thinking of doing something a litle more simple, with only 4 servos, controlling 4 control surfaces at the bottom. This should be like any other singlecopter 1M4S, exept for the outer shape.

My setup is 1 turnigy d2830, 1 SF 0947 prop (this combo gives me 730g thrust according to ecalc), 1 5A UBEC, 1 30 amp afro esc(simonk flashed), 1 hobbyking 6 ch rx, 4 turnigy 9g servos, 1 1200mah 3s1p lipo (20C rating), and finally one HK multiwii 328p flight controller. I built a fame out of foamboard, got the servos directly linked to the control surfaces(i believe it will be more responsive, and have less slack this way) and have an all up weight of 380g- This means thrust isnt an issue (thrust test showed the same results, it has enough power).

I set everything up, i programmed the board, and tried it with the fabric PID settings. I have still only flown it attached to a string on top, so it is hanging, and i test its stability when throtle is almost enough to lift. The thing is, when it is almost flying, it starts going in circles wider and wider, this looks like a typical case of PID settings too high(mainly the P one). So, i tried setting them lower, but even when the setting are very very low, this is still happening...

Anyone can give me some advice on something they think can help me? If someone who has built a singlerotor successfully (or not) can share a piece of advice on these machines, i would be very grateful.

PS: the CG is just a litle bit on the TOP of the meridian(1-2 cm), so the control surfaces can produce enough torque to control it.

Thank you.
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Last edited by ustary; Oct 14, 2013 at 05:16 PM.
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Old Oct 16, 2013, 05:30 AM
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After a bit of PID tuning i can now hold it in the air. There is still much to be done, i still feel too much control coupling (when i try to give it pitch motion, there is an unwanted roll and yaw motion), but i belive further tuning may fix it. More importantly: if i give almost enough throtle to levitate, and i drop it from height with accelerator mode, the aircraft will slowly descend to the floor in a head position, that is, it will not rotate around any axis, and will controlabily "land".
One of the most impoirtant things now will be to strenghen the structure, as i believe the foamboard isnt holding up quite as well as it should. In unexpected encounters with objects the foamboard tends to crack, and the aircraft is almost more tape right now than foam board. Im thinking of maybe getting some carbon fiber strips, or maybe more inexpensive aluminium, and gluing these around the edges of the sphere, so these make up the first contact surface.
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Old Oct 16, 2013, 09:34 AM
Ran D. St. Clair is offline
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ustary,

We are not trying to be rude by ignoring you. There just isn't much to say. What you have is a flying wing that happens to be round. the laws of physics make such a design difficult. For aerodynamic stability in a hover you want the center of lateral resistance ahead (above) of the CG. For forward flight, it needs to be the other way around. A single propeller has a lot of gyroscopic precession, so pitch becomes yaw, and yaw becomes pitch, and roll tends to be all over the place depending on airspeed. The flight controller can compensate for a lot of this, but it's still fighting the basic design. Control authority varies dramatically depending on throttle, so theoretically you need different PID settings for climbing, hovering, and backing down. All of this can be overcome with careful design, tuning, and more control surfaces, but it's fundamentally a difficult situation.

The one I built was nothing but carbon fiber hoops, so it even bounced to a certain degree, but it was never that much fun to fly. It had a ring wing, so wasn't quite the same thing, more of an airplane than a helicopter, and it had no flight controller. Lots of pilot skill required to hover...

You are doing incredibly well for a 2nd RC aircraft. If you are having fun, that is all that matters.
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Old Jul 16, 2015, 06:44 AM
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Hi
I love single copters and I built one of them but it doesn't work good.in fact it hovers for a few seconds after that loose the balancing and it can't balance and level itself.the flight control is multiwii 2.3.I attached an image of it.please see it and give me your suggestion.
Do you think multiwii firmware is not enough and bad or the aerodynamic and the place of motor and servos are not good??
Thank you
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Old Jul 16, 2015, 10:15 AM
Ran D. St. Clair is offline
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Quote:
Originally Posted by rclove97 View Post
Hi
I love single copters and I built one of them but it doesn't work good.in fact it hovers for a few seconds after that loose the balancing and it can't balance and level itself.the flight control is multiwii 2.3.I attached an image of it.please see it and give me your suggestion.
Do you think multiwii firmware is not enough and bad or the aerodynamic and the place of motor and servos are not good??
Thank you
I don't see any aerodynamic reason why it should not fly. I assume there must be a problem with the setup of the flight controller. I am not saying anything bad about the Multiwii, but I use the KK2 board with OpenAero-VTOL firmware. You probably don't need VTOL firmware though as I doubt you will ever go into forward flight. At most it will hover and translate sideways fairly slowly. Regardless, we can't see your control setup.
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Old Jul 16, 2015, 10:03 PM
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We built this singlecopter a few days ago as an exercise for my students and got it to work fine. Like RD says above, there was significant coupling between pitch and roll and it would precess like a top at first but we figured out how to set the volumes on the pitch and roll servos to counteract the precession. At first it would also move diagonally when giving a pure pitch stick input, but the volume mixing fixed that too.

We have not had a chance to transition it yet, but I see no reason why it shouldn't work as the CG is at ~1/4 chord.

I will post the OAVtol Excel file with settings tomorrow when I can get to my computer.
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