Warp Drive - Square Tip Vibrations

Hope this is the right thread. Apologies in advance for this lengthy post.

I have a Kitfox IV Speedster, bought this past summer, with a Rotax 912 ULS 100HP, three-blade square-tip nickel-edged Warp Drive prop. Don't know the prop model or serial number, nor blade-length off-hand, but will find out on return to hangar.

Had the aircraft into a Rotax shop this past week for swapping out engine shock mounts, fixing a coolant leak, and, wanting more cruise speed, a blade pitch-angle increase, and while I was at it, prop dynamic balancing. Figured, since the aircraft had been Colorado based, with high density altitudes as normal, that I could safely reduce the climb performance at sea level here in Western Washington and gain some cruise speed.

I don't know the starting nor resulting blade pitch angle; will measure it next time at hangar, but had asked for a 1 degree increase. Static runup speeds dropped about 300 RPM according to the A&P and on my flight home I seemed to notice an increase in cruise, but didn't have the best conditions to measure that. I was told that the initial values on dynamic balancing were close to .4 and end values were something like .07. Sorry that I'm not quite sure what exactly those values represent, but I know you'll help me.

Here's the rub.

Formerly, on reducing engine power from about 5000 RPM, I might have noticed on a few occasions a mild vibration. I had attributed that to possible tired and worn shock mounts which were obviously in need of replacement.

On the flight home, from Arlington to Apex, about 30 NM, on power reduction from 4000 RPM I noticed a quite stronger vibration, enough that I found it troubling. The A&P had mentioned, while I was picking up the plane, that, during the final static runups, he'd found a "prop flutter" in mid-range RPM's, but that it went away towards the higher RPM's.

I called him after I put the plane to bed, and we discussed whether the new vibration was because of the pitch angle increase adding a bigger load at lower RPM's, which seemed like the logical answer to me. He mentioned the possibility of carbs being out of sync, but my guess is that the single variable we've changed was prop pitch, and I should look to that for the answer.

Here's the advice I'm seeking.

1. After I've got the model and serial number and pitch angle, is this something that I should call the Warp Drive factory and seek advice.

2. I have the correct torque values and tools for the prop bolts, plus a digital level and have read more than a few forum posts on changing blade pitch, plus the factory procedures online. But, I haven't done this before. Should I just re-adjust the blades to the previous angle and see if that addresses the issue? Will me doing that have any effect on the dynamic balancing assuming I seat the blades properly? I'd prefer not to have to spend any more money on A&P's doing this.

3. Are there any other obvious factors that you guys are aware of that I'm missing?

Thank you gentlemen for taking the time to read this.

Before I moved anything on the prop, I would check to make sure the blades are all at the exact same pitch now. Adjust later, but it would be real good to know what is causing the vibration. Say one blade was off now, but you moved the pitch, and got them all set right and it was smooth, you would probably think that adding pitch caused the vibration, not poor workmanship. Not saying that's the cause, but I would eliminate it first. If blades were all good, I would next check the torque on all the bolts. Jim Chuk

In addition to Jims recommendation, a complete inspection of the prop would be prudent and also checking to see if the engine mounts are installed correctly and torqued correctly. Also to note, you now have new, more pliable engine mounts that may be making a vibration you had before more noticeable now.

It would also be handy to report back your findings and current blade angles, prop diameter and reduction drive ratio.

Greg

Jim and Greg, thanks for the solid advice. Will check what I can, like blade pitches and torque settings, but not quite sure how to check if mounts are installed correctly, nor drive ratio. Thoughts on that? On first glance the mounts look as if they're similar to the previous ones. Will also report back the values.

Jeremy,

Read this service bulletin: http://www.kitfoxaircraft.com/suppor...etins/sb54.htm

It doesn't give the torque values, but I'm sure someone here with IV has them or John McBean at the factory should know.

Greg

Greg, thanks for the service bulletin. I've read it.

Can't figure out how to check that the engine mount spacer is the right length, or that all the washers etc. are in place without dis-assembling each mount, which of course I'd like to avoid, as that's why I paid the local Rotax A&P to do it, because I didn't feel competent. Hmmmm, I'll work on that.

I here ya! If I were you, I'd be asking the A&P if the work complied with the SB and also ask him to note such in the log book. Just because it went to a Rotax A&P doesn't mean he's aware of the Skystar SB unless you specifically told him.

Prop flutter - if that's what it truly is, is kind of rare with these (it's a well used combo) unless there's damage and it's also potentially dangerous. I'm suspecting it may be something else causing vibration and not really "flutter". In any case, I'd probably try to nail it down "static" and not up yonder!

Greg

By replacing the engine mount rubbers, this should have tightened up the flexibility of the mount which will exaggerate any vibrations already there.

A square tip, nickel leading edge Warp Drive prop is at the absolute maximum if not beyond the MMOI (Mass Moment of Inertia) values for the 912 gearbox. This is putting stress on the gearbox beyond what it is designed for. But, because you have the 100hp ULS, you have a slipper clutch in the gearbox which lends some "fudge" factor.

How many hours are on the engine in this airplane? Most service facilities will agree that the gearbox needs to be "serviced" every 300 hours. This service consists of disassembly and inspection of the internals, usually with replacement of the spring washers. A heavy prop will shorten this service interval.

The nature of the IV engine mount system, with the reversed carburetors, will also lend to exaggerating any vibrations.

(I have moved this to it's own thread for future reference.)

Quote:

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Originally Posted by happiestflying

Greg, thanks for the service bulletin. I've read it.

Can't figure out how to check that the engine mount spacer is the right length, or that all the washers etc. are in place without dis-assembling each mount, which of course I'd like to avoid, as that's why I paid the local Rotax A&P to do it, because I didn't feel competent. Hmmmm, I'll work on that.

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I just ran onto this discussion. When I was building my first Model IV, I noticed that when the mount was assembled and torqued, there was a significant gap between the mount and the rubber isolator - something like, I could insert a credit card I to the space. Being not too smart, I decided that was not quite right and ignored the dimensions of the spacer, grinding them a bit shorter to give a snug fit. My guess now is that you could do an external inspection checking for a slight compression of the rubber donuts. Not a precise check, but my eyeball fix worked perfectly for me for 900 hours. I am curious if the mod recommended in the bulletin needs to be a that precise.