Vibration during cruise descent - Rans S6 / 912

[aeroturbo]

I've got a Rans S6-116 with a Rotax 912UL, driving a Warp Drive 2 blade prop. It has done about 800 hrs, about 50 of which have been in the last 18 months since I bought it.
I've found an increasing tendency of the aircraft to vibrate at certain rpm / airspeed combinations. Typically it cruises at 100 mph IAS and 5000 rpm. If I throttle back to say 4500 rpm and let the nose drop to maintain 100mph, I get a sudden increase in noise, apparently coming from in front of me, which persists until I drop the rpm below 3500. If I then power up slowly, the vibration starts again at about 4000 rpm and stays until I exceed almost 5000rpm. If I do this faster, say 110mph IAS, then the rpm figures are higher, and vice versa for lower IAS. It doesn't seem to occur at all below 90 mph IAS.
I've mentioned this to Rotax experts and have followed their advice:
1) Checked prop balance - seems OK ( I have limited facilities to do this but the previous owner's inspector who's a Rotax engineer balanced it not long before I bought it)
2) Moved prop round 1-bolt - no difference (gearbox is an odd ratio to spread the firing pulse anyway so I didn't expect this to work)
3) Replaced both carb needles and seats - no change
4) Checked balance of carbs - were OK
5) Looked for cracks in engine mount - none found
6) Checked float chamber levels - were OK
7) Looked for loose / broken mountings and bolts - nothing found
8) Tried a Woodcomp 3-blade prop. This showed a big improvement, the vibration was less and the "band" of rpm prone to vibration was less. However I still got it at about 4000 rpm / 100 mph
9) Checked plugs for signs of uneven running - all nice light brown.
10) Looked for things loose behind the firewall - nothing found, everything touched was not vibrating unduly.
11) Removed cowls and anchored aircraft so that it could be run at all rpms and observed from outside - nothing seen. It's worth noting though that no vibration increase is noted when static at any rpm - it only occurs at the stated airspeed.

Has anyone else experienced this sort of vibration?
Can anyone suggest what might be the cause?

Thanks

[Monocock]

Hi there

I had a similar problem once.

Can I suggest you check the prop's tracking.

1. Park the aircraft in a hard surface and out of any breeze.
2. Double check the ignition is 'off'.
3. Rotate prop' until one blade is pointing straight down.
4. Find something like a jerry can that you can place in front of the prop so it is JUST touching the tip.
5. Rotate the prop by hand to the next blade. Check distance from the jerry can of the tip.
6. You need to make sure the difference in tracking (if there is one) is within acceptable tolerances.

I had a GSC prop once that vibrated under exactly the same conditions that you describe. I checked the tracking and one blade was a quarter of an inch out. I tried a different prop and the engine was as smooth as silk again.

What was happening was that at full power the difference was forced out by centrifugal force. At lower power the prop speed was too slow to vibrate. It was at around 4300 to 4900 where it vibrated.

Hope that helps.

[Genghis the Engineer]

What fuel are you using?

G

[aeroturbo]

Thanks for that. I did look at that when I was observing it running, and it looked OK, but I didn't check it static, so I'll do that next time I'm there. I don't know the tolerance for a Warp drive, but I'll find out - a few mm I guess.

[aeroturbo]

I use Mogas all the time.

[Genghis the Engineer]

It may not help, but I would suggest trying a tank of Avgas. Some years ago we found a Jabiru vibration problem was alleviated by higher octane fuel - and eliminated by that and a wooden prop. It certainly will do no harm apart from perhaps necessitating a plug clean.

I do however also agree with Mono about checking the tracking.

G

[aeroturbo]

Yes, I guess so, that's an idea. Running 2 carbs in parallel is open to odd effects I suspect. We don't like putting leaded fuel in 912s of course, so maybe I can try some 97 or 99 octane mogas - it is available in some petrol stations here.

[aeroturbo]

It's just occurred to me that I've never checked the blades are at the same angle. Again it was set up just before I bought the aircraft, but it might be off. I've made a blade protractor and will try that too.

[phiggsbroadband]

Hi, at a certain speed we get a rapid clicking noise from the radio headsets, which was traced to loose aileron cables hitting a resonant frequency. If we slowed down or speeded up the rattle stopped, Or if we banked a little L or R. Maybe you have something similar, it might not be with the prop...

[aeroturbo]

Thanks for that new idea. next time. My sound only comes and goes with rpm and airspeed, not with bank, but next time I'll try to think "what else?".

I've checked the tracking and blade angles today - alas the tracking was good to within 1-2 mm, and the blades were the same angle to within 0.5°. So, I'm still scratching for ideas.

[aeroturbo]

I've had the outer rubber doughnut mounts out today. They were a bit deformed to the shape of the washer and the ring, but otherwise OK. I put the doughnuts in backwards which should have made the mounts a bit stiffer, but it didn't change the vibration. I've ordered some new rubbers anyway as they are over 5 yrs old.

I also tried tightening the bolts in the noseleg to see if it was vibrating - no change noticed.

I also checked to see if the vibration caused rpm variations (no) and if the vibration varies with switching off each ignition in turn (no again).

[aeroturbo]

I flew it yesterday with the exhaust pulled back to ensure it couldn’t rattle on the side of the hole in the cowl that it was closest to (and could move towards if pushed) but no, it made no change. I also flew it well out of balance with the vibration occurring and it made no difference, so odd airflow effects seemed to have been eliminated. I also tried with the nose wheel slider bolt tightened a bit to check it wasn't vibrating, but hadn't got round to devising a brake to stop the nose wheel rotating, which someone had suggested.


Running out of options, I decided to change the prop pitch to see what effect that would have. When I checked them , one blade was about 2° more than the other. I had checked this previously and thought they were pretty much the same as each other, but several repeats yesterday suggested that there was this error. Simple protractors with a weight plumb-bob have their limitations! So I took the lower blade up and after tightening it was then 1° more than the other, but I tried it anyway. The result was a sluggish take-off and climb, but it got there and when tested – no vibration at all could be caused no matter what rpm and airspeed I used.

So I had a couple of more flights tweaking the setting, ending up with both blades about 1-1.5° more than the lower one had been, which is about 0.5-1° less than the higher one had been, and got a good compromise – good takeoff and climb, very slightly quicker than it had been (max power straight and level 115mph IAS @ 5500 rpm, cruise 104 mph IAS @ 5000 rpm, climb at 5400 rpm, static 5200rpm, climb ~1000 fpm) and only a tiny bit of vibration could be found at 3800 rpm ±200 rpm at 100 mph. Overall it was so much better, almost any power setting at any speed was OK, and performance seemed to be better. Happy pilot!

Thanks to all who read and offered helpful comments.

Stuart