Replies to This Discussion

Permalink Reply by Luke Olson on October 15, 2012 at 12:59pm

I think that will make a little difference but a much bigger difference would be to mount the battery (usually the heaviest component) on the top of everything instead of underneath everything.

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Permalink Reply by Will Snodgrass on October 15, 2012 at 5:49pm

I was thinking that too, but there is a half sphere plastic dome that covers everything on top.

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Permalink Reply by John Githens on October 29, 2012 at 6:34am

Well, it looks like you would be shifting the plane of the props by as much as 2 inches. I would guess that would make a noticeable difference, but I don't have any test data to back that up. Also, you would be lowering the CoG by shifting the weight of the motors and props. At least it appears that you can flip the rotors without too much trouble (on my frame, the props would hit the bottom if I did the same change.). If you continue, it would be great to hear if the difference was significant. Not sure how to approach measuring "significant" in this context.

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DeveloperPermalink Reply by R_Lefebvre on October 29, 2012 at 6:55am

I think it's an interesting idea to try as well.  I'll keep tabs.  I wanted to build my Octo this way, but as it's ultimately designed to be a camera ship, I just couldn't make it work out practically.

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Permalink Reply by Will Snodgrass on October 29, 2012 at 5:57pm

Monroe is the one that sparked this idea...(oh and there is EXACTLY 4 1/2" of difference) I cant put the battery on top because this quad has plastic dome that negates that possibility...... Although only 1/4" of the battery would be below the props.

I should be able to just flip all of the motors and make no other physical changes right? Would it Require any APM mods though?

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Permalink Reply by Will Snodgrass on October 29, 2012 at 5:59pm

Monroe, I sent you a friend request. I wanted to run something by you in private....

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Permalink Reply by Tim Wilkin on November 19, 2012 at 6:14pm

This thread is a few weeks old, so my comments might now be unnecessary now...

Moving the CoG above or below the thrust plane will diminish maximum torque about the centre of mass and thus reduce the dynamic response to attitude perturbations. It won't actually make the quad more stable with regards to exogenous forces (such as wind gusts)... just less sensitive to small variations in thrust from the motors (so perhaps easier to control in manual flight). A good transmitter with an exponential rate on the stick would be a better alternative.

If you want to enhance stability in hover, tilt the motors toward the central vertical axis of the quad. You'll suffer a slight decrease in maximum thrust, but introduce a natural balancing moment whenever the attitude is perturbed away from level. You'll obviously need to re-tune your control loops to accommodate the changes to linear and angular accelerations at various throttle settings.

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