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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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