Engine/Propeller efficiency

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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.
C-75 @ RPM=2275    prop=CL7349    IAS=102    TAS=110
                   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