XP-39: pros cons

[GregP]

A backfire can be caused by either too rich or too lean a mixture. The mixture WANTS to be at about 14.7 : 1. Anything with a mixture less than 14.7 : 1 is rich. If it gets below somewhere around 11 - 12 : 1, it can backfire. Anything with more than 14.7 : 1 is lean. Again, if it gets above 17 - 18 : 1, it can backfire.

Most pilots in WWII wanted to cruise slightly rich, at about 13.5 : 1 or so. It has only been since digital fuel injection that we cruise lean. Without the computers, it is way too easy yo ruin an engine with lean cruising. You have to be alert to changes in altitude, altimeter setting, and cylinder head temperature. Generally, you lean as you go up and adjust to the richer side as you descend. You can do it with cylinder head temperature alone, but a deliberate descent or climb or a noticeable change in atmospheric pressure will make you look at mixture, too.

Once you as an operator learns the right position of the mixture lever for proper operation, you can do correctly it with your eyes closed.

Here is a page from an Allison engine manual that shows how to tell by looking at the exhaust if it is rich or lean.

 

[Zipper730]

A backfire can be caused by either too rich or too lean a mixture.

With over-priming a problem, I assume on starting, rich would be more likely to cause trouble?

Most pilots in WWII wanted to cruise slightly rich, at about 13.5 : 1 or so. It has only been since digital fuel injection that we cruise lean.

Wait, I remember hearing that later in the war (1944-45) you'd see auto-lean used at high altitudes?

 

[GregP]

Usually rich from over-prime, as you say, when starting, but not necessarily.

As far as "auto" settings, yes ... you CAN let it run in auto-mixture. But, you can also adjust the mixture to suit yourself manually, just as you can in a typical light aircraft today.

Charles Lindberg was famous for coming home from a long flight in a P-38 with WAY more fuel than his mates. Using his leaning techniques, the range of the P-38 was considerably lengthened in the Pacific. But, it worked just as well anywhere else. He was cruising at higher manifold pressure and lower rpm. Up to a point, it was fine and did no harm. The trick is knowing when that point has been reached. At the time, it wasn't well-known. Today, it is and we let the run modern general aviation piston engines on the lean side of peak EGT with very accurate digital temp gauges. Back in the early 1940s, that was either a great recipe for winding up in the water or a good way to fly long over-ocean flights. Lindberg knew the difference.