RPM Wobble

RPM Wobble

Anyone with  O-300 experience. I have a small fluctuation  (plus/minus) of  5 (or so) RPMs at cruse power settings (wobbly tac needle).Engine runs smooth .Run-up and static test with in specs.

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Re: RPM Wobble

Check the tach cable.

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Re: RPM Wobble

When was tach and cable ever checked, how long has cable been installed tach hours and calendar time? They can become worn and bind due to lubrication. Try using a digital handheld tach and compare readings. Don't be surprised if mechanical tach reads maybe 50 to 100 low as they can bind.

Bill

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Re: RPM Wobble

Thanks , I'll do some testing on the tac and cable.

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Re: RPM Wobble

install a digital tach
some are primary replacement
all are very precise
they are coupled to both magneto and thus can give you indications when something is not perfect on mags,this in addi8tion to giving you a lighted andprecise reading.
mecanicals tach are things of the past

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Re: RPM Wobble

you can check a great unit
the p1000
at          www.horizoninstruments.com

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Re: RPM Wobble

Good stuff Daniel I'm on it!

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Re: RPM Wobble

My 2 cents. i just installed an Electronics International Electronic tach in my Cessna 175. I did all the work and my mechanic inspected and handled the STC paperwork. My problem was an oil leak where the tach cable connected to the tach drive housing on the engine. Installed new gaskets, sud broke, lots of fun, still leaked figued out tach cable coupling was the leak, new cable, still leaked, pulled housing ( again ) new seal, still leaked. My mechanic thought the shop that installed the new seal in the housing put it in bacwards. By then I had enough, bought elec. tach, removed cable and old tack and sealed that housing off with a cap nut. Installing the new tach was lots of labor laying under the panel, pretty straight forward. wires to the mag switch p leads under the panel, wire to ground wire to power ( got to install a new circuit breaker in lieu of a fuse with the STC ) and done. Everything worked like it is supposed on the first run up. My mechanic was concerned about the tach time, I bought mine thru Chiefs and provided my tach time, which is entered into the tach. it stays there, if you loose power it is still there, very accurate. I really like some of the features, on a mag check you can zero the reading then switch to one mag and it shows digitally the actual drop, then switch to the other mag, same thing, way cool. Not cheap but it fixed my ongoing problem . Tim

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Re: RPM Wobble

Mechanical tachs are nothing more than cheap speedometers with a spiral wound cable that turns the instrument pickups thru a flexible housing.  They sometimes "wobble".  They sometimes can be fixed with pulling the cable and lubricating it with graphitic grease or replacement.
In any case, keep this in mind.  The instrumentation is not critically accurate, nor is it necessary that it be.  It's just a rudimentary way to generally tell what kind of power setting you're pulling.
If it really bothers you, you can throw a little money at it, ...or a lot of money at it.  It won't make your engine run any better.
(While I don't like "wobbly" indicators, I fix them, but I don't usually just jump into it with a full wallet.  I usually just replace the broken/weak part with similar parts.  My own experience with "digital" instruments is that they are a tedious detail.  Who cares if your EGT is 1437 degrees and "wobbling" between that at 1439 and 1428 and on and on and on ad distractum.  And who cares if your mag drops 47 on a digital ...or about 50 on an analog that costs 1/4th as much.)
But that's just my nutty, inexpensive, opinion.

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Re: RPM Wobble

We have the P1000 and it does the job.. Although, I wish the thing would count in 10s instead of 1s because it is never satisfied to just be still.. It is like a kid - Always jumping around and won't sit still..

I guess the only + feature that I would give it is that it will have a little red light illuminate if you lose a mag.. Other than that, it is expensive and seems to be nervous.. The old iron works OK for me.. as long as it dosen't wobble.

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Re: RPM Wobble

if you can Dave ,get a digital tach.unless you are from a realy old school you will appreciate it .                         .i love all my digital instruments
we have found out,that  in the demanding skydiving operations ,that having precise readings in tach, egt ,cht,we can find the best operating procedures and extend cylinder life.
i dont know in states but here in canada we have to have a yearly check on rpm precision if its out by 100 you have to repair .
in mecanical  tachs the more you wear the more imprecise you get .

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Re: RPM Wobble

Hey!  Who are you callin' "old"??? (grin)
Digital equipment is fine, except in this case you'd be measuring it again with a micrometer when all you have to cut it with is a hatchet.  (The O-300 engine is far more agricultural than deserves a digital tach, except in the minds' eye of the beholder. IE, it's a personal thing.)  But digital equipment doesn't readily indicate trends or limits.  (Quick!! What's the low end of the green arc on the original tachometer?)

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Re: RPM Wobble

euh ! 400 rpm?
no sorry thats my harley.                                                            i have no idea.
do i have a pilot licence ?                                                           let me check that too

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Re: RPM Wobble

General consensus  seems to be the tach and not the actual rpms of the engine, "comforting" I'll have it checked  .
daniiel   I fly the B.C. west coast.

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Re: RPM Wobble

Answer: 2200 RPM -2450 is the green ard,  2450-2700 is the yellow arc, and 2700 is the redline.  Digital tachs don't display that info.

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Re: RPM Wobble

dave
canadian west coast  is on top of my  list of places to visit eventualy with my plane
happy flying !

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Re: RPM Wobble

George,  you may have just come up with an idea that will make your fortune. 
Perhaps you should approach the digital manufactures with lights for the various rpm ranges. 
If I had a digital tach, it would be nice to have colored LED
lights to represent the ranges, in addition to a digital rpm readout.

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Re: RPM Wobble

on digital tachs theres a red light ,wich is  on most units, pilot programable .

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Re: RPM Wobble

Hmmmmmmm-  Sounds like I may want to investigate these digital tachs a little more.   I have not had the oppertunity to see one in action.
Thanks for the info Daniel.

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Re: RPM Wobble

As I recall, Horizon Intruments digital tach has three lights across the top.  A GREEN, YELLO and RED one depending on which RPM range your running in.

<http://members.aol.com/horizonins/p1000.htm>


      Dave

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Re: RPM Wobble

youre right dave
when you order at horizon they requesy for wich very specific airplane and unless you tell them differently they are programing the gauge for you .
the green ,
+yello(as you say )+RED LIGHTS WILL BE ON DURING THE RANGE SPECIFIED BY THE MANUFACTURER .
in addition they will write on the face of the instrument the rpm range for each of the categories (.ex.green 2200.2500)

the thing also act as a timer for different purposes,including time of cureent flight etc etc .

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Re: RPM Wobble

I've experienced digital speedos, tachs and clocks. (I think we all have)
It seems to me that the analog type is quicker to interpret in one's mind.
You can glance at it or not even have to look directly at it.  You can tell if the needle is in the "right place"
The digital types are cool to looka at...very modern looking and may or may not be more accurate, depending on the unit.
The digital ones, you have to do a quick mental calculation.
Maybe my synapses are mucked up...
I think one or two knots, or, 5 or 25 RPMs shouldnt be a very critical issue.
Michael

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Re: RPM Wobble

I put the Digital in and enjoy the benefits of both -

I find it easier to maintain Altitude because the digital picks up on minor changes.

I like the indicator lights for the mags - if you have an open ground on a mag - weather it be on the runup (intentional)  or some other time (unintentional)  You get an immediate red light.

I also like the analog or dial and have retained it - I've gotten used to seeing the dial in certain areas, and on initial power changes I like to see the dial position.

The Digital has the lights for rpm ranges so they'll remind You of operating ranges green-yellow----RED- just in case you weren't paying close enough attention to the dial at that moment.

They both have their merits - so I'm retaining both.
- and could recommend the same to anyone else

Ken Wanagas

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