Our Legacy has had a pitch vibration in the stick that comes and goes. It seems to be related to about 50% fuel load or less and reduced but stable power. The plane has about 275 hours and this has been going on since the first 40 hours. When it is vibrating in level flight (approximately 2 cycles per second and the stick is visibly moving in a pronounced shake) it will continue to vibrate when pitched up at about 1.5 G. After the pitch up, when pitched down to less than 1 G, the vibration will quit. After working thru many of the same contact points firewall forward and the gear doors Paul Miller had, I still had the vibration. All the while, I'm thinking, this is a great way to initiate flutter. I might add, I very carefully rebalanced the control surfaces after painting.
I mentioned the problem to Chris Zavatson at the Lancair Breakfast Fly-In at Willows, CA.. Chris asked if I had checked the control and trim hinges for play. That was easy, I check them every pre-flight. Only not the way Chris checks them. He pushes and pulls looking at the hinge. See video http://www.n91cz.com/HingePlay/hinges.htm
I replaced the Pitch Trim Hinge Pin with Stainless Steel (3/32” - .09375) welding wire. Now, no play can be seen in the push pull test. We have put 20+ hours on with the new SS hinge pin and no more stick shake. I included Paul’s post of problem areas (see below) since I had some of them too.
Steve Colwell Legacy RG 310 hours
Sent: Monday, April 12, 2010 6:47 AM
To: lml@lancaironline.net
Subject: [LML] Legacy vibrations resolved
I had written about vibrations in the 300 hour Legacy engine
compartment. Many wrote about their fixes. I am happy to report that
the following worked for me:
1) Changed engine isolators last year (no difference)
2) enlarged cutout on nose gear door stiffener. After repeatedly seeing new witness marks I did a retraction test and saw the interference is much larger than I expected. I trimmed another 1/8 inch from the cutout. This cutout prevents interference from the bolt attaching the linkage to the nose gear on both sides. The co-pilot side needed a larger cutout. White primer was used to show the marks after each flight. I think this was transmitting a lot of cowl vibration.
3) I sprayed white primer on the areas contacting the baffling. I
found three hard hit areas and this is using the factory white powder coated baffling. Two areas at the front of the #5 cylinder needed bending away from the cowl. Another area just above the oil cooler on the top side of the cowl was hitting. The #5 EGT probe was hitting.
The #5 forward rocker cover was hitting on the bottom and was peened about a 1/4 to match the form of the cowl. I removed the rocker cover to check for clearance with the rocker arm.
4) Baffling repairs were made to areas that were not tight against the
cowl. This significantly reduced the CHTs. I used the cooling and
drag reduction data on lml from owners to help make small but helpful changes in the baffling.
5) The prop governor cable was a hard hit on the metal baffling and was trimmed.
6) The nose gear doors had a slight overlap which was trimmed so they close tight with no gaps.
7) Both main gear inner and outer doors were adjusted to be flush and tight when retracted. This took a significant amount of time involving small hinge adjustments and testing of each individual attachment link on the outer doors to get the right length before attaching both links for the final test. I say this because the two links are slightly different in length and geometry. I found one was a little longer than required and actually twisted the door slightly open when both links were attached. This was not evident until I retracted the gear using only single links.
8) the large circular baffle behind the prop is cracked in two places and remains that way until a fix can be made. This appears to be from a tight fit and needs to be measured for a new smaller piece. This is a large crack that could easily be flopping around in flight and somehow interfering with the cooling process.
9) removed the quick drain for oil and replaced with the factory plug.
I have lots of clearance and may put the quick drain back in.
The net result is now I have a smooth engine compartment. My symptoms
included a difficult propeller balancing as we could never exactly home in on a solution. The vibration was small at 2700 rpm and takeoff but increased significantly with rpm reduction. I attribute that to the higher torque twisting the engine closer to the left side of the baffling. The vibration was evident in the cockpit, cowling and when looking at the wings. The next project will be to replace the silicone baffle with new material and replace the circular baffle plate behind the prop and replace the left side metal baffle to better fit the cowl line near the #5 cylinder. Many thanks to all the suggestions that made this easier to do.
Paul Miller
N357V Legacy RG