PA32-260 Tail Vibration

Natembt

I have a 1966 PA32-260 with a tail "vibration" that is related to both power setting and engine RPM. The "vibration" exhibits itself in a visually discernible movement of the stabilator. The vibration is not correlated to the control yokes, panel or interior components of the airframe.

The vibration is transient, it does have repeatable characteristics. The higher the RPM the lower the amount of movement. Typically tail is free of movement between 2650 and 2500 with power setting ranging between 60-80%. Movement is most intense between 2300-2400, (same power settings).

The vibration has remained throughout all the changes listed below.

Here is what has been done:

Engine overhaul with components weights match, crankshaft balanced.
New engine mounting basket by AIW
New engine vibration isolators by both Lord and Barry
New exhaust system
New baffling
New starter, alternator and carburetor.

Stabilator removed, striped, inspected. No damage was found other than a small piece of sheet metal bent from "hangar rash". New hinges were installed on the trim tab. The stab was painted and balance checked. The trim cable and and jack screw spool were replaced. All new bearings and hardware were used to re install the stabilator.

New bearings in the ailerons, hinges inspected. Aileron balance checked.

The prop is a new Hartzell 3 blade, balanced and tracking checked. The condition existed also with the original 2 blade propeller

As to points of contact from the engine to the airframe, at least 10 very good mechanics and AI's have looked to find a point of contact.

What we have found most recently is that the tail is completely free of vibration if the main tanks are full of fuel. Once fuel in the mains is used below 7 gallons the vibration begins very slowly, on reaching the level of 5=7 gallons the vibration is most prominant.

I have flown one other PA32-260 with the purpose of determining if the vibration observed in mine was common to the model, it was not. The Cherokee 6 that I flew was in the worst mechanical condition that i have seen, it stabilator was however absolutely smooth through all power, rpm and airspeed ranges.

Comments are welcome.
Nate

kzadora

Just a low-time pilot here, so take it with a grain of salt, but my bet is this is an “aerodynamic” problem, and not a “mechanical” one. In other words, at certain speeds and angle of attack turbulent air from the propeller hits something attached to wings or airframe and is causing the stabilator to flutter slightly.

Given the extent of repairs and improvements you have done to the powertrain and propeller it is unlikely that mechanical vibration is being transmitted from the engine or prop all the way to the tail. You also have overhauled and balance-checked the stabilator itself, so most likely in itself the stabilator is not the source of the issue.

The fact that the problem is most severe at certain RPMs and weights (low fuel) suggest an airflow/angle of attack relation. My suggestion would be to look more closely at flaps (check the rigging), landing gear, and anything close to, or at the fuselage that might interfere with the airflow coming from the propeller and hitting the tail.

Hope this helps. Good luck and keep us posted!

Scott Sherer

I have shipped this question off to our A&P Erich Rempert. Stand by

Scott Sherer

Here's a response from our IA/A&P Erich Rempert:

Look up "Natural Resonance" and "Natural Frequency" and how they relate to aircraft structures.

I've got a gut instinct that there are a number of modifications to this airplane that interrelate in a manner that is setting up a frequency issue.

The cause could be aerodynamic, or it could be from the engine; it sounds like it's more likely to be from the engine.

That's not to say anything is wrong or incorrect, it's to say it introduces a frequency that resonates in the airframe easily and becomes noticeable.

Tesla (the Russian) did an experiment that almost caused a building to collapse with a small device that would tap on a beam lightly but at just the right moment as to eventually cause the building to sway. Same idea here.

Every object has a natural frequency; buildings, bridges, guitars, conveyors, and Aircraft. Helicopters are actually very suseptable to rapid unscheduled disassembly during hard landings because of the phenomenon.

The point you mentioned about the fuel level affecting the vibration is what caused the lightbulb to go off for me.

If I was an engineer working for an Aerospace company, and the airplane had an Experimental R&D Airworthiness Certificate; I would try to find a way to dampen the frequency out of the airframe system. Perhaps I'd take shot bags and attach them inside the wing where they couldn't move or interfere with things out near the tip to see if that would deaden the ability for the vibration to propagate. Of course I'm not, and that's just an idea, that could also cause some unintended consequences as well!

It is an IA's responsibility to determine if an alteration has any adverse interrelationships with other modifications previously made to an airplane. There was a case of a Turbine Cessna Centurion that had every legal mod known to man including wing tip fuel tanks that spun out of the clouds in IMC and killed several people. It was found that each of the modifications alone didn't pose adverse effects on handling, but when combined they caused unforeseen problems.

You may have some similar combination existing that is allowing this frequency to resonate and become noticeable.

I'd be reluctant to operate the airplane extensively in the problem range for concern of fatigue. I'd also be careful about exploring the problem via trial and error as you may find something that exasperated the issue rapidly to the point of failure.

And I'd be reluctant to add shot bags in the wings as they may create some issue you'd never think of like added yawing inertia that could cause the airplane to be unrecoverable from a spin like the 210....

What I'd wonder about is was there a point when this issue emerged, and what was done just prior? It could be something you'd never expect like wing root fairings, or an Avionics installation (CG change/location of weight)

It certainly sounds to me like the wings are where the vibration begins and it transmits to the tail given a full fuel load dampens the vibration from occurring. Bear in mind the vibration could be CAUSED by the engine still, which resonates in the wing causing it to vibrate and then ultimately shake the tail...

Weird one for sure, you may want to call Piper and explain it to them and see what they say.

~Erich Rempert, IA Consultant

Natembt

Erich,

The airplane was purchased without any real flite time being put on it due to some registration/de-registration issues. During the time that the feds were getting the paper work done we did a handful of things, avionics and the addition of wing root fearings, wheel pants, etc. It was on the first flites after putting it back in the air we discovered the issues.

Subsequently we removed all of the airframe mods to determine if they were the cause.

From what we know of the airplane it was for most of its life a test bed for Narco Avionics. There were marks on one wing paint where they had mounted a radar pod. I am wondering if they did something to the wing that would have changed its structure. We have done the "normal" inspection for annuals and not observed any thing that appears to have been altered.

Does anyone have a contact at Piper?

Nate

Scott Sherer

Here's how you get it started at Piper: https://www.piper.com/contact-us/