Why Balance Your Prop?

Properly balanced engine installations offer a number of advantages over unbalanced equipment.  The most significant advantage is in the longevity of components associated with the power plant.  In many cases high vibration levels have led to premature failure of critical engine components. 

The most widely known damage from excessive vibration includes:

  • Propeller
  • Engine Crank
  • Engine Accessories
  • Engine mounts
  • Firewall
  • Instruments
  • Passengers

Let’s look at each one of these.

Propeller

The propeller is probably the single greatest influence on airframe vibration.  The propeller is a relatively large mass distributed across a large distance.  The diameter of the propeller distributes the weight of the propeller and the thrust that the propeller generates. Because of the large diameter (relative to other rotating components such as the crank), any mass or aerodynamic irregularities along the propeller cause a rotating imbalance that shakes the entire engine and airframe once per revolution.

These imbalances cause the propeller, spinner, and crank attach bolts a great deal of cyclical stress, which over time can lead to crack generating and propagation, leading to component failure.

Engine Crank

The next component in the vibration chain is the engine crank.  Vibration from the propeller, pistons, rods, and the crank itself, are all transferred through the crank to the engine casing.  The engine crank transfers a considerable amount of stress from individual pistons and rods to the propeller and any propeller vibrations or unbalanced piston/rod combinations to the case of the engine.  It is for this reason that engine cranks require special servicing procedures such as dye penetrate, magna fluxing, etc, to ensure that no crack has started due to excessive vibration.  We have all heard stories about cranks breaking inflight, allowing the propeller to fly off, leaving the aircraft in a tricky position.

Engine Accessories

Engine accessories have potentially the worst position in the vibration chain.  Since the accessories are hard mounted to the engine case, they must endure the full spectrum of engine vibrations.  Between the propeller, crank, and case, there is little material, besides a very think layer of oil, to reduce the peak amplitudes of vibration.  Therefore, the components mounted on the engine, such as alternator, magnetos, vacuum pump, must all endure the engine’s vibration. That vibration leads to failed magnetos, disentigrating vacuum pumps, failed alternator bearings, etc.

Engine Mounts

Engine mounts are the next piece in the vibration chain. Typically the engine is mounted on vibration isolators in an effort to reduct the amount of vibration that is transferred to the rest of the aircraft.  Unfortunately, vibration isolators work best at very high frequencies, removing a majority of the piston spikes, but unable to remove the lower frequency once-per revolution dynamics.  Poorly balanced engine installations can usually be expected to destroy the vibration isolators and lead to engine mount weld cracking, causing expensive repair bills.

Firewall

The Firewall then absorbs any remaining vibration transferred into the engine mount.  The firewall will distribute the vibration into the remainder of the airframe to be absorbed and disappated at cyclical stress, causing noise and metal fatigue.  In most tractor style aircraft the firewall is also closely coupled to the instrument panel, further transferring energy into expensive panel mounted equipment.

Instruments

The instrument panel is an excellent indicator of residual vibration from the engine.  On a Cessna 172 that was recently inspected, the vibration from the propeller imbalance can be seen in the artificial horizon, the fluid compass tends to give errant readings, and needles jump, making it difficult to get accurate information.  Following dynamic balancing, the evidence can literally be seen on the instrument panel, a change in the way the gauges behave is immediate proof of the advantages of dynamic balancing.  For gyro equipment, this particularly troublesome, since the gyro consists of a mass rotating at high speed on bearings, the vibration from external sources can quickly lead to premature failure, resulting in costly repairs at best, partial panel IFR at worst.

Passengers

Passenger comfort is greatly improved with a well balanced aircraft power plant.  The smoothness of balanced aircraft could lead to more flight time!

Questions? Contact us at sales@rpxtech.com