Dynamic prop balancing - benefits!
I just re-installed my Catto 3-blade prop after having the pitch increased. I thought this would be a good time to do a dynamic balance of the engine/prop combination so I set up the appointment for this morning. I've had a number of little items I've worked on for the past few months while my prop was in the shop, so I was busy this week getting everything in the engine area back up to snuff and ready for engine start. I finally got the prop back on, tracking straight and the spinner re-installed just as 8am rolled around and John showed up for the balancing. He installed his equipment on the top of the engine; an accelerometer to measure the movement of the engine/prop mass and an optical sensor to measure the RPM and clock the blades. A small computer uses the information from the two sensors to determine imbalance and recommend the location and amount of mass to add to offset the imbalance.
After the sensors were installed I hopped in and started the engine. Blonde Streak hasn't had her engine run hard since January so I intended to baby her a bit. She ran real rough - more so than ever. After about a minute of this John told me to cut the engine and said he spotted some fuel leaking. We traced the leak to the new fuel flow divider I had installed - apparently I had only hand tightened the fuel lines at the flow divider and it was leaking from all four lines. We quickly solved that problem by tightening the nuts and then did a re-start. Much better! I let her run for about 5 minutes while the oil temps came up and then gradually increased the throttle until she was at max. I noted 2150 RPM and a fairly good vibration which I have been accustomed to since the beginning. I nodded to John and he took the readings on the machine and a few seconds later he gave me the signal to shut her down.
The first reading was .69 inches per second - a little higher than John said he usually sees (about .4) but within the limits of balancing. John said the goal would be to get less than .2 and a reading over 1.0 is an indication of issues that needs more investigation before balancing is attempted. We were right in the middle of the range. The computer told him where to add the mass so he installed 3 large area washers plus an AN bolt and nut to the starter ring. I hopped back in a wen through the process again. A realized a very noticeable improvement in the vibration and the static RPM rose as well - this time to 2215! I hadn't expected that, but John said it's very common to see an increase in RPM after balancing. It makes sense - a smooth running prop and engine is going to take less effort to turn so the extra power is going to show up in RPM gain. Great!
After the second run the reading came to .29 - still higher than the target, so we added two more washers to the same location for a total mass of 51 grams. I hopped back in, started her up and ran the engine up to 2270 RPM this time. Another increase and much less vibration again. After shutting down John said the reading was now .09 and good enough to stop. We removed all the sensors, wrote up the paperwork and made a logbook entry and called it a success.
I'm thrilled with the results! Much less vibration and an increase of over 120 static RPM. The RPM increase was a pleasant surprise and one I'm grateful for. Since I had the prop re-pitched I knew I would see a drop in static RPM which would likely result in longer takeoff roll. The price you pay for higher top in speed with a fixed pitch prop. Well, the old prop used to run 2350 static RPM so I lost a net of 80 RPM so I'm not complaining at all. I know the top end performance will be better with the new prop and the smoother ride will be more enjoyable. I guess I'll have to go fly now to see if I'm right!
Bottom line - if you haven't dynamically balanced your prop I would highly encourage you to do it right away. The benefits of the smooth ride, less wear and tear on the airframe and higher RPM's will translate into a much better performing airplane.
Last edited by Brian DeFord : 04-17-2006 at 01:57 PM.