Hello,
I have a vibration problem with Evo60NX I am using in my RToucan (modified Score).
The propeller and spinner are balanced so these vibrations may come only from the engine itself or its mount.
It is rather unlikely the engine parts are at fault as they are original factory parts (I am assuming of course the crankshaft counterweight is properly selected by the Chinese manufacturer but there is no way to check it with them) so the engine mount is the suspect.

The engine is mounted using two glass reinforced "T" bars provided in the Score ARF. I have rebuilt, stiffen and reinforced the front of the fuselage and the "T" bars are mounted now to the 5/16" thick epoxy laminated custom made ply rib that replaced the old fuselage front rib.

I started suspecting that the "T" bars cantilever beams to which the engine is mounted are too flexible in bending and must be stiffened.
The problem is how to do it. 

Please see the attached. Many details shown in this attachment are different now but the "T" mounts remain the same.

Any suggestions?
Thanks,
Matt

 

Jim,
The box like front part of the fuselage is no longer flimsy.
Like I wrote in the original message, I rebuilt and reinforced it, including the new tank compartment enclosure that carries now the in-plane shear generated by torsion much better than before. The torsional and bending stiffness of the front part of the fuselage is much higher than before so I have to exclude this as the source of "flimsiness".

Motorman,
This is called the first mode resonance and of course there is at least one RPM causing it but I do not see nor feel it in the entire engine operating RPM range (8,600-10,800). The vibrations amplitude seems to be almost the same in the RPM operating range and this indicates strong damping effect of the supporting structure. Specifically, the "T" mounts must provide sufficient internal damping for the damping ratio to be such that the system responds with the steady state forced vibrations I see (ref: https://en.wikipedia.org/wiki/Damping_ratio).

The problem is quite difficult as stiffening the "T" mounts in bending, what is technically possible, may only decrease the amount of damping present in the system, leading to more pronounced resonance.

The best idea is add more damping to the system until the damping ratio approaches 1 but this may be difficult and technically rather complex.

Hmmm...I should have used standard hardwood crutch while rebuilding the front of the fuselage. I will try different prop. but doubt it will help in this case.

Thanks,
M



Any other ideas?



 

 

Different motor mounts?  It sounds like they're a typical RC-ish motor mount -- are there other RC motor mounts that you could use?

Lots of hits on a search for engine mounts from Tower Hobby.

http://www.ohio-superstar.com/dave-brown-product-line/vibra-damp-soft-mounts

     I can't tell you how much I recommend you DON'T try soft-mounting the engine! There may be some vibration issue here, or not, but some of the most frustrating months of troubleshooting the West Coast Sekrit Kabal (AKA David, Ted, Brett, and Bill) were trying to make David's soft-mounted PA51 work. Elastomers of every possible compliance were tried from dead soft rubber to pretty high durometer urethane. We consulted with the originators extensively, David has a degree in Aeronautical Engineering, and I have been analyzing the dynamics of flexible aerospace structures of various types for 33 year on the most critical systems that exist. Nothing we tried worked even a little bit. Finally, David replaced the isolators with aluminum spools to hard mount it, and instantly, it ran perfectly.

   Brett

 

Brett,
Thanks for your comment. 
I doubt that David's model has the same structural characteristics as the Score:)
Also, I suspect that the side mounted engine on this particular Score isn't helping the problem.

One of the Score's that our local group has had vibrated no matter what we tried, different engines, props, mounts....
Mine is several years old with an inverted PA 61/header muffler, Xoar 13x4 and has no vibration issues, go figure:)

Jim 

Brett,
Thanks for your comment. 
I doubt that David's model has the same structural characteristics as the Score:)
Also, I suspect that the side mounted engine on this particular Score isn't helping the problem.

     I would not jump to that conclusion. The airplane has a firewall mount and the rest of the front end was built assuming there was a soft mount, i.e. not very sturdy by stunt standards.   The solution - replace the rubber with solid metal spools - led to several other issues, in particular, he started with 3/32 6061-T6 backplate mount, and it worked for 6-ish months with that. When he hard-mounted the engine, this plate started cracking almost immediately, a few flights. He replaced it with 1/8" aluminum, it broke in a week or two, then 3/32 mild steel, that broke after 40-50 flights, then 1/8 mild steel. That made it last long enough to get through a few contests, but still had to be replaced multiple times. At that point, he was concerned about the rest of the front end coming apart due to the light construction, and the airplane was retired in relatively short order.


 I can think of multiple solutions to this issue that don't involve soft mounts. I cannot emphasize how much trouble various people have had with soft mounts on stunt engines.  The *only* time I have seen them work in competitive airplanes was with the OPS40 which are remarkably smooth to begin with and probably didn't need it. And had issues with the custom built "flex header" cracking.

     In all other cases, it didn't even give a *hint* of working. That includes some very capable individuals, including experts on soft mounts in other events and full-scale aerospace applications.

  Before anyone points it out, the B-17 was a different issue, where the engines were interacting through common graphite/epoxy structure, and in that case, the "isolation" consisted of mounting the engine to a 1/8 aircraft ply firewall with an RC mount. That was a funny series of phone calls, with Howard calling me up at work, trying to beat Paul to the phone - to try to get me to talk him out of it when he did call. Howard explained the issue to me, and how Paul wanted to cut the carbon mounts off, and how bad it was. I said "I don't know, that seems like a pretty good idea to me", and, it was very clear that this was not the answer Howard was looking for!


     Brett

I would abandon the sidewinder engine config. and put as much soft balsa as I could fit into the tank compartment, grain spanwise, well bonded to both fuselage doublers. I believe some of this might be due to the glue used to laminate the ply doublers to the fuselage sides...maybe contact cement? While this stuff makes a strong bond, it is not a rigid bond like epoxy. Try a couple of test panels to see if you agree?  Steve

Are the ' T ' mounts replaceable ?

Al's aluminum ones , thinking alloy'd have a differant resonance .

Getting off track , the ' soft mounts ' - rubber .

The NORTON was quite scientific , also theyed done a lot of testing / calculations ( Dunlop ? )
on the rubber to match the amplitude & magnitude of the vibes . Set up right theyre turbine smooth - PAST 2.000 rpm ,

Around 1800 they can get a resonance - stagger .

ALSO the ' shim system ' is to control SIDEWAYS movement , in our case it'd be longitudeinal movement .

Theres a ' Three Point ' system used in the Racing Machines ( std. top is just mini exhaust rubbers )
The Three Points give a PLANE . as in geometric stability . The Side Shimming is said to be Ten Thou .

Below 2 thou clearnace they BIND . ( due to alignment / frame dimensional considerations , and so on .
They steer well , at 2 1/2 to 4 1/2 side clearance. Over that theres discerable lateral slack in the handling,
On Lean - and weave on undulating surfaces .

THUS , with ' soft mounts ' the rubber bushes would need to be compatable with the engine .
A lathe and various grades ( colours ) or urethaNE RUBBER ( NOLATHANE ) would be of some use .
If the longitudeinal gap wasnt controled , thered be corkscrew missalignment under use , at the spinner .

Hello All,
The reduction of the steady state forced vibrations achieved by using my "weird" arrangement is either zero or so small that it cannot be seen.
I have made seven full pattern flights yesterday, each time watching the vibrating engine for at least 30 seconds on the ground before going to the handle.

One more reason to go electric.

I will fly this thing until it falls apart due to fatigue stress cracking and then build something lighter with standard hard wood beams epoxied into the carbon composite reinforced fuselage.

Thanks,
M

Matt,
Another possibility is pre-ignition.  Try adding one or two head shims and/or using a slightly cooler glow plug.  That will retard the timing a bit and hopefully cure your vibration.
Fred