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  #1  
Old 11-26-2017, 08:04 PM
tridge tridge is offline
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yaw torque issue in large vectored tailsitter

I'm having trouble getting the yaw torque right for a large dual motor vectored tailsitter.
On a smaller vectored tailsitter we're not having any problems. The two motors are set to opposite directions (one clockwise, one counter-clockwise) and it flies really well.
The larger vectored tailsitter behaves quite differently. The way it flies looks like both motors are turning in the same direction, but they are set to opposite directions and I've confirmed the directions by starting the motors in the vehicle editor and seeing which way they spin.
You can see the problem in this video:
https://www.youtube.com/watch?v=HO4XeMSih1A
starting at around 1:50. The model is hovering in no wind, and should have a small total yaw torque, with the motors pointing straight up. Instead the motors are at a large angle in order to hold the yaw.
For comparison, here is our smaller aircraft with the same setup:
https://www.youtube.com/watch?v=W2OIl7i3EaI
that one behaves as expected, with the motors only needing to tilt when you command some yaw rate.
Can anyone suggest why the larger aircraft behaves like this?
All the model files are here:
https://github.com/ArduPilot/SITL_Mo...idge/CUAV_TVBS
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  #2  
Old 11-29-2017, 12:08 PM
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Jeremy Sebens Jeremy Sebens is offline
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I reviewed both the CUAV_TVBS model and the Caipirinha.

I don't see any obvious flaw in the TVBS model from a theoretical standpoint, but...

I wonder if the problem might lie in the use of symmetric components in the TVBS. The Caipirinha uses two separate movable pods as motor mounts, while the TVBS is using a symmetric motor mount component. This should work OK, but it's possible that you've exposed a bug in the tracking of symmetric prop effects.

I'd recommend trying a setup like the Caipirinha, with individual physics for the left and right mounts and their children. If that works, then we know there is an issue with symmetry and V-pitch propellers (at least) and I can file a bug report.
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Old 11-29-2017, 06:22 PM
tridge tridge is offline
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Quote:
Originally Posted by Jeremy Sebens View Post
I reviewed both the CUAV_TVBS model and the Caipirinha.
thanks!
Quote:
I wonder if the problem might lie in the use of symmetric components in the TVBS. The Caipirinha uses two separate movable pods as motor mounts, while the TVBS is using a symmetric motor mount component. This should work OK, but it's possible that you've exposed a bug in the tracking of symmetric prop effects.
I've actually tried it both ways with the same result.
It also doesn't seem to matter what propeller I use, so it doesn't seem related to using the vpitch props.
What I think I need to do is slowly morph the Caipirinha to be the CUAV_TVBS, testing at each stage to see how yaw torque is handled and hopefully find the key difference that way.
Cheers, Tridge
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  #4  
Old 11-29-2017, 06:58 PM
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flexible2g flexible2g is online now
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Just took a look see. Two things to start with.
First-you must have defferant collar schemes on top and bottom, otherwise it is unfly able. I made a solid black plane, a newbie pilot, it lasted only 3 minutes.
Second- even on low rates, it is no fun. I would make high rates, 60% of the present low rate, and make the low rate, 25% of the present low rate. Be sure to 60% expo on high rate, and 40% expo on low rate.
Do these things, and we can take another look.
If you wish to try these things, post it here, as beta.
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  #5  
Old 11-29-2017, 08:52 PM
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legoman legoman is offline
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Quote:
Originally Posted by tridge View Post
I'm having trouble getting the yaw torque right for a large dual motor vectored tailsitter.
On a smaller vectored tailsitter we're not having any problems. The two motors are set to opposite directions (one clockwise, one counter-clockwise) and it flies really well.
The larger vectored tailsitter behaves quite differently. The way it flies looks like both motors are turning in the same direction, but they are set to opposite directions and I've confirmed the directions by starting the motors in the vehicle editor and seeing which way they spin.
You can see the problem in this video:
https://www.youtube.com/watch?v=HO4XeMSih1A
starting at around 1:50. The model is hovering in no wind, and should have a small total yaw torque, with the motors pointing straight up. Instead the motors are at a large angle in order to hold the yaw.
For comparison, here is our smaller aircraft with the same setup:
https://www.youtube.com/watch?v=W2OIl7i3EaI
that one behaves as expected, with the motors only needing to tilt when you command some yaw rate.
Can anyone suggest why the larger aircraft behaves like this?
All the model files are here:
https://github.com/ArduPilot/SITL_Mo...idge/CUAV_TVBS
Check to see if both of your motors are running similar RPM's, pitch, and throttle settings. A High pitch prop running at a low rpm will require different torque than low pitch High rpm. I have a feeling that your auto pilot is lugging one motor with high pitch to balance the thrust of the high rpm low pitch

I am running manual controls with fixed v-pitch props and I can not recreate the problem in your video
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  #6  
Old 11-30-2017, 12:57 AM
tridge tridge is offline
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Paul Riseborough has had a look at this now as well, and discovered that the key factor is the "Prop Wash Factor" of the motors. I had that set to a higher number on the CUAV_TVBS than I had it on the Caipirinha. When I lower it on the CUAV_TVBS the "yaw torque" issue disappears.
Jeremy, can you explain a bit about what the Prop Wash Factor does? The docs in the vehicle physics editor help text don't give any hints as to why this would produce a yawing effect like we observe.
Paul had some other air flow modelling issues he noticed while experimenting which he will post separately.
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  #7  
Old 11-30-2017, 01:01 AM
tridge tridge is offline
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Quote:
Originally Posted by flexible2g View Post
If you wish to try these things, post it here, as beta.
I should explain that this plane isn't intended to be flown manually in RealFlight. It is setup for flying with the new LINK feature, with ArduPilot controlling it.
Dual-rates and expo just break the linearity of the aircraft for the autopilots control.
Regarding the colour scheme, I know it isn't great (especially the mirror-like effect on the tail). I'm a beginner with 3ds max and am still learning how to control things like that.
Cheers, Tridge
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  #8  
Old 11-30-2017, 01:02 AM
tridge tridge is offline
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Quote:
Originally Posted by legoman View Post
Check to see if both of your motors are running similar RPM's, pitch, and throttle settings.
yes, they are all the same on both motors.
See the comment I just posted about the prop wash factor being the key difference.
Cheers, Tridge
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  #9  
Old 11-30-2017, 10:07 AM
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flexible2g flexible2g is online now
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You must have a force to control yaw. Newtons' law.

http://teachertech.rice.edu/Particip...uviere/Newton/
I don't see how you accomplish this on this vehicle.
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Old 11-30-2017, 11:55 AM
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Quote:
Originally Posted by tridge View Post
Paul Riseborough has had a look at this now as well, and discovered that the key factor is the "Prop Wash Factor" of the motors. I had that set to a higher number on the CUAV_TVBS than I had it on the Caipirinha. When I lower it on the CUAV_TVBS the "yaw torque" issue disappears.
Jeremy, can you explain a bit about what the Prop Wash Factor does? The docs in the vehicle physics editor help text don't give any hints as to why this would produce a yawing effect like we observe.
Paul had some other air flow modelling issues he noticed while experimenting which he will post separately.
Quote:
Originally Posted by tridge View Post
yes, they are all the same on both motors.
See the comment I just posted about the prop wash factor being the key difference.
Cheers, Tridge
The prop wash Factor increases or decreases the airflow coming off the propeller. with the airflow blowing up or down on the wing it negated most of the roll athourity of the thrust vectoring. I did notice the roll rate not increaseing with thrust vectoring with the manual controls on both TVBS and the caipirinha. It was my second guess
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  #11  
Old 11-30-2017, 12:59 PM
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Jeremy Sebens Jeremy Sebens is offline
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Quote:
Originally Posted by tridge View Post
I've actually tried it both ways with the same result.
It also doesn't seem to matter what propeller I use, so it doesn't seem related to using the vpitch props.
What I think I need to do is slowly morph the Caipirinha to be the CUAV_TVBS, testing at each stage to see how yaw torque is handled and hopefully find the key difference that way.
Makes sense. I'll be interested to hear what you find out!

<edit> looks like I was slow on the uptake and results were already reported! I'll dig in below.

Last edited by Jeremy Sebens; 11-30-2017 at 08:50 PM.
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  #12  
Old 11-30-2017, 04:44 PM
tridge tridge is offline
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Quote:
Originally Posted by flexible2g View Post
You must have a force to control yaw. Newtons' law.
It does work fine, there are several people flying real vehicles of this type. Roll comes from differential thrust. Pitch comes from pitching the motor tilt. Yaw comes from differential tilt in the motors. Thrust from the motors.
Here is a video of the same type of aircraft (just smaller) in RF8:
https://www.youtube.com/watch?v=W2OIl7i3EaI&t=320s
Here is the same aircraft, but the real aircraft this time:
https://www.youtube.com/watch?v=nIXR...index=26&t=71s
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  #13  
Old 11-30-2017, 04:49 PM
tridge tridge is offline
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Quote:
Originally Posted by legoman View Post
The prop wash Factor increases or decreases the airflow coming off the propeller.
I think it is a physics bug in RealFlight. This type of vehicle does fly very well with a lot of yaw authority in practice.
Here is a nice video showing just how much yaw authority this sort of vehicle has in a real vehicle:
https://www.youtube.com/watch?v=koOqenMpvck&t=84s
We can just set the prop wash factor to zero and we can fly it in RealFlight, but I suspect that is just hiding the issue. Hopefully Jeremy can work out whats going on.
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  #14  
Old 11-30-2017, 05:10 PM
priseborough priseborough is offline
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I am using a variant of the Caipirinha model the battery moved back c.g. and motor pivots moved back closer to the wing. It flies OK in hover with Prop Wash Factor set to 50% with no noticeable roll trim required, but if the factor is set to 75% there is a large moment about the X body axis that cannot be controlled.

https://drive.google.com/open?id=13x...8sKANIjgZVzQgG
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  #15  
Old 11-30-2017, 05:14 PM
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flexible2g flexible2g is online now
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yaw

This screen shot shows the problem. The motors are not set up for movement in RF.
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