C-85 Don's stroker mod. Need advice on prop

[Robert Lees]

I've completed flight testing of my C-85-8 similar to the Don's Dream Machine mods (C-90/0-200 crank, rods & pistons). The installation & flight tests in our BC12D Taylorcraft went well, and I would welcome some advice on the pitch & diameter for our propeller.

For the flight tests, I've borrowed a MacCauley Klip-Tip 1B90 7143 (actually 70" dia). It comes from a standard C-85-8 equipped Taylorcraft, and works just fine on that aeroplane, in that it would just red line in Straight & Level cruise, but only just. This prop was cut down from a 7443 long ago.

With this same prop fitted to my modified engine, however, the static rpm is right at the upper limit of the static rpm permissable for the normal C85 (2350 rpm), and in S&L cruise it wants to leap through red line at full throttle. so it's over-revving. I think I need a coarser prop, but how much coarser should it be? 1" or 2" or more? And is the diameter right?



For info, my engine dyno'd at 93 hp at 2475 (100 rpm below the 2575 max). We might be getting close to 100 hp at the full 2575 rpm.

Any & all feedback / suggestions / experiences welcomed. Any Cub drivers with similar engine configuration care to comment?


I have a fresh Mac 7443 (from our old A-65) ready to be clipped & repitched. Then I can give the test flight prop back to its original owner.

Rob

[Ragwing nut]

Re: C-85 Don's stroker mod. Need advice on prop

Don;t you dare clip your 74/43. Worth too much money and too hard to find that are still 74" long. Put your prop on it and fly it. Then go from there

[Doc]

Re: C-85 Don's stroker mod. Need advice on prop

I still have a yellow tagged 1B90 7138 in the hangar.... Doc

[Ragwing nut]

Re: C-85 Don's stroker mod. Need advice on prop

I still have a yellow tagged 1B90 7138 in the hangar.... Doc

That is a Lycoming prop. Can't be twisted enough and may also have a different bolt circle.

[gcgilpin]

Re: C-85 Don's stroker mod. Need advice on prop

1B90 7138 will bolt to the small continentals (C85, C90, 0200). It would work well getting off the water / climb. Will definitely over-rev in cruise.

I used to use a 1B90 7140 on a C90, it worked well.

[Robert Lees]

Re: C-85 Don's stroker mod. Need advice on prop

1B90 7138 will bolt to the small continentals (C85, C90, 0200). It would work well getting off the water / climb. Will definitely over-rev in cruise.

I used to use a 1B90 7140 on a C90, it worked well.

That's the problem I'm having...overrevving in the cruise. And overrevving static. Need suggestions for how much additional pitch to put on it.

Don;t you dare clip your 74/43. Worth too much money and too hard to find that are still 74" long. Put your prop on it and fly it. Then go from there

I have no other prop to play with (and at 74" long, it's too long for the 85).

[NY86]

Re: C-85 Don's stroker mod. Need advice on prop

How fast do you want to go? If you had a full 71" I'd proably go minimum 46 on the pitch. You're torquing like a C90, so whatever prop is used on a C90 for your airframe is probably a good place to start. Maybe even as high as 50 or 52.

Stay with 71" length as longer has not been shown to be free of nasty vibrations...

What Mike said: Don't cut a 74 down. No one has come up with a way to cut it back longer....

McCauley considers the 1A90 and 1B90 to be equivalent.

[gcgilpin]

Re: C-85 Don's stroker mod. Need advice on prop

If you like good takeoff / climb performance I'd leave your prop as is.

If you want to get a little more cruise speed add 2 or 3 inches of pitch (should reduce RPM by 50 to 100 rpm).

Depends what you use the plane for, if you're on floats it sounds perfect.

[Robert Lees]

Re: C-85 Don's stroker mod. Need advice on prop

No, we're not on floats (I wish!)

I just feel that the engine isn't working hard enough (with a tendency to overrev). I'm not looking at needing a huge cruise speed; but I think I can sacrifice a bit of the extraordinary climb rate (900 fpm at 1280 lbs gross) for more efficient cruising at sensible power settings.

Thanks for the input, all. I'm quite happy to swap our 7443 prop for the right one for this engine, but

1. I need to find the "right prop" and
2. Shipping to the US would make a swap uneconomical and
3. There are too few Taylorcraft in the UK to find someone who needs a 7443 (most have one already).

Rob

[akndrifter]

Re: C-85 Don's stroker mod. Need advice on prop

I agree, don't chop the 74''er. I don't have any experience with C-85s, but I know on my A65 it really makes her preform. I was getting 1000fpm with two guys and 5 or 6 gallons in the nose at about mid 20s to 30*F. I do have to throttle back in cruise so I don't go past redline. But I'm not in a hurry to get anywhere, still building time.

[dmcneil]

Re: C-85 Don's stroker mod. Need advice on prop

Hi Rob,

If you look at what McCauley recommends on the stock BC12D-85, it should give you some guidance. The Univair catalog reprints the manufacturer's recommendations for the BC12D-85:

Standard... CM7148
Cruise....... CM7150
Climb........ CM7146
Sea-Plane. CM7144

It would seem your 7143 is not nearly enough pitch for the C-85/O-200 STC. A friend has a Don Swords rebuilt engine in her Luscombe and she is using a McCauley 7151... same prop as she was using before the rebuild on the stock C-85. It gives her great climb and cruise performance. The McCauley recommended C-85 props for the Luscombe are as follows:

Standard... CF7151
Climb........ CF7148

For the C-90 on the Luscombe:

Standard... CM7152

John is right on about the 71" diameter on the C-85.

Dan

[Jim Herpst]

Re: C-85 Don's stroker mod. Need advice on prop

Robert, I have a polished MAC 71/49 that I am using on my A-65-8 equiped Baby Ace. Overall it works well in this plane with the A-65 BUT I am not getting redline no matter. I have not checked my tach for accuracy (but will) but I believe when it it registering 2300 flat out its more likke 2250. I would thinke your orginal prop (43 )pitch would work well for me and my prop would be perfect for you..... want to possible swap?

[Robert Lees]

Re: C-85 Don's stroker mod. Need advice on prop

Jim, the shipping costs UK to US and vice versa would outweigh any advantages.

Incidentally, a pal of mine gave me a nice program that determines engine rpm based on sound. You need a laptop (or similar portable device to take to the airport) with a separate microphone. It works by listening to the exhaust pulses, and gives a digital and graphical readout of the engine rpm. We used this program both on for ground runs and in the air using a laptop, and it gave excellent indications of rpm. In fact we used it to calibrate our Tacho.

UnZip the attached file and load the included files into the same directory on your computer, and run the exe file.

[Jim Herpst]

Re: C-85 Don's stroker mod. Need advice on prop

Robert,

You never fail to amaze me .First with the pictures you share. Then always the unique answers.

Thank you. Best with the prop situation

Jim

[Doc]

Re: C-85 Don's stroker mod. Need advice on prop

I still have a yellow tagged 1B90 7138 in the hangar.... Doc 

...procurred this prop from an estate sale...probably used on a floatplane with a Continental. Doc