The International Cessna^® 170 Association

Folks, I'm at the end of my rope with this one....

I've had (4) different props on my 170. I own (3) of them
(!@#$%^). All of them have been in the shop (two shops
to be precise), both shops say they're all good, balance well,
and 2 of them are well within overhaulable limits, etc.

Two of the props (one I own, yet another I borrowed) have to
be "convinced" to go onto the flange with a plastic mallet.

The other two kina just slip right onto the flange, but there
isn't any significant "play" to speak of.

I've dialed the crank flange, run-out is about 1.5 thousanths.
I've measured the drive bushing bores in two of the props (that
shake) and they're all dead-nuts at .6375 (I think that's the right
value, I'd have to check my notes).

The crank center flange/boss is 2.2500
The center bore in both props (that shake) is 2.2505.

The prop mating surfaces, spinner back-plate, crank flange
are squeaky clean with no paint, burrs, etc. on them.

I'm at a loss to figure out why 2 of the props turn turbine
smooth (unfortunately, these 2 props are below overhaulable
minimums) and two of them shake like a wet dog at "mid-range"
rpms (about 1900-2200. One is noticeably worse than the other).

I've tried every indexing scheme out there, flipped the blades
180 on each one.... (worn out 3+ sets of prop bolts!).

I'm thinking the only way I'll get this to work is to have a dynamic
prop balance done. One of my local prop shops (the only one
which "offers" dynamic balancing) won't do it on my 170 because
he says the spinner back-plate is too thin (I think it's .062, I also
think that's BS).

Anyway, I'm looking for ideas on what to do next.

I thought I'd look up the tolerance limits for the crank flange prop
drive bushings to see what the allowable tolerance is (I've measured
mine, but don't know what the acceptable limits are).

Long story short, I have (3) props in my possesion now. One is
below limits (the price was right though.... was given to me for
free by a good friend), I have the prop that came with my airplane,
which, after having been in the shop twice, has improved but is
still not perfect, and I have another used, full length/width/thickness
prop I bought recently which also shakes like a wet dog....

I flew a '53 for awhile which had an O-300D and that thing ran
smooth as a turbine. I know the crank (counterweights) is
different on my C-145-2, but I would still think a small bore
6 cyl Continental should run darned smooth through-out the
operating rpm range.

Any direction (what to look for next) would be greatly appreciated.

Bela P. Havasreti
'54 C-170B N170BP

Bela,

My 170 with an O-300A shakes in that 1900-2200 range as well. My buddy's luscombe with an O-200 (2/3's O-300) shakes there as well. I'd love to fix it, so I'll keep listening, and ride on your frustrated coat tails.

I have balanced quite a few propellors for business and the standard practice is to balance it at the RPM you fly the most (cruise). With a fixed pitch prop that means you actually have to balance it in flight. Any way the point is that it will be balanced at cruise RPM and will probably still vibrate when in the reduced range. After all it feels totally balanced at cruise right now if its like mine. It might get better but I don't think it'll be perfect like you want.

Have you checked the track of the prop?

Kelly

Hy,
I would suggest to contact the prop manufacturer, they might have had a similar problem reported to them
Cheers Eric

A local mechanic dynamcally balances props here. He did the one on my 170A and got the vibration down to less than 0.02 inches per second, by using an AN bolt/nut and a couple of washers through the backplate of the spinner.

One of the tricks he said was correcting over painting -- many props have been painted numerious times. So much that the paint is quite thick in places and thin in others. The varying paint thickness can change the props airfoil and cause vibration. Before dynamic balancing he strips and repaints the blades first. This can make a big difference in the amount of counterweights needed to balance.

The first thing I would do, is check the prop for paint accumulation, if it has too much paint, strip it repaint and try it again.

A prop shop balances a prop in static only. That means that one blade weighs about the same as the other. The problem is that one blade may have it's center of gravity near the hub, while the other has it's center of gravity nearer the tip, so once you place the thing in motion it each blade has a different moment than the other and it'll vibrate. If it's been re-pitched,...that's still another problem...as it only averages out the pitch for the two blades to be equivalent...yet in motion one may have more of it's thrust near the hub while the other may be nearer the tip. It won't run as smooth as one with identically profiled blades (like it probably left the factory.)
A dynamic balance is in order. Even then, the prop may not become "silky" smooth. But it may improve.
Your dampened crank is unlikely to be the culprit. Those dampners are for a different type of vibration known as "order" vibrations that can actually be destructive...not just annoying.
Exactly which make, model, etc. props have you got?

Thanks for the replies folks. As far as paint goes, all 3 of
my props are polished (flat-black painted on the backs
of the blades).

All 3 of them are 1A170DM76 McCauleys.

One is a 53 pitch (has been repitched twice, from a 47, to a
50 to the current 53). The other is a 51 pitch (re-pitched
once from a 53) and the 3rd one is a 48 pitch (don't know
what the original pitch was, but I know it was re-pitched
prior to my obtaining it).

The 53 pitch prop is full length, the 51 pitch is 75.25" and the
48 pitch is slightly below the minimum length.

I like the performance of my '51 pitch prop the best, so
based upon your info on what could be wrong, I think I'll
have a dynamic balance done on that one. I've been told
re-pitching a prop "ruins" it's efficiency, but I didn't know
the CofG of the individual blades could be compromised....
That would explain a lot!

For what it's worth, the 51 pitch did get better after the 2nd
visit to the prop shop. What made it better was taking some
material off one side of the hub (so that it balanced perfectly
vertically about the hub axis).

Thanks again,

Bela P. Havasreti
'54 C-170B N170BP

For what it's worth, Paul Nyenhuis at Aviation Inspection & Repair, Arlington WA does "Chadwick dynamic prop balancing". 360-435-4411

Before balancing a rough prop, make sure that the specified angle is correct at each station from hub to tip. George Drummond at Merrill Field Propeller ( 907-272-2368 ) overhauled my factory new IA175DM 8046 several years ago, and when he was done it ran smooth as silk. His experience with both factory new and in service props is that often these station angles vary all over the map, This can cause the left and right sides to pull unequally and can cause part of one side to attempt to advance faster than the rest. All of this inefficiency leads to vibration. McCauley has a spec. sheet which shows the correct angle for each prop every three inches from the hub out. This topic has been discussed at length on the supercub.org website.

Thanks Eric,

I'll give Galvin & Paul Nyenhuis both a call and see who has
the better deal.

N4517C, where can I get my hands on the McCauley spec
sheet you mention?

Bela P. Havasreti
'54 C-170B N170BP

I called McCauley Textron and product support faxed the spec. ( 937 890 5346 ) It came as a chart covering several props.

Thanks, I'll call McCauley and ask for the same spec
sheet.

BTW, Galvin wanted $350-$450 for a dynamic balance.
The outfit Eric Taylor suggested (Aviation Inspection &
Repair, at Arlington) said $130. I have an appointment
with the latter on Saturday. Thanks Eric!

Bela P. Havasreti
'54 C-170B N170BP

I figured AIR would be way cheaper--Paul doesn't have near the overhead to support that Galvin does (coutesy cars,red carpets,free cookies,etc...).
What time you headed over to Arlington,Bela? maybe I'll wander over that way & catch up with you.

Eric

Yeah, everything at Boeing Field is $$$$$$$.
Supposed to meet with Paul at noon. I'll keep
an eye out for ya.

Bela P. Havasreti
'54 C-170B N170BP

Folks, I'm happy to report that my prop vibration is GONE!
I hereby highly recommend Paul Nyenhuis at Aviation
Inspection & Repair in Arlington, WA (thanks again Eric for
the tip, and George for pointing me in the right direction).

Paul stuck with it for darned near 3 hours, putting different
amounts of weight in different places until the Chadwick balancing
machine was happy. Best $130 bucks I ever spent on an airplane....
And to think the previous owner(s) flew it with that shaky
prop on there for 35+ years.....

Anybody want to buy my spare 53 pitch, full length 1A170DM prop?
I bought it right off this list just recently because I didn't know what
else to do with my original (shaky) prop.... I'll be honest and say this
53 pitch prop I just bought didn't turn turbine-smooth on my airplane
either... but that's nothing a dynamic balance can't fix!

Bela P. Havasreti
'54 C-170B N170BP

Happy - Happy, Joy - Joy !!
Great news, Bela! And now that it runs smoother you'll have fewer avionics failures and sheet metal cracking, not to mention exhaust cracking, etc. etc.
Glad it worked out.