Richard Harris Posted October 30, 2011 Share Posted October 30, 2011 Hi Chaps, I have a bit of a problem with a new model which was test flown today. The model in question is a Multiplex Gemini that I bought new/second hand. The motor is a brand new Turnigy aerodrive jobbie that was already fitted. When I get close to full power I am getting a really bad vibration/noise, almost like the fuselage is resonating. I have changed the prop and prop driver off another model which is perfectly fine yet I am still getting the vibration. Checked the motor and its mount and all seems good bar from the motor was only mounted using two M3 bolts, i have now added the rest and its solid. Anyone have any ideas? vibration and a plastic mount is obviously a worry for me! I made a video of its second flight, you can hear the noise in some instances as the throttle is opened. Rich Quote Link to comment Share on other sites More sharing options...
Vecchio Austriaco Posted October 30, 2011 Share Posted October 30, 2011 Saw a Gemini today at our field - and I have to say it also had vibration problems. May be not as loud as yours - but too much for my taste. What kind of prop are you using? Did you try to turn it relative to the engine in small steps? sometimes this can compensate the prop / engine excentricities Cheers VA Quote Link to comment Share on other sites More sharing options...
Ben B Posted October 30, 2011 Share Posted October 30, 2011 One of my planes does that when the grub screw holding the front of the outrunner onto the rear mounting plate comes a little lose. But fundamentally what you're hearing is something flexible (probably a control surface) being hit with a resonant frequency from a source of vibration- it could just be a slightly out of balance prop. But I'd check the motor and motor mount well first of all, especially because the last time this happened on my plane I ignored it and the front of the outrunner flew off and ripped all the wiring out of the motor. Only a cheapy £10 motor but still annoying.... Quote Link to comment Share on other sites More sharing options...
Richard Harris Posted October 30, 2011 Author Share Posted October 30, 2011 VA, I am using an APC copy, but did try other props and drivers. The bigger the prop the worse it gets. I have not tried turning the prop but will do this so thanks on that. Just seems really odd, could take the motor out and try and clock up the shaft but that is clutching at straws. Rich Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted October 30, 2011 Share Posted October 30, 2011 Could be the prop is exceeding its limits and fluttering - I've had similar noises from GWS slowfly type props on relatively powerful motors - and heard of them failing if persisted with! Does it happen more when going to full revs from fairly slow flight? The prop may be stalled in this condition and not when it's flying faster - I think I heard some evidence of this on the video and this can dramatically increase noise and vibration. Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 30, 2011 Share Posted October 30, 2011 How is it with the prop OFF? Maybe, just maybe.... you have a loose / missing magnet. Or, Is the shaft perhaps slightly bent?? Quote Link to comment Share on other sites More sharing options...
Chippy Posted October 30, 2011 Share Posted October 30, 2011 Rich, Are all the side thrust setting screws making contact with the side sections of the motor mount? I think the mount is the same as the acromaster, mine suffered from bad vibration but turned out to be the prop nut I also swapped the APC prop for a xoar wooden one There was an extra safety sheet with my acromaster stating that vibration could be caused by the side parts of the mount not being glued properly to the fuselage sides and that the cross brace at the back of the mount HAD to be fitted securely to reduce vibration problems. Quote Link to comment Share on other sites More sharing options...
Vecchio Austriaco Posted October 30, 2011 Share Posted October 30, 2011 Chippy, As you mention the Acromaster - my son had one some time ago - and it was terrible about vibrations. This was the plane where I started to move the prop relative to the engine to find the spot where the thing makes less noise.... You can see the guy doesn't look too happy Back to Rich - may be its also a kind of Multiplex design problem Cheers VA Quote Link to comment Share on other sites More sharing options...
Chippy Posted October 30, 2011 Share Posted October 30, 2011 Posted by Vecchio Austriaco on 30/10/2011 23:12:10:Back to Rich - may be its also a kind of Multiplex design problem Maybe it is.. quick search on err... *another forum* suggests that its quite common with these 2 models, seems to be quite a few different fixes too. VA, He seems very happy... they do fly well. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted October 30, 2011 Share Posted October 30, 2011 I think Tim's suggestion is a good one - take the prop off and see if it persists. I'm wondering if it might be control surface flutter caused by prop wash? If it goes away without the prop then at least you can clear the motor and its mounts and can focus in on the prop/prop nut etc. either mechanically or aerodynamically. Another thing to bear in mind is that while no vibration is "good" some of these very light weight electric models do act like perfect amplifying resonant chambers and can make even a quite mild vibration sound much worse than they are! BEB Quote Link to comment Share on other sites More sharing options...
Tom Wright 2 Posted October 31, 2011 Share Posted October 31, 2011 Rich . Some motors do this when slightly over propped ,might be worth propping down a pitch or diameter, If the engine mount system is anything like the Cub then the problem is most likely with the aluminium plate mounting an adjustment screws ,this set up is notorious for vibration troubles , set them up correctly and they will run smoothly for a while then revert to horrible vibration again ,a close look at the the way things are done up front should inspire a mod. Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted October 31, 2011 Share Posted October 31, 2011 rich-i would suspect the engine mount--- i know of one that came out in the air.....i know it has adjustments in it that i would suspect...and double check..... i have seen a acromaster have the same symptons.....the least little knock or nose over and they start to play up....maybe wrong but worth double checking... ken anderson ne..1.. multiplex dept.. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted October 31, 2011 Share Posted October 31, 2011 I would go for a possible lack of rigidity in the motor mounting......I had a similar problem a couple of years ago with a front mounted outrunner that vibrated & resonated over half throttle....it was so bad it threw the prop off (which hit the garage door with a resounding CLANG) I had used 3mm birch ply but the mounting to the model was less than brilliant I have to admit. Laminating another 3mm layer & beefing up the mounting points solved the problem & it is smoother than a very smooth thing now..... The fact that (as you say) you hear of similar problems with these models perhaps points to an inherent design flaw. Can you beef up the mounting in some way? Possibly even replace the mounting plate with something more rigid? Decent ply or even aluminium perhaps..... Quote Link to comment Share on other sites More sharing options...
Richard Harris Posted October 31, 2011 Author Share Posted October 31, 2011 Chaps, Thanks for all the sound advice I have had a play this morning, first I removed the motor and checked the best I can for loose magnets , all seem to be in the correct position. I haven't a DTI to hand so couldn't check the shaft accurately but when viewed side on the prop tips seem to be in alignment when rotated with each other. Motor has been run up with no prop or prop driver and it is vibration free. The motor mount was tightened up as much as I dare (including the thrust adjustment bolts) and I did a static test. First attempt with the prop it had on yesterday and the second try with a wooden prop. I moved the props about on the driver but it seemed to make no difference. To me it seemed a bit better but if you touch the model you can feel vibrations still. I will make up an alloy mount to replace the standard plastic type and try again later in the week. If anyone else has any ideas please let me know, it all seems a bit odd to me! Here's the 'ground' test. Rich Quote Link to comment Share on other sites More sharing options...
Tim Mackey Posted October 31, 2011 Share Posted October 31, 2011 Seems that its just a flimsy mount then Rich. Even with a well balanced prop, the cheaper motors ( and I have no problem with the turnigy stuff in the main ) will still have some run out, and this is amplified by the airframe - and especially so if the mount is a little "soft". Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted October 31, 2011 Share Posted October 31, 2011 I see the problem.....you have the throttle on the wrong stick...... Seriously though I'm going with a bit too much flex in the motor mounting as per my earlier post..... Can you use the existing mount & fix this to something nice & solid (like a bit of 2 x 4 clamped into a workmate..... If it still vibrates then maybe a new mounting plate is required. If all is smooth then I would look at how the mounting plate attaches to the model & see if this can be made more rigid. Plastic & foam are not known for their stiffness (oo-er vicar) under load....... Other options...can you rear mount the motor on the models "firewall" perhaps? I have found this to be the most rigid method..... Quote Link to comment Share on other sites More sharing options...
DH 82A Posted October 31, 2011 Share Posted October 31, 2011 I take it that all the props tried have been balanced ? Quote Link to comment Share on other sites More sharing options...
Richard Harris Posted November 7, 2011 Author Share Posted November 7, 2011 Chaps, A big thanks for all the info regarding the vibration problems. I made up a new motor mount from aluminium and took her up the field yesterday. All vibrations have completely disappeared and I was able to increase the prop diameter for unlimited vertical performance without the worry of ripping the model apart! I have to say I am a little surprised to have found this with a Multiplex model, but its sorted now and that's all that matters. Thanks again Rich (proper mode flier ) Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted November 7, 2011 Share Posted November 7, 2011 Excellent news Richard....glad you got it sorted.... I work for a German company & my colleagues tell me there is no word in German for "overlooked".....seems the Multiplex boys might just have to invent one..... Seriously though I wonder what they would say if you fed this info back to them via Mr Upton......especially as you say it seems to be fairly common.... Quote Link to comment Share on other sites More sharing options...
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