Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)

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Hi P-39 Expert.

My P-39Q POH manual says 87 gallons. It's a pdf available online. Here is the address:

https://airandspace.si.edu/webimages/collections/full/Pilot's%20flight%20manual%20for%20P-39%20Airacobra.pdf

Look on page 21. 87 gallons for the P-39Q. That's why I used 87 gallons. I just picked a manual, I didn't deliberately cherry-pick one with less fuel.
 
I checked my P-39N-0 and N-1 manual. It says 87 gallons internal, too. Then I checked my P-39K manual. It says 104 gallons internal + 16 gallons internal overload, for 120 gallons.

Let's see. They built 9,558 to 10,092 P-39 Airacobras. The P-39N accounted for 2,095 of them. The P-39Q accounted for 4,905 of them. That's 7,000 P-39s in the Q and N models alone, or 73% of them if you use the 9,558 build total.

They built 210 P-39K models. That's 2.2%. Not exactly the "mass-production model," is it?

The vast majority of P-39s that got sent anywhere had 87 gallons of internal fuel and about 2% of them held 120 gallons internal fuel if you look only at the K models. I decline to look any farther into the other 25% of P-39's. Feel free. But, and here's the thing to notice, the 87-gallon models were built AFTER the P-39K's were built.

Maybe they found out that the 120 gallons of internal fuel were just too much for the airplane to fighter well with? Whatever the reason, the later P-39s did not hold 120 gallons.

Methinks you are cherry-picking again, trying desperately to use the absolute lightest P-39 you can find, with the highest internal fuel tankage you can find, and then use the highest speed and climb numbers you can find along with the longest range you can find.

The things is, these things above did not happen on the same model P-39. You need to choose a single model, accept the weight, accept the fuel specified, and accept the numbers that aren't in column 1 of the cruise charts. You also need to accept that the P-39 didn't fight very much at 25,000 feet. It was in its element at 12,000 feet and below. Maybe 15,000 at the highest. It was mainly seen within 175 miles from the departure point.

But, I'm pretty sure you will ignore this and continue to claim 120 gallons of internal fuel. After all, it's in the simulator, right?
 
Well, having received an answer (not a particularly good one since, again, it missed a key part of the question) for my post #2746, I'm now counting pages since my last question at post #3011....three have gone past so far.

I'm reminded of the great quote by the author Douglas Adams "I love deadlines. I love the whooshing noise they make as they go by." Can we implement a whooshing noise as each new page is added to a thread we're watching?
 
BINGO!!!!

 
The "D"s all had 120 gallons, the K & L & M all had 120 gallons.

The Q-1 had 87 gallons.
The Q -5 had 110 gallons (?)
The Q-10/-15/-20/-30 all had 120 gallons.

120 gallons was quite doable. (leave out the wing guns for weight compensation )

However the whole thing is based off the "N" and one extraordinarily good test and a one or two strange numbers in the flight charts.

A P-39 with a drop tank, no matter what model, is not fast enough at altitude to do escort work. The P-38 and P-47 could cruise at over 300mph at 25,000ft with drop tank/s attached and not use column 1 on the charts.
A P-39 without drop tank, no matter what model, is 20-40mph slower than the P-38s and P-47s of mid 1943 and slower than the 109s. at the altitude the bombers are flying at.
A P-39, no matter what model, at 20,000ft and above has the worst power to weight ratio of the available allied fighters and of the defending fighters.

the P-39 is a low drag airplane and can reach a pretty good speed if given enough time. But it can't bleed off speed in maneuvers and get it back quick.
P-39 as a bomber escort for B-17s would be of less use than the Bf 110 was to the Germans in 1940.

Heck, stick a 25imp gallon tank in the back of a Spitfire IX and a 90 gallon tank under it and you would have a better escort fighter than a P-39.

They did wind up sticking a 41 gallon and a 33 gallon tank in the back of Spitfire IX so it was possible. Maybe not a good idea but possible.
 
A P-39 that just dropped it's tank was 10mph slower at all engine settings than a P-39 that never had one fitted, stated in the frequently quoted tests)

The main tanks of a Spitfire were increased from 85 to 95 gallons with the two speed supercharger. They could and did also put 25 gallons in the wings (12.5 in each) plus as you say a rear tank which was fitted in some especially the MkXIV. So 120 gallons plus is easily possible in the Spitfire. BUT any reading even on wiki explains that the extra 10 gallons in the Spitfire main tanks didnt increase range or endurance, it just did everything more quickly. Only the P-39 gets the free lunch where it cruises at 95% of its top speed and uses 1/3 of its maximum power/fuel consumption even with a huge tank strapped underneath.

Are any of the performance figures at 25,000ft with 100gallons of fuel on board, as in external tank just dropped?
 
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