Rotax 447 vibration diagnosis

[109jb 181]

Not really Kitfox or Avid Flyer related because it isn't either but you guys have helped before with tuning on my 582 powered Kitfox so thought I would give this a go here.

I sold my 582 powered Kitfox 4 and in its place I bought a Rotax 447 powered Hipps J3 Kitten that I've nicknamed "Patches" because of all of the fabric patches I had to put on to make it what I consider airworthy.

Enough introduction, now to the problem. The engine seems to be running well except that it vibrates. Power is good and getting 6200 RPM in a comfortable climb. The vibration happens at pretty much all RPMs but it is more noticeable at mid-range RPMs. This Rotax 447 is a single carb and the new digital EGT gauge I installed shows the EGTs are running in the 1000 to 1100 deg F range at everything above low speed. The EGT spread between the 2 cylinders when cruising and vibrating is only about 20-30 deg F, and CHTs are about 350 (analog gauge) and appear to be very even. These things have been done or are just other information. New BPR8ES plugs (inverted engine) gapped to 0.015", 2 blade ground adjustable GSC propeller 60" diameter.

This is what I plan to check next:

1. Both prop blade pitch angles
2. Prop balance
3. Prop tracking
4. Cylinder compressions for good and even

Any other ideas?

 

[1avidflyer 1983]

If it runs good, the prop or prop shaft are good places to look.  If that is a new prop because of a prop strike, maybe bent prop shaft in the gearbox??  Maybe put a dial indicator on it when the prop is off.  Two blade prop will run rougher that a three blade as well.  I have a 1" shaft I run through the prop that I rest on two real small drill bits to check for balance.  I've been tempted to buy a J3 kitten at times.  I was at the Hipps "factory" once, when we were visiting Greenville SC.  It's was about 20 miles from there in Saluda NC  JImChuk

[Allen Sutphin 705]

I would also check the shock mounts between the engine and plate. My Fox vibrated pretty bad with a Hirth 3202 until I replaced the rubber mounts. This can be done after checking what Jim said. Normally with bad mounts, the vibrations will increase with RPM. Mine was so bad you couldn't read the gages.

[109jb 181]

Thanks. I will check those things out.  Overall I'm really happy with the airplane. It flies really well although it does have a couple little quirks. Visibility over the nose is fantastic on the ground requiring no s-turns at all, but the flat attitude has a drawback. It sits flat enough that a full stall landing would really "hook" the tailwheel and slam the mains on. I'll just have to figure out how tis airplane "wants" to be landed. It wheel lands really well and the ground handling is a piece of cake. In-flight it handles really well except has a lot of adverse yaw which may be due to the non-plans aileron design, but it is easily manageable. Overall I'm really pleased with the airplane because I got it pretty cheap. I'll do what I can with the 447, but eventual plans will be to put a 1/3 Corvair engine on it. I happened to have a Corvair engine project that was for another airplane that didn't pan out, so I'm going to use the Ultravair plans available online and build the 1/3 Corvair for the J3Kitten. I'll be flying it with the 477 until then though.

Thanks again for the tips. I will let you know what I find.

[wypaul 448]

Have you checked the Service Bulletin on the GSC prop?

https://www.gsc-systems.com/ppg-propellers-technical-information/

Edited 13 Oct 2022 by wypaul

[ChrisBolkan 130]

I vote the prop. We had a vibration in a 503 powered Ridge runner. It looked very similar to your plane. Anyway, it had a vibration. The GSC prop tracked well and it looked brand new even tho it was something like 15 years old. The vibration turned out to be the prop. We replaced the prop with a very affordable composite prop by Ultra Prop. The vibration vanished and it was smooth as butter. We returned the prop to GSC for inspection and re-balance. it was returned to us but the plane was sold before we could try the reconditioned GSC prop. With what I know now, I would just buy a prop from ultra prop and put it on and fly. I was very impressed with the company and the construction of the propeller. SUPER light weight, no warping, no balance issues and VERY reasonably priced. 

[1avidflyer 1983]

I actually have a two blade IVO currently for sale.  It's an ultralight model, two blade 72".  Bit to long for a 447, but they can be cut down real easy.  IVOs are very simple to adjust as well.   I would probably shorten it to 64" and see how it did.  I had a two blade 60" IVO on my Himax which had a 447 in it, and it was two small at that length.  JImChuk

[109jb 181]

The current prop is 60 inches and the ground clearance is marginal in my book. Another reason for slightly larger tires. 64" would be 2" close to ground and would not work IMO. 

 

[109jb 181]

Well, vibration is better, but still there to some extent. I took the prop off and re-varnished the blades, checked balance and set pitch on very flat table. Prop flange checks good on dial indicator and tracking with prop installed is within 1/16". Through all of this, I found that the initial tracking was off 3/4", and balance was a little off but brought into balance using a few additional washers on the hub. Ordered new engine mounts, and it looks like the aluminum angles bolted to the engine block have been rubbing the steel tube mount bolted to the firewall.  I will take a burr and die grinder and put a little relief in when I replace the rubber mounts and see what happens.

[nlappos 413]

109jb,

Do you have a prop balancer? I used a static motorcycle wheel balancer and it made a difference, It could tell if I put one extra coat of varnish on one blade. HF used to carry them but I think they dropped it, ebay has them.

[109jb 181]

Yes. I have a balancer I made for GSC props. One 1/4" washer 6 inches from center can be detected. 

[109jb 181]

So I was able to fly it again after changing the mounts, balancing the prop, and fixing the prop tracking issues. Vibration is still there but definitely much less. On inspection I see that one of the started bolts can touch the firewall with very little effort. I'm thinking this may be the issue and will address.

I do have a question though. When flying yesterday, operating at about 4000 rpm descending from downwind, the EGT on the front cylinder got much too hot for my liking. Wound up chopping it, descending, and then bringing in power but with more load on the prop to get it down. The descent was not unloading the prop much at all, and on my 582 powered Kitfox would not have been an EGT issue.  After flying I lowered the clip one notch on the jet needle and will try that, it is now at the lowest notch, but I remember on my 582 I went from 11G2 needles to 11K2 needles to correct the midrange lean spot in the 582. Is there a similar change for the 447? It currently has a 15K2 stock needle and stock 2.70 needle jet. perhaps I need a 2.72 needle jet?