this thread is like mud wrestling with a pig.
eventually you realise the pig actually likes it !
eventually you realise the pig actually likes it !
Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)
- Thread starter greybeard
- Start date
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wuzak
Captain
So, 20 Gallons to get to the cruising altitude, whether that is 5,000ft or 30,000ft?
Shortround6
Major General
Again, figures from the take off, climb and landing chart have nothing to do with range. The takeoff allowance for range calculation is 20gallons for this particular plane in clean condition.
And that 20 gallons gets you to 5,000ft. If you want to fly higher to do not pick and attitude and the range connected with that altitude and magically (pixie dust?) get it. You want to fly at 10,000ft or 20,000ft you have to expend the energy needed to lift the plane's weight to that altitude.
That's for combat at 15000ft. Are we cruising at 25000ft? Then combat should be at 25000ft. I figure combat GPH in column I by converting normal power to combat power by dividing the GPH by 2600 (max cont. RPM) and multiplying that by 3000 (combat RPM). At 25000ft GPH is 62 divided by 2600 X 3000 = 72GPH. 20 minutes at 72GPH = 24gallons.
Problem is you are being too simplistic. change your engine rpm by 400rpm and the impeller in the supercharger changes by 3840rpm. Power required by the supercharger changes with the square of the speed. Likewise the boost the supercharger supplies changes with the square of the speed of the impeller. Chart says at 15,000ft the plane cruises at 103gph but the Military power is 147gph at 14,600ft (close enough) so military power uses 43.7% more fuel per hour or minute. This may not be accurate for a number of reasons but it is a lot closer than simply comparing the rpm. This can be seen to be happening by comparing column II to column I. both columns use the same rpm but use different manifold pressures.
So we are back to 39 gallons for cruise range/radius.
And that 20 gallons gets you to 5,000ft. If you want to fly higher to do not pick and attitude and the range connected with that altitude and magically (pixie dust?) get it. You want to fly at 10,000ft or 20,000ft you have to expend the energy needed to lift the plane's weight to that altitude.
That's for combat at 15000ft. Are we cruising at 25000ft? Then combat should be at 25000ft. I figure combat GPH in column I by converting normal power to combat power by dividing the GPH by 2600 (max cont. RPM) and multiplying that by 3000 (combat RPM). At 25000ft GPH is 62 divided by 2600 X 3000 = 72GPH. 20 minutes at 72GPH = 24gallons.
Problem is you are being too simplistic. change your engine rpm by 400rpm and the impeller in the supercharger changes by 3840rpm. Power required by the supercharger changes with the square of the speed. Likewise the boost the supercharger supplies changes with the square of the speed of the impeller. Chart says at 15,000ft the plane cruises at 103gph but the Military power is 147gph at 14,600ft (close enough) so military power uses 43.7% more fuel per hour or minute. This may not be accurate for a number of reasons but it is a lot closer than simply comparing the rpm. This can be seen to be happening by comparing column II to column I. both columns use the same rpm but use different manifold pressures.
So we are back to 39 gallons for cruise range/radius.
Shortround6
Major General
I am using the 20 minute combat allowance because that is what the Air Force in their charts for operational radius. I am not "splitting the difference" but trying to estimate the military power fuel burn at 25,000ft.
Well our "expert" claimed the P-39 could be used to escort 8th Air Force bombers if only the high command hand not been so stupid. So it has to fly as high and as fast as the planes that were used to escort the bombers. Not putz around at 5,000ft 250 IAS at 5-6,000 providing target practice for AA gunners and low time Luftwaffe pilots. (British learned about low speed low altitude cruise in 1941)
Well, it is quick and easy
the calculator uses the 0.02 estimate correction and as long as we are all aware of it we are close to being on the same page.
The Air Corp jumped around a bit using true airspeed on some charts and IAS on other and not only mixed them up on the same chart but sometimes mislabeled them.
The P-39Q flight manual being a case in point. Column I being labeled IAS but unless you believe the P-39Q could cruise at 330mph IAS at 25,000ft (495mph true
)something is way off and that is the easiest explanation.
I use True Airspeed Calculator
simply because it is quick and easy, making no claims for accuracy for actual navigation purposes. Somebody else uses something different and the answer is off by 10-15mph out of 300 mph I am not going to argue.
Peter Gunn
Master Sergeant
The best way to sway an argument/debate with a real life pilot when you're not a real life pilot.
"A" pilot, what about a squadron?
Bifff - you taking notes here?
So you don't use any fuel going from 12,000ft to 25,000ft? And the chart is apparently wrong showing a range of 210 miles whereas you come up with 395 miles?
So you contend that in 1943, the P-39 was not only capable of long range high altitude escort deep into Germany, but for taking on German interceptors on more than an equal footing? The movie Fantasia called, they want their dancing hippos back.
Per Joe in his post above "how are you calculating this without OAT? Are you using a standard lapse rate in the calculator or a 0.02 estimation correction? If we are doing a model with no real world conditions (actual temp, atmospheric pressure winds) you can almost plug in any number you want."
Also you mentioned that since we're cruising at 25,000 ft, combat should be at same. Uh, have you ever read encounter reports? Mustang Encounter Reports
I would love to close this thread, but then another thread would get hijacked.
FLYBOYJ
"THE GREAT GAZOO"
And no matter how you slice it you're still burning a substantial amount of fuel if you take off and go straight to altitude, and please don't cloud the issue with some operational procedure or event.
OK - so doing it this way save you about 3-5 gallons from mine and SR calculations, I'll give you that, but during this simulated combat scenario you burnt up 1/4 of your fuel. Combine that with the fuel that got you to 25,000 feet and you better have a short trip home!
This is 100% wrong - the P-39N flight manual CLEARLY shows the internal fuel at 87 Gallons in the weight and balance report. Here....
So maybe this stealth 120 gallon fuel capacity you keep mentioning is "the fuel being made" when you climb as SR pointed out?
OMG, SMH - so the fuel burnt during that time just "goes away"? You magically put that fuel back in the tank and press on? CLIMB GRADIENT?!? That's the only thing I added in my calculations - ever heard of it???
And can it effectively fight at these altitudes? NO. If that was possible General Kenny would have kept the P-39 in the SWP and never messed with the P-38!
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FLYBOYJ
"THE GREAT GAZOO"
Agree 100% and that's why for simplicity I used IAS - our friend doesn't seem to grasp that temp and air pressure are major factors in computing IAS to TASWell, it is quick and easy
the calculator uses the 0.02 estimate correction and as long as we are all aware of it we are close to being on the same page.
The Air Corp jumped around a bit using true airspeed on some charts and IAS on other and not only mixed them up on the same chart but sometimes mislabeled them.
The P-39Q flight manual being a case in point. Column I being labeled IAS but unless you believe the P-39Q could cruise at 330mph IAS at 25,000ft (495mph true
something is way off and that is the easiest explanation.
I use True Airspeed Calculator
simply because it is quick and easy, making no claims for accuracy for actual navigation purposes. Somebody else uses something different and the answer is off by 10-15mph out of 300 mph I am not going to argue.
In fact the last kill of the Leningrad area PVO (air defence) was made by a pair of Airacobras from 103 GIAP with a pair of La-5s from 11 GIAP intercepting a Ju 88S at 9000m (29500 ft) on 8 March 1945, the German crew was taken prisoners.
FLYBOYJ
"THE GREAT GAZOO"
Juha - do you know how many claims P-39 were recorded by the VVS? I'm trying to find data for Soviet P-39 claims vs Luftwaffe claims of P-39s
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
This thread is sort of like a porch light with little P-39s buzzing around it...
P-39 Expert
Non-Expert
The 20 gallons gets you to 5000ft, then climbing to 25000ft toward the target nets additional range. Let's say it takes 15 more minutes to 25000ft and average climb speed is 220mph (170mph IAS) then the climb toward target adds 55miles to range. I don't think the pilot climbed to 25000ft over the base and then vectored toward the target, he vectored toward the target asap after takeoff. All that is figured into the chart.
Figuring combat GPH is an estimate since that figure is not provided in the chart. I believe my simplistic method of converting max cruise RPM (2600) to combat RPM (3000) is close enough at that altitude. Using your example the 62GPH at 25000ft converted to combat power would be 89GPH. I wish it were so but I just don't think it's possible. I'll stick with my figure, we agree to disagree.
Glider
Captain
That's the bit I like. It takes the P39N 1.45 mins to 5,000ft approx. 3.15 mins to 12,000ft and 8.4 mins to 25,000ft. According to P39 this extra 5+ minutes climb and the fuel you use in that time to get to 25,000ft quote 'have nothing to do with range'
I know that I am much happier in a Glider where this isn't concern, but I wouldn't trust my life in the hands of someone with this belief.
GregP
Major
Hi SR6,
I was hoping, but I suspected your very complete post earlier would not be enough. So, I fall back on one particular definition of insanity as "doing the same thing over and over while for a different result each time."
Perhaps some distraction?
1) What do they pack styrofoam in when they ship it?
2) Statistics will show that 1 out of every 4 babies born in the world is Chinese. So, if you have 3 kids and your wife is pregnant, do you ever worry about that?
3) If you're not supposed to drink and drive, why do they have drive-through liquor stores?
4) I bought some dehydrated water, but I didn't know what to add ...
ba da boom ...
I was hoping, but I suspected your very complete post earlier would not be enough. So, I fall back on one particular definition of insanity as "doing the same thing over and over while for a different result each time."
Perhaps some distraction?
1) What do they pack styrofoam in when they ship it?
2) Statistics will show that 1 out of every 4 babies born in the world is Chinese. So, if you have 3 kids and your wife is pregnant, do you ever worry about that?
3) If you're not supposed to drink and drive, why do they have drive-through liquor stores?
4) I bought some dehydrated water, but I didn't know what to add ...
ba da boom ...
Dear me, when will this end? Figuring out this stuff isnt an estimate. Read this. AC Eng Perf Analysis at R-R Which is about the RR XX Merlin in the main but also includes discussion of the Allison V 1710. The people at the time knew the far end of a fart about this "stuff". If you understand it all you are a better man than I, I just understand the principles. I KNOW the people at the time knew the actual and theoretical consumption of their engines at all engine speeds, altitudes and boost setting with a lot of other variables thrown in too.
Edit, from what I know the consumption related to the swept volume/2, the RPM, the boost pressure, and the mixture setting. The difference between max economy and max short term power consumption is typically 1:3. so 50gph and 150gph on a Merlin (in the roundest of round numbers) This is why a Spitfire could be running low on fuel after a short time in combat at 30,000ft.
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Glider
Captain
So you plan sending your fighters into the attack one at a time? Don't you think forming up is a good idea?
Simplistic is one word that fit's, that I grant you.
buffnut453
Captain
Could always ban him for the sheer monotony generated?
Yes, I know...that's neither fair nor nice. I'm beginning to realize that he simply doesn't know what he doesn't know, hence he thinks everything he does know is correct (if you get my drift).
FLYBOYJ
"THE GREAT GAZOO"
So I guess there's no such thing as a climb or descent gradient? I guess my C172 has just turned into a helicopter! Oh but wait, it doesn't matter because I have reserve fuel that's specifically for take off and landing! 
buffnut453
Captain
Just had to post this...in case there are any Blackadder fans out there:
It reminded me of someone....
It reminded me of someone....
Glider
Captain
It goes to show that if someone has a belief you cannot discuss things with them, because that have a belief. That belief encourages them to make wild assumptions and statements that support that belief because they need something to support the belief. To them it's logical and simple. How many times in the threads have the changes to armour, COG, installing new engines been described as 'simple' and 'obvious' and any evidence to the contrary more, or less ignored.
The evidence is irrelevant as they have a belief. When the evidence is against them, they often change the subject and ignore the unanswered questions. Partly because they know that they cannot supply the answers, but to recognise that would dent the belief and that cannot be allowed to happen.
You will notice that at the start of all this there was a recognition from P39 that the aircraft was better at low/medium altitude than higher up, which is true, it is better lower down. The problem was that he believed that it was supreme at low / Medium altitude which it clearly isn't. Now that has been proved beyond any shadow of doubt, he is now trying to pretend that it was better at 25,000ft, which a) it clearly isn't and b) Is contrary to his initial premise that it was better at lower altitude.
The belief that the P39N was some kind of wonder weapon ignores the fact that was little more than a twice around the airfield before it runs out of fuel fighter, ( a little harsh I know but you get the picture). The fact that a Typhoon could lug 2,000lb bombs further that the P39N can go clean sort of proves the point. I would expect the P51 to do something similar and the P40 probably has a better payload range. In fact the P39N doesn't really have much of a payload.
Why was the P39Q pretty much the last production version of the P39 if the P39N was so good?. I don't actually know the detailed reasons but we can assume it was because it was an improvement including the increase in fuel. The users were happy to accept the lower performance for those advantages, which tells me that there was something wrong with the P39N.
The evidence is irrelevant as they have a belief. When the evidence is against them, they often change the subject and ignore the unanswered questions. Partly because they know that they cannot supply the answers, but to recognise that would dent the belief and that cannot be allowed to happen.
You will notice that at the start of all this there was a recognition from P39 that the aircraft was better at low/medium altitude than higher up, which is true, it is better lower down. The problem was that he believed that it was supreme at low / Medium altitude which it clearly isn't. Now that has been proved beyond any shadow of doubt, he is now trying to pretend that it was better at 25,000ft, which a) it clearly isn't and b) Is contrary to his initial premise that it was better at lower altitude.
The belief that the P39N was some kind of wonder weapon ignores the fact that was little more than a twice around the airfield before it runs out of fuel fighter, ( a little harsh I know but you get the picture). The fact that a Typhoon could lug 2,000lb bombs further that the P39N can go clean sort of proves the point. I would expect the P51 to do something similar and the P40 probably has a better payload range. In fact the P39N doesn't really have much of a payload.
Why was the P39Q pretty much the last production version of the P39 if the P39N was so good?. I don't actually know the detailed reasons but we can assume it was because it was an improvement including the increase in fuel. The users were happy to accept the lower performance for those advantages, which tells me that there was something wrong with the P39N.
