I am 99% sure we've finally eliminated the vexing vibration we've experienced with our Legacy RG. We have an IO-550 (stock compression, mags, etc.) and a Hartzell 3-blade propeller installed. I say 99% because one of the more vexing parts of this vibration problem has been its intermittency; I thought I had it fixed before, but it came back. Since applying what I believe to be the ultimate fix we've flown nine flights without a relapse. The aircraft is MUCH smoother at all the usual power settings.
What was the problem?
Our Legacy has had an intermittent pitch vibration in the stick. It seemed to happen more frequently with a 50% fuel load or less, and at reduced but stable power settings. Up to 460 hours on the Hobbs the vibration had been occurring since the first 40 hours. When in level flight the vibration at it's worst was approximately 2 cycles per second, causing the stick to move visibly in a pronounced shake. The vibration would continue when pitching up at a rate of approximately 1.5G, and would quit when releasing stick pressure and allowing the nose to pitch down to less than 1G. My primary concern with this vibration was the possibility of initiating aerodynamic flutter. If you are unfamiliar with the concept, aerodynamic flutter is an increasing (undamped) oscillation of structures exposed to aerodynamic forces (wings, stabilizers, control surfaces, even bridges and other structures) which can lead to structural failure. Flutter frequency can increase slowly, or it can increase rapidly, giving the pilot little time to react before disaster.
How did we fix it?
I read about Paul Miller's similar experiences in taming airframe vibration on the LML. He claimed to have alleviated much of his vibration problems by eliminating contact points between the engine/cowl and ensuring the landing gear doors were fitted properly and functioning correctly. Chris Zavatson opined that play in the pitch trim tab hinge might be a contributory factor. He suggested replacing the hinge pin with stainless steel welding wire of an appropriate size. Here are all the vibration mitigation actions we tried:
- Carefully fitted nose gear doors
- Found/fixed numerous contact points under the cowl
- Replaced pitch trim tab hinge
- Prop balanced—twice
- Prop aligned per Hartzell recommendations
- Rebalance control surfaces after painting
After a great deal of work and many test flights, Paul's and Chris's recommendations failed to eliminate the vibration (although now I'm certain the engine is not touching the cowl and my gear doors are perfect). I next sought help from Hartzell. Hartzell says a 3-blade metal prop is a very good gyroscope. That is, it's very stable when spinning at speed, and will magnify any force that might be causing vibration. This is particularly so if it's attached to a stiff, carbon fiber airframe. In our case, the lower the rpm, the worse the vibration. We typically cruise between 12,500’ and 17,500’ altitude at 2300 rpm, full throttle and lean of peak. I informally polled other Legacy and IV pilots; some felt they too were experiencing more vibration than they felt they should. This led me to contacting Continental Motors and others experienced with the IO-550/Hartzell combination. Ultimately, I ended up at Lord Mounts. Lord representatives told me Mooney owners had a similar problem they solved by using engine mounts with varying elasticity; specifically one hard and three soft mounts. Although it seemed like a long shot, at this point I was willing to try almost anything.
Do you want the good news, or the bad?
First the bad: If you plan to replace your engine mounts you should know the new Lord mounts were a bear to install, at least in our Legacy. They are about ¼” thicker (before the mounting bolts are torqued) than the original mounts, which means you must compress them before the bolt threads engage. Alternatively, you may be able to use slightly longer bolts provided the threads don't bottom before they are fully torqued. The good news is our engine is MUCH smoother now, and has been for the last nine flights. Although my seat of the pants sensor says this is the fix, I plan to reinstall vibration sensors and repeat a test card with the new mounts installed. I want to put a few more hours on the plane with the new mounts, after which (assuming no more vibration) I'll be 99.9% certain we've solved the vibration problem, and I'll publish the part numbers for the new mounts.LOBO members Steve and Claudette Colwell share PIC duties on their Legacy RG, N15SC.