Q: I recently installed a new three-blade propeller in my Comanche 250 with an 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. This did not change anything. The engine still vibrated.
I contacted McCauley and they had some engine mounts that they thought would fix the issue. It did not. The mechanic checked out the airframe and we corrected all movable surfaces that were loose. 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 to find out it now has the same issue. My thinking now is if the engine is running rough or incorrectly?
I do notice that if I retard the throttle back to 10-12 manifold pressure, the vibration smooths out (still 2400 rpm). When I apply the power again, the vibration returns.
A: Jeff, it looks like you’ve got a good one for us here. After reading what you’ve done and what you’ve ended up with, this may be quite a challenge. I must admit, it certainly raises several questions in my mind.
My first question has to be: What else was done at the time or just prior to the installation of the new prop? Was there a regular scheduled maintenance event, such as an annual inspection or a 100 hourly inspection?
Since you’ve already gone to the trouble of discovering that both props exhibit the same vibration, I’d bet there is nothing wrong with either prop. I’m leaning towards something else that may have changed unknowingly that you’ve just overlooked up to this point.
Was the aircraft recently put up on jacks during a routine maintenance to check the gear operation? There have been cases in the past where, during this maintenance procedure, the gear doors have been disturbed and fail to close to their proper position. If one or more gear doors are not fully closed, sometimes during flight the airflow will cause an airframe vibration at certain power settings and certain attitudes. I’ve also heard of a vibration being caused by the cowl flaps being misadjusted or suffering from worn bushings.
You mentioned that when you apply power the vibration occurs. That may indicate that we’ve got something that is coming in contact between the engine and the airframe due to the torque of the engine. If you’ve ever watched an engine during a ground run-up with the cowling removed, you’ve probably noticed that as the power is increased or decreased, the engine appears to roll in the mounts.
This may be the case in your situation, so you’ve got to look very closely in the engine compartment for anything that may be making contact as the engine operates at higher rpms. Inspect the exhaust system closely and try to observe it during ground operation. Also, check any oil cooler lines, air induction tubing, etc.
Is there anything other than reducing power that changes the condition? What happens if you switch to left or right magneto versus running on both mags? What happens if you pull carburetor heat on? Were there any new avionics antennas installed or changed recently? How about the gap seals between the wings and the fuselage?
Jeff, I must confess, it’s troubleshooting like this that typically generates more questions than answers, but I’ve found in the end, the problem will be found and it’s usually something simple that was just overlooked.
I wish I could be of more help, but for the moment, I’ve given this my best shot and I hope it helps you, or at least points you in the right direction.