Alternating lift on rotor blades?

[NightmanX]

Hi folks,

I'm new to the forum but playing for a while now but now I ran into something very weird. After I got Breaking Ground I started a new game and at some point I decided to make a helicopter/drone using the new motors

I watched some tutorials and then tried to do my own but I ran into something I find very weird. As I increased the torque the propellers did spin faster but no lift, only a lot of vibration. I then opened the debug tools and enabled display of aero forces. The weird thing I noticed was that as the propellers were spinning the lifting force kept alternating up and down instead of only being up. At some point I got one that worked ok but no clue how to do it again or what's wrong. I tried with Elevon 5, FAT-455 Aeroplane Control Surface and even a few wings with the same thing happening

Also how can I add screenshots on this forum? I only found a button to Insert image from URL

 

Any help is greatly appreciated!

 

[pincushionman]

5 hours ago, NightmanX said:

…Also how can I add screenshots on this forum? I only found a button to Insert image from URL…

Can’t help with robotics stuff (didn’t buy it yet), but I can help with the pictures…kinda. You’re right, you only have an “insert image from URL.” The forum doesn’t host anything other than the BBcode of the messages themselves; all other files you share must be linked from elsewhere. Imgur works pretty well and is free; try posting your images there and use the linking tools.

[NightmanX]

Thanks, I uploaded to Imgur but can't use the link...Insert Image from URL just turns red when I paste the link (only the edit box turns red not the whole dialog window)

[James Kerman]

Welcome to the forum @NightmanX.

I just copy and paste the "direct" link from imgur directly into your post. It should auto embed.

[NightmanX]

Finally! 

[bewing]

Deploy direction and deflection is based on the offset from the CoM of the craft to your control surface -- relative to the current orientation of the control surface. As your rotors spin, they are moving closer to, and then further away from the CoM. This causes their deflection angle to change (and perhaps even reverse) which is causing your lift vectors to change.

What you want is to preset your control surfaces to the proper deflection with the rotate gizmo in the editor, and then reduce your authority limiter down to almost nothing. This will give you lift vectors that are always proper, plus allow you a small amount of additional control.

 

Edited June 18, 2019 by bewing

[NightmanX]

19 minutes ago, bewing said:

Deploy direction and deflection is based on the offset from the CoM of the craft to your control surface. As your rotors spin, they are moving closer to, and then further away from the CoM. This causes their deflection angle to change (and perhaps even reverse) which is causing your lift vectors to change.

 

I put the motor right on top of CoM and still the same thing...there must be something else going on, how do other people make functional helicopters?

[OHara]

You might be seeing a problem that frustrated us in making variable-pitch props before robotics.  I suspect that if you rotate the prop very slowly, with blades 'deployed' you will see them snap quickly to the opposite direction and then correct their deployment direction by moving at their usual response-speed.

Somehow,  after rebuilding stock props many times, the problem would disappear for me.  I have not made the problem disappear in Breaking Ground robotics helicopters, except by centering the prop on the CoM (which worked for me, but I see you are building a quad-copter so that is not a solution for you).  I do not remember seeing a solution in the stock props thread (link) but it is a long thread with clever people.

The other option we have now is to put a small servo at the root of each blade, and use that for the pitch.

[Guest]

6 hours ago, OHara said:

The other option we have now is to put a small servo at the root of each blade, and use that for the pitch.

This has the additional immense benefit of letting you control collective.

[OHara]

The OP was using the new-with-1.7.2 ability to adjust the deployed extent of control surfaces with an axis group, to control collective.

That used to work with pre-robotics stock props (with a right-click menu open to adjust the pitch) with only a  little frustration from the momentary flipping of deployed direction.  But there, the prop was a separated craft, so the blades tended to be perfectly symmetric about that crafts CoM.

With the new rotors, the blades are part of the whole craft, and it is difficult to arrange them symmetrically enough about the whole-craft CoM to avoid the glitches.   Deployed control surfaces are therefore not very useful on rotating parts.

Edited June 18, 2019 by OHara
the term is now 'pre-robotics' stock props

[NightmanX]

Thank you for all your insights!

I am a little confused as to why do these work. Shouldn't they have the same issues I am having?  (I basically copied the design and made 4 instead of 2, I also tried with 2 as in the first video and it still didn't work the same way) As I understand based on your answers the behavior I'm seeing is as expected

[OHara]

13 hours ago, NightmanX said:

the behavior I'm seeing is as expected

Well, expected for those of us who have seen the glitch before, but not wanted.   
It did not cause much problem before Breaking Ground, but will cause frustration now.  I entered a bug report (https://bugs.kerbalspaceprogram.com/issues/22932)

You make a good point about others' successful helicopters.  If their KSP behaves the same as ours, which has almost always been a good assumption in the past, then
1) James Baxter did not 'Deploy' the control surfaces, but set their pitch with the rotation tool, and
2) Kergarin's very symmetric craft has the CoM always on the same side of the actuation axis of each control surface (from the point of view of that control surface).