Lean Of Peak circa 1941-1942

by Ernie Ganas

I was reading Voices of My Peers, Clipper Memories which is a collection of stories from old time Pan Am Airways pilots, engineers and other staff. Not well polished since many of them were in their late 70's when they put pen to paper, but stories of overcoming adversity and flying the big boats etc. are really interesting if you are interested in aviation.

I came across the following which I'm going to quote from an engineer/pilot who joined Pan Am in 1941 and is now writing about his experiences with Pan Am Air Ferries which set up the ferrying of planes across the South Atlantic and even to Dutch East Indies in early 1942.

Here Jack Wahle (Cal 1941 in Aeronautical Engineering) is talking about a trip from Miami to the Royal Dutch Navy at Trincomalee, Ceylon which was done after he was part of a B-314 Clipper crew that took off from San Francisco and had to fly around the world to get to New York (due to the start of the war).

The next leg to Ascension Island, some 2,600 nautical miles, was the longest we were to face and with the prevailing headwinds tested the extreme range of the PBY. Pappy and Ken were very serious during the weather briefing and carefully studied the course and altitude options, not only for the best airplane performance, but also for clear skies to ensure accurate celestial navigation. They were also aware that I had a few tricks up my sleeve that I had picked up in the Pacific.

I drew up a "howgoesit" chart and explained how it worked, especially the graphical way it determined the "go" - "no go" decision on the basis of actual rather than forecast conditions.

I also explained some of the theory of engine operation in the lean mixture range. The received wisdom among pilots in those days was that leaning out the mixture made the cylinder head temperature go up. Period. This was because manual mixture controls were designed to put out a rich mixture so that excess fuel would help cool the engine at high power. As the airplane climbed to higher altitude, the thinner air eventually made the mixture so rich the engine would lose power. Leaning out the mixture would restore the power, but the temperature would increase somewhat due to less cooling by unburned fuel. Alarmed by the rising temperature, pilots would be reluctant to lean out any further. Actually, in a normal piston engine at constant altitude, further leaning out will cause the temperature to peak out and then decrease as the excess air now takes over the cooling job. The most efficient mixture in terms of horsepower as a function of fuel flow is well down on the lean or "air cooling" side of the power vs. mixture curve.

Pappy and Jerry were a bit skeptical, but in view of my recent conquest of the throttle lock, let me have my way. As it worked out we arrived a little late, but a little ahead on fuel so I gained a little more face.

Now since he was taught this between the time he joined Pan Am as a Engineering Officer in June 1941 and started flying in the fall of 1941 and the article is about flights in early 1942 it appear at least Pan Am knew about LOP operations in 1941.

Continuing the conversation..

by Stan Stewart
35-A33 KSAC

Must not have had "Auto Rich" and "Auto Lean" positions on the mixture controls like some of the R-1800 engines on the DC-3/C-47 airplanes had. Could Lindberg have made it to Paris running his engine ROP? I think I read somewhere Lindberg leaned at night by observing the length of the exhaust flame coming out of the exhaust stacks.

by Old Bob Siegfried

We often leaned by the color of the exhaust, though Lindbergh had the benefit of some study done by a professor at New York City University. He had developed a chart for Lindbergh to use that told him how much RPM drop he would get when the engine was operating at Best BSFC. that was explained to me by a student who worked with the professor who developed the numbers. The student's name was Johnny Miller.

PS, The Auto Rich and Auto Lean positions were both on the rich side of peak EGT. Auto Lean provided very close to Best Power mixture which is normally about fifty degrees Fahrenheit richer than peak EGT. To get to best BSFC, we had to manually lean the engines. On the later engines that were equipped with BMEP gauges, we used those instruments to set the mixture. That knowledge was widely available during the thirties, but it tended to get lost during the war expediency style training for WWII.

Happy Skies


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