Super Decathlon intermittent vibration in cruise

S

Stan Johnson

New member
I purchased a new 8KCAB with Hartzell C/S prop from my friend Bobby Younkin in 2005 and now have 300 hours on the plane. Bobby was right when he said "It's a great airplane".

Problem is I have always experienced intermittent vibration at cruise power 70%. I have used several different methods of leaning including the methods outlined in the Lycoming manual. It's not so bad at 2500 but gets progressively worse when you back the rpm's down in the 2400 range. I'll be flying along and it's like you turned on a switch and it sometimes quits vibrating as suddenly as it started.

The EGT always peaks on #2 and in cruise setting the #4 is usually about 100 degrees cooler, often times indicating a red light on the scan. Interestingly the #4 CHT is always the hottest. Could this have something to do with the intermittent vibration?

Any ideas?

Stan Johnson
Fayetteville, AR
 
When it occurs, try squeeze some rudder on, or change the pitch slightly - I get similar on my 7GCBC and I am sure it is fabric drumming under the belly at certain settings - seems to be setting up a harmonic.
 
Mike,

Thanks for the reply. I've been changing the pitch to find a sweet spot but have never thought to apply slight rudder pressure. I'll try that.
 
I've experienced the same thing.
We are new to this forum and new Super Decathlon owners.
I took a relatively long x-country from Seattle to Reno and noticed this vibration coming and going. It felt to me at the time more like an engine induced vibration so I tried rpm and pitch changes but neither affected it. I'll try the rudder pressure now also.
 
Larry,

I tried the rudder thing but got no results. I have found that if I lean to peak EGT at 75% power (or lower) and stay around 2500 rpm this minimizes the intermittent vibration. But I will still experience occasional intermittent vibration. This always happens in cruise settings straight and level.

When I back the prop down below 2500 it's much worse.

I've had the timing checked, prop dynamically balanced, checked the spark plugs, talked to everyone I know who is airplane wise but still have not truly figured it out. I talked to Matt Younkin who ferried the plane from the factory to Arkansas. His Dad had been an ACA dealer for a few years and Matt had previously ferried a few Super Decathlons. Matt told me that when he picked up my plane and started home the vibration was so different from others he turned around and went back to the factory, thinking something must be wrong. He asked the chief test pilot for ACA to go up with him. Matt said the ACA pilot went up with him and experienced the vibration and told him it was the friggin Hartzel prop - as if to say what do you expect? Everyone I've talked to says "if it's intermittent it's not the prop"?????

Good luck with yours
 
Larry,

I'm curious if you have the Hartzell or MT prop on your plane?

I have the Hartzell C/S. One of the hubs always leaked a small amount of grease since new. When it had about 200 hours on it the seal blew and I had to have the seals replaced. This repair had no effect on the vibration issue.
 
Vibration is a dynamic senario so nothing can be specifically set in stone.

Suffice to say, aluminum props and 4-cylinder Lycomings always have some range of vibration. A little out of nominal but still in tolerance can have a significant effect on where that range is.

It may sound s little "rub some dirt on it" but I would just avoid those RPMs. Going to an MT would be a nice upgrade, but its a lot of money.
 
Jerry,

Do you think my intermittent vibration has anything to do with the note in my first post where I state that the #4 EGT is around 100 degrees cooler than #2 (hottest)? Are the fuel injectors on my plane (2005 8KCAB) adjustable?

I have now logged 300+ hours on this airplane and have figured out pretty much where the sweet spots are on power, mixture and rpms. But sometimes in cruise and straight/level it just starts vibrating. At that point I can usually adjust rpms up or down just a bit to abate the vibration- but sometimes it just will not go away???

I'd love to have a MT prop. The only thing holding me back is the money.
 
Stan and others,
I have the Hartzell. Ours (my wife calls it hers) is a 2001 with 250 hrs TT. I have been cruising around 2350 rpm; this being driven only from the standpoint that the tach time is 1:1 at that rpm. I'm hearing and experiencing however, that the IO-360 is more comfortable with a little higher rpm - so I'm beginning to cruise at 2400 - 2450 rpm now. I agree with you, if money was no object I would seriously consider getting an MT (I just bought a new MT for the Rocket I have under construction). Bur it doesn't quite make sense that an intermittent vibration would be induced by the prop ??? Seems anything induced by the prop would be constant with regards to rpm, load, pitch, or some other factor.
I posted a question asking if anyone knows of a competent mechanic in the Northwest familiar with these aircraft .... and no reply yet. Anyone ???
 
Larry,

I had no idea re: "tach time = 1:1 at 2350 rpm".

I'm no engineer but it makes sense (to me) that for a given power/mixture setting the engine has to work harder at a lower rpm since the prop is taking a bigger "bite". Jerry makes a good point when he suggests avoiding those rpm's which produce problems.

Good luck and let me know what you figure out with yours. I'll do like wise if I ever truly figure it out. Just keep experimenting with power settings, mixture and rpm. You should be able to find the sweet spot. And don't wait too long to figure it out. I've had to repair chaffed strobe wiring in the wing, replace alternator/voltage regulator and replace transponder cable - plus new seals on the prop. Who knows how much was attributable to the intermittent vibration but I believe it may have played a part in all of it.
 
The cylinder temperature differential has more to do with the abnormal baffling design on the product line. Long story.
 
Jerry,

My post above dated 10/12/07 is referring to EGT not CHT.

I routinely see a range of 30F to 40F between hottest and coolest CHT in cruise settings.

What would explain the +100F difference in EGT between # 2 and #4?
 
Doh, I need to learn to read.
(Or fight off the 2 year old off my laptop better)

Yeah, that usually leads to a mixture issue. I dont think its anything to be alarmed about unless its close to limits. You can always look into it (read: throw money at it) but I can't be certain that it will change much.
 
Jerry,

The engine monitor in my plane is programmed to flag a red light when there is a +100 degree difference in hottest EGT vs. coldest EGT and I do, at times, experience the red light (in cruise). When at 75% and peak EGT, # 2 will always be hottest (usually around 1450 to 1480). # 1 and # 3 are usually within 40 to 50 degrees but then comes # 4 at around 100 degrees cooler. I've read where this can simply be explained by a subtle difference in the sensor location?

Are the fuel injectors adjustable? I've had them disassembled/cleaned to check for any abnormality but they check out good.

Is this something I should discuss with Lycoming?

Thanks for your help.
 
You certainly could. #3 and #4 are usually the hottest.

They injectors are not adjustable, to my knowledge. When you had them checked, did they all flow evenly? The distributor (spider) might be the snag.
 
Jerry,

When you say "#3 and #4 are usually the hottest" are you referring to CHT or EGT? Because #3 and #4 are always the hottest CHT on my plane. It's the EGT where the big difference is on #4.

Thanks for your help. I will contacat Lycoming re: the fuel injector system.
 
Larry,

I just spoke with Lycoming about my intermittent vibration. He said it's probably something to do with the valves or valve guides and it almost never happens in newer engines. There is a Lycoming mandantory service bulletin 388C recommended at 400 hour intervals. He suggested that I have my mechanic try the procedures outlined in this bulletin - looks complicated. He also said not to worry about the +100 degree spread in CHT but to check the baffling anyway.
 
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