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 !
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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.
I computed a 10 minute combat at max power (147 GPH) in my calculations assuming a quick powerful encounter, this was a subjective choice. Why are you splitting the "combat allowance" between max continuous and military power? Is this subjective or do you have a reason for this?
If this is being used for a cruise, unless you're time pressed, why not use Column V for Maximum range? I mean, isn't the purpose of this exercise is to squeeze every mile we can out of this dog?
Using a IAS to TAS calculator - 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.
Please allow me to clarify the Flight Operation Instruction Chart (range chart) for you.
The range chart is all the pilot needed to plan a mission. To compute range for a mission the allowance for warm-up, takeoff and climb to 5000ft (20gal for P-39N) was deducted from total fuel then that net fuel was divided by gallons per hour and multiplied by TAS (not IAS). That's how all the ranges in each column are computed. For example look at column III at 15000ft. 70net gal divided by 67GPH X 300 TAS (242 IAS) = 313miles. The graph shows 300miles but the actual miles were rounded down a little as a safety factor. Same for all the columns at all altitudes (except column I). Gallons divided by GPH X TAS = range. The Fuel from Sea Level on the Take off, Climb and Landing Chart was not used since it already included the 20gal allowance for Warmup, Takeoff and Climb to 5000ft and didn't account for miles gained toward the target during climb.d
To get combat range simply deduct the 20min combat reserve and 20 minute reserve for landing in addition to the 20gal reserve for takeoff. To get combat radius take half of the combat range. The reserves could vary if you wanted a longer combat or landing reserve.
The range figures for column I (max cont. power/normal power) vary widely depending on what altitude is used. Right above the range figures in column I it says "At 12000ft Only" meaning the range figure is accurate only at 12000ft. This is because the gallons per hour (GPH) vary from about 100GPH up to 15000ft and then reduce steadily to only 62GPH at 25000ft. So range at 25000ft would be 70gallons divided by 62GPH X 350mph TAS (233mph IAS at 25000ft) = 395mi instead of the 210mi shown on the chart. This is the result of lower GPH coupled with higher TAS at 25000ft than at 12000ft. The other columns II through V show what throttle/propeller/altitude settings will result in the range for that column. Column I fixes the power setting at max. continuous (2600rpm) so the GPH and TAS will vary significantly as altitudes increase. Naturally I prefer to use 120gallons internal instead of the 88.
As to whether the P-39N will fight at 25000ft, the cruising speed in column I was 350mph (233 IAS), the top speed was 370mph and the rate of climb (clean) was 1950fpm per the Wright Field performance tests. Climb at 25000ft was better than any P-40, P-38F/G, P-47, P-51A, F4U, F6F, Zero, Ki-43 or Typhoon in 1943. About the same as a Me109G. But not nearly as good as the Spitfire IX. P-39N was certainly competitive in 1943.
There is a IAS-TAS conversion chart in the P-39Q manual that curiously is not in this P-39N manual.
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.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.
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!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.
This is 100% wrong - the P-39N flight manual CLEARLY shows the internal fuel at 87 Gallons in the weight and balance report. Here....120 less 60 = 60gallons for cruising.
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???Again, you don't use any figures from the Takeoff, Climb and Landing Chart in computing range.
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!Actually for the P-39N service ceiling was 38500ft, combat ceiling was 31000ft (climb at 1000fpm) and the Flight Operation Instruction Chart (range chart) says it will cruise at 30000ft.
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.
Wait... a P-39 fighting at 25,000 feet...
...
BWAHAHAHAHAHAHHAHAHA
You're kidding right? Right?
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.
This thread is sort of like a porch light with little P-39s buzzing around it...I would love to close this thread, but then another thread would get hijacked.
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.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.
So, 20 Gallons to get to the cruising altitude, whether that is 5,000ft or 30,000ft?
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.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.
So you plan sending your fighters into the attack one at a time? Don't you think forming up is a good idea?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.
Simplistic is one word that fit's, that I grant you.I believe my simplistic method of converting max cruise RPM (2600) to combat RPM (3000) is close enough at that altitude.
I would love to close this thread, but then another thread would get hijacked.