| Author | Message | ||
     Steve Adams New member Username: Stevepadams Post Number: 2 Registered: 05-2011 |
Well guys I have been lurking for awhile, flying my new(to me) S-90(176PA). I have been fighting vibration in climb and at higher RPM's. Usually at cruise it is better, but not gone. In decent and when I lower the power it goes away completely. It is a bit transient in cruise but usually not gone. So far we have, removed the nose gear fairing(but not the strut fairing) and then reinstalled it, replaced all the motor mounts, balanced the original 3 blade hartzell(multiple times, balances great), now we have put a 2 blade loaner prop on(got about 10mph in cruise!)balanced it out, still no luck. I removed the strut fairing and nose fairing and flew for 30 minutes mid afternoon yesterday. It felt better, but not sure it's gone. Gotta fly from AZ to San Diego on Thursday so I'll know for sure. But I figure I should throw it out there and see if anybody has had the same issue. The stock nose fairing is about 10" wide and it was only attached at 4 points along the nose strut. I wouldn't consider it loose, but there was some play at the trailing edge. It seems like a strong vortice off the prop could get it vibrating pretty good. Time will tell, but I am open to ideas. S-90, IO-540 C4B5, had a 3 blade, now a 2 blade prop. Interesting though, the 2 blade took my cruise TAS from 165 to 173kt. with no noticeable change in takeoff or climb performance. Not sure I want to give it back, but that ugly Cherokee 235 spinner has got to go. I love the plane, it flies awesome, but this slight vibration has got to be fixed! | ||
| Jimmie Kevin Alderman New member Username: Vikingpilot Post Number: 6 Registered: 07-2011 |
Is it a high frequency or low frequency vibration? My viking-IO520D Continental- had a high freq vibration- it was noticeable primarily through excessive noise in the cabin. I had the mechanic balance the engine/prop combo at speed while on the plane- what a difference. | ||
| Steve Adams New member Username: Stevepadams Post Number: 3 Registered: 05-2011 |
It is a fairly low frequency vibration. When it is bad, I can look out and see the ailerons vibrating. We haven't balanced either prop in the air, never knew that was possible. We have been using a dynamic balancer but it only works at 2k rpm, not the 2650 redline. | ||
| Mike Mc daniel New member Username: Mikemcdaniel Post Number: 2 Registered: 03-2008 |
Steve, Sorry to hear that you are having in flight dynamics problems. Over the last few decades I have had a fair amount of flight dynamics vibration mitigation tasking. Indeed the first thing almost everyone points to is the prop / engine combo however if you think about it it is seldom these mid to high freq events that are what you are really feeling in the seat of your pants. It's only normal for the uninformed pilot to point to the engine / prop as the problem however in the low freq range you need to ask your self this simple question......just what major mass component in the engine is rotating at .5hz to 3hz (30-180 RPM)?? The answer to this simple question is the simple answer that there is no large mass in the engine rotating at such low speeds. One of the things that drives pilots nuts is that, especially on multi-engine aircraft the prop spinner(s) are likely to be observed to be deflecting in a .5hz to 3 hz rage and often the spinner tip is tracing an orbit of considerable size. Almost everyone would testify that the engine / prop has to be the problem when in actuality the engine and prop is really a massive gyro that is isolation mounted and the airframe is the actual source of the critical dynamic. Many years ago I was contracted to solve a vexing problem on a fed gvt multi engine airplane that had a very troubling low freq flight dynamics problem with 3 NEW engine and 11 NEW props tried ( no wonder our taxes are so low). I had a heck of a time convincing anyone that the engines / props were not the problem. Try as I might everyone was fixated on the darn spinner that was tracing a orbit. I a final frustration I obtained obtained permission to captain a test flight and conducted a test where I would shut down each engine and while obtaining vib spec recording to disk. Everyone got a bit up tight when I feathered to last engine and the darn spinner was still tracing a orbit with ALL ENGINES stopped in a glide!! At the end of the day I was able to trace the .7 hz spectra to a combo of a loose elevator trim hinge pin/ rod end and trim actuator with enough accumulated tolerance to freate a low freq dynamic. In this case the only ting actually somewhat static was the right engine while the rest of the airframe was actually driving everyone crazy. I guess at this point I can tell you that so far everything you have done is normal...... balancing, rebalancing, re-re-rebalancing the prop to absurdly low levels.....changing props....This is all the norm but of course no amount of corrective measures will yield results unless yo address the actual problem is addressed. The thing thats a bit frustrating about airplanes is that us humans are somewhat restricted to the cabin in-flight so often we often need to use remote sensors and data collectors to zero in on problem locations. Perhaps I might be able to give you a few ideas that might help you get on track to solve your bad-vibs... feel free to give me a ring in the evenings at (541) 388-7862 or email mmcdan3189@aol.com Regards, Mike Mc Daniel | ||
| Steve Adams New member Username: Stevepadams Post Number: 4 Registered: 05-2011 |
Mike, I appreciate the offer, and advice. I am sitting in San Diego after a very smooth flight this morning. So it seems that the wheel pant/gear fairing might be the right place to start looking. I'll start putting stuff back on and keep everybody posted as to what we find. Thanks! |