Porterfield Airplane Club

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Hello, this Joel Harris former owner of NC32368. I a considering getting another Porterfield and am looking for data on hanging an 85-8 on it. I spoke with Joe Rankin years ago about it and do not fully recall the all the details of the conversion....has anyone here done that or have an airplane that was converted? My memory does recall Mr. Rankin telling me it was done a field approval.
Anyone out there able to give some feedback on this?

Best Regards....Joel

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As far as I know, Joel, Joe Rankin's Collegiate is the only one with a C-85.  Back in the mid-50's, my Collegiate, a FP-65 was upgraded to a Franklin 90, but using the accessory case of the original Franklin 65 to still be able to use the existing mount (so no starter & generator).  The major concerns the CAA had were: weight & balance; adequate fuel flow at climb attitude at full power; and adequate carb heat, which required rerouting the cabin heat duct from its muff to the carb airbox.  The irony is that Joe could have produced enough supporting data, as the TC holder, to have the C-85 added to the list of acceptable engines on the TCDS, but he lacked the funds and the enthusiasm.  I've never seen his drawings for his C-85 installation, so I have no idea how it differs from a stock CP-65.  Joe's son is sorting through Joe's documentation for Porterfield (Rankin) Aircraft, but he admonished me that he and his bride are also preoccupied with running their FBO/maintenance facility/flight school, so it will take a while before Joe's field approval documentation is available for you to use as a basis for your C-85 conversion.

I can scan and send you the documentation for 41Vermont's Franklin 90 upgrade, and maybe you could use that as a basis for your upgrade, but the FAA has been hesitant to grant field approvals for the last decade or so...but maybe your MIDO is more interested in antique airplanes than the Boston MIDO, which seems terrified of them.

Soft landings,


Morning, I see....thanks for the info. I am expect it may be a challenge.
I will keep you posted.

Thank you,


Trevor Bange said:

Joel and Andy,
I have a 35/70 Porterfield Sn. 242 here in Australia not far from Brisdbane with a Continental 75 in it. It was the original Le Blond radial but in 1950 to 1953 it was converted to a A75 Continental. In 1996 I upgraded to the C 75 - starter and generator, everything was approved nearly - the engineer did the approvals for the mounts, mogas use etc but forgot to add C 75 to the A75 approval. I have a hassle with this now as it does not come across as compliant for a change of egistration I want to do. I tried getting information on the original approval for the A 75 but the approval records were destroyed and the CAA has no suitable records eith. If someone can help in this area it would be appreciated.  The C 75 can be upgraded easily to the 85 with just a change in the carby throat I understand. It would make a big difference to performance.
My direct email address is : trevorbange@bigpond.com
Many thanks.

You're partially correct, Trevor, in that the C75 & C85 are very similar, with a few critical exceptions.  The connecting rods in the C85 are drilled and the pistons have more rings than the C75 to safely allow the 300 higher RPM the C85 can spin up to (2275rpm vs. 2575rpm).  This mimics the approach Continental took in upgrading the A75 to the A80. 

Converting a 35-70 to a 75C (with an A75-8) is pretty straight forward, as though they're certificated under different type certificates, the airframes are the basically the same.  However, adding the additional weight of starter, generator, etc. associated with a C75-12, will require a much different engine mount and cowl, as you've learned with #242. 

Where is the engineer who neglected to adequately proofread his approval request?  Can you find him and have him resubmit the approval request?  He should do it under warranty as his previous typo rendered his original work to be of little merchantable quality.  It might be that CASA wouldn't approve a C75-12, so he changed it to an A75-12 just to get it through.


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