Bob Sanders on Ercoupe Propeller Performance
June 12, 1950
Dear Ercoupe Owner:
We have been asked several times for more information about cruising speeds, recommended fuel consumption, and engine life. It has, therefore, occurred to us that the enclosed table might be of interest to you.
The tables are made up for both the 75hp and 85hp Ercoupes, using the standard pitch McCauley propellers in both cases. There will be some variation between individual airplanes, depending upon condition of the wings, antenna installations, etc., as well as the actual propeller and carburetor characteristics.
A check two or three points with your own airplane should tell you how close your Ercoupe is to the standard one tested. All variations should be proportional except that fuel consumption and. range at sea level cannot be used to correct altitude values and vice versa.
In checking your speed, fly over a straight road or railroad, at a given engine speed and. altitude, between two landmarks at least three miles apart and time your flight with a stop watch. Fly in both directions, compute your ground speed for each direction and average the two speeds. This will give you-true air speed. Check this against your airspeed meter reading. Remember (1) your airspeed meter may no longer read correctly, and (2) your air speed meter is only supposed to read your true air speed under standard conditions at sea level. You can use any of the popular computers to convert your air speed meter readings to standard conditions if tests are made at non standard condition. This will let you compute your air speed meter error for its reading at the time of your tests.
Tachometers are also subject to losing their accuracy: they may be easily checked by a local automobile speedometer shop.
The “Normal maximum” condition on these tables is the recommended cruising speed for average use. At these speeds the engine should give excellent life and little trouble. The “Conservative cruise” speed will give even longer engine wear and should be used when speed is of less importance or under conditions of extreme heat. The other conditions are maximum range for varying winds and will be helpful whom you are trying to stretch out your fuel to the next airport.
Note that at 80 mph with no wind at sea level you can get 23.4 miles for each gallon, but if you must buck a 30 mph headwind you will do better to fly at 97 mph when you get only 15.6 ground miles for each gallon. With a quartering wind, use only the component along your course (either head or tail wind) when figuring your best cruise speed and range.
Remember, if you put on a high pitch “cruise propeller” or a controllable one, you can get a higher cruise speed for a given engine RPM; but you are using more power to do it. Since most approved propellers have about the same cruising efficiency (the thinner metal ones are somewhat better than the thick wood), the fuel consumption and life of your engine will be about the same for a given airspeed, regardless of the propeller pitch, so long as you stay below the engine redline PPM. One exception to this is that extra fuel will be used with a steep pitch propeller when operating near full throttle, since the carburetor is designed to provide this extra fuel near full throttle to prevent detonation.
Sincerely, Bob Sanders
Additionally the data from flight tests by the Alon Company with a Continental C90-16F engine and a McCauley 1A90CF 71/52 propeller.