What’s Shakin?

By Matthew Dock, RPX Technologies

The worst thing about complacency is that you don’t know it is happening. We are conditioned to ignore what is normal and only worry about things that change. As a pilot that has flown a 1963 Cessna 150 from the Bahamas on the east coast to the Channel Islands on the west coast, I know that I am relaxed as long as the sound and feel of the engine doesn’t change. However, any variation in RPM or vibration will immediately cause me to jump.

Unfortunately we are tuned for change, not the constant drone of engine and propeller; even though most damage is related to fatigue, the constant vibration of people and metal. Pilots and passengers are very susceptible to fatigue from aircraft vibration. It happens frequently that after measuring excessive vibration on an engine, I ask the pilot about their feet becoming numb or if they have eye fatigue due to monitoring a shaking instrument panel.

The military has done numerous studies on the impact of vibration on the human body. The whole body vibration frequency is approximately 4-8Hz, or 240-480 RPM. This is one reason that helicopter pilots often complain about back problems; the whole-body vibration matches closely with the main-rotor blade rotation frequency. For small piston engine aircraft operating at 2400 RPM, this causes a 40Hz vibration. That 40Hz vibration causes fatigue in the lighter portions of the body; the hands, feet, and eyes. The vibration often causes numbness or tingling, usually in the feet due to the proximity to the firewall.

While pilot and passenger vibration is tiring, airframe vibration can be dangerous. Excessive engine vibration can lead to a broad series of failures that often appear unrelated. Exhaust cracks are one case in point. An engine vibration near the FAA limit of 1.2 Inches per second (AC 20-37E) can easily generate displacements of 0.060 per inch. That doesn’t seem like much, but imagine something like an exhaust pipe that is held at one end and then shaken at 0.060″ at 40Hz for hundreds of hours. All the weld joints and sharp bends will experience high stresses as the unsupported weight of the exhaust works against the vibration. This high stress causes fatigue cracks that lead to failure of the exhaust system, spilling exhaust air into the cowling and possibly the passenger compartment. This is
one reason that high-end exhaust manufacturers require dynamic balancing for their warranty. At best you need a new exhaust, at worst you experience hypoxia due to the fresh air inlet being positioned directly behind the cracked exhaust pipe; like I did, in my Cessna 150.

Vibration Costs Money

A cracked spinner backplate

That cracked muffler on a simple Cessna 150 can cost hundreds of dollars to repair or replace, plus the cost of disassembly and assembly. Other vibration induced failures can be equally expensive. The panels of older aircraft contain numerous electrical- and vacuum-operated gyros that operate on small bearings. Bearing life is reduced by the third power of acceleration. Meaning that for every l0X increase in vibration, there is a 1000X reduction in bearing life. Just the directional gyro or artificial horizon in a Cessna 150 can cost thousands if purchased new. In addition to exhaust and panel issues, there are engine mount and firewall cracks, leading to costly repairs. The spinner and spinner back-plate take a lot of abuse as well. All these components add up to significant repair bills over the life of the aircraft.

The life of all these components can be improved by vibration reduction.

Top 10 Vibration Causes
Here is a list of the top 10 things to check when experiencing vibration:

  1. Spinner Imbalance
    Run the engine with the spinner off and see if the vibration changes. A lot of spinners are out of balance and will cause damage to the spinner back-plate.
  2. Blade Track
    Pull the sparkplugs so the engine will freely rotate. Use a fixed object on the floor to check that all the blades are within 1/16 of an inch.
  3. Blade Pitch
    There are inexpensive digital protractors that will allow you to measure the pitch of the blade; usually best done at multiple locations along the blade.
  4. Compression
    Check the differential cylinder compressions for any weak or leaking cylinders.
  5. Intake Leaks
    Check for intake leaks that may cause one cylinder to run lean and misfire.
  6. Ignition System
    An old magneto or fouled plugs may provide a weak spark, causing poor ignition.
  7. Engine Mounts
    Damaged or compressed motor mounts can transmit additional vibration. Also verity the installation of the motor mounts. Different engine/airframe combinations may require a very specific installation.
  8. Wheel Pants
    May sound odd, but sometimes the wheel pants or other loose accessories can start to vibrate in-flight, causing airframe vibration.
  9. Engine / Cowling Contact
    The engine, exhaust, intake, and all other components mounted to the engine should be free and clear of contact on the cowling. Look for rub marks on the inside of the cowling.
  10. Check the Dynamic Balance
    Even a statically balanced propeller can generate significant vibration, just from the installation variations. Dynamic balancing should be done often to insure smooth flying.

How to Reduce Vibration

Dynamic balancing kit used to reduce vibration

The best way to reduce airframe vibration is to have the airplane dynamically balanced. Good dynamic balancing will have multiple steps provide the owner/operator with a smooth aircraft. Any dynamic
balancing service will start with a good pre-balance inspection that addresses the items previously listed. After inspection, the mechanic will install an accelerometer and a phototach to measure the vibration and the position of the propeller. The mechanic will run the engine three or more times to add weight and reduce the vibration. Dynamic balancing is typically a simple, straightforward process. If difficulties are encountered it can be due to loose equipment or damaged components, something that should be addressed immediately.

Editor’s note: See the article that follows for how EAA 1541 chapter members can get their aircraft’s prop balanced using the equipment pictured.

FAQ
FAA Advisory Circular 20-37e, Aircraft Propeller Maintenance, is a great resource for information. It discusses dynamic balancing and general maintenance procedures for aircraft.
Who can perform dynamic balancing?
For certified aircraft, when following FAA approved procedures, dynamic balancing is performed by an “appropriately rated mechanic”, a power plant mechanic.
Is dynamic balancing a major repair?
Dynamic balancing is a not a major repair when using FAA approved procedures.