My plane has a mild vibration. Usually occurs above 130mph. Sort of like riding down a dirt road.

Controls giggle a little and you can feel it in the rudder pedals. Last week I felt it in the seat of my pants as well for the first time. Today, not so much. I have my approach plates attached to my yoke and you can see them flutter a bit. Not so bad that they are unreadable but they are moving with the vibration.

Does not seem to be overly speed dependent that I can tell. I have had it up to 170mph and did not see any significant change in the vibration.

Also does not see to be prop/engine dependent. I have manipulated the rpms from 2100 to 2400 with no real difference. MP from 24" down to 18"

The tach oscillates around 50-75 rpm consistently. Fuel flow on Shadin also oscillates about 1-2gph.

Plane flies true when trimmed in all three axes.

This plane has the main gear slippers installed. Soon to have the leading and trailing edge wing fillets installed but holding off till I understand this vibration a bit more.

Engine has 650 hours on it. Carbureted 250. Three bladed Hartzell prop. Wing tip tanks.

Vibration has been there since I bought it. Previous mechanic said it was normal but I don't think so.

Any ideas where I should begin troubleshooting this?

I see you are in Florida. Make an appointment with Matt Kurke over in Naples and have him go fly with you and check out the tail, landing gear, etc. Matt is an engineer as well as an A&P.

Richard,

One odd source I found the hard way was poorly torqued engine mounts - perhaps a thought.
I'd go to Matt 1st though.

It may be the 3 bladed prop. I had a 63 Cessna 310 that had 3 bladed props installed . It seems that when Cessna offered the 3 bladed prop as a factory option in 64 they had to change the motor mounts because of adverse vibration. I changed the motor mounts and added a prop synchrophaser ($15K) to solve the problem. I know it's an issue with IO-360 Mooneys.

Have you checked the cable tension on the control cables ? Especially the stabilator cables ?

T-6 Driver

Thanks everyone. Appreciate all the feedback. Gives me ideas where to look.

Make sure that all of the cowling mounting hardware is there and properly adjusted (hold down straps, lower mount bumpers etc). Check the nose gear door hinges. They should not have excessive play. Engine mounts if they haven't been replaced in the last 3 years/500 hrs should be done on principle alone. This sounds to me to be a vibration transmitted from the engine compartment. It may also be aerodynamic, but that would probably be easy to find as there would be something seriously misalligned or missing to cause that. My bet is it is engine relate vibration that is getting transmitted to the airfram due to something shifting under load and coming in contact with the vibrating items (engine prop and everything bolted to same).
-Zach

Thanks. I was wondering about the gear. One flight I was barely making 120 knots at 3000ft and the winds were reported calm at the surface. I was told if anything was amiss with the gear I would not get a gear up indication but I am not so sure that applies to the gear door. I'll get under there tomorrow and see how much slop there is. My climbs are typically about 700fpm at 110mph.

I just went through the entire engine/airframe logs a week ago and I don't recall seeing where the engine mounts have ever been replaced though visually they look in good shape. The engine was overhauled 650 hours ago so I presume they were replaced then. The Tips mentions a few things about using the wrong mounts or installing them the wrong way. Unfortunately, I am not very mechanically inclined so I can't tell if they are right or not. Probably not a good idea buying a 55 year old plane and not being mechanically inclined. Makes my mechanic very happy though.

So here is the situation with the gear doors. Left main is solid, no play at all. Right main has about 1/2-3/4" play. Nose gear has 3/4-1" of play.

Having no experience with Comanches (but getting plenty of it now) is this within tolerances? Looking at how the doors are connected I am not sure how you would tighten them up but I will certainly leave that to a mechanic.

Richard, the specs for adjusting gear doors are in the service manual on this website.

http://www.comancheflyer.com/NS/index_serv_manuals.php

It goes something like, when the aircraft is on jacks with gear in wells, a 9 lb weight should deflect the door down by 1/8 or 1/4 inch ... don't quote me on that as I did it from memory but it should be close