So...I have a '55 Cessna 170B, with a 165hp Franklin engine conversion. I'd really like to put a different propeller on it.

The STC for the engine conversion specifies the "standard" 1A170 McCauley 76 incher, repitched to 56 to maintain the static rpm in the TCDS.

There are plenty of 170's out there that have 80/42 props, or the McCauley repitched for higher static rpm, either through STC or field approval.

I'm wondering out loud how difficult it will be to do the same on my bird, with the different engine configuration. I was thinking of an 80 inch propeller pitched to something appropriate, or maybe re-tweaking my existing propeller back to the original 53. Any Stinson drivers out there who have put a different prop on the 165 hp Franklin?

We are stuck with two propellers for our ships. Stentich or airomatic. The airomatic is a selfpitching model that works on aoa and relative wind. If you would like specifics drop me a pm.

Dane

The McCauley 1A175DM8042 (or 43?) props are hard to find used, all you Alaska guys seem to have them up there. Spendy when you find one, even spendier new.
I'd just send the 76 incher off for a little flattening. I think that 3 inches less pitch you're talking about just might make enough difference to be worth the trouble. The prop guys can also repitch based on how much more static rpm you want.
I doubt anyone's ever gonna check to make sure the static rpm is per spec anyway. Does the engine STC specify the actual propeller pitch, or just the min/max static rpm?

I'm running the stock McCauley 76" pitched at 53, which is the stock from the factory setting. I can't imagine a takeoff running lower rpm on climbout than it provides, but some guys do for the sake of higher cruise speeds. What kind of max rpm can you acheive with the combo of your Franklin and a 56" pitch?

FYI, for the longest time I thought the 53 or 56 meant degrees of pitch, but I recently learned that it means forward linear displacement per revolution. 56" forward per revolution. Must be at standard atmospheric conditions??

I recently learned that it means forward linear displacement per revolution. 56" forward per revolution.

Yup, but measured as if the propeller is travelling through a "solid medium"... or you could say "assuming 100% propeller efficiency".

zane wrote:I'm running the stock McCauley 76" pitched at 53, which is the stock from the factory setting. I can't imagine a takeoff running lower rpm on climbout than it provides, but some guys do for the sake of higher cruise speeds. What kind of max rpm can you acheive with the combo of your Franklin and a 56" pitch?

FYI, for the longest time I thought the 53 or 56 meant degrees of pitch, but I recently learned that it means forward linear displacement per revolution. 56" forward per revolution. Must be at standard atmospheric conditions??

Well, I would only be guessing (and I suppose I was) why the engine STC specifies a particular propeller pitch on the McCauley. I am getting the same static RPM on my 56 inch pitch, as most 170's would on their C-145/O-300 with a 53 inch pitch. At sea level and around 60 degrees I was getting between 2250 and 2300 rpm (it would lift the tail off the ground!) I can only assume that is why the STC specified that particular pitch (56) - to maintain the same static RPM in the original TCDS - or else the developers of the STC were happiest with that range of performance numbers, and were more concerned with cruise than with takeoff.

As the poster mentioned above the reason I want to repitch it (or look at a "seaplane" 80 inch prop) is to have access to a greater percentage of available horsepower on takeoff IOW higher RPMs on takeoff IOW improved takeoff performance. I realize that I will lose some in cruise, but I figure a 76/53 prop on a 165 horsepower engine will 1) have significant takeoff performance advantage over a stock 170, and 2) at the very least match the cruise performance of a stock 170.

I could live with that. My typical flight is less than two hours anyway. My concern is that a 337 with the repitch, on top of a STC that specifies a particular pitch, may not fly with the FSDO even in AK. Opinions?

Simple answer: Don't ask, don't tell. So far, it's worked for the Navy, I'm told.

This one would be next to impossible to detect in any kind of inspection.

Normally, I'm totally agin non approved installations, but.....good grief, people pitch props all over the place, pretty routinely.

If you are really concerned about making certain it is squeecky clean, approval wise, check with the mechanics around, and find out who a reasonable FSDO maintenance inspector is, and present to him or her a "hypothetical" scenario. As in, "If I were to have this prop repitched, would it constitute a Major alteration?" If he or she says nope, go for it.

I'd say it's a minor. Even though the pitch is spelled out in the specs, I doubt anyone would care if the pitch differed by an inch or two.

And, how would they tell?

MTV

This would be an automatic find in any annual inspection that's worth a damn!!! The mechanic is REQUIRED to run the airplane and confirm that it conforms to the Type Certificate, or proper Suplemental Type Certificate. If you repitch it, it will be stamped with the NEW pitch, therefore not conforming to the STC anymore... so insurance is no longer any good, your annual is no longer any good, and if it goes to any kind of reputable IA, he wont let it out the door either. Your only option is to get a field approved alteration to do the repitch... sorry, but that's what the regs say.
JH

Thanks everyone, I'm going to munch on the problem for a little while. I just want to get a little more RPM on takeoff.

What JH says is technically true.

That said, I have never seen a shop pull a fixed pitch prop during an annual, or the spinner, to see what it says on the hub.

In fact, I bought a Super Cub once, ran it for several years, and several annual inspections, at two very reputable shops.

Finally stuck a ski into a big drift in flat light, bent the prop. We took the prop off to have it re-bent. Some engraving on the prop hub: "Hmmm, what does that say?". "Let's see, it says Unairworthy--do not use". Oops.

But, as JH says, it could cause issues, so take that into account.

I'd bet dollars to doughnuts that you can get a field approval for a pitch change out of the ANC FSDO. It's worth the asking. Find a good mechanic who deals with them all the time, and ask him to "research the subject" for you. If he finds an inspector who'll sign it-you are golden.

To me, prop re-pitch on these things is pretty much a no brainer, and a lot of mechanics and FAA types view it the same way.

MTV

I bought a new 74 x 44 prop for my Champ but wasn't happy with the take-off and climb performance. I pulled it off and took it to Maxwell Propeller Service in Minneapolis. They looked in the book - Type Certificate info - and mentioned that the 44 pitch was the lowest one there. I said I knew, but wasn't getting the maximum static rpm, so I wanted 2 inches taken out of it. The tech groaned a bit but went ahead with my request.

The A&P who signed the log simply stated that the prop was re-installed after being repitched. He didn't check to see if it complied with the TC.

I'm going in for an annual in a couple of weeks; we'll see what happens, but I'm pretty certain that nobody will care. I also doubt that they'll do a run-up to see what the static RPM is.

Jon B.

If they don't do a static runup when performing an annual, they're not worth wasting your money on. What that tells me is that your engine isn't making the horsepower that it should... aren't we flying back country strips and mountains???? Don't we want full power??? Disregarding the legality of signing it off when it's not complying with the TC, who in their right mind says "my plane has too much power for these strips/situations... I want LESS power"??????!!!!! Airplane engines "talk" to you..when they're not right, they tell you... you just have to listen...
As far as a "reputable" shop never looking at the prop, there's something drastically WRONG with that... and you've never been in my shop either! That's basic airworthiness... I'd never go back there.
Just my opinions, and worth what they cost...
JH

John,

I can show you a dozen aircraft of one make and model, all with the same fixed pitch prop, and they will all show slightly different static rpm. That's why the manufacturers give a range, not a specific rpm.

As to mechanics checking fixed pitch props, you're saying that you pull every spinner off every prop, and check the numbers during EVERY annual inspection?

Good for you. You're probably the only one on the planet who does.

That's not a bad thing, but.....

MTV

MTV... I know I'm not the only one... but I also know alot of guys don't... but it's REQUIRED at every annual or 100 hour inspection to check the "bolts for improper torquing and lack of safetying" (from FAR 43 Appendix D, (h)(2))... so how do you do that with a spinner on? How can you check torque on a bolt if it's not loosened and re-torqued?
It's also REQUIRED as part of an annual or 100 hour inspection to determine that the aircraft conforms to the original type certificate, and if not, that it conforms to approved data such as STC's or field approvals that allow it to deviate from that type certificate. ALL inspectors are required to have access to that information before they can do the inspection.
As far as a range of static RPM, you're exactly right..every engine/prop and airframe is going to be a bit different, not to mention variance in the reading of the tachometer, and that's the reason they supply a static rpm range.... but from what I read he was talking about installing an 80 inch prop or repitching his back to 53"... both of which are not allowed by that STC. If the propellor setting called out for by that STC didn't give a static rpm reading that fell within the approved range, and the engine checked out as far as compression, timing, etc., then I'd have the prop repitched to get that rpm, as he said the STC called for the original type certificate's rpm range.
FAR43.15 (c)(2) states that before approval for return to service, you "must run the engine(s) to determine satisfactory performance, in accordance to the manufacturer's recommendations of (i) Power output (static and idle rpm)."
When I hear of a mechanic that doesn't think they need to do this sort of thing, the first thing that pops up in my mind is, what else are they deciding they don't need to do??? Are they checking control cables for frayed strands, checking the linkage to see if the throttle goes all the way open, checking to see if the venturi or carb heat flap are about to be swallowed by the engine? It's pretty basic in my mind.
Besides all that, like I stated before, if something happens and a problem arises, and they find the aircraft isn't in compliance with all the airworthiness stuff, it throws away all your insurance, and everything else, as technically you're operating an "unairworthy" aircraft. With the way things go now days, you've just opened yourself up to a stack of lawsuits, not to mention a stack of violations, if the inspector wants to "make a name for themselves".
This post has gone on longer than I had intended, and I doubt that I'll change any minds with it, but I hope everyone that reads it may stop and think just a little, about what's being "done" at your annual. Remember that the owner or operator of an aircraft is primarily responsible for maintaining that aircraft in an airworthy condition.
I've said way too much... if you want to discuss my ideas or views, please do so by PM.. I'm not angry or anything, but this is getting off the topic and wasting space, so just holler at me by PM.
Thanks,
JH