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My recent Coupe, a C85, came with a 7351 prop on it, I guess left over from
when the engine was a C75. At the time, I was not aware that it was not the
correct prop. I did right away realize it just wouldn't spin up like
previous C85s I've had . In level flight it wouldn't turn over about 2325,
full throttle. I'd pull it back some and still get 2300 for cruise and
about 108 mph TAS. I had the prop cut and pitched to 7148 and could then
full throttle it up to about 2525. Cruising at 2400-2500 then gave me about
104 mph TAS. The extra length before cutting it kinda follows the load
issue you mentioned. roger
----- Original Message -----
From: "John Cooper" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>; "Ctech" <[email protected]>
Sent: Tuesday, February 28, 2006 10:46 AM
Subject: RE: FW: [COUPERS-TECH] C-85 STC, Performance, Props
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advice in this forum.]----
At 06:37 PM 2/27/2006, Ed Burkhead wrote:
Here's a thought that might contradict your assertion above: The C-85 on
a
Coupe with a 1A90 prop has the middle of the static rpm range at about
2125.
Yet most planes can reach 2575 at full power in level flight.
That's about a 450 rpm gain from static to full speed.
But the top end of the static range on the C-75 is 2100. It would lead me
to believe that the prop/engine combination *could* reach 2400 or 2500 at
max power, delivering over allowed power.
The answer is somewhere in the middle. The 73" diameter prop presents a
greater load per RPM, so to speak, and takes more power to spin up than
the 71" prop. At these ranges, an inch of diameter absorbs more power
than an inch of pitch.
But, if the engine/prop combination delivers 75 hp at 2275 rpm at sea
level,
then that prop which gives static rpm of 2100 would almost certainly be
able
to cruise at 2275 rpm, delivering 75 hp and a good cruise at 1260 lb.
The only time the engine produces 75 hp at 2275 is if the throttle is
firewalled, pressure is 29.92 and RPM is constant. If it's cruising at
2275 at part throttle, HP is less than rated.
Since we rarely cruise at sea level, the power is reduced a little or a
lot
from 75 hp, even if we were to cruise at 2275 rpm, so, maybe we wouldn't
be
running the power over what the engine can handle. (That engine is
identical to the C-85 and 75% of 85 hp is 63.75%)
What do you think?
My only real concern with operating over the red line, aside from the
legal issues, is McCauley's concerns with 73" props on C85s. They didn't
make that restriction because they wanted to sell shorter props.
BTW, I have a C90 with a 7152 prop that turns about 2125 static and maybe
2500 flat out on a very good day. I simply can't understand how a smaller
displacement, lower compression, lower HP engine with a smaller carb
throat could swing a bigger prop as fast. (Calibrated tach)
John Cooper, A&P
Skyport Services
PO Box 249
4996 Delaware Tnpk
Rensselaerville, NY 12147
518 797-3064
Fax 518 797-3865
www.skyportservices.net
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