stunthanger.com
Electric Stunt => Gettin all AMP'ed up! => Topic started by: Rick_Huff on July 27, 2013, 07:44:18 AM
-
I'm flying an RD1 electric with an Eflite 15 motor. Its front mounted, as you can see in the picture. The mount is made from stainless steel (the original was aluminum, but I felt I needed more strength). The problem I had was with vibration. I think I hit a resonant frequency with the rpm I wanted to run (10,000-10,500) and the nose shook violently in that range. I decided to add a rear bearing to it, which you can see in the 2nd photo. Its a real simple addition, just a piece of basswood with a 5mm bearing pressed fit into it. I had to move the shaft back 1/8" to engage the bearing, but it works perfectly. No noticeable vibrations at any rpm. I would be a lot happier with a longer motor shaft, so I could get full engagement into the bearing. I was wondering if a shaft from an Eflite 25 (which is about 1/4" longer) could be used in an Eflght 15?
Thanks,
Rick
-
The stainless motor mount is 0.43" thick. I may have been lucky with the alignment, but it seems dead on. I checked on the battery usage before and after the bearing installation, and there's no change. (Well, compared with the usage before the crash that necessitated re-building the nose). Your point is well taken though. As long as some part of the shaft stays in the bearing, it doesn't need to be completely engaged, and it may help if there is any misalignment
Thanks,
Rick
-
If you are getting a vibration , there is most likely something wrong. either your shaft is bent from the crash, or you have a bad or damaged bearing. You should not need a third bearing to stop vibration. Also your prop or spinner may be out of balance or alignment. Read this http://www.apcprop.com/v/html/tech_support_notice.html#holealign and this http://www.apcprop.com/v/html/balancing.html both are very important. As far as using a longer shaft I don't see why you cant as long as the diameter is the same.. I've even made my own shafts fro 5mm music wire. Most people feel that the extra bearing may not be needed. Most people that I know that are using this motor are not using a support.
-
Rick,
I like the mount design however being only 0.043" thick could be part of the problem. Early on we tried using thin firewalls to do the front mount and found that the motor would move and needed the rear bearing to stabilize it. Now I use 3/16" mounted with plywood doubles an small screws through the ply into the sides plus angled gussets. With this mount I have not needed the rear bearing.
It could be you hit a harmonic point at the 10K rpm point, have you tried to take it to 11K as a test? If the rear bearing solved the problem it should be OK. Do check the balance of the prop and the alignment of the prop hole to make sure it is centered and tight on the shaft.
Best, DennisT
-
William,
Thanks for your thoughts. I believe I balanced the prop correctly, but I did try another prop with the same results. I removed the spinner and tried it with just the prop: same results. The crash was pretty violent. The lines caught on the grass in takeoff, it came at me, then went straight up, then straight down into the ground. The spinner, motor and 1/3 of the battery were buried. I was totally surprised that the shaft appeared straight. I suppose it doesn't have to be off much to cause vibrations at 10,000 rpm. I'll order another shaft and try it.
Thanks,
Rick
-
If that manufacturer also makes another motor in the same series but is longer, you can get the replacement shaft for the longer motor and it will have the extra length you are looking for. Just check the specs for both motors to make sure the shafts are same diameter, but they typically do in the same series.
-
Rick,
Here is my solution to a rear bearing. This was done back in 2009 and still going strong. If you have vibration like you describe, then look elsewhere.
-
Hello Rick,
Nice solution to the problem. Let us know if that shaft from the 25 fits.They are cheap enough, I'd think.
The flexibility of the mount coupled with gryroscopic precession will, indeed, cause all sorts of vibrations.
Its also stresses the prop. Isn't amazing how a bearing in a block of basswood calms things down?
regards,
Dean P.
-
Dean,
I received the E-Flight 25 shaft and installed it in the E-Flight 15. It fit perfectly and the shaft extends 0.200" behind the motor, which is perfect engagement into the rear bearing. I'm pretty sure the motor mount was not stiff enough, but the rear bearing takes care of the problem. Thanks to all for your help.
Rick
-
For what it may be worth, I had the bell on one of my motors deform (fell to the concrete floor from its hook on the wall). The motor was rear mounted and hit (plastic spinner first). The next time I flew it, I had a nasty vibration that was transmitted down the lines. Turns out the shaft was not bent, but the aluminum bell (end where the shaft exits) was (very slightly). I chased that vibration for a couple weeks before checking the bell itself for runout. I managed to ease it back to true by rigging a runout tell tale on the bench and gently nudging the case until it was reasonably true again. The vibration went away. Put a feeler gauge in the gap between the bell and the fixed end of the motor to check it. The gap should be the same as your turn the bell through a full rotation. It doesn't take much.
Bob