Chasing Vibration

Sure, piston-powered airplanes are inherently vibey and while we go through great lengths and expense to smooth them out, you can only do so much. But when unusual amounts of vibration set in, it’s worth your full attention. Before showing up at the shop, logically troubleshoot on your own to help move the process along.

How Much Vibe?

When talking about small-airplane vibration, it must be looked at as a condition that falls into one of two separate categories: normal and abnormal. For the Cessna 182 with that freshly overhauled and nicely balanced Continental engine, vibration is a mere whisper of the exhaust trail burbling under the right belly pan.

On the other hand, the Beechcraft Baron equipped with a big, two-blade Hartzell paddle prop or the Cessna P-210 with the wide-chord McCauley fan, rumbling along in climb, is a common occurrence. Every make and model of airplane is different. To make matters even more complex, individual aircraft sometimes develop personal characteristics common to dynamic entities flying in close formation.

Aside from the constant spinning, pushing, whirling, and twisting done in the average airplane, just forcing air out of the way in an effort to stay aloft creates some vibratory noise—even gliders vibrate. Since everything around us vibrates and sings at different levels, identifying the source of a particularly annoying vibration can be difficult. The engine that purrs like a kitten on the ground can act like a blender in flight. The propeller that runs smooth in flat pitch can literally throb with a minor blade angle change.

Impact air against the airplane, flexing of the fuselage, and pressure-box buildup within the cowling can reposition inspection covers, baffles, cowl flaps, and doors. Anything and everything attached to the airplane can cause some form of vibration or be caused to vibrate in response to a tension-release input.

Consider that the following items are the most likely areas to find vibration issues, starting with the propeller, of course.

Propeller Vibration

Here’s a scenario to ponder. The overhaul of the small McCauley propeller from the nose of the fixed-gear Cessna Cardinal was accomplished without incident, but upon installation and ground running of the engine, it became apparent that something wasn’t right. There was no vibration to speak of—at least not in the instrument panel or glareshield—but something was happening well out of the ordinary. The curious nature of the ground run became a serious concern during a short test flight.

The aircraft and engine felt as if they were connected with a series of loose ball bearings running in elliptical orbits connected by a single thread. The freshly overhauled propeller was again removed and sent to the prop shop, where it was discovered that the static balance portion of the overhaul was not accomplished. At least that’s what was reported. The severely out-of-balance prop was creating an elliptical wobble that tried to tear the engine loose. All propeller manufacturers establish balance criteria for their props. 

Across the board, regardless of make and model, the propeller is considered in balance when it can rest evenly on a knife-edge beam without dipping a blade. The tolerances for static balance are plus or minus one weight. The gram equivalent will vary depending on the propeller, and the weight slugs can vary in size, but getting it close is the main idea.

Aside from outright balance, propellers are checked for blade track and angle. All props will have slightly different specifications. Your local prop shop should be consulted for the particulars on your aircraft. Most propellers can be positioned on the crankshaft in one of two ways. Typically, the number-one blade on the prop is placed at the top center mark on the crankshaft flange. Three-blade propellers are often installed with the number-one blade up or down, depending on balance requirements and the aesthetics of having both props positioned the same way on shutdown. Regardless of the make and model, current wisdom dictates the use of a dynamic propeller-balancing machine to eliminate prop vibration. This may be the very thing needed for the airplane that rattles through the air, but it should be the last thing accomplished in the vibration troubleshooting process.

It does no good to dynamically balance the propeller if you have a partially plugged injector or if your exhaust stack is hitting the bottom cowling. There are also some combinations of propeller and engine that don’t seem to want to get along. Surprisingly, a lot of the problems we hear about from shops seem to involve three-blade aftermarket props such as on an older C-310 or Mooney, to name two examples. Both of these aircraft have had problems with conversions, even when factory-new props are installed.

Vibes in the Engine Bay

An avalanche of marketing and media hype has been written on the topic of engine balancing, with many fine experts touting the use of electronic spin-balancing equipment from the automotive industry. Along with this, we have seen the redefining of balance weight margins and a rush to equalize cylinder volumes, all in an effort to get the lowly piston engine to run smoothly. The endeavor has been rewarding in many respects, but the process has been oversold. Industry pressures exerted by the overhaulers in this business have forced a balancing war of sorts, balancing pistons and matching rods. It makes sense, but too much emphasis can be placed on engine balancing when it can all be rendered null and void by peripheral complications.

Engines inherently vibrate during operation because the rate of pressure rise in the combustion chamber is fairly rapid. Combustion chamber design and location of the ignition source will greatly determine the speed with which the pressure rises. For the most part, it isn’t much of an issue. It is why all engines are subject to some amount of vibration. When an engine develops an abnormal vibration, it’s usually the result of something external to the rotating masses within the crankcase. A plugged injector, a failing spark plug, low compression on one or more cylinders, and magneto failure in the secondary circuit can all create vibration problems.

Aside from subsystem deficiencies, externally mounted components can wreak havoc on your smooth-running engine as well. Baffling that rubs inside the cowling, exhaust stacks that hit the lower cowl openings, crossover intake pipes that interfere with nose bowl structures, and loose accessory mountings will produce a variety of noises and shudders. Alternators and generators can also produce high-frequency vibrations, e.g., poorly balanced rotors or failing bearings. Of the many vibration ills in the engine compartment, the most overlooked problem is the “shanked out” engine-mount bolt.

One such problem surfaced for a Cessna P-210 that shuddered and shook for years while the owners paid thousands to isolate and repair the problem. Turns out the left front engine-mount bolt was tight against its threads but not against the leg. The looseness in the pack caused a heavy, dull vibration felt mostly in the climb. The engine rocked against this play and the vibration was transmitted throughout the airframe. A change to the appropriate bolt with the correct spacers eliminated the vibration. It is important to pay attention to bolt length as well as the assembly of the shock mount itself. Hard, sagging shock mounts will not function as vibration isolators. They merely transmit the vibration.

Check all hard lines and tubes for security and attachment, and pay particular attention to heat exchanger shrouds and exhaust pipe heat shields. Internal engine components can also cause engine vibration—especially when the engine has had to endure ham-fisted pilots. For cripes sake—feed the power and prop in gently!

Counterweight Issues

Rapid counterweight pin and bushing wear can occur when the throttle is jerked around with reckless abandon. It can also happen to any dampened engine when the throttle is reduced in a descent to the point that the engine is being driven by the forward motion of the aircraft. This is a situation in which the engine is “free-wheeling” and under no load. Crankshaft counterweights (or dampers) are heavy chunks of steel loosely attached to the crankshaft. They are designed to absorb vibration and shock loads from the expanding combustion gas and resultant push on the piston. While they greatly reduce the stress applied to the crank, they also protect the propeller blade tips from excessive and destructive vibration.

When an engine is producing a vibration caused by some deterioration in the damper assembly or has been assembled with rotating component parts that have not been match-weighed, the vibration will increase with engine rpm. For engines that experience an increased vibration at low rpm with high manifold pressures, the problem is usually associated with some imbalance in the combustion process. This could be a problem with low compression in one cylinder, broken piston rings, or an unusually slow or unusually fast burn rate in the combustion chamber.

Dynamics-of-Flight Vibration

Airframe vibration can be very difficult to pin down since the smallest of vibration sources can be amplified by the “drum effect” of wing and tailcone recesses. Of concern are cowl flaps, landing gear doors, wing and tail fairings, and doors and windows. Generally speaking, an increase in speed will bring about an increase in the pitch and frequency of the vibration—but not always. Sometimes, deck angle will impact an ill-fitting nosegear door in the climb, causing a buffet in the nosegear wheel well.

Piper main gear doors are particularly problematic because of the poor fit caused by warping of the fiberglass door material. Adjusting the rod end to pull the door closer only aggravates the problem by cinching up the one side while the other sags with the strain. Cowl flaps are a common vibration source because of the wear occurring in the hinge-pin area. When the flap is pushed open, the air load holds the door in one position, regardless of the amount of hinge-pin wear. But when the cowl flap door is closed, the control cable pulls upward on the rear of the door, allowing the forward hinge and hinge pin to “float.” This allows the hinge pin and cowl flap to vibrate, causing even more pin wear. Replacing cowl flap door hardware when wear is detected will virtually eliminate the problem. Also, make sure the doors are rigged for proper position and that they clear any nearby vent and drain lines.

Poorly secured fairings and loose weather-strip sections can seem fine on the ground but pull open in flight. Make sure all fairings, especially long runs, are well secured and any gaps are sealed with some type of foam or rubber self-adhesive strips. Cabin doors and windows are notorious for causing wind noise and water leakage, but the burble created by a poor fit between the door and the frame can cause a “drumming” against the fuselage and vibration in the thin windows aft of the door. Keeping door and window seals in good shape and rigging the door hold latches will help—to a degree.

Occasionally, no amount of rigging will get the door to sit flush in its frame. Years of flexing combined with the warping that occurs every time the wind grabs an open door makes cabin doors and windows difficult to fit. Wood blocks placed between the door and frame will allow for strategic manipulation. Use care, though. Leaning heavily on a door with the intention of “bending it back” can result in cracking the window or weakening of the door structure. Make several small changes rather than one big one.

Haphazardly installed antennas are another source for strange sounds. They must be placed with both antenna function and proper installation in mind. Use the procedures outlined in the antenna installation guide or AC 43.13-2B. We’ve seen a number of installations done without the proper skin doublers.

Fluttering Controls

Vibration created by worn control surface hinge points and out-of-balance control surfaces can be subtle but increasingly destructive. Balancing controls upon repair or painting of a surface is a requirement. Even a slight unbalance of a control can cause a high-frequency oscillation that is dampened by the control cables but can be felt in the control wheel. This condition caused the aft fuselage of a Cessna Cardinal to literally “sing” with the humming of a tight stabilizer cable. Many manufacturers are strict with trim tab free play. The concern is for control surface flutter, which can lead to hinge-point failure and control loss.

The rattling of several loose tabs can be unnerving. While difficult to feel, a slight increase in control wheel or rudder pedal “buzz” can be found by increasing the speed toward the red zone. Don’t wait to make repairs. Vibration caused by loose trim tabs is not only annoying, it’s dangerous. Replace all hinge points, rod ends, bushings, and bolts, as needed.

Sound-Damping Vibration

Even when all the necessary precautions have been taken, fuselage vibration and noise during flight can be substantial. Many years ago, Cessna provided a Service Kit that allowed for the installation of 1/4-inch-thick, sticky-back foam sheets on flat sheet metal surfaces between formers and bulkheads. Made of dense foam covered by a layer of thin foil sheeting, the sound-deadening  material was cut to fit and glued down on forward and aft bulkheads, belly pans, and aft fuselage skins. The material worked well to keep the aluminum skins from vibrating due to air loads and exhaust burble. While the Cessna kits are no longer available, foam sheets can be purchased from aviation sources for approved material. Fire rating is the issue to be concerned with.

Aircraft Spruce has approved foam available in multiple thicknesses up to 1 inch thick by 48 inches wide. They also have 6-inch-wide tape with a foil back for homebuilts. Other aviation sources will have foam as well. If you elect to use stuff sourced from your local hardware store, understand that installation of commercially available sheeting is not approved and, as such, would require a field approval by the FAA. That may prove difficult, but if you elect to install the soundproofing anyway, make sure to choose a material that won’t attract moisture and won’t burn.

Take a small sample and expose the foil side to a propane torch. If it flames up aggressively, pick a different type of sheeting. If it smokes a little but only scorches the foil and burns the foam, then it should be OK for use. Be sure to leave fuselage belly drain holes and air vent cutouts free and clear during the installation.

The most effective placement of sound-deadening panels is on the aft cabin bulkhead and fuselage tail section. Glued to wide expanses of aluminum skin, the foam inserts will absorb virtually all the vibration and noise caused by the “drum effect.” If there is any secret to success, it’s working in a planned, methodical fashion. Look for the easy things first, but keep in mind the effects of air loads on airframe components. Why make the process any harder than needed?

We’ll look at propeller balancing in an upcoming video field report in The Smart Aviator.