Dynamic Propeller Balancing Dr. Andy Elliott, CFI, ATP Servicios Aereos, LLC EAA Flight Advisor 10/22/2015

Do your instruments look like this in flight?

Overview What is dynamic propeller balancing? How is it done? Why/If you should have it done? When should you have it done?

What is Dynamic Propeller Balancing? “Dynamic Propeller Balancing is a process where an electronic balancer (computer) is used to measure the vibration (magnitude & phase) produced by a running engine. Balance weights are then added to the propeller/ engine assembly to reduce the synchronous (1/rev) vibration due to mass imbalance to the lowest level practical.”

Magnitude & Phase? Magnitude refers to the amount of shaking caused by the mass imbalance. Units are inches per second, root mean squared (ips RMS), a kind of averaged shake velocity. Measured with an accelerometer. Phase refers to the time between the peak acceleration caused by the imbalance and the passing of the reference propeller blade. Units are degrees of propeller rotation. Measured with an optical tachometer.

Example Magnitude=2 Phase=15°

How is it done? Top cowling removed. Any existing weights removed. Step #1 – Setup Top cowling removed. Any existing weights removed. Accelerometer attached to engine near front bearing and pointed at it radially. Reference blade marked with reflective tape; optical pickup mounted on cowling or engine. Sensor wires tied down and connected to balance computer. Set-up computer.

How is it done? Step #2 - Baseline Run (typical GA engine) Check brakes (!); close canopy; center controls, start engine; stabilize at ~1000 rpm. Run engine up to ~2100 rpm; stabilize. More important to get ~same rpm each run than exactly some number. You get a tach check for free! Take vibration measurement. Confirm. Reduce rpm; cut off engine.

How is it done? Step #3 - Reference Run Add estimated weight at computed position on flywheel or spinner screws. For previously done engine-prop- airframe combo, computer usually makes “OK” guess. For each run, results help the computer “learn” how weight affects the vibration magnitude and phase. Repeat engine run and data collection.

How is it done? Steps #4, 5, 6, … - Trim Runs Computer determines where and how much weight to add to minimze vibration. Weights may need to be split between mounting locations. Repeat engine run and data collection. Cycle to allow computer to refine the engine/prop combination and fine tune weight and location computation.

How is it done? Last Step - Final Run For A/C without flywheel, final weights usually attached to spinner backplate. But removing/replacing spinner may cause changes.  Remove instrumentation, torque everything up, replace cowling and fly away smoooooth and happy! 

How is it done? Worst CJ I’ve ever done! Actually gave 2.2 IPS RMS, and I had to reduce RPM for the 1st 3 runs! Total installed weight was 220+ gms! Typical O-320 or 360 will use  35 gms. Final reading <0.04.

Why should you have it done? Say you just bought 4 brand new premium tires for your Escalade @$400 each. Would you mount them and drive away without having them balanced? Same applies for your plane. All the drive system components may be manufactured under close tolerances, and may even be statically balanced individually, but the whole assembly still needs to be balanced. Operating speeds for aircraft engines (35-45 Hz) are outside the range that human beings sense well. You cannot reasonably differentiate levels of vibration at those frequencies.

Why should you have it done? Average value for unbalanced aircraft > 0.40 (ips RMS). Typical value after balancing = 0.04 ips! 10X reduction in fatigue load to airframe and engine = happy airplane! 20 db reduction in perceived vibration level in operation = happy co-pilot! 1+ knot speed increase at WOT.

When should you have it done? Never been done. Usually, this amazes the owner (and his wife!). New engine or new prop After any work on engine which effects moving parts. After any work on prop. Or once per year at annual, or every 100-200 hours.

Final Notes Inability to get a good balance is indicative of other problems with the engine/prop. Will not eliminate aerodynamically-induced or 4-cylinder torque-variation vibrations. But still can make a 4 feel much like a 6, and make a 6 feel like a turbine! Questions??

Special Pricing! EAA-only prices, if you come to Falcon Non-turbo’ed Lycomings, clones, other aircraft with prop extensions and easily accessible flywheel - $125 Non-turbo’ed Continentals, other aircraft where you need to drill and attach weights to the spinner back plate - $150 Turbos, radials, auto conversions, etc, – quoted on request. Guarantee: If I can’t get it to  0.07 ips - $0! And, if your plane measures  0.10 ips at start and you choose not to proceed - $50.