Forgive my multiple post - I originally posted this in the "quadcopter" forum, and got very few views, no comments. I'm hoping sending to this group might net me a little more feedback :-)
Today I'm beginning the thought process in earnest towards building a 3rd generation frame for my quadcopter, I'd love to know your thoughts on potential ideas, flaws, etc. I'd be happy to try and document the build-up here over the next week or so as I get it together.
~My first iteration was the 3DR quad - enough to get me off the ground & learn basics, but a somewhat fragile frame & expensive to fix (given my early learning curve - smile)
~Second iteration was a homebuilt 3/4" aluminum arm tubing based design with aluminum sheet, plexi, nylon motor mounts and rivets as most fasteners.
Version 2 worked great. I got all sorts of awesome videos out of it - nearly no unwanted vibrations, very robust, simple, super rigid... but a couple weeks ago I had an unfortunate crash where at very low RPM, during fast descent, one of the motors locked up & couldn't overcome the relative wind - the other 3 motors spun up, as the locked one screamed (siezed) resulting in a death spiral to ground (topic for another thread! - My Q: What brushless motors can overcome reverse relative wind from low power?).
...So, I'm ramping up for a version 3 build. Of course it needs to be at once: strong, light, easy to fix, build, and find parts for. I have a lot of bike parts and raw aluminum & other metals.
Enter: The Bike Wheel. I have several floating around the garage/shop. And it seems like a sweet compromise of strength & lightness... with only a few spokes necessary to hold a central electronics/battery hub taught. Here's a mock-up sketch:
Each motor would be mounted with one M4 screw with a locknut through one spoke hole with an "L" bracket of 1.25"x.125" aluminum. I had some angle aluminum laying around so I just trimmed up a piece and it could look like this. I imagine I'll need to simply cut some grooves to accept the bottom of the rim and keep the angle bracket square to the rim (green line):
What do you think? Has anyone tried this? Images/links/videos appreciated!
My version 2 was 24.25" from prop hub to prop hub. This version would be approx 24" (mountain bike wheel diameter is 22.25"D). If I decide to move to a Hexa (6) prop copter I could simply scale it up with a road bike rim (24.5" OD - ~25.75" prop-prop).
Bike wheel rim weights can vary wildly. Mine is pretty heavy (extremely strong/rigid) at 440 grams - but it's free, in the garage. Much lighter ones exist for pennies at the bike recycler in Santa Rosa.
Big question for me is if I put a plate in the middle (see sketch) - will it resist oscillation? Will it have some undesirable harmonics? Maybe even desirable isolation? What might be the best mounting object to use in the middle (mount battery, APM, telemetry, camera gyro, etc)? A plate of aluminum with spoke holes drilled? a tiny plate of steel to tense spokes a lot?
Appreciate any feedback you might have for me - and thanks in advance!
Well if you got most all of the parts I say go for it. Shield your fc well in case of crash. I'm sure the lighter the rim the better. and just think you can run around the outside of the rim with led lights. What a great effect.
You might consider replacing the metal spokes with wood or something that may transmit less vibration. If you have room on the rim a hex would be very kool.
Well good luck. We'll watch for your video's.
why not you can put 6 8 moters i whould bolt x mount to the rim with u bolts
Thanks for your replies, guys. Last attempt to comment got lost by the system, it would seem.
I also had the idea about LEDs around the perimeter... might make seeing directionality from the ground extra easy.
I'm really curious about harmonics of a plate suspended in the middle - how spoke tension, lacing, etc might affect that. Also perhaps isolating the mount plate with neoprene or something of the likes.
Interesting idea, what about lacing the rim using heavy nylon the same stuff as in tennis rackets, laced with some tensioner on one end, maybe even a heavy zip tie. Im no engineer but perhaps it would transmit less vibration and less chance of harmonics - or maybe it flexes too much and drives the IMU crazy? Another possibility might be to mount the spokes to the rim on a rubber gasket, each with slightly different tension - maybe that kills the harmoics (closet MTBiker speaking here)
I've got the frame into almost ready to mount everything mode. 8 tensed spokes make for a pretty darn solid frame. The aluminum plate is pretty solid. Some of you might recognize the clecos I'm using to hold the motor mounts down for predrilling the brackets. Pics: