Ask Paul: What’s causing vibration?

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.

JEFF ODELL

 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.

Comments

  1. George Canale says

    Can you explain the vibration better, I have a similar issue with my piper six but it’s more of a shudder upon leveling off while reducing map. The shudder lasts about 6-7 cycles, you can hear it and feel it. It’s like when you shut down a plane and you hear a dud dud dud dud.
    George

  2. Kenneth Hetge says

    Some late input to the conversation but thought it may help. I ran into a situation very similar to this on a Doyne Conversion IO-540 powered Travelair. We had a vibration in one engine that we could just not track down. We repeatedly installed the balancing kit and balanced the prop. We installed new motor mounts. The mag check on the ground was always good and the timing was good. The plugs were all cleaned and tested a couple of times during the fight. We even pulled the prop and sent it to the shop for balancing.

    Our particular fix was related to a single plug in the #3 cylinder that seemed to always look different (black carbon-ish compared to other plugs). We also noticed a loosy goosy contact spring in the mag harness that screws into the spark plug along with some carbon tracking in the plug ceramic. Remember, our plugs were cleaned and tested on the bench several times and the mag check was good. A new spark plug in this position along with a new plug end on the harness wire going to this position cured the in-flight vibration at high power. This was a real head scratcher and the fix was not obvious and almost accidental. Good luck and try to keep your sanity. These types of problems can drive you crazy and cost a lot of money to chase!!

    • Mark Zeiler says

      The McCauley prop on my Comanche gave a similar vibration at around 2300 RPM, not severe by any means, but enough that it was noticeable, given the remarkable smoothness above that speed. During a tear-down inspection for a prop strike (my most humiliating aviation moment), I mentioned the roughness and they told me a change in counterweights is suggested with the upgrade to the Black Mac prop. Since the case was cracked anyway, they swapped for the new weights.

      While I haven’t had a chance to fly the airplane again, yet, I am hopeful that the change will be noticeable. I understand that the Macauley prop is signed off as a direct swap-out and cracking the case is not an option, but I eagerly look forward to checking for the change when the aircraft is flying in another couple of weeks.

  3. Pat Barry says

    “The Comanche aircraft accidents are full of this type of accidents where the tail gets destroyed by flutter”

    There is no evidence that supports this statement. The Comanche is an outstanding aircraft, stable, and the tail is efficient.

    Like any aircraft, the c of g shifts with loading and airspeed, and if the trim system is not maintained (worn links or loose trim drum) then flutter of the trim tab can occur as the stabilator and trim tab is ‘unweighted’ – typically at higher airspeeds that are encountered in descent.

    Poor maintenance is not exclusive to Comanches so to single out the Piper Comanche is inappropriate. If you ride in an aircraft again that has flutter it should be inspected for loose components which allow the flutter. With respect to the weight procedure for the stabilator, the correct balancing of the stabilator with weight on the balance weight arm is well described in the service manual and should be followed. As for patches not permitted on a stabilator – you are correct. It sounds like the aircraft about which you speak was poorly maintained.

    The International Comanche Society is alive and well, and has a team of technical advisers that can be accessed through the ICS website. There is also a link on Delphi Forums to Airworthy Comanches site where a chat room of experienced Comanche owners and mechanics share information. Between the two sources I’d say the Comanches may be better represented than any other type group.

    As a general comment, like any aircraft, once an owner gets in front of the maintenance curve the Comanche is a straightforward plane to maintain. It is a complex aircraft and, like other complex aircraft, there are several systems which need review, but once each system is corrected it is not a difficult plane to maintain. There are numerous STC items which improve the performance of the Comanche, and parts are available from Piper and aftermarket sources.

  4. Donald Hyatt says

    Boy you got my hair up on the back of my neck and I use to be fearless flying anything. First of all you have two things that really throw up an immediate red flag for me. One you are flying a Comanche no really. The Comanche tail both on the single and the twin is very critical. I have ridden in a Comanche that had work on the tail talk about a vibration. The plane went into tail flutter maybe 60 knots below it’s red line airspeed. Thought we were going to loose the tail off the plane. The Comanche aircraft accidents are full of this type of accidents where the tail gets destroyed by flutter. Here are two key things to know. One the tail has to be balanced per the manual. Go read up on it. If I remember right on the single Comanche you disconnect the horizontal stabilizer cables and the stabilizer should set dead level, if you were working on the twin Comanche you have to put a 2.2 pound Piper weight tool over the stabilizer balance tube weight and again the cables are disconnected and the horizontal stab must sit perfectly level. I forget but if you look it up go confirm this first, the plane must be level fore and aft and side to side per the manual when you are doing these stab balance checks. Also two no repairs are allowed on the horizontal stabilizer. This means no patches only whole skin replacements. Three you said your mechanic fixed the loose ness in the control surfaces. Which surface and what did he or she do to fix the problem. I hope they only used approved parts or parts made to Piper drawings specifications and drawings. Fourth there use to be a Comanche society or association. They had a very knowledgeable technical guy I forget his name something like a Pierre French name if you can find him or some one that use to see his articles written for the Comanche society they will point you in the right direction. I have survived the dreaded Comanche flutter. The plane was way out of balance on the horizontal stab and also had a patch on the horizontal stab skin. Here is the disclaimer anything I stated above should not be used to perform maintenance on the aircraft until you go confirm the latest procedures in the Piper Comanche service manual. I would also suggest you go read all the service bulletins on the Comanche there were definitely some on the tail and balance weights. Of course you could also have something really simple a loosed inspection panel will put all kinds of vibrations in the airframe and scare the be geezers out of you. One other thing I have seen. This also would go back to the tail. Some folks try to do too much of a good thing and put an erosion strip of tape on the leading edges on tails. If it is there remove it and go fly. I have seen the erosion tape actually make a plane right at touch down flare stall the stabilizer and make the plane pitch nose down. We’ll be careful and fly safe.

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