Engine/Propeller efficiency
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We
have no professional or expert qualifications of any kind.
It's fully up to you to check any information you
find here with
standard aviation industry sources such as
aircraft maintenance
manuals, flying instruction books and, above all,
FAA regulations!
Paul Prentice published tables in his 1991 book "Fly About Adventures and
the Ercoupe." His tables have checked out pretty well, from my experience with
measured speeds during in-flight observations. Ed Burkhead
The following material is used by permission of the author and copyright
holder, Paul Prentice. Paul may be contacted at
Flyabout1@cs.com
to request
permission for any other use. All rights reserved.
For each engine, these are at the "recommended cruising RPM according to the
"Operators Manual" by Teledyne Continental Motors Form X30012 FAA approved
Dec. 1980.
All numbers @ 5.1 degrees Celsius @ 5,000 feet.
prop=ST7351 IAS=106 TAS=114
prop=CR7353 IAS=111 TAS=119
C-85 @ RPM=2400 prop=CL7148 IAS=104 TAS=112
prop=ST7150 IAS=108 TAS=116
prop=CR7152 IAS=112 TAS=121
C-90 @ RPM=2350 prop=CL7150 IAS=106 TAS=114
prop=ST7152 IAS=110 TAS=118
prop=CR7153 IAS=114 TAS=123
O-200 @ RPM=2500 prop=CL6948 IAS=103 TAS=111
prop=ST6950 IAS=108 TAS=116
prop=CR6952 IAS=112 TAS=121
For comparison: The C-85 with a 50 inch pitch prop. The 50 inch pitch means it would theoretically travel 50 inches forward during one turn.
113.6 miles/hour = (50 inches * 2400 rev/min * 60 min/hour ) / (12 inches/foot) / (5280 feet/mile)
ENGINE PROP COUPE AVG CRUISE TIAS TAS MAX IAS TAS
TYPE SIZE MODEL EFFICIENCY RPM SEA LVL 5000' RPM SEA LVL 5000'
C-75 7351 C-D 97 2275 106 114 2275 106 114
C-85 7150 CD-to-G 95 2400 108 116 2575 116 125
C-90 7152 F1-F1A 95 2350 110 118 2475 116 125
C-90 7153 A2-A2A 97 2350 114 123 2475 120 129
O-200 6950 D-G 91 2500 108 116 2750 118 127
These are the best table I've seen.
For a long time, the passed on wisdom was, with a C-85 engine,
7148 climb
7150 normal
7152 cruise
That may well have worked OK for Coupes limited AND FLYING with no more than 1260 lb. gross weight.
At higher weights, the climb performance wasn't there. At higher weights with the 7152 prop, it was possible to have terrible climb AND terrible cruise speed as the engine couldn't turn the prop enough to get up to good cruise speeds. I encountered Coupes that could barely climb and could only cruise at 90 mph at near full power.
Perhaps if those Coupes had stayed at a very light load they would have flown well.
At any rate, we made a loose consensus revision to
the "normal" props for the C-85 engine to:
7146 climb gives about 100 mph @2400 rpm
7148 "normal" gives about 104 mph @2400 rpm
7150 cruise gives about 108 mph @2400 rpm
I personally had my prop repitched to 7146 by accident (as I couldn't remember the "correct" number when the A&P called on the phone during the prop overhaul). I was happy with that prop.
With the 7146, I easily operated out of short fields down to 1800' (no obstacles) and based at 2400' with trees some distance past the field end. I was always off in the first half of the runway and had climb even at the higher gross weights. Over the Central States/Midwest, I could always get up to 12,500' even though it took an hour or so to get up there.
I went out West one time and flew at about 1320 pounds gross or so. On that trip, I started flying at dawn or before and made the last takeoff at 10:30 most days in the high desert. Even at the 6,700' field, I was off in the first 1/3rd of the runway and, though climb at 6,700'-12,500' was anemic I could climb.
On one 10:30 a.m. takeoff, the high desert up and down drafts exceeding 1,000fpm over powered any climb the airplane had so I found up drafts and circled in the thermals for climb.
At all times, I was able to get to 12,500' with the gross at about 1320 lb. - well, all but once. One afternoon in the high desert, I did an afternoon leg when the worst of the thermals were getting milder, down to turbulence I'd call moderate. In the moderate remaining turbulence, I was only able to get to 11,500' and cruising along, spun my whiz-wheel and found I was at 14,500 density altitude.
I tended to limit my rpm to 2400 in cruise because I'd read something saying that was the maximum recommended continuous cruise rpm. A number of mechanics told me I could go ahead and spin it up and it wouldn't hurt the engine at all. Considering that, with the flat, climb prop, I wasn't straining the engine at cruise, I probably could and would now consider spinning it up to cruise at the red line.
For me, cruising at 7146's 100 mph (carefully calibrated) was quite adequate to go to the Atlantic, the Pacific, the Canadian border, the Gulf of Mexico and a bunch of places in between.
Ed