I recently installed a new 3 blade propeller in my Comanche 250 with a o-540 engine. After the first flight I noticed a vibration when I decrease the rpm to 2400 or below. So I had the prop dynamically balanced. We got the prop to .07ips This did not change anything. The engine or airframe still vibrated. I contacted McCauley and they had some engine mounts that they thought would fix the issue. It did not. The balance A/P checked out the airframe and we corrected all movable surfaces that were loose or worn. This did not help. I have checked and rechecked if something from the engine was touching the cowling but I don't see any evidence of this other than the rubber baffling and felt seals in front of the engine baffle.

I put on the old propeller in frustration as I remember my plane used to be super smooth now to find out now the old 2 blade propeller also now has the same issue. My thinking now is the engine is running rough or incorrectly? Have you ever heard or run into this? I do notice that if retard the throttle back to 10-12 MP the vibration smooths out (still 2400rpm). When I apply the power again the vibration returns. The vibration is not severe but not smooth like it used to be.

Any help where to go would be appreciated.

Jeff

Hi Jeff,

Sounds like you've had a tough time of it. I am certainly no expert in this area and I own a -180 with the old two blade prop but one thing that fixed the symptoms you describe in my aircraft was changing from massive electrode plugs to Tempest fine wire plugs.

I was told that engine 'roughness' (actually felt like vibration to me) in association with high power settings could be caused by weak and therefore uneven ignition. I guess it is the same principal as leaning the mixture too far and experiencing engine rough running? Anyway, changing my plugs to new Tempest fine wire plugs made a big difference to the engine smoothness at high power. I guess checking your mag timing and ignition leads may also be a good idea...

Good luck...

James.

 

Jeff,

The report from the prop dynamic balance may give you some direction. It has been a decade but IIRC there are 9 parameters on that report - at least there were for the Twin. The shop couldn't tell me what the meanings of the results past the first one were so I called the manufacturer of the device - today it is probably on the internet. After much research, it was the engine mounts that had been improperly torqued by the shop that handled the remove/install of the engines. Now I have the torque checked every annual and at the most recent annual had the mounts changed after 4 years. Both husband and I noticed there was less vibration after the mounts were changed - so something else you might check.

I credit Zach Grant with the advice to change mounts every four years when he ran the CPPP course that is now CPTP - worth every penny. http://www.comanchetraining.com/

I think none of the 4 cylinder Lycomings have counter weights. You would have to call Lycoming with your data plate info of check the engine parts book to get it. A guy at the airport put an STC'D 3 bladed prop on a Mooney with an O-360 and ended up going back to the 2 blades due to excessive vibration when compared to his 2 bladed prop. As said in this thread it could be the motor mounts. I had a 63 310 that had 3 bladed props(per stc) and you could feel the vibration through the floor boards. It had the wrong motor mounts installed when the previous owner upgraded to a colemil conversion (IO-520) and the 3 bladed props were installed. In 64 Cessna changed the motor mounts(bigger footprint) when the 3 blade prop was a factory option to eliminate this problem. Bernie

I stand corrected. After doing some homework it appears the 200HP IO-360 has counterweights and the O-360 (180 HP) doesn't. Just a thought. bernie