What Prop?

[barnstmr]

Re: What Prop?

Does the O200 crank STC specify a different prop than the C85? After the mod can you run the same props as the O200? Please explain.

Here's the FAA answer. NO. You must go by the engine rating (i.e. the approved operating limitations).

In the case of the STC being sold by Aircraft Specialties to install the O-200 crankshaft in a C85 does not alter the FAA approved engine ratings. (i.e. it is not approved to operate at different power/RPM than the original C85. I am guessing they demonstrated "equal or better" performance.) Therefore, all of your operating limitations remain the same if you install this engine.... including max RPM. So this means you cannot legally use a different propeller than is already certified for the C85 is approved with on your airplane, unless you are prepared to do the necessary engineering work to demonstrate to the FAA how the prop and engine combination comply with the airworthiness requirements.

Sure... technically the hybrid C85 "stroker" engine will produce more horsepower at certain RPM levels than a stock C85... this is not difficult to reason or to demonstrate. But there is a whole lot more to gaining FAA approval to operate that way.

So this means any propeller that allows the engine to operate (turn up) to a higher RPM/power level must be scrutinized against all of the applicable requirements. This would likely involve testing and/or analysis work. Or if you want to propose a propeller that is different than already certified with a C85, but does not cause a difference in the RPM/power levels... it can be done. Same requirements, but a lot less compliance work- since there would be no change to to the limits.

[T-Rex]

Re: What Prop?

Barnstormer, that all makes sense but what these guys and you were talking about a few posts ago was using a Mac 1B90 74-43 or 45 on the plane with a C-85 with the O200 crank conversion. When I was looking for a prop for my plane with the C-85 I went to the TCDS for the Prop and to my knowledge that length was not listed on the TCDS. So if you wanted to use that prop on that engine you would have to go through all the testing and analysis you mentioned because wouldn't you have to prove that there was not destructive harmonic vibration with the prop. If you could put anything on that fell within RPM/power output limits why did everyone who had a Sensenich M74CK-? have to cut their props down to 72 inches when they upgraded from 65 to 85 hp. Look at Note 9 on page 5 of the TCDS for the MAC 1A90 and 1B90 props. So if you wanted to go longer than 72" on the DCO you could possibly field approve a longer prop that mates with the C-85 as long as you meet the propellor requirements of the CAR's that the plane was certified under.

[crispy critter]

Re: What Prop?

Guys,
I don't know about the 85 but I am running the 74/45 McCauley on a straight 0200 Cont not an 85 stroker. The 74/45 are approved on the 696 type cert and is also approved for install on 0200 as per thier type cert. The probem was that the 74/45 was never approved on an 0200 under the 696 type cert. Of course thier was never an 0200 approved under the 696 either. The only reason I had to get a DAR approval was because my FSDO was being way too picky. Even thou the prop was apporved on both the engine and current type cert,it was never approved on both together so they told me I had to get a DAR to sign off on it. All this to replace a missing AWC!!! Even my FSDO told me that if I had an AWC already that it would probably just be a log book entry and a 337 but because I had made so many other mods to the airplane they wanted everything that wasn't type specific to have it's own approval.

[T-Rex]

Re: What Prop?

The guy, Joe, that bought T-Rex from you now has a O200 powerd BC12-D and he runs the 1B90 75-45 or 43. He's on Baumann amphibs so we don't see him as much any more but will be in for his annual this winter. That engine and prop combo is also used on the Cessna 150's as the seaplane prop. It's to bad some of these FISDO guys cannot connect the dots on stuff like this and save some people time and money. It,s not like your the first and or only guy to ever do this. The guy we work with at the MSP FISDO is great, but he lived and worked in Alaska for some time so he seems to have a realistic approach to stuff like this.

[flyguy]

Re: What Prop?

OK, while we are discussing this I have a question.

The float guys I understand, longer prop, lower speeds, flatter pitch, more thrust.

I had a 7446 on a 65 that had a 85 venturi and main jet in the carb. It would spin right on past 65 redline in level flight and climbed very well. Suspect might have been about 75 HP, who knows.

My read on putting a 74 on an 85 is that if the pitch was selected correctly it would aways climb better. For sure the 2.5 extra square feet of prop swept area, out at the tip, would provide more thrust (with the right pitch.)

I also suspect that cruise could be improved with the correct selection of pitch, but I do wonder, as the 74, to wind up further would require a flatter pitch than say a 71. Would the flatter pitch be less efficient at cruise, even with the longer prop. Which characteristic has the most effect at cruise?

Edit: thinking on this more I wonder if the limit on getting a good cruise with a 74 would be that a good cruise pitch on the longer prop, for a higher speed, would not be able to turn up enough static rpm's and would not climb very well. Am I close here?

[barnstmr]

Re: What Prop?

Barnstormer, that all makes sense but what these guys and you were talking about a few posts ago was using a Mac 1B90 74-43 or 45 on the plane with a C-85 with the O200 crank conversion.

My mistake... I thought the question was about the O-200.

[Dano"T"]

Re: What Prop?

I can tell you from experience though, that a 74" prop won't turn up well with a STOCK C85...I tried it. I took the Sensenich 76AK-2-40 prop from my C90 T-craft and tried it on my C85-8 Tcraft.....barely got 2000 rpms static....a 76AK-2-40 prop measures about 75" tip to tip on a tape measure and mine has a 40 pitch.....It did not work well....My C85 has a McCauley 71-42 and it works awesome.....on the C85. Perhaps the O200 stroker C85 will turn the longer prop better.....but again...it must still comply with TC.

[T-Rex]

Re: What Prop?

We have a guy here that has an 85 hp J-3 on aqua 1500's wit the M76AK2-38 and it just barely makes minimum static RPM. With those heavy floats and the slow turning engine it really seems to be a mutt.

[Ragwing nut]

Re: What Prop?

some of you guys I think are getting confused. the original poster has an L-2, not a bc12d, which is a different tcds and different limitations, and flies completely different so what works or is legal on a bc12d may not necessarily apply

[T-Rex]

Re: What Prop?

When he installed the C-85 he deviated from the original type design. Now he just cannot use the original propellors that were certified with the plane because it was originally certified with the 65 and not with the 85. Even though the Mac is listed on the TCDS that does not apply because it was certified and tested with the 65, and now it has a C-85 witch is not listed on the TCDS for the L-2. When he put the 85 in per STC all of the applicable props on the TCDS go out the window and he now has to follow the operation limits of the C-85. If you look at note nine on page five of the TCDS for the MAC 1A90 and 1B90 props, that gives the results for testing that was done dealing with harmonic vibration. The fact that it is an L-2, BC12-D, Champ or J3 does not matter because that info in note 9 pertains to engine and propellor combos. In that note it lists the C85 with the 1A or 1B90 props at a max length of 71" and a min. of 69". The Sen. TCDS has the same table and that lists the M74CK-2-X so thats a 72" Sen. CK series prop or the M76AK-2-X thats a 74" AK series prop. This is in my opinion why for floatplane opps. the C85 is not the best engine. You get descent power but are limited in the propellers and lengths to a 72" Sen CK or a 71" MAC or a 74" SEN AK prop that the engine can't really turn. This all pertains to metal props not sure for wood. When I purchased my prop I looked at every prop application chart I could find and nowhere not on any plane equipped with a C-85 is any mac prop approved at a length longer than 71". You could do all of the vibration analysis and possibly prove with the O200 innards that there is no destructive harmonic vibration but I can only guess that would be expensive. If you go by the vibration analysis that Mac and Sen performed it should be an easy field approval as long as it meets the clearance requirements spelled out in the CAR's that it was certified under. If you do get the 74" or 75" 1A90 or 1B90 prop approved on the C-85 I would like to know because that would be a great floatplane prop.