Re: [COUPERS-TECH] C75 to C85

[roger anderson]

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Ed and Ed,

 

Excellent information.  I also have a 7351 on my C85.  I had already thought about the trimming process.  However, this prop was reconditioned only a 100 hrs ago, many years after it had already been used on and the engine had been upgraded to a C85 (done back in the '50s).  So I guess other owners at least accepted the prop and its performance as ok.  I get a static runup of about 2100+, so I'm in that ballpark.  I get a TAS of 108 at 2300, which is still partial throttle and about 65% on you other chart.  I wasn't aware that 73 is not an approved prop length for the C85.  Trimming without repitching might be a good way to get a little better climb performance while being able to spin it up to 2400 and keep the 108 TAS maybe.  At the moment the climb is ok but the wx. since I've had it has been cool.   Again, thanks for the info.   Roger

----- Original Message -----

From: Ed Burkhead

To: Joseph E. Robbins ; Coupe-Tech

Sent: Tuesday, February 24, 2004 9:28 PM

Subject: RE: [COUPERS-TECH] C75 to C85

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Ed,

 

I’m not a real propeller expert but here’re my thoughts for what they’re worth.  I’m going to ramble some trying to say what I want to say.

 

First, I’m guessing your plane is now a 415-C and has been (or should legally have been) flown at 1260 lb. gross weight.  If so, your 75 hp engine probably had the oomph to pull your plane through the air at a pretty good speed.  Referring to Paul Prentice’s chart in Fly About Adventures and the Ercoupe, I’m guessing you may have been cruising at 2275 rpm and getting about 106 mph.

 

According to Aircraft Specification A-718, for your C-75, a McCauley 1A90CF or 1B90CM is allowed with a length of 71-73 inches.  However, when you change to C-85, the same prop isn’t allowed to be more than 71 inches.  That’s so (as I understand it) the engine can spin the prop faster to develop the higher horsepower.  Also, on a C-75, the allowed static rpm range is 1950-2100.  After changing the engine, the prop has to have static spin in the 2025-2225 rpm range.

 

You’ll notice that the static rpm ranges overlap.

 

Again, as I understand it, if your static rpm is low, you have a prop that’s too steeply pitched.  This may be good if your plane is light and the prop can get you up to high speed for a high speed cruise.  If your plane is heavy, you’ll never get to that high speed and you’ll lug.  You’ll have terrible climb and terrible cruise.

 

If your static rpms are in the high end of the allowed range, you’ll get better climb but you may not get as fast a cruise.  With my 7146 prop, I got 2225 static but only got 100 mph at 2400 rpm.  I had a friend with a 7152 prop who got horrible climb and 90 mph cruise.  I could climb to 12,500’ anytime I wanted and once topped out at 14,500’ density altitude.  I don’t think my friend could get above 7,000’.

 

By type certificate A-718, Propellers and Accessories 6. (which applies to 415-C), or

By type certificate A-787, Propeller and Propeller Accessories, 4. (which applies to 415-D)

your McCauley prop MUST be 69.5-71” and you MUST have static rpm 2050-2225.

 

Bottom line, you have to get your prop length trimmed or get the FAA to sign off on a form 337 to keep what you’ve got.  I’d trim the prop because they had a reason for requiring the shorter length.  When I had mine reconditioned and re-pitched (20 years ago) it wasn’t that expensive.

 

When you get it trimmed, you can have it re-pitched if you want, down to 50” or 48.”

 

If you are a high-flyer or want to go mountain flying, or want to fly off hot, short fields, you might consider 46.”  I’ve been told 46” is “illegal” but no one can cite a reference.  The type certificate REQUIREMENT is to not exceed that static 2225 rpm.  My prop had this pitch and turned 2225 by my tach and I liked it – but that’s me.

 

Again, from my sources:

52”       High cruise – results undefined and may be bad

50”       “Normal”           108 mph cruise

48”       “Climb”             104 mph cruise

46”       “Extreme climb” 100 mph cruise (accurately measured by me)

 

 

Ed Burkhead

http://edburkhead.com/

[EMAIL PROTECTED]    (remove the QQQ)

 

-----Original Message-----
From: Joseph E. Robbins [mailto:[EMAIL PROTECTED]
Sent: Tuesday, February 24, 2004 5:46 PM
To: Ed Burkhead
Subject: Re: [COUPERS-TECH] C75 to C85

 

Ed...

I noted with great interest your response to Roger  re: props.

 

I currently have a C-75 that swings a CM 7351. What is your understanding regarding this prop? The engine will be a C-85 in a few days and I therefore will need to re-pitch.  Would appreciate  your thoughts.

 

According to McCauley the 7351 is the standard prop for C-75. 

 

Ed Robbins

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