Propeller Pitch for C85-12 with 0-200 crank and rods

[skyboltone]

The STC for the C85 substitution in the BC12-D says: "Meets the specifications of the BC12-D-85."

TC A-696 for the BC12D-85 Says: McCauley 1A90 with the following limits: Static r.p.m. at maximum
permissible throttle setting: not over 2350, not under 2170. No additional tolerance permitted.
Diameter: not over 71 in., not under 69.5 in.

So, uh, for the 64 dollar question what pitch is that? Assuming we are shooting for 2350 RPM. The airplane has a McCauley 71" on it. The engine is a C-85-12 with all the accessory pads blanked off and a 0-200 crank and rods installed.

Thanks
Dan

[Mark Julicher]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

It will be about 46 inch pitch. Maybe 47. I don't have that exact setup, so maybe somebody with the 85 will chime in here.

[Ragwing nut]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

71-48

[skyboltone]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

71-48

Thanks. That seems about right. I don't know why they limit the length to 71".

[NY86]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

Thanks. That seems about right. I don't know why they limit the length to 71".

Because McCauley has determined that longer props on that engine may exhibit vibration issues.

[otrcman]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

Because McCauley has determined that longer props on that engine may exhibit vibration issues.

John,

Not trying to be argumentative here, but I have been interested in this longer prop question on C-85's for some time and have tried to do some research. So far I've never gotten a definitive answer.

Has McCauley or Teledyne actually determined that there may be some vibration problems with props longer than 71 inches, or have they simply never done any testing to verify that the longer props are OK? Do you have some actual correspondence stating one answer or the other ? I've asked this question of both McCauley and Teledyne and no-one that I've asked seems to know. Apparently the certification work was done many years ago and the people working there now don't have sufficient data to answer my question.

What really muddies the waters is the data in the 1B90CM Type Certificate Data Sheet, P-842. It shows the C-75 as legal with the longer prop (up to 74"), but of course the C-75 has a lower redline RPM (2475 vs 2575 for the C-85). So we know that the rotating machinery and propeller combination have been cleared to the lower RPM. Then there's the O-200 engine, which shows that same prop as legal up to 75" diameter and 2750 RPM.

What are we to take from this ? The 0-200 crank, rods & pistons are not the same as a C-85, so maybe there is some resonance with the C-85 rotating machinery and the longer prop. Of course the O-200 crank conversion muddies things still further. Now we have a "C-85" with all the same internals except camshaft. Do the prop limitations change to be the same as an O-200? It's pretty clear that neither McCauley nor Teledyne have addressed the O-200 crank conversion.

The FAA views propellor approval as an engine-propellor-airframe combination. That's the legal way to look at it as far as they're concerned. But as we get into modifications like the O-200 crank, it sure would be nice to know what are the implications for prop usage.

Dick

[skyboltone]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

Dick: I wouldn't worry to much about being argumentative here. That's what we do when we're not not arguing. I've been thinking about calling Don Swords. I think he is the owner of the 0-200 crank mod. I spoke to him one time and in reply to a specific question; "Does the FAA consider this a C-85?" his answer was in the affirmative. I seem to recall that the seaplane option includes a longer prop. I wonder if Terry Bowden has considered this question as owner of the Harer STC and a DER to boot. Terry?

[otrcman]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

I asked Don Swords about this some time back and he gave me the same answer that you had. The FAA still considers it a C-85 after the O-200 crank mod and all C-85 specifications still apply. I had the feeling that Don didn't want to open a can of worms by bringing up the prop issue with the feds.

Terry B. may be the best person to tackle the prop question, but I suspect that nothing would change until someone did some testing. And testing would no doubt be too expensive to consider doing.

Dick

[barnstmr]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

The problem is this.... because the engine rating and LIMITATIONS are not changed by the STC, I cannot substantiate on paper any propeller that allows the engine to operate outside the FAA approved LIMITATIONS of a C85 engine. However we know that the STC modified engine actually does not operate on the same power curve as a stock C85. I think it is safe to say that for a given RPM, the engine is producing more horsepower than a stock C85.

Give me a call if you want to discuss the options.

[jim cooper]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

Terry...I had a Cessna 140 8 years ago with the 85hp don swords conversion. This was supposed to produce an honest 100hp. It also had new millinium cylinders. Is this hp upgrade correct or am I truly senile. JC

[barnstmr]

Re: Propeller Pitch for C85-12 with 0-200 crank and rods

Jim - There is no doubt that the STC modified engine is capable of producing more HP than a stock C85 at any given RPM. But Swords does not publish a power curve for the STC modified engine. Neither does Aircraft Specialty Services. The FAA approved paperwork for both of those engine STCs does not allow operation at a higher RPM than a C85. So we are left with the same propeller requirements as for a C85. And we are left guessing at the actual power being produced for a given RPM.

Now consider DAN's original question on this thread. Pick a 71 inch propeller with a pitch that will limit the engine to 2350 rpm (STATIC). IT IS GOING TO TAKE A GREATER PITCH TO LIMIT THE MORE POWERFUL STC MODIFIED C85 ENGINE THAN A STOCK C85 ENGINE. You should see some improvement in takeoff performance over a stock C85 due to the additional power... but you should see a dramatic improvement in cruise due to the greater pitch. Remember that also in flight, the pilot is required to restrict the RPM to a redline of 2575 RPM which is the same limit as a C85. We are left to guess exactly how much HP is actually being produced at these power settings.... but one thing for sure... it is more than a stock C85.

I believe there are some folks out there with a misunderstanding of the engine STC limitations and are using a prop that allows the engine to spin faster than the C85 limits. I am sure those folks are seeing some dramatic performance improvements, but are technically operating the engine beyond its certified limits.... illegally. It may or may not actually cause a safety issue... but I am certain the engine's will not last as long when operated at higher RPMs. It is a durability issue in my mind.