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Old Sep 16, 2014, 10:13 PM
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Originally Posted by tom stanton View Post
The next thing I noticed is that the gains seem quite high when hovering. In the video you can see the servo shaking like crazy when in a hover. I have tried to reduce the gains, but it seems to only reduce the stability (though it was quite windy when testing). The last thing I noticed was that the motors are angled slightly downward in FF, this especially doesn't help with this aircraft as the CG is so low, it naturally wants to push the nose down anyway. So angling the motors up slightly should help with FF

https://www.youtube.com/watch?v=E95rNzCjLow
The shaking might be vibration related. I suggest going to GWS 3 blade props as 2 blade props have a dramatic vibration mode when connected to a motor on a sloppy pivot. Always balance your props carefully of course, and even balance the motor bell housings using the cable tie method. Most importantly, set the MPU6060 LPF: to 21 Hz, or even 10 Hz.

I doubt the nose down angle of the motors in FFF has much effect. The angles of the various aero surfaces will dominate at that point. It's easy to fix or experiment with though.
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Old Sep 17, 2014, 02:37 AM
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Quote:
Originally Posted by Ran D. St. Clair View Post
The shaking might be vibration related. I suggest going to GWS 3 blade props as 2 blade props have a dramatic vibration mode when connected to a motor on a sloppy pivot. Always balance your props carefully of course, and even balance the motor bell housings using the cable tie method. Most importantly, set the MPU6060 LPF: to 21 Hz, or even 10 Hz.

I doubt the nose down angle of the motors in FFF has much effect. The angles of the various aero surfaces will dominate at that point. It's easy to fix or experiment with though.
I was thinking about ordering 3 bladed props as I think you may have mentioned it somewhere before, but I just don't know what size would match with a 2 bladed prop. The only size I can get locally to me is 9x4.5 3 bladed (currently using 10x4.5 2 bladed), would this be worth a try?

The reason I think the motors being angled down has an effect is because it flys quite a bit better when the motors are off, I'll test it and see.

Thanks again!
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Old Sep 17, 2014, 09:27 AM
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Originally Posted by tom stanton View Post
The only size I can get locally to me is 9x4.5 3 bladed (currently using 10x4.5 2 bladed), would this be worth a try?
The 9x4.5 3 bladed props should be worth a try. They are not expensive or anything. You will lose some raw thrust but you may have enough thrust margin to make up for it. I think you said you were planning to upgrade the motors, at which point the GWS 10x6 might work well. Normally we want lower pitch for something that hovers, but VTOL aircraft are always a compromise between hover and forward flight and the 6" pitch isn't as high as it sounds since it will turn at a lower RPM.

I can't fully explain the physics of it, but a 2 blade prop has a "whirl instability" which is a well known phenominon going back to the early 1900s. On my VTOL-Trainer (Bixler) 2 blade props are unusable but 3 blade props run smoth. This is true even if the two blade props are carefully balanced. it has to do with an interaction between gyroscopic precession and the slop in the tilt mechanism. The motors start banging back and forth within the slop range making a horrible vibration that would quickly destroy the servo and possibly the motor mounts. It kicks off at a specific RPM which in my case happens to be at about hover power. Your issue may or may not be related, but it is easy enough to try some 3 blade props and see if it helps.
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Old Sep 17, 2014, 11:23 AM
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I think it's a matter of mass distribution and rotational inertia. As you tilt the prop perpendicularly to its rotation axis the varying inertia will be converted in vibration.
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Old Sep 17, 2014, 05:42 PM
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Hopefully the 3 blade props will help with the wobble as I think that may be the only problem left to fix

Here is a video of some aerobatics and stall testing:
RC VTOL V2 Aerobatics + Stall Testing (6 min 43 sec)


Please excuse my Dad's poor camera skills haha. It might be hard to see from the video, but the aircraft stalls perfectly. It just drops the nose and the wings stay level. This is great for transitions from FF to hover, I can just glide it in and increase the power as the plane is about to stall without having to worry about tip stalls.

I have now painted it grey to look more scale!



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Old Sep 17, 2014, 06:21 PM
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Very nice. And it handles a bit of wind as well....
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Old Sep 20, 2014, 12:12 PM
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3 Blade prop tests

Unfortunately I managed to break one of the props before getting any videos of it, so I'll have to explain how it went. I now have the new 1250kv motors, so I tested them with 2 bladed props first to see how they performed. The 2 bladed props are 10x4.5 and the 3 bladed props are 9x4.5

First thing I noticed about the 3 bladed props is that it hovered at a lower throttle. But it seemed to be less stable in a hover, especially on the roll axis. The only reason I can think of why this is happening is because the heavier props aren't decelerating/accelerating quick enough for roll control. Even with gyro gain tuning, it still wasn't as good as the 2 bladed props. Also pitch stability was slightly worse, not by much, but noticeable. I'm guessing this was because the props are heavier, so they produce higher a gyroscopic effect, which doesn't help with this style of bicopter setup.

One huge problem I discovered with the 3 bladed props, which eventually caused it to crash, was that they suddenly lost a lot of thrust when the aircraft had no airspeed. This was a huge problem when transitioning from FF to a hover as it would almost tip stall because one prop had "cleaner" air than the other. Not sure if this is a know problem with multirotors, but it happened a few times. I'm guessing this is down to the blades being closer together, so as soon as it needs to produce static thrust, there is a lack of "clean" air. So when moving forward, air is forced through the prop, so each blade doesn't lose so much thrust when following the next blade. But in a hover, the air from the leading blade has a larger affect on the trailing blade. Because of this, the 3 bladed props were a lot better in FF, almost too good. It would cruise at about 30% throttle and pulled some speed at full power!

Ultimately, 3 blades would be better if I could find large diameter and less pitch props because it would probably reduce the wobble in the servo play. But all the props I can find are "?"x4.5. So for now, I'll stick to the 2 bladed props as they seem to work pretty well and I have loads of spares!

Luckily the crash happened at a low speed and about 10ft high. It landed in a thorn bush, so just a few dents in the wing and some thorns in the nose. But apart from that, its fine
Hopefully this long explanation made some sense... please feel free to suggest any reasons other that what I've said as to what is causing the problem with the "tip stalling" effect on each motor.

Also, I've now draw on a scale looking window. It's starting to look like a V22 now
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Old Yesterday, 03:48 AM
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Hi Tom
Very cool project!
I'm thinking of building one following your build.
I'm sure a lot would be very interested in your KK2VTOL settings for the project. It would make a very nice starting point. Could you please tell us aboud your settings used.
Thx
Roger
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Old Yesterday, 05:29 AM
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Originally Posted by xband View Post
Hi Tom
Very cool project!
I'm thinking of building one following your build.
I'm sure a lot would be very interested in your KK2VTOL settings for the project. It would make a very nice starting point. Could you please tell us aboud your settings used.
Thx
Roger
Hi Roger,

Thanks for the interest!

I get a lot of messages similar to yours, asking for KK setup details. I will be creating a specifications video in the next few days (should be uploaded before next weekend).

VTOL aircraft are one of the hardest aircraft to get flying, so if there was no challenge in building them, everyone would have one and this project wouldn't seem as interesting! Which is why I'd recommend you do it yourself as you'll learn far more and it will be far more satisfying! Also have a good read of the manual to get familiar with the OpenAeroVTOL firmware so that you can tune/change settings to match your aircraft.

This isn't just directed at you, I've basically sent a similar reply to everyone that has messaged me. This aircraft isn't like a quadcopter where it should fly straight away as long as motors/props are on the right way. There are so many different variables that you'll need to tune yourself. I had to go through quite a bit of trial and error to get it to work, but isn't that expected when looking at how long it took to get the real V22 Osprey working?

Hope this didn't come across wrong. In the upcoming video, I'll be explaining the basic volume settings I'm using on my aircraft and which axis gyros I'm using for hover/FF. I'll probably put together a spreadsheet with all the settings in the KK board, but I won't have time to explain everything!

Thanks
Tom
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Old Today, 07:20 AM
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Hi Tom

Thanks for the quick reply.

As my version of the plane most likely won't be an exact copy of your one the settings might be different. I'm just looking for a starting point and I think your next video will do very nicely in providing that. I'm looking forward to it and I'm sure it will clarify lots of my questions. Otherwise I'll ask again ;-)

\Roger
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