What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Prop vibration analysis and balancing

Vansconvert

Well Known Member
I'm curious if anybody has had their prop analyzed for vibration and subsequently balanced. If so, was there a noticeable difference? Who does that and what does something like that cost?
 
I paid $250. I think it helped but you have to realize that it's only balanced at one RPM - mine was 2300, IIRC. At other rpms, you still get some vibration.
 
I'm curious if anybody has had their prop analyzed for vibration and subsequently balanced. If so, was there a noticeable difference? Who does that and what does something like that cost?

Practically every aircraft can benefit from Dynamic balancing, the procedure is recommended by practically every prop and engine manufacturer. Plenty of posts on the process if you take the time to do some searching of the archives. I also have info posted on my website.
 
I dynamically balanced my prop with a Dynavibe. It reduced vibration from about 2.5 IPS to 0.15 IPS, using a bolt on the starter ring flywheel. I haven’t yet tried to refine that by using other weights. It did produce a noticeable reduction of vibration. It’s not perfect, but better. A particular hole on the flywheel seemed to work best, as indicated by the Dynavibe phase readout. It should be noted that this can work for weight imbalance, but cannot entirely solve other imbalances such as aerodynamic imbalance of the prop. An aerodynamic imbalance causes longitudinal vibration as well as lateral.
 
First saw this tech applied to GA in the 1980s, when it was current- value twice as expensive as today. Was good value then, better value now. Focuses on a single rpm, but will help across entire band. Once you see/ feel the diff, you won't go back; less vibration means less wear/ deterioration throughout the airframe. [And less fatigue of oldpilots.]
Had an IO-320/ Hartzell balance from orig 0.3 to less than 0.02 g/inch -- butter smooth, and airplane happy-happy! Cost typically $250-350; search these forums for vendors near you.
 
As I previously mentioned I used a Dynavibe to balance my prop. One thing that was confusing at first was that the phase angle changed with rpm. After thinking about it, that is what is likely to happen. The engine, it?s mounts, and prop is a mechanical system consisting of mass (the engine), a spring force ( rubber mounts) damping (rubber mounts also), and driving force ( prop imbalance). That is a classic driven spring-mass-damped system, which is mathematically well known. This system has a resonant frequency. Resonance occurs at a phase angle of 0. As we change rpm, and go off resonance, the phase angle will change, either leading or lagging, depending on whether + or - rpm change. We don?t necessarily operate at resonance anywhere in the normal rpm range. Hopefully not! I never saw a phase angle near 0. This is a phenomenon studied in engineering and the math that goes along with it. Things are probably more complex than this since there are multiple driving forces (engine pulses, aerodynamic effects). But at least the varying phase has an explanation.
 
I've had a few props dynamically balanced over the last few years. Here's some thoughts

- If you have an iphone there's a number of free "vibration" apps that will measure vibration levels for you. This may be slightly arbitary, but you can get some insights as to different RPM levels or compare objectively between aircraft.

- A bit obvious, If you choose to get the balancing done the improvement (which is almost inevitable if it's not been done before) will depend on how much out of balance you already are

- Before you get the work done I'd recommend marking your flywheel, spacer, prop, backplate and spinner to make sure that they always go back in the same orientation as any subsequent changes will negate your efforts

- Following on from the above, do your spinner holes allow any variation in the exact fitment of the spinner ?. Even a tiny amount of slop will throw things completely out after say a prop bolt re torque.

- Check the "trim" of your prop pre balance (I use a tiny bit of coloured tape, red on 1 blade, blue on the other - ground run then engine and view side on, if the pitch trim is perfect you see green, otherwise you see red & blue bands - the order of which tells you the relative pitch difference)

- When you get to actually doing the balancing be ready for multiple engine starts with very little battery re charge time ... my B&C lightweight Pitts set up could not keep up and a flat battery slows things right down.

Overall, I've always gained from the balance work (particularly when mounting video cameras) - it's generally well worth the effort.
 
Optimum balance at only one RPM ?

I frequently read here about props balanced and are "turbine smooth" at cruise RPM. I assume this is with CS props ? My FP won't come close to cruise RPM on the ground.

I have a Dynavibe and have been considering fabbing an ugly top cowl for inflight readings, or a hole in the current cowl with a cover like the oil door :eek: Has anyone addressed this issue ?
 
Any recommendations on the iPhone vib measuring apps?

George

I use one by diffraction Limited - it's called (imaginatively) "Vibration"

If you do search the app store I'd do it when no ones watching ..... not all "vibration" apps are ones you'd care to be seen researching !
 
Back
Top