windhund116
Senior Airman
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- Jul 3, 2017
Yeah. That is what I was taught by a mechanic. Convection action.
XP-39 II - The Groundhog Day Thread
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Yeah. That is what I was taught by a mechanic. Convection action.
I don't know, I'm a historian by training without any formal technical training, being only interesting in also the technical side of warfare. One Soviet graph attached.
P-39 Expert
Non-Expert
Gosh, this report stated that lightened P-39Ks could fight up to 27000'. Only reduced by 600lbs. Wonder who came up with that idea?
You need to read that entry again. The statement was that after reducing the weight, the P-39K had its SERVICE CEILING increased to 27,000 feet. It doesn't mean the aircraft could fight there. It just means the aircraft could finally GET THERE. I wonder how low the service ceiling was before weight was removed.
I can't quite read the number but it looks like either 630 or 650 pounds.
- Ivan.
GregP
Major
The weight would likely be the 30-cal wing guns and installation kits, plus a little extra. I did notice that the weight saved by changing the 37 mm cannon for a 50-cal was negligible and not worth the engineering effort. Removing the wing guns would not have affected the CG much, either.
The P-39C had a service ceiling of 33,300 feet, already comfortably above 27,000 feet.
The P-39D had a service ceiling of 31,100 feet, already comfortably above 27,000 feet.
The P-39N had a service ceiling of 35,900 feet, already comfortably above 27,000 feet.
The report cited was about P-39Ks, and I can't find a test report on a P-39K just now. I'm assuming it was as good as a P-39D and not better than a P-39N. I am surmising that the P-39K could, in fact, get to 27,000 feet, but not in time to stop the Japanese raiders with the warning time they had available.
Taking out the wing guns would have made it climb better, but not by all that much. But losing 650 pounds WOULD have made the climb slightly better at the expense of losing firepower. Sounds like what they wanted at the time, but I'm having trouble getting my brain around how a slightly better climb would allow them to get up to the bombers without knowing the warning time they had for the raids. We'd have to know their warning time to see how much climb difference would have resulted in a good outcome. But, it seems that whatever climb improvement they got, it made a difference.
It still would not have made the P-39 dogfight with a Zero at 27,000 feet (or anywhere else, for that matter), but it could give them a firing pass at the bombers.
This is an interesting report, but it doesn't have enough meat in it to make an intelligent analysis of what they removed to save 650 pounds or how much change in climb it made. The P-39K had a 1,325 hp Allison instead of a 1,200 hp unit, and removing the wing guns, installation kits, and ammunition was the likely source of most of the weight.
If you look at the P-39N (same engine hp), the rate of climb at 25,000 feet was 1940 fpm. From the data I have ...
The Allison should have been making about 1,050 hp at 15,000 feet, where it had a climb rate of 3,340 fpm at 7,274 pounds. If we remove 650 pounds, we get 6,624 pounds, where the climb rate SHOULD be about 3,668 fpm, an improvement of 327 fpm or 9.8%. The time to climb to 15,000 feet was 4.15 minutes. Shaving off 9.8% makes it close to 3.75 minutes. If we assume about a 9.8 % gain to 25,000 feet, we go from 8.04 minutes stock to 7.25 minutes when 650 pounds lighter. This is, admittedly, a 1st-order approximation and is likely a bit optimistic.
So, logically, a change of somewhere around 45 seconds to 27,000 feet made the difference. That assumes the P-39s were unopposed and could hit the bombers when they got there. Sounds a bit unlikely, but this report would indicate that it was the case.
All I can say is that if a change in climb of 45 seconds made the difference, then maybe removing the wing guns was a good answer. I just have never seen much indication they DID that on a large-scale basis in the USAAC/F in WWII.
The P-39C had a service ceiling of 33,300 feet, already comfortably above 27,000 feet.
The P-39D had a service ceiling of 31,100 feet, already comfortably above 27,000 feet.
The P-39N had a service ceiling of 35,900 feet, already comfortably above 27,000 feet.
The report cited was about P-39Ks, and I can't find a test report on a P-39K just now. I'm assuming it was as good as a P-39D and not better than a P-39N. I am surmising that the P-39K could, in fact, get to 27,000 feet, but not in time to stop the Japanese raiders with the warning time they had available.
Taking out the wing guns would have made it climb better, but not by all that much. But losing 650 pounds WOULD have made the climb slightly better at the expense of losing firepower. Sounds like what they wanted at the time, but I'm having trouble getting my brain around how a slightly better climb would allow them to get up to the bombers without knowing the warning time they had for the raids. We'd have to know their warning time to see how much climb difference would have resulted in a good outcome. But, it seems that whatever climb improvement they got, it made a difference.
It still would not have made the P-39 dogfight with a Zero at 27,000 feet (or anywhere else, for that matter), but it could give them a firing pass at the bombers.
This is an interesting report, but it doesn't have enough meat in it to make an intelligent analysis of what they removed to save 650 pounds or how much change in climb it made. The P-39K had a 1,325 hp Allison instead of a 1,200 hp unit, and removing the wing guns, installation kits, and ammunition was the likely source of most of the weight.
If you look at the P-39N (same engine hp), the rate of climb at 25,000 feet was 1940 fpm. From the data I have ...
The Allison should have been making about 1,050 hp at 15,000 feet, where it had a climb rate of 3,340 fpm at 7,274 pounds. If we remove 650 pounds, we get 6,624 pounds, where the climb rate SHOULD be about 3,668 fpm, an improvement of 327 fpm or 9.8%. The time to climb to 15,000 feet was 4.15 minutes. Shaving off 9.8% makes it close to 3.75 minutes. If we assume about a 9.8 % gain to 25,000 feet, we go from 8.04 minutes stock to 7.25 minutes when 650 pounds lighter. This is, admittedly, a 1st-order approximation and is likely a bit optimistic.
So, logically, a change of somewhere around 45 seconds to 27,000 feet made the difference. That assumes the P-39s were unopposed and could hit the bombers when they got there. Sounds a bit unlikely, but this report would indicate that it was the case.
All I can say is that if a change in climb of 45 seconds made the difference, then maybe removing the wing guns was a good answer. I just have never seen much indication they DID that on a large-scale basis in the USAAC/F in WWII.
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Service Ceiling - means as high as it can fly with operational stability, not effectively engage and fight.
The same goes for the often mistaken Maximum Range, which means flying from Point "A" to Point "B", not Combat Radius, which is a much different range.
For example, the Fw190A-8 had a service ceiling of nearly 34,000 feet, but it was at a disadvantage in combat at those higher altitudes.
The same goes for the often mistaken Maximum Range, which means flying from Point "A" to Point "B", not Combat Radius, which is a much different range.
For example, the Fw190A-8 had a service ceiling of nearly 34,000 feet, but it was at a disadvantage in combat at those higher altitudes.
Hello GregP,
The P-39K and P-39D-2 used the V-1710-63 engine which performed pretty much the same as the V-1170-35 engine in P-39D above the 12,000 feet critical altitude. The superchargers were the same but the -63 could run more boost down low and that is where it gained its extra power.
From the combat reports that I have read, it appears that the P-39 in various models never seemed to perform quite as well in the field as specifications and test reports would indicate. Maybe this is yet another case.
Another possibility is that the P-39K was carrying a drop tank.
- Ivan.
The P-39K and P-39D-2 used the V-1710-63 engine which performed pretty much the same as the V-1170-35 engine in P-39D above the 12,000 feet critical altitude. The superchargers were the same but the -63 could run more boost down low and that is where it gained its extra power.
From the combat reports that I have read, it appears that the P-39 in various models never seemed to perform quite as well in the field as specifications and test reports would indicate. Maybe this is yet another case.
Another possibility is that the P-39K was carrying a drop tank.
- Ivan.
Hello Juha3,
My credentials are fewer than yours.....
What document did that page come from? There are a few odd aspects of the M-82FN that I am interested in and perhaps I have the document you are referencing. I don't like to read through a bunch of Russian manuals unless I know I will find something.
From what we KNOW:
The critical altitude of the M-82FN was 4650 Meters.
From what various sources state:
The maximum speed has been given as low as 625 KPH and as high as 648 KPH.
The critical altitude for the La 5FN has been given to be as low as 5000 Meters and as high as 6250 Meters.
How do we reconcile all these?
Here is what I BELIEVE. (Emphasis on Believe but it seems to fit all the data points.)
With an engine making its best power at 4650 Meters, I believe a very good example of the La 5FN could reach 648 KPH at 5000 meters. This would be more typical of expectations for ram effect.
By 6000 Meters, its speed may have dropped to 635 KPH which is also pretty reasonable.
If you believe that speeds were 10 KPH slower at each altitude, I won't argue. Production quality did vary quite a bit.
- Ivan.
GregP
Major
Hi Ivan.
I show the P-39D with the V-1710-35. 1,150 hp at 12,000 feet and 3,000 rpm
I show the P-39K with the V-1710-63. 1,100 hp at 13,800 feet and 3,000 rpm. I am assuming it dropped about 50 hp from there at 15,000 feet. The P-39K with the -63 also had a WEP rating of 1,580 hp at 2,500 feet and 60" MAP, but that would not have still been in use at 27,000 feet since it was a 5 minute rating. I based my estimate on Military power, not WEP.
I fully realize that the P-39 was not considered a great airplane, but the performance of the P-39N at 15,000 feet from wwiiaircaftperformance shows 3,340 fpm climb. That's about P-51D territory. At 26,000 feet, a test on a P-51D shows 2,780 fpm climb. The P-39N shows 1,940 at 25,000 feet. Clearly, the P-51D is better up high.
But at 17,400 feet, the P-51D shows 2,965 fpm while the P-39N at 15,000 feet shows 3,340 fpm. Different story. Down near 15,000 feet, the P-39 is right in there with the P-51D. At least, the test airplane was. The dog of the group seems to be the P-39D which, for some unexplained reason, shows poor climb perfomance that was "recovered" in later models. That tells me there is something wrong with the P-39D test. I just don't quite know what it is and the interest level is dropping fast since the P-39 was never considered a great fighter by the U.S.A. ... only the Soviet Union loved it, and they did great with it.
Maybe we needed to look at their operations!
I show the P-39D with the V-1710-35. 1,150 hp at 12,000 feet and 3,000 rpm
I show the P-39K with the V-1710-63. 1,100 hp at 13,800 feet and 3,000 rpm. I am assuming it dropped about 50 hp from there at 15,000 feet. The P-39K with the -63 also had a WEP rating of 1,580 hp at 2,500 feet and 60" MAP, but that would not have still been in use at 27,000 feet since it was a 5 minute rating. I based my estimate on Military power, not WEP.
I fully realize that the P-39 was not considered a great airplane, but the performance of the P-39N at 15,000 feet from wwiiaircaftperformance shows 3,340 fpm climb. That's about P-51D territory. At 26,000 feet, a test on a P-51D shows 2,780 fpm climb. The P-39N shows 1,940 at 25,000 feet. Clearly, the P-51D is better up high.
But at 17,400 feet, the P-51D shows 2,965 fpm while the P-39N at 15,000 feet shows 3,340 fpm. Different story. Down near 15,000 feet, the P-39 is right in there with the P-51D. At least, the test airplane was. The dog of the group seems to be the P-39D which, for some unexplained reason, shows poor climb perfomance that was "recovered" in later models. That tells me there is something wrong with the P-39D test. I just don't quite know what it is and the interest level is dropping fast since the P-39 was never considered a great fighter by the U.S.A. ... only the Soviet Union loved it, and they did great with it.
Maybe we needed to look at their operations!
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Hello GregP,
The big difference in the later models of the P-39 was that their superchargers were different.
They couldn't run the run the 70 inches Hg boost levels that the early (-35, -39, -63, -73) engines could without damage, but they also had higher critical altitudes.
For these early engines, WEP didn't matter past 12,000 feet because there wasn't enough capacity in the supercharger left to provide the extra boost. They could only provide boost for the Military rating up at 12,000 feet and boost dropped after that.
The engines in the P-39 were all single stage, single speed engines, so critical altitude was never very high in any version.
With the P-51D, you have a TWO Stage supercharger and at 17,400 feet, it still is WAY BELOW its critical altitude in high blower which I believe was around 21,000 to 22,000 feet but also way above its critical altitude in low blower which was only 8,500 feet.
- Ivan.
The big difference in the later models of the P-39 was that their superchargers were different.
They couldn't run the run the 70 inches Hg boost levels that the early (-35, -39, -63, -73) engines could without damage, but they also had higher critical altitudes.
For these early engines, WEP didn't matter past 12,000 feet because there wasn't enough capacity in the supercharger left to provide the extra boost. They could only provide boost for the Military rating up at 12,000 feet and boost dropped after that.
The engines in the P-39 were all single stage, single speed engines, so critical altitude was never very high in any version.
With the P-51D, you have a TWO Stage supercharger and at 17,400 feet, it still is WAY BELOW its critical altitude in high blower which I believe was around 21,000 to 22,000 feet but also way above its critical altitude in low blower which was only 8,500 feet.
- Ivan.
P-39 Expert
Non-Expert
Please expand above.
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Peter Gunn
Master Sergeant
Waaaaaiitt...
I thought I read somewhere that for every pound of weight removed you got an extra 1.2fpm climb performance (as a guess).
Or 650lbs means 780fpm added (as a guess).
Now you come along with these "calculations" with all your fancy formulas and aviation knowledge and only show an increase of .50fpm, or .50307 to be more specific for removing 650lbs. What kind of shenanigans are these?
P-39 Expert
Non-Expert
Please expand above.
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GregP
Major
So, this P-39K shows a true airspeed of 353 mph at 25,000 feet, and the rate of climb is about 760 fpm or so. But, if you look at the P-39N test, at the same altitude, the rate of climb is 1,940 fpm and the speed is 370 mph.
Interesting, to say the least but, thinking of the attachment above and the statement that the P-39s could not reach 27,000 feet, the actual tests show ANY of the P-39s could have gotten to 27,000 feet, but the particular model of the airplane has a LOT to do with what the climb rate and top speeds are when you get there. I am left with the dictinct impression that my earlier premise was correct, the P-39 could get to 27,000 feet, but not within the time allowed for by the early warning system in the above report. In that case, perhaps removing the wing guns would actually make the mission possible. It would NOT make the P-39 a better mount against the Zero, maneuverability-wise, but could make it possible to actually intercept bombers at 27,000 feet and interrupt their mission.
Most Japanese bombing missions were NOT at 27,000 feet, if you go look at the after action reports that I have seen. Things don't always stay the same, and the Japanese apparently didn't always come in at low altitudes. At least the P-39 had a good cockpit heater!
Thank you Mike Williams for saving these performance test data charts and reports!
Nice to have test data when you look at specific questions.
Interesting, to say the least but, thinking of the attachment above and the statement that the P-39s could not reach 27,000 feet, the actual tests show ANY of the P-39s could have gotten to 27,000 feet, but the particular model of the airplane has a LOT to do with what the climb rate and top speeds are when you get there. I am left with the dictinct impression that my earlier premise was correct, the P-39 could get to 27,000 feet, but not within the time allowed for by the early warning system in the above report. In that case, perhaps removing the wing guns would actually make the mission possible. It would NOT make the P-39 a better mount against the Zero, maneuverability-wise, but could make it possible to actually intercept bombers at 27,000 feet and interrupt their mission.
Most Japanese bombing missions were NOT at 27,000 feet, if you go look at the after action reports that I have seen. Things don't always stay the same, and the Japanese apparently didn't always come in at low altitudes. At least the P-39 had a good cockpit heater!
Thank you Mike Williams for saving these performance test data charts and reports!
Nice to have test data when you look at specific questions.
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P-39 Expert
Non-Expert
Please expand above.So, this P-39K shows a true airspeed of 353 mph at 25,000 feet, and the rate of climb is about 760 fpm or so. But, if you look at the P-39N test, at the same altitude, the rate of climb is 1,940 fpm and the speed is 370 mph.
Keep in mind that in the test the K was climbing at normal power 2600rpm and the N was climbing at combat power 3000rpm. The K was still under the 5 minute limit until mid 1942. After mid year 1942 the K would be able to climb at combat power 3000rpm for 15 minutes. K still wouldn't climb like the N since the N had a more powerful engine, but it would climb better than the test (3000rpm vs 2600rpm). The curved lines between 12500' and 15000' on the climb test attached shows the 5 minute mark when the rpm was reduced from 3000 to 2600. The blue dots estimate the climb rate at 3000rpm. That would show a climb rate of 1000fpm at 27000' with NO weight reduction. (The red dots are for the A6M2).
Interesting, to say the least but, thinking of the attachment above and the statement that the P-39s could not reach 27,000 feet, the actual tests show ANY of the P-39s could have gotten to 27,000 feet, but the particular model of the airplane has a LOT to do with what the climb rate and top speeds are when you get there. I am left with the dictinct impression that my earlier premise was correct, the P-39 could get to 27,000 feet, but not within the time allowed for by the early warning system in the above report. In that case, perhaps removing the wing guns would actually make the mission possible. It would NOT make the P-39 a better mount against the Zero, maneuverability-wise, but could make it possible to actually intercept bombers at 27,000 feet and interrupt their mission. As the attached chart shows, at normal power 2600rpm the K model would only be climbing at 500fpm at 27000', too low for combat. At 3000rpm the K model would still be climbing at at least 1000fpm, commonly know as combat ceiling (not service ceiling 100fpm). Again, before ANY weight reduction.
Most Japanese bombing missions were NOT at 27,000 feet, if you go look at the after action reports that I have seen. Things don't always stay the same, and the Japanese apparently didn't always come in at low altitudes. At least the P-39 had a good cockpit heater! Japan's best bomber, the G4M Betty bombed at between 18000' and 22000'. I don't know of ANY Japanese bomber capable of bombing at 27000'. Turbocharged B-17 and B-24 bombers flew at only 25000'. A 27000' combat ceiling was plenty.
Thank you Mike Williams for saving these performance test data charts and reports!
Attachments
GregP
Major
Found all these charts myself, but thanks for posting. It tells me the P-39 was exactly waht we all thought, basically a low-to medium altitude fighter. Once you get above mid-teens, it starts running out of power. Since single-stage engines do that, where did 66 pages of posts come from?
Cheers everyone.
Cheers everyone.
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Hello GregP,
Well, there are plenty of single stage but multiple SPEED or variable SPEED superchargers that do a lot better than that.
Most of the German fighters used single stage superchargers.
The single stage but two SPEED V-1650-1 installed in the P-40F and P-40L was a lot more capable at altitude than the single stage single SPEED Allison.
66 Pages of posts???
The Title for this thread should actually be the same as the Dr Seuss book:
"The Places You'll Go".
We seem to cover a lot of ground in this thread and you never know where we will end up next!
- Ivan.
Dash119
Senior Airman
Any reference to Dr. Seuss gets a Bacon rating...
Peter Gunn
Master Sergeant
66 Pages of still trying to turn a sow's ear into a silk purse.
Lots of coulda' shoulda' woulda' mumbo jumbo about trying to turn a mediocre low altitude fighter into a world beater.
Lots of coulda' shoulda' woulda' mumbo jumbo about trying to turn a mediocre low altitude fighter into a world beater.
GregP
Major
Perhaps it was more of trying to make the interim fill-in fighter perform a bit better since they were flying it anyway. "A bit better" by shedding sonme dubious weight ain't gonna' get you 1,000 fpm climb rate, even at critical altitude and below. But, it LOOKS like shedding some DID allow the airplane to make an interception where it couldn't before shedding the weight, assuming you read between the lines as they were intended to be read.
If the report REALLY says the P-39 could not GET to 27,000 feet, then I have to question the truth of it. Clearly, all variants could. So, I'm making the assumption that shedding the weight allowed it to make an intercept within a given time frame that it couldn't make before shedding the weight. While that seems logical, it wasn't much used by P-39 units in the Pacific as a tactic for perrformance enhancement.
If the report REALLY says the P-39 could not GET to 27,000 feet, then I have to question the truth of it. Clearly, all variants could. So, I'm making the assumption that shedding the weight allowed it to make an intercept within a given time frame that it couldn't make before shedding the weight. While that seems logical, it wasn't much used by P-39 units in the Pacific as a tactic for perrformance enhancement.
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