Vibrations are bugging me

GyroRon · #1 10-17-2005, 06:50 PM

Hello all, Want to share a problem I am having and see if there is any cures or ideas for cures.

I have a RV-4 with a Lycoming O-320 160 hp engine. Prop is a Sensenich fixed pitch metal prop. The prop is a model number 70cm7s9-0-81 and in the paperwork for it, it states to avoid operations above 2600 rpm. My engine mount is a dynafocal type mount and also worth noting is the plan was first flown in 2003 and only has about 240 hours total time BUT.... the engine is a first run engine put in service in 1986 on a Cessna 172 and had 1971 hours total time at the time it was removed from the Cessna and put on the RV-4 in 2003. So it has not been overhauled and now has over 2200 hours since new. Compression checks on cylinders at last oil change was 70#, 75#, 75#, 71#, over 80.

Okay the problem......

When I start the engine and run it at idle it is reasonably smooth, in other words there is some vibration but not enough to bother me. Run the engine up around 900 RPM up to about 1600 RPM and the vibration is quite pronounced. Above that rpm and the vibration is again not too bad, it is there but just not that bad. At RPMs close to 2500-2600 it is reasonably smooth.

I took the plane to a mechanic who put on equipment to do a dynamic balance and after the 1st runup the computer called for weights placed at a certain degree, and the mechanic used washers on the spinner for this, then we ran a 2nd runup and the computer would again call for more weight at a different degree, so more weight added and a 3rd runup with the same results... more weight needed in a new position. During this process as I did the runups, the low rpm vibrations worsened each time more weight was added. In frustrationwe took off all the weights and put the cowling back on and said forget it.

On two occasions people have told me my prop does not appear to be tracking in plane. One of those times was by a aerobatic pilot who was flying extremely close off my right wing, and the other time was a pilot who was standing next to the plane while the engine was at fast idle on the ground. The mechanic tested the prop by putting a dab of paint on the tips of the prop - a different color dab on each blade - and had me run the engine while he stood behind the prop and touched the tips of the blades with a piece of tape on a stick and then looking at the tear in the tape and color transfers. He also did another test where he put a ladder in front of the prop and rotated the blade to a fixed position and measured the distance to the ladder and then turned the prop to the other blade and measured the distance again and in both tests he claimed it was in perfect track.

I did not build the plane, I bought it from the builder with about 150 hours on it. In the planes log book there is notes about these vibrations from as early as the first flight and all it appears he did to resolve it was to remove the prop and reinstall 180 degrees to where it first was.

The engine does not seem to be sagging on the mount and I can not see any cracks in the mounts or any signs of the mount moving on the firewall, but my eyes might not be trained well enough to spot a problem on these areas..... The rubber cushions between the engine and mount look to be in good shape with no obvious rotting or cracking. I have not checked anything with a torque wrench - I don't even own one... - so I can not comment on that.

The mags are slicks and turning one mag or the other off seems to have no effect other than the drop in RPM.

The only other thing I can think of worth mentioning is there is a noise coming from the firewall area when taxiing over bumpy ground or even on smooth ground if I taxi at the rpms the vibration is bad. I had the cowling off a few weeks ago and had someone bounce the plane up and down while I listened and looked at the firewall and there is a noise coming from a aluminum plate that the starters relay is bolted to and the plate is riveted to the firewall, a few of those rivets are loosened up and was making a noise as the plane was bounced. I assume this is what I hear when I taxi, although I believe I have heard the noise on climbout after taking off.....

So.... sorry for the long post, but any ideas or hints or whatever would be appreciated.

svanarts · #2 10-17-2005, 06:55 PM

Have you tried a different prop? Have you contacted Sensenich? Do they have any suggestions? I'd try to rule out the prop first.

GyroRon · #3 10-17-2005, 07:25 PM

Yes that was my first desire for trying to find a fix. Another pilot based at the same airport as me has a RV-4 with a 150 hp lycoming, and he has a spare wood prop he said I can try out. I just haven't had the time to go out there and see if it will bolt up to my engine yet.

jclark · #4 10-18-2005, 06:44 AM

Ron,

First, I am sorry that I have not been able to followup on our earlier conversation.

If the prop tracks true on the ground BUT ***seems*** to be wobbling when the engine is running, there seems to be an implication that the engine really is shaking a lot more than desired. And this points to the isolation mounts as a potential area of concern.

Are the rubber isolation mounts as "new" as the plane (~250 hours)? I would assume this is the case.

They really should be checked for torque (and make sure there is a cotter pin in there).

Also, I would fix that plate with the solenoid. Remove all ancillary noises and vibrations.

James
... lots of time spent chasing "vibrations"

penguin · #5 10-18-2005, 07:33 AM

Ron,

Last Christmas I pulled my O-320-E2D with 2500 hrs on it (first run) as it was making metal and had it overhauled. I was feeling annoying vibrations at all rpms from run up upwards. Working on the exhausts reduced the vibes to some extent (pointed the tail pipes away from the floor), balancing the (metal Sensenich) prop did nothing. After the overhaul everything is much, much smoother. Its a radical step to take (cost me $11,500), but the overhaul was the single largest vibration reducer for me.

You could try indexing the prop around one hole at a time on the crank to see if that makes a difference, I would also try the wooden prop (if you fly with it check the weight & balance). If your mags have not been inspected for 500 hrs or so it may be worth getting that done also. How old are your spark plugs? Do you have a multi cylinder temperature monitor? - can be very useful in finding out if there are vast differences between cylinders at the problem rpm.

Yours, Pete

Gordon · #6 10-18-2005, 10:56 AM

I had the very same RPM vibrations with my new Sensenich propeller on my RV-6A and reclocked the prop to one position past TDC clockwise facing the airplane. It worked for me. That is the postion that my Cessna 172 manual recommends.

gmcjetpilot · #7 10-18-2005, 03:40 PM

Regardless of engine mount: Conical or Dynafocal GET LORD engine mounts. Berry Controls are not as good. I don't have part numbers but call them and they will help you. Going to Lord engine mounts helped my RV-4 Vibration problems.

Also not all prop balances are equal. I had may prop balanced 3 times but two different mechanics before I found the sweet spot. Metal props are not as smooth as wood, but performance should be better with metal props due to the thinner airfoil.

Also remember the RV-4 is a compact airplane with the engine mound bolting to aluminum stringers that go a sort distance to your insrument panel and next to your arms. Also any cables/wires can transfer some buzz into the cabin. G