XP-39: pros cons

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.

GregP said:

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

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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.

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

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.

Sorry for necroing the old thread

Shortround6 said:

We may run into the planned operational endurance vs the ferry range endurance/fuel capacity.
A lot of US fighters carried more fuel than they were estimated at/tested at/approved for for ferrying.
XP/YP-39 was very overweight. Performance figures with 200 gal on fuel on board?
Estimates do vary
Neither plane actually flew with the 'paper' engines that the estimates were based on (1000hp).

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XP-39 was light. Vee's says 5500 lbs for the XP-39. K-5054, Spitfire's prototype, weighted 5332 lbs, while not having the turbo, no nose wheel, and was with small fuel tankage. Speed figures with 200 gals on the board will barely change (in case the prototype was tested with half tankage), climb is another question.

The notion that XP-39 was overweight is worthy of the same treatment as the 390-400 mph speed figure for the XP-39, ie. a good wooden stake in the heart.

Tomo, how about adopting the USN's XFL as a land based fighter?

GrauGeist said:

Tomo, how about adopting the USN's XFL as a land based fighter?

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It was with worse aerodynamics than the P-39, mostly due to the 'exposed' radiators and a bigger & thicker wing. Barely beating 330 mph in it's un-armed form.

Just having the Allison company making a reliable 'fast' drive to the impeller (9.60:1 instead of the 8.80:1) by, say, early 1942, would've meant a lot to the whole P-39 program. See here the P-39C with a few guns removed and such the S/C drive making more than 400 mph (ie. some 30 mph better than the standard P-39C).
Even better if Allison makes a variable speed drive to their S/C, like they did for the auxiliary S/C on their 2-stage V-1710s, but this time just for the single S/C on the engines found on the P-39s, P-40s and P-51s. Again, by early 1941, and with the drive giving the variable drive ratios between, say, 7:1 (for lower altitudes and take off) up to 10:1 (for higher altitudes).

tomo pauk said:

XP-39 was light. Vee's says 5500 lbs for the XP-39

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tomo pauk said:

The notion that XP-39 was overweight is worthy of the same treatment as the 390-400 mph speed figure for the XP-39, ie. a good wooden stake in the heart.

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Unfortunately the myth is true.

5500lbs is the proposed gross weight.
The XP-39 weighed 4,495lbs empty when first inspected. Unfortunately it was supposed to carry a 1550lb useful load.
The Gross weight had reached 5855lbs before the plane was finished and it weighed in at 6104lbs when it got to Wright Field

Weights are from pages 84-85 of "Cobra" by Birch Mathews.

They may have been carrying ballast, there were certainly no guns fitted. Granted the XP-39 was fitted with heavy accessories like a constant speed prop which the Spitfire prototype was not cursed with
The YP-39 was tested at 6592lbs. and the P-39C was tested at 6689lbs.

This contains an interesting tidbit. While the speed and climb were done at 6592lbs they got landing speeds at 6300lbs and at 7000lbs
P-39Cs had no armor or self sealing tanks.

tomo pauk said:

It was with worse aerodynamics than the P-39, mostly due to the 'exposed' radiators and a bigger & thicker wing. Barely beating 330 mph in it's un-armed form.

Just having the Allison company making a reliable 'fast' drive to the impeller (9.60:1 instead of the 8.80:1) by, say, early 1942, would've meant a lot to the whole P-39 program. See here the P-39C with a few guns removed and such the S/C drive making more than 400 mph (ie. some 30 mph better than the standard P-39C).
Even better if Allison makes a variable speed drive to their S/C, like they did for the auxiliary S/C on their 2-stage V-1710s, but this time just for the single S/C on the engines found on the P-39s, P-40s and P-51s. Again, by early 1941, and with the drive giving the variable drive ratios between, say, 7:1 (for lower altitudes and take off) up to 10:1 (for higher altitudes).

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Well, how can we clean up the XFL?

Yes, the wing was slightly larger in width and area than a P-39D, but it was a bit shorter and weighed (empty) about 300 pounds less than the P-39D and this does not take into the consideration of the arresting gear, either.

The stated weight also includes two .30 MGs and a single .50 MG as far as I am able to tell, if it had the 37mm installed, the weight would be more, of course.

FWIW In my notes I have the P-39C weighing 7090 lbs TOGW with 171 lbs of armour and full ammo and fuel (170 USgal). The fuel tanks were not self-sealing and the radio installed was the SCR-283. I suspect that the 6689 lbs gross weight was with the 'normal' 104 USgal fuel load as listed at the time.