Prop Balancing Question

[AaronWhitaker]

Daryl and I just got the Dyna Vibe that has been making the rounds this summer. We went to the hanger and set it up but I don't think we are doing it right. We are getting readings of 3.7 IPS on the Dyna Vibe. The instructional chart you download says that 1.25 IPS is extreme.

We have just over 300 hours on the engine and it has run great. Haven't really noticed any extreme vibrations. We have the 3 bladed Ivo prop. There are indicator tapes across the joints between the three blades. The tapes have never shown any indications of tearing. I would think that if the prop was extremely out of balance they would have torn long before now.

The attached picture shows how we set it up, which is just like the picture we got from Dorsal.

What I am wondering is if we have read the tape charts right. The charts give a tape width for a certain RPM. Is that engine RPM or prop RPM? The rotax we have is geared down about 2.6 to 1 if I remember right.

Any suggestions?

Thanks!
Aaron Whitaker
N730WH

[n85ae]

I had the same thing with mine, it suggest 2.1 initially, and mine never had
any serious vibration. When I added weight and got the IPS reduced I do notice
a big improvement. My guess - Is that that sensor is a little bit whacky, and
the signal output is not linear.

It still works though, and well enough that I was glad to use it.

Regards,
Jeff

[n85ae]

Actually your posting the high IPS, is good, as I have been a bit worried about
it. However mine static balanced fine in the past, and I could not understand
how the DynaVibe could be saying I had a serious problem. I did get mine down
to .07 and I notice a big difference in flight. Pretty sure it's the sensor output
is the culprit.

Regards,
Jeff

[AaronWhitaker]

I think that maybe we should rebalance the carbs just to eliminate that possibility. The Dynavibe website says not to try to balance the prop with a rough engine. Still, we would have to add an awful lot of weight to get it down as far as the Dynavibe says to.

[n85ae]

3.7 according to what I can find from googling ought to pop the fillings out
of your teeth ... Maybe it is that bad, so probably worth investigating.

Jeff

[jamesmil]

hi guys, would like to chime in on this as I used the dynavibe about 6 mo. ago on 4 different airplanes and it worked great.as i look at the picture if I am looking at it right it appears that the mount for the prop pick up is an L shape with a along arm forward with the pick up on it. is this arm mounted solid with no movement ? if not it would tend to act like a tuning fork giving very large ips readings. the first thing I would do is remount the pick up securely and take another reading. hope this helps.

[AaronWhitaker]

That is the conclusion we had come to for the accelerometer. I have made an new mount for it. Apparently the stiffness of the optical pickup makes a difference as well? Also, what about the length of the tape? We are using the 1.9" because we are about 6" from the hub and assumed that the tables for selecting tape width were based off engine rpm which is 5,000 rpm. However, the Rotax is geared down 2.43 to 1 which means the prop is only turning about 2,000 rpm.

[AaronWhitaker]

Okay, we went back and tried it again today. We made stiffer mounts for the accelerometer and the optical pick-up. We adjusted the tape width to match the prop rpm, not the engine rpm. We made sure the leads were well away from the spark plug wires. We even replaced the batteries in the Dyna Vibe. The read out was rock steady, but no difference. Still reading 3.5 to 3.7 ips. That kind of vibration should be about shaking the plane apart right? At 4,000 rpm there is no visible motion of the engine. Is it possible the Dyna Vibe is out of calibration?

[Dorsal]

Don't know about how sensitive the unit is to loosing calibration. I suggest you try balancing as n85ae did and see where that gets you.

[n85ae]

I saw several at Oshkosh with similar amounts of hardware added for balance
as I did (i.e. AN-3 bolts with washers) inside the backplate. Which sort of
reassured me that I wasn't nuts in adding them. (well at least not alone)
I've been pretty happy with the results.

Likely refinish my prop next season, and look forward to using the DynaVibe
again at that time if it goes around again.

I think the sensor output has some gain, but is probably accurate as you
get lower in vibration. Even if the IPS value is incorrect, it doesn't really
matter as you just tweak for the lowest possible value. Which clearly is a
lot better than not using it.

On the other hand, the tool did find a cracked prop prior to my using it.

Great Tool.