Vibration during cruise descent - Rans S6 / 912

[Stuart Ord]

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 100 mph, 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 5000 rpm. If I do this faster, say 110 mph 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
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

Stuart

[mikehallam]

My similar a/c has the two blade ground adjustable Warp Drive propeller. It too vibrates when throttled back, not always the same rev's though, seems more at random. I just got used to it over the years assuming it's because at reduced throttle, same speed means the airflow is partly driving it and the blades shake a little.
One thing you didn't mention & may have absolutely nothing to do with it, is prop tracking. I used a stick near the tip to see what it did and rotated it to find about 4 mm. fore aft blade to blade, changing it 180 degrees on the hub was much the same error. I then used a sheeet of writing paper over ~ 1/3rd of the backplate on one side to correct it. [and re-use it in the same position every Annual, because the prop comes off to remove the cowling nose bowl].

BTW the asi with LAA pitot and panel static exaggerates mightily at 100 mph. Other Rans users have tried alternative staic positions to get the top end readings nearer the truth. GPS tells all !

mike hallam.

[Stuart Ord]

Thanks Mike. I put the same query into PPRuNe and got a very similar answer! I'll have to try that as whilst it looks OK when running, I wouldn't like to say to less than a cm.
My problems is usually very repeatable, although a couple of times (usually after I've made a change) it seems less and i think I've cracked it, only to be disillusioned on the next flight!
Yes, my pitot is a bit of a fibber too. I reckon 100 mph IAS is in reality closer to 90 mph. As you say, GPS tells all and doing reverse runs gives this sort of error, as well as unexpectedly late arrivals at destinations! I can even get mine to read zero in a "stall" with full flaps, so it's not just top end to doubt. doesn't stop the enjoyment of a good little aircraft, though, it's just "character".
Stuart

[Captain Pulsar]

As a wild guess, and assuming you have a nosewheel airframe, I would check to see if various combinations of prop wash and slipstream are causing your nosewheel to spin during flight. If the wheel is out of balance then it may provide the vibration you mention although you don't say if the vibration is related to RPM directly. Do you feel the vibration as well as hear it?

If you have a spat on your nosewheel, then the underside half of the tyre is exposed to slipstream and might exaggerate the effect. To resolve the matter, I guess you need someone to fly alongside with some binoculars to see whats happening, if anything. Best of luck!

[Stuart Ord]

Hi Robert,

Thanks for that idea. It is nosewheel, but doesn't have a wheel spat. Yes I do feel the vibration, but not through the pedals or stick, it's noticeable through the seat, but it's the sound that's striking.

The onset and cessation of the vibration is really quick, so I doubt it's the wheel changing rotational speed, but I'm just guessing. The Rans nosewheel is spring loaded, so maybe it's resonating somehow, that could only happen in flight obviously. If only I had a brake on the nosewheel! I guess I could improvise one for a test (shhh). I'll ponder that after checking the tracking.

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.

Stuart

[Captain Pulsar]

Hmm, thinks!!

Another possibility would be to check exhaust can mountings. Bearing in mind that the more thrust produced the more the engine will strain against the rubber mounts, (assuming you have them). This might place stresses on your exhaust mountings and some wear may have resulted.

I had a similar thing a few years back where a vibration noise would occur when changing power settings, but seemed to be at slightly different RPM settings from one day to another. It turned out that with a slight reduction from cruise power the exhaust can was resonating within the alloy strap that supports it, however the problem could only be reproduced at near to full power. I tied the aircraft down and ran various power settings with the cowling off whilst I inspected the exhaust close up, (being careful of the propellor obviously). It was by doing this that I noticed that the engine actually moves forward by a few millimeters at near to full thrust and the play in the exhaust mount was only occurring in a narrow rev range. A tiny alteration to the position of the exhaust mount fixed it. Do let us know if you find the problem.

[Stuart Ord]

Hmmm, maybe. I noticed that the exhaust is quite close to the hole in the cowl, and pushing the exhaust moves it as much as a cm. At one extreme of movement the exhaust does touch the cowl. Having the Rotax ball and socket joints and springs, it is a bit moveable and I guess I'll need to restrain it a bit to make sure it's not vibrating there at high power. However the noise does disappear at max power (anything over 5000rpm stops the noise) so I doubt it will help, but as Sherlock said, "When you've eliminated the impossible, whatever remains must be true". Something like that, anyway.

I did test the tracking and blade angles today. Alas, they were close to perfect, so that's not the problem. The tracking was within 2mm, and the blade angles were within 0.5° of each other. I did measure the backlash at about 3mm at the prop tip, but I gather that's normal for a 912UL with gearbox and slipper clutch.

Stuart

[Stuart Ord]

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).

[Bill McCarthy]

Have you thought about engine timing and fuel quality ?

[Stuart Ord]

Hi Bill,
I've not tested the timing - I've not looked into doing that on a 912. I've got a Xenon lamp, I presume there are timing marks on the crank somewhere? Sounds a bit hazardous to me with a prop on! Can it be done?
Fuel - I am using fresh unleaded. Someone suggested using a tank of Avgas to get the 100 octane, which I'd rather not do. I have 40 litres of 97 octane mogas waiting for my tanks to get low enough to do the temporary upgrade.
Stuart

[McAulay]

Have you tried having the prop Static Balanced? That can make a difference

[Stuart Ord]

Thanks, yes, I did it myself - had to make a larger version of the balancer I use for model aircraft props! It was as good as I could ascertain. Not as good as dynamic balancing. The previous owner's inspector checked it on something more sophisticated a couple of years ago and it was OK he had told me. I can't find any damage to the prop. If tomorrow's experiment (restraining the exhaust a bit) then I might have to get it dynamically balanced.

[Stuart Ord]

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

[Ian Melville]

one blade was about 2° more than the other

That is a lot, do you mean 0.2 degrees? I would have though 2 degree diff would rattle your fillings out

[Stuart Ord]

Hmm, I was at the dentist yesterday......
Yes, 2.0 degrees. Having said that I'd put the accuracy of the protractor ±0.5°, so it might have been less than this. However the improvement is marked (and very welcome).
Stuart