Dynamic Prop Balancing

[skippydiesel]

Hi All,I have just fitted another engine in the Sporty and I found this vibration around 3000 rpm when coming back to idle, you don't seem to feel it as you take off and all the other the other rev rages it is smooth, it is just around the 3000 rpm, the old engine never had this vibration that I have now which seems so strange to me, same prop.

Dieter the L2 has checked everything else and now it seems to be this balance issue, so to eliminate this and hopefully fix it who is the best person to get this prop balance done and how much does it cost approx?

 

David

Hi David,

For best results

 

Static balance your prop as best as you possibly can

 

Same goes for pitch adjustment.

 

(You need to be obsessive about the above - I use small increment digital scales for weighing & digital protractor for pitch, double check the protractor with a Warp Drive bubble unit)

 

Then get your dynamic balance done.

 

I used Superair @ Armidale Airport, NSW Ph (02) 67725055 - they are aircraft engineers, servicing all classes (inc turboprops) of smaller aircraft including choppers. Their prop balancing equipment is independently calibrated & certified and they are very thorough when using it. They are not cheap but you know you are getting a professional job - call them for a + GST quote.

 

You will find that the "focus" of the calibration is on cruise rpm. The power setting/prop speed experienced over the longest time (cruise) is where most pilots want the smoothest running. This may mean you still experience vibration at other power settings. In my case the dynamic balance seems to have benefited the whole rpm range.

 

 

[Peter Anson]

For more information on propeller balancing, here's a link to an article by Brian Rebbechi that first appeared in "Airsports"

 

http://sonexaus.wikispaces.com/file/view/Propeller%20Balancing%20-%20Airsport%20Winter%202016.pdf/609193595/Propeller%20Balancing%20-%20Airsport%20Winter%202016.pdf

 

 

[Bruce Tuncks]

Thanks Peter, thats a real good article. I never really understood "inches per second" before or the theory of the trial weight..

 

I had a bad experience with dynamically balancing my jabiru 2.2 when I lost an exhaust valve-seat due to overheating.

 

In the end,it was all worthwhile because it led to instrumenting all the cylinders for cht and egt and evening them up.

 

But next time I'm going to do the prop balance in the air even though it will be more work to set up.

 

 

[skippydiesel]

..........................I had a bad experience with dynamically balancing my jabiru 2.2 when I lost an exhaust valve-seat due to overheating.

In the end,it was all worthwhile because it led to instrumenting all the cylinders for cht and egt and evening them up.

 

But next time I'm going to do the prop balance in the air even though it will be more work to set up.

Hi Bruce,

 

Can't get my head around your suggestion of a dynamic prop balance somehow being involved with a subsequent malfunction of an exhaust valve seat and the need for CHT & EGT.

 

In those immortal words - "Please Explain" ??

 

You are absolutely correct in saying that an attempt to do a dynamic prop balance in the air will be "more work". A correctly done dynamic prop balance at or within the nominated RPM range (usually cruise RPM) will require several readings after each weight adjustment. I presume you will land for each weight adjustment??

 

 

[M61A1]

Hi Bruce,Can't get my head around your suggestion of a dynamic prop balance somehow being involved with a subsequent malfunction of an exhaust valve seat and the need for CHT & EGT.

 

In those immortal words - "Please Explain" ??

Overheating while ground running...

 

 

[skippydiesel]

Overheating while ground running...

WOW!! when I had mine done (912 ULS) it was quite quick. (I am assuming you had prolonged ground runs) They started very soon after I landed, so no need to warm engine. I helped with cowl removal and "tying down" the aircraft. One person fitted the various instruments to the engine/prop, another did a quick check on prop pitch and asked me my preference for best RPM range for balancing (4800 - 5200 engine 1975-2134 prop RPM). I sat in the cockpit and made throttle adjustments on their signals. Each run up was followed by a brief shut down for analysis of readings and weight adjustments. Don't think the whole process took much more than 20-30 minutes. From landing at Armidale to departure - max 45 mins. Take off brought an immediate smile to my face, which only got wider in cruise - turbine smooth.

 

 

[Bruce Tuncks]

I was pretty quick too, but you have to run the engine at cruise rpm ( as you stated ). I was watching the only cht I had ( on no 4) and stopped before it red-lined. The whole story ( with pics of the valve seat ) is on this site.

 

One reason for the no 4 getting so hot was that the EGT went over 800 degrees on that cylinder, more than 100 degrees higher than the rest.

 

I agree with prop balancing, and I also reckon getting the EGH and CHT even between cylinders is a good idea.

 

And yes it will take several flights to get the job done with balancing. I am considering a permanent small "vibration" meter for the panel. If this works, anything which makes the reading less would be a good idea.

 

Experienced pilots who fly in my plane reckon its pretty smooth, but I am capable of imagining things and would find a meter reassuring when flying over tiger country. Well at least it would be reassuring if the vibration level was normal.

 

 

[skippydiesel]

......... I am considering a permanent small "vibration" meter for the panel............" It would be interesting to know what meter/cost you manage to find.

 

".......... I am capable of imagining things and would find a meter .reassuring when flying over tiger country. ......."

I think you probably echo the thoughts of many a pilot, while over the dreaded "tiger country".

 

 

[Peter Anson]

I was pretty quick too, but you have to run the engine at cruise rpm ( as you stated ). I was watching the only cht I had ( on no 4) and stopped before it red-lined. The whole story ( with pics of the valve seat ) is on this site.One reason for the no 4 getting so hot was that the EGT went over 800 degrees on that cylinder, more than 100 degrees higher than the rest.

I agree with prop balancing, and I also reckon getting the EGH and CHT even between cylinders is a good idea.

 

And yes it will take several flights to get the job done with balancing. I am considering a permanent small "vibration" meter for the panel. If this works, anything which makes the reading less would be a good idea.

 

Experienced pilots who fly in my plane reckon its pretty smooth, but I am capable of imagining things and would find a meter reassuring when flying over tiger country. Well at least it would be reassuring if the vibration level was normal.

If you read Brian's article linked to my earlier post you will see that it is not necessary to run the engine at cruise revs, and while it is possible to get a small improvement with in-flight vibration measurements, the gains are small and probably not noticeable in the cabin because the engine mounts damp out the vibrations. I fly a Sonex with a Jabiru 3300, so close-cowled and prone to overheating on the ground, but this presents no problems for doing a balance run on the ground. It also takes just 3 runs of about 30 seconds: one run to get the base data, one run with a trial weight attached and a check run with the final balance weight. We did the engine runs at 1900 rpm, a fairly arbitrary figure that is above engine mount resonance. In truth, my crappy Azusa brakes are barely capable of holding the Sonex at higher RPM.

 

 

[skippydiesel]

Hi Peter,

 

I agree with "... it is not necessary to run the engine at cruise revs...," If the revs you choose are the ones (range) you want the balancing to be most effective in.

 

However most pilots will be looking for a smooth cruise.

 

Static Balancing & accurate prop pitch adjustment (for CS & ground adjustable props) will give benefits throughout the RPM range. This is a prerequisite, if you are to get any benefit from Dynamic balancing.

 

Dynamic Balancing does not claim to remove all anomalous vibration, at all RPM's. It is all about reducing vibration within a chosen RPM range, hence my choice to go with my cruise RPM range. There may be benefits higher & lower (as was the effect in my case) but this is not particularly looked for.

 

I have difficulty with "....takes just 3 runs of about 30 seconds: one run to get the base data, one run with a trial weight attached and a check run with the final balance weight."

 

True this would be the minimum runs required, but I suggest not typical of a well done dynamic balance service.

 

In my case, from suspect memory, a total of five runs were done certainly more than three. After the base data run - 4 more, each refining the last, until an acceptable reading was achieved.

 

The aim to achieve an acceptable reading under .14 IPS (inches per second) and usually to less than .07 IPS. at your chosen engine/prop speed with the MINIMUM of additional weight added.

 

 

[cooperplace]

I started the jab this morning, haven't flown for a couple of months, the prop has been balanced in the meantime, and my first thought was, my god, it's smooth, like a different engine.

 

 

[Peter Anson]

Hi Skip,

 

The reason it is not so important to run the engine at cruise speed during the test run is that the aim is simply to find the vibration amplitude and phase. That vibration will obviously always be at engine speed even though what you feel in the cabin will probably be different at different rpm. In my case, there was a bad vibration in the cabin at 1500 RPM as I pulled the power back. The propeller out-of-balance wasn't worse at 1500 RPM, it just felt worse in the cabin because that is probably the resonant frequency of the engine mounts. The picture below is the vibration spectrum output of the vibration analyser. It plots peak inches/sec against frequency (plotted as RPM) while the engine was running at 1850 RPM. You can see the obvious large peak at 1850 due to prop out-of-balance, as well as a smaller peak at 5550 RPM, (3 x engine speed) due to 3 firing pulses per rev, and a smaller peak at twice engine speed which is possibly due to propeller blade beats.

 

 

Fortunately most of that vibration doesn't get through to the cabin. The picture below shows the vibration spectrum measured by a second transducer attached to the instrument panel. Presumably if we had run the test at 1500 RPM the vibration would have been much worse.

 

 

The following picture shows the vibration spectrum with a "near enough" set of weights fitted. Brian was able to tell me the optimal weights and locations required, but the Sonex is a little unusual in that it doesn't have a conventional spinner and backing plate. The only place to attach weights is under the propeller bolts. I used a combination of aluminium spacers and AN washers to achieve an approximate balance. Even this rough balance made a dramatic improvement. My 1500 RPM shake was completely eliminated.

 

 

Later I made and fitted the accurate weights Brian specified. We got together a couple of months later and recorded the following vibration spectrum at 1825 RPM. Note that the vertical scale is larger than in the earlier graphs. The peak IPS value at engine speed is down to just 11% of the starting value. The other peaks are not worse, just magnified by the graph scale.

 

 

This didn't take multiple runs and gradually zeroing in on a minimum value. The analyser came up with a suitable correcting mass and location and it worked. The only way to use a smaller balancing mass would be to locate it further from the centre, not an option on the Sonex. The balance mass had to be on the propeller bolt pitch circle. In my case, the correct balance mass location was part way between two bolts. Again, Brian was able to tell me exactly how much mass to attach to which bolt.

 

 

[skippydiesel]

Hi Peter - very interesting & informative.

 

I started from a different perspective - I was pretty happy with my static balance and perceived vibration but as a self confessed obsessive, I wanted more/better.

 

Did a bit of research - service providers/ cost / accessibility and availability and ended up wit SuperAir, Armidale ( I just happened to me visiting friends in Moree, NSW, so a small diversion to Armidale, worked in very well.

 

The results were very pleasing but hardly as dramatic as your problem vibration at a certain RPM.

 

 

[Peter Anson]

Hi Skip - I had done what I thought was a careful static balance but still had the bad shake at 1500. It could be that my prop had an out-of-plane couple, the sort of effect you'd get with a badly tracking propeller but wouldn't show up with a static balance. More recently with a new prop, (my old one got very out-of-balance after a ground strike), my static balance proved good enough so that I didn't have to change any weights.

 

 

[skippydiesel]

Hi Peter,

 

My prop is a compost, 2 blade, ground adjust. Static balance & pitch adjustments fairly easy to do (with the right equipment).

 

"......new prop,.................... my static balance proved good enough so that I didn't have to change any weights."

 

This seems counter intuitive. Would have thought a new prop would have needed a fresh Dynamic Balance.

 

 

[Peter Anson]

My prop is a compost

Been doing some recycling? My new prop was checked dynamically but we didn't need to change any weights from my static balance as the peak IPS was less than 0.1 I guess sometimes you can just be lucky.