Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (1 Viewer)

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P-39 Expert said:
Because the only difference in fuel consumption at 25000' is RPM. Throttle set at full, mixture at auto rich. No other changes except going from 2600rpm to 3000rpm. Your figures at 14000'-15000' have no relevance to fuel consumption at 25000'.
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Do you realise how funny that is, you may think changing RPM is a small thing, it changes everything. If a supercharger, engine and propeller are past optimum altitude, increasing RPM just makes the problems worse not better, like flooring the throttle when your wheels start spinning on snow.
 
Shortround6 said:
Hmmm, the figures at 14000-15,000ft were ALL at full throttle, mixture set at auto rich. They do show a pattern. So will every other aircraft engine.

Don't tell me I am wrong, show me I am wrong. Tell me where the power to overcome the increased internal friction and the increased power to turn the supercharger comes from?

The figures from the mid teens show a 26.6 % increase in fuel consumption going from 2600rpm to 3000rpm for a 12.5% increase in power.

power at 25,000ft for an Allison with 9.60 supercharger gears and no RAM at 2600rpm was about 670hp. You want a10.4% to 15% increase in power for a 15% increase in fuel consumption. NO other aircraft engine I know of could do that.
Click to expand...

It would appear that someone hasn't heard of the Second Law of Thermodynamics.
 
GregP said:
Hey pben.

He gets it from the P-39Q manual, page 25, chart for 8100 to 7200 pounds ,with 75 gal external tank. First set of columns, there is an entry for 25,000 feet. It says 2600 rpm, 267 mph T.A.S., Full Throttle, 62 U.S. gallons per hour (max continuous power). The climb data from the same manual says it will climb at 750 fpm at 135 mph in a combat climb and 650 fpm at 140 mph in a ferry climb at 25,000 feet. Combat missions were at 3000 rpm and 44..5 " MAP. Ferry mission were at 2300 rpm, 31" MAP (page 23).

When he gets to 25,000 feet, the pilot has to reduce to max continuous (2600 rpm, F.T.), level off, and accelerate to 267 mph. He'd better be quick about it because the range is not much, so he has to turn around rather quickly.

Range is unknown because the range tables are at S.L. and 12,000 feet. Ain't much performance at 25,000 feet. An old Spitfire from 1941 could climb at 3 times the rate, was faster, and had better range.

Let's look at it for the P-39Q. Assume for the sake of argument we are on the drop tank from engine startup, to save arguments and get the best possible result.

87 gallons internal, 75 gallons aux tank. Total = 162 gals.

Climb to 25,000 feet at combat settings takes 13.1 minutes and eats up 39 gallons, leaving 123 gallons, per the manual. Assume we average about 140 mph (goes from 160 mph to 135 mph along the climb for best rate). That moves us forward about 29.5 mile. Let's call it 30 miles.

Assume we cruise at 25,000 feet at max continuous (2600 rpm, F.T., 62 gph burn). We have 36 gallons of aux tank fuel left. That's enough to go for a while 34.8 minutes at 267 mph T.A.S., if we instantly accelerate from the 135 mph climb speed to cruise speed. 34.8 minutes at 267 mph gets us 155 miles farther from home. We are now 155 + 30 = 185 miles from home and we drop the aux tank because it is EMPTY.

Sine we now have no external stores, we use the no external stores chart. Here's where it REALLY gets interesting. We don't change power because we are at full throttle, but our 267 mph T.A.S. with the external tank now magically jumps to 330 mph I.A.S. (if you believe the P-39Q manual). On a standard day, 330 mph I.A.S. becomes 498 mph! So dropping the 75-gallon tank somehow magically makes us accelerate from 267 mph T.A.S. to 498 mph T.A.S. , according to the P-39Q manual.

I think some marijuana was somehow involved in writing this manual. I'll make the assumption that they MEANT to say T.A.S. in both places. So, we now accelerate from 267 mph to 330 mph. Again, pretty doubtful, to me. But, let's run with it.

OK, we now run into our 10 minutes of combat at 25,000 feet and 62 gph and somehow, we manage to engage in this combat at 25,000 feet without losing any height and without getting any close or farther from home. Doubtful, but what the heck, let's ty it. The 10 minutes will eat up 10.33 gallons, leaving us 76.67 gallons of fuel, still at 25,000 feet and 330 mph T.A.S., which takes us 408 miles before we are a glider pilot.

But, suppose I want my half hour of reserve so I'm NOT a glider pilot. That means we only can use 45.6 gallons of the 76.67 gallons we have remaining, still at 25,000 feet and 62 gph, still at full throttle. That means we can only go 243 miles instead of 408 miles.

So, we went 185 miles, had combat for 10 minutes, and we now have 243 miles of range left. Our total distance we can travel will be 185 + 243 = 428 miles, so our combat radius range is now 214 miles and we end up over home base at 25,000 feet with 31 gallons of fuel remaining. Great.

We went from less than 190 miles range from my earlier post all the way to 214 miles range. We gained 24 miles * 2, or 48 miles. Nice.

But, to do that, we needed to be on aux fuel from takeoff (likely against procedures), needed instant acceleration from climb to cruise and instant acceleration from cruise with tank to cruise without tank, needed an instant turnaround at 214 miles, and we needed to have combat joined sometime AFTER we used up the entire aux tank, and also needed the combat to stay at 25,000 feet and get no closer or farther from home.

Offhand, I'd say we needed an absolutely ideal P-39Q mission to get a whole 214 miles from home. Definitely a niche airplane. And my earlier post with 170 - 185 miles of radius would be about right. And I don't think a P-39Q will cruise at 330 mph at 25,000 feet unless there really IS a flying pig somewhere in regular passenger service. At least you don't have to clear the plugs because you're already at full power.

This is futile, but was a bit fun, considering the 498 mph T.A.S. thing. A 498 mph P-39Q? Naahhhhhhhhh ....

Cheers.
Click to expand...
You went to a whole lot of trouble to quote combat radius on 87gal internal and a 75gal drop tank. All P-39s would hold 120gal internal and carry a 110gal drop tank, same as what was at the fighter bases in east England.

And don't climb to 25000' at combat setting (careful, Flyboy doesn't know what that means, he can't find it quoted anywhere), use the ferry setting so that it takes you 31min and you don't burn up your engine. And you will have traveled 110mi (170IAS average = 220mphTAS). But don't figure that into your range because you may not be heading to your target.

Learn how to use the Flight Operation Instruction Chart (range chart). It doesn't include any figures from the Takeoff, Climb and Landing chart. Total fuel 230gal, less takeoff and climb reserve 20gal, 20min combat at 25000' 25gal, and 20min landing reserve 10gal leaving 175gal. Divide that by 62GPH = 2.8hrs flying time x 267mphTAS = 748mi. Divide by 2 for radius 374mi. Warmup and takeoff on internal, switch to drop tank as soon as gear/flaps are up and climb speed is reached. Simple form up as lead pair take a wide turn to target vector and the 7 following pairs make progressively narrower turns to form up the 16 plane squadron and vector to target. Cruise starts even before the 20gal T/O/Climb allowance has gotten you to 5000' as your climb to 25000' is on target heading. Drop tank fuel 90gal (110gal less 20gal T/O reserve) gets 387mi (90 divided by 62gph = 1.45hrs x 267mph). When the drop tank runs dry switch to internal and start home. If combat occurs and the drop tank is dropped before it is empty then combat radius will be shorter, just like any fighter carrying a drop tank. If combat occurs just as the drop tank runs dry then deduct combat allowance 25gal and landing reserve 10gal from the 120gal internal fuel (85gal left) and come home at 330TAS (85 divided by 62GPH = 1.4HR x 330mphTAS = 452mi).

Learn how to use the correct chart, it's much easier to use and more accurate.
 
P-39 Expert said:
You went to a whole lot of trouble to quote combat radius on 87gal internal and a 75gal drop tank. All P-39s would hold 120gal internal and carry a 110gal drop tank, same as what was at the fighter bases in east England.

And don't climb to 25000' at combat setting (careful, Flyboy doesn't know what that means, he can't find it quoted anywhere), use the ferry setting so that it takes you 31min and you don't burn up your engine. And you will have traveled 110mi (170IAS average = 220mphTAS). But don't figure that into your range because you may not be heading to your target.

Learn how to use the Flight Operation Instruction Chart (range chart). It doesn't include any figures from the Takeoff, Climb and Landing chart. Total fuel 230gal, less takeoff and climb reserve 20gal, 20min combat at 25000' 25gal, and 20min landing reserve 10gal leaving 175gal. Divide that by 62GPH = 2.8hrs flying time x 267mphTAS = 748mi. Divide by 2 for radius 374mi. Warmup and takeoff on internal, switch to drop tank as soon as gear/flaps are up and climb speed is reached. Simple form up as lead pair take a wide turn to target vector and the 7 following pairs make progressively narrower turns to form up the 16 plane squadron and vector to target. Cruise starts even before the 20gal T/O/Climb allowance has gotten you to 5000' as your climb to 25000' is on target heading. Drop tank fuel 90gal (110gal less 20gal T/O reserve) gets 387mi (90 divided by 62gph = 1.45hrs x 267mph). When the drop tank runs dry switch to internal and start home. If combat occurs and the drop tank is dropped before it is empty then combat radius will be shorter, just like any fighter carrying a drop tank. If combat occurs just as the drop tank runs dry then deduct combat allowance 25gal and landing reserve 10gal from the 120gal internal fuel (85gal left) and come home at 330TAS (85 divided by 62GPH = 1.4HR x 330mphTAS = 452mi).

Learn how to use the correct chart, it's much easier to use and more accurate.
Click to expand...
After combat, can you land and put the tank back on, to keep your combat radius? I am thinking outside of the box here.
 
P-39 Expert said:
You went to a whole lot of trouble to quote combat radius on 87gal internal and a 75gal drop tank. All P-39s would hold 120gal internal and carry a 110gal drop tank, same as what was at the fighter bases in east England.
Click to expand...
My understanding was that the P39N (the one you always seem to quote) normally carried approx 80 gallons internally instead of the 120 gallons to reduce weight and improve performance.

So make up your mind. Either :-
a) Use the smaller tank when discussing range or
b) Use the lower performance when discussing performance
c) Try and respond to some of the many questions people have asked you
 
P-39 Expert said:
And don't climb to 25000' at combat setting (careful, Flyboy doesn't know what that means, he can't find it quoted anywhere)
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Yep - because it's a BS term made up by you "(in balance", "tail cone", etc.) Show us in the Flight Manual where that term is?!?!?!?
P-39 Expert said:
Learn how to use the correct chart, it's much easier to use and more accurate.
Click to expand...
I think you've been shown by at least 4 people on here that it's YOU who doesn't know how to use the chart and I'm still waiting to hear about a cross country flight you planned and flew using similar charts!!!
 
Last edited:
P-39 Expert said:
You went to a whole lot of trouble to quote combat radius on 87gal internal and a 75gal drop tank. All P-39s would hold 120gal internal and carry a 110gal drop tank, same as what was at the fighter bases in east England.

And don't climb to 25000' at combat setting (careful, Flyboy doesn't know what that means, he can't find it quoted anywhere), use the ferry setting so that it takes you 31min and you don't burn up your engine. And you will have traveled 110mi (170IAS average = 220mphTAS). But don't figure that into your range because you may not be heading to your target.

Learn how to use the Flight Operation Instruction Chart (range chart). It doesn't include any figures from the Takeoff, Climb and Landing chart. Total fuel 230gal, less takeoff and climb reserve 20gal, 20min combat at 25000' 25gal, and 20min landing reserve 10gal leaving 175gal. Divide that by 62GPH = 2.8hrs flying time x 267mphTAS = 748mi. Divide by 2 for radius 374mi. Warmup and takeoff on internal, switch to drop tank as soon as gear/flaps are up and climb speed is reached. Simple form up as lead pair take a wide turn to target vector and the 7 following pairs make progressively narrower turns to form up the 16 plane squadron and vector to target. Cruise starts even before the 20gal T/O/Climb allowance has gotten you to 5000' as your climb to 25000' is on target heading. Drop tank fuel 90gal (110gal less 20gal T/O reserve) gets 387mi (90 divided by 62gph = 1.45hrs x 267mph). When the drop tank runs dry switch to internal and start home. If combat occurs and the drop tank is dropped before it is empty then combat radius will be shorter, just like any fighter carrying a drop tank. If combat occurs just as the drop tank runs dry then deduct combat allowance 25gal and landing reserve 10gal from the 120gal internal fuel (85gal left) and come home at 330TAS (85 divided by 62GPH = 1.4HR x 330mphTAS = 452mi).

Learn how to use the correct chart, it's much easier to use and more accurate.
Click to expand...
1626909570676.png
 
WOW...ok, I did a bit of searching on the interwebs for this mysterious throttle setting called "Combat Power" and lo and behold, I found it!

In a wargaming forum for IL-2 Sturmovik.

About the 20th post down, the guy is roasting the P-39 for poor performance (gasp) and this quote is from their post:
Dogfighting on combat power is equally frustrating, and I constantly find opponents slipping away in situations where I would certainly have caught them in a Yak or MiG.
Click to expand...
(See the full conversation here: Thoughts on the P39...)

So now it seems rather clear how "someone" got their credentials with the Bell Product.
 
GrauGeist said:
WOW...ok, I did a bit of searching on the interwebs for this mysterious throttle setting called "Combat Power" and lo and behold, I found it!

In a wargaming forum for IL-2 Sturmovik.

About the 20th post down, the guy is roasting the P-39 for poor performance (gasp) and this quote is from their post:

(See the full conversation here: Thoughts on the P39...)

So now it seems rather clear how "someone" got their credentials with the Bell Product.
Click to expand...
Amazing! I bet you can also find "Flux Capacitor" in there as well!
 
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