Prop dynamic balance report

From day one I have been moving fuel injectors and ultimately replacing a fuel spider in an effort to run LOP and smooth out my engine. While I have come very close to achieving my goals I never really noticed much improvement in vibration so I decided to dynamically balance the prop.
I don't have many contacts in the area but was able to borrow a Dynavibe from a Socal member in San Diego (Thanks Bill!). I had purchased an MT 3-blade prop with a rebuilt hub and new blades so I figured the system would be well balanced and perhaps I was wasting time.

I am now a true believer. All my work balancing the fuel system was nothing compared to the vibration reduction I was able to achieve.

Keep in mind I had never watched someone balance a prop but it was a breeze. In my case the initial run showed 0.25 IPS. Second run I was able to cut that in half and with a little math, down to 0.03 IPS on the third run!

What a difference. Even if you have a new prop I would highly recommend this. Two hours of work, three runs and I'm flying a different plane!

That's a huge difference. Evidently MT doesn't balance their props before shipping ? For what they cost, I'd say that's a failure on their part. I'm sure 0.03 IPS feels like a turbine after 0.25 IPS. That's a lot. I always have mine dynamically balanced after any major work on the engine. It makes everything better (avionics component life, engine accessories, etc.). It's good you sorted that out. Happy flying.

Anyone know of a balancer...

In North or South Carolina? Have heard great things about results and think I may be missing out on turbine smooth.
Thanks,
Patrick

BlackhawkSP said:

That's a huge difference. Evidently MT doesn't balance their props before shipping ? For what they cost, I'd say that's a failure on their part. I'm sure 0.03 IPS feels like a turbine after 0.25 IPS. That's a lot. I always have mine dynamically balanced after any major work on the engine. It makes everything better (avionics component life, engine accessories, etc.). It's good you sorted that out. Happy flying.

Click to expand...

Larry,
A static balance on a prop is what is typical from any factory prop. Not unlike the old tire bubble in the window static balances.
This has nothing to do with an on plane dynamic balance which takes into consideration the entire "system" to include prop, engine, isomounts and airframe vibration with the plane running.

Kahuna:
I guess that makes sense. I assumed that a prop would show up new with a spin balance like a new tire/rim combo from the speed shop. I guess that if the prop was static balanced and was a bit off, and it was installed on the engine at the clocked position for the worst outcome, that it could be off by quite a bit. I would hope that the engine would not be out that much by itself. The great thing about the dynamic balance is that it takes the whole "shebang" in to account.

Dynamic prop balancing can only correct a mass imbalance of the rotating propeller assembly, this would include the flywheel, prop, spinner, spinner bulkhead.

Dynamic balancing cannot correct or compensate for internal engine imbalance (may be caused by pistons/rods/crank), engine mounts or anything else.

No matter how well the prop was balanced from the factory, when you add the spinner assy all bets are off.

I use a vibration spectrum analysis plot during dynamic balancing to isolate vibration frequencies, this can be used to help pinpoint specific sources of vibration.

For more info on this topic please visit my webpage which discusses these things and much more:

http://www.expaircraft.com/PropBalance.htm

In specs

Walt:
Good website. From your figures it was good to learn that my prop was not significantly out of the norm before I began and in very good shape after.
I understand that dynamic balancing only covers the rotating parts on the front of the crank but wouldn't your vibration analysis cover the complete airframe/system? What type of problems external to the engine do you find? Engine mount or isolators etc?

The spectrum analysis (SA) shows the level of vibration at different frequencies or speeds, in the case where the vibration pickup is attached to the engine (like during prop balancing) it will only show engine vibrations, and yes some of those vibrations will get transferred to the airframe via the mounts.

The airframe can vibrate for many reasons but has to be excited by an outside force like the engine or the air passing over it. Measuring airframe vibrations is little different than the engine and the sensor must be attached to the airframe to do this. On occasion I have done flight tests measuring airframe vibrations, but this is a very difficult area to pinpoint problems with as there is no "standard vibration signature" to compare anything to and every airframe will be different. Almost any airframe component can vibrate (usually doors, fairings, gear, etc) but there is no specific frequency associated with airframe vibrations so finding problems can be hit or miss.