P-39 Expert
Non-Expert
Go to Mike Williams site and look at the official government/military tests. It didn't matter how much horsepower an engine developed, what mattered was how the airplane performed.
XP-39 II - The Groundhog Day Thread
- Thread starter Shortround6
- Start date
Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules
- Status
- Thread starter
- #122
Shortround6
Major General
lift of the wing actually doesn't have a whole lot to do with climb. This is counter intuitive but I believe it is true.
Best climb is achieved at the point where the induced drag curve crosses the profile drag curve.
Like this.
with the plane flying at the minimum drag speed the rest of the engines power can be used for climbing. This assumes that the minimum drag speed has a comfortable margin above the minimum control speed.
Lift goes up with square of the speed, which is why the induced drag (or drag from creating lift) goes down so quickly, at least to a certain point. However the profile or parasitic drag also goes up the square of the speed so trying to fly faster to create more lift to increase climb doesn't work very well. You have less power left to actually do the work of lifting the plane.
You can estimate climb to some extent if you have (or can reasonable guess) the power needed to fly at the minimum drag speed and figure the power to weight ratio of the plane using the excess power available.
Any of our members who know more about aeronautics ( a bunch of them) are free to correct me on this.
However we can also make the same mistakes than many designers of the 30s made. Those inaccurate estimates of drag and/or induced drag that often under estameted the power needed to fly at minimum speed (prop efficiency can also screw things up).
When you have planes that outperform similar aircraft by a large margin you really have to start looking for the answer. Not always easy to find and make sure you are comparing the same things.
The US was one of the few countries that used military power for the first 5 minutes of a climb, Both Britain and Germany used a much longer time period rating. The US also, at some point during the war on certain aircraft ran take-off power for 5 minutes and then some WEP rating for the rest of the climb unless the engine overheated.
Sometimes a Non US test did use full power for a climb but it is almost always noted.
British tests for the Spitfire for instance used 2600rpm for the early Spitfire with Merlin III engines, later Spitfires used 2850rpm for the climb. Later on some spits were allowed to use 3000rpm at the higher altitudes where the supercharger could not provide full pressure. But finding a test for a Spitfire where they used 3000rpm and full boost from the ground to 25,000 ft or more is going to be a very rare test.
The P-39N in those tests seems to outperform, and by a large margin, the P-39s that came before it and the P-39 that came after it (the Q).
It was also running over the allowable temperature limits.
Best climb is achieved at the point where the induced drag curve crosses the profile drag curve.
Like this.
with the plane flying at the minimum drag speed the rest of the engines power can be used for climbing. This assumes that the minimum drag speed has a comfortable margin above the minimum control speed.
Lift goes up with square of the speed, which is why the induced drag (or drag from creating lift) goes down so quickly, at least to a certain point. However the profile or parasitic drag also goes up the square of the speed so trying to fly faster to create more lift to increase climb doesn't work very well. You have less power left to actually do the work of lifting the plane.
You can estimate climb to some extent if you have (or can reasonable guess) the power needed to fly at the minimum drag speed and figure the power to weight ratio of the plane using the excess power available.
Any of our members who know more about aeronautics ( a bunch of them) are free to correct me on this.
However we can also make the same mistakes than many designers of the 30s made. Those inaccurate estimates of drag and/or induced drag that often under estameted the power needed to fly at minimum speed (prop efficiency can also screw things up).
When you have planes that outperform similar aircraft by a large margin you really have to start looking for the answer. Not always easy to find and make sure you are comparing the same things.
The US was one of the few countries that used military power for the first 5 minutes of a climb, Both Britain and Germany used a much longer time period rating. The US also, at some point during the war on certain aircraft ran take-off power for 5 minutes and then some WEP rating for the rest of the climb unless the engine overheated.
Sometimes a Non US test did use full power for a climb but it is almost always noted.
British tests for the Spitfire for instance used 2600rpm for the early Spitfire with Merlin III engines, later Spitfires used 2850rpm for the climb. Later on some spits were allowed to use 3000rpm at the higher altitudes where the supercharger could not provide full pressure. But finding a test for a Spitfire where they used 3000rpm and full boost from the ground to 25,000 ft or more is going to be a very rare test.
The P-39N in those tests seems to outperform, and by a large margin, the P-39s that came before it and the P-39 that came after it (the Q).
It was also running over the allowable temperature limits.
You must have the same book I do. Mine's packed away for moving, but from memory, I believe Koga still had engine power available, as the oil wasn't lost catastrophically, and he didn't have to push it very hard to reach his emergency field. His wingmen made no mention of a dead stick landing. On teardown the bearings were burned but not seized. The pistons were seized, but that was attributed to post-crash corrosion.
Good point. On an American aircraft (other than Grumman) it probably would. US landing gear trunnions tend to be hell for stout, but Horikoshi was a master of lightweight construction, and likely created a design that was strong for vertical loads, but less so for sheer. Also, his one-piece integral wing design didn't have the weak points of detach fittings for the wing panels. "Sheet metal forensics" can generally determine if a fabricated structure of complex shape, such as a wing, has been deformed. The trauma usually wrinkles skins and pops rivets. We did a little of that in mech school.
What I wonder is why didn't Koga make a gear up belly landing? He probably would have walked away, the plane burned, and Naval Intelligence frustrated.
Spot on! People get over fixated on Coefficient of Lift and Total Lift because they're simple single-value functions and sound impressive. What really matters in most all aircraft performance regimes is Lift/Drag. As Shortround points out, best climb occurs at or very near the best Lift/Drag ratio, tempered slightly by powerplant concerns such as prop efficiency or tailpipe deflection angle. This is where thrust available exceeds thrust required by the greatest amount, and thrust required is, of course, tied to L/D. The excess thrust can be converted to linear acceleration in the horizontal plane, or by maintaining airspeed constant, find an upward inclined plane that achieves the maximum defeat of gravity by that excess thrust.
- Thread starter
- #125
Shortround6
Major General
Even into the early 30s there were people who should have known better (like editors of aviation magazines) who thought that biplanes would outclimb monoplanes because they had more LIFT. This totally ignored the higher drag of the biplane and given equal engines the resulting lower surplus power/thrust at a given speed that could be used for climb.
It is possible to muck things up if you go too far though. The clipped with P-63s used in post war air racing might not have climbed very well, they sure couldn't turn very well.
But that may be a case of needing either a high angle of attack (high drag) to generate lift or needed to fly faster (higher drag) to generate the needed lift.
According to at least one pilot, whatever they gained on the straightaway they lost in the turns having to take a wider (longer) track around the pylons.
It is possible to muck things up if you go too far though. The clipped with P-63s used in post war air racing might not have climbed very well, they sure couldn't turn very well.
But that may be a case of needing either a high angle of attack (high drag) to generate lift or needed to fly faster (higher drag) to generate the needed lift.
According to at least one pilot, whatever they gained on the straightaway they lost in the turns having to take a wider (longer) track around the pylons.
FLYBOYJ
"THE GREAT GAZOO"
In the series "Air War" by David Jablonski, it was mentioned the area Koga chose was a predetermined emergency landing site and it was thought the surface would be acceptable for an emergency landing, it turned out to be soft muddy tundra. Lt. Michio Kobayashi who was flying a Kate actually scouted the landing spot and witnessed the crash. It was also mentioned that he was leaking fuel. Photo from wiki
Some more on this...
The Akutan Zero – the first intact to be captured by the US in 1942 and the last flight of Tadayoshi Koga – WW2Wrecks.com
Last edited:
fastmongrel
1st Sergeant
The fuel remaining in the tanks would have been removed for analysis. I wonder if there was any facility to replicate the C3 fuel so that tests would be more realistic.
A minor point but at this time there was no British fuel all production of Petroleum products had been standardised. Unless the RAF had a secret refinery tucked away somewhere.
P-39 Expert
Non-Expert
Couple more things. I'm not convinced that the Airacobra was a miracle airplane. Just that it was a much better plane than we all have been lead to believe for the last 75 years. All the reference books, every single one listed the performance figures for the P-39 well below the figures and graphs in wwiiaircraftperformance.org (Mike Williams site). Those were the official military tests. William Green, America's Hundred Thousand, all the reference books were pretty accurate except for the P-39. The information on Mike Williams site for the P-39 were released in 2012. Makes me wonder if his information had been somehow lost for the past 75 years. My whole contention is that the P-39 in most all its models was a better performing plane than previously published books indicate. This also helps explain why the Soviets were so successful with it.
And the FW190 did climb about the same as the Hellcat and Corsair. Look at the climb graphs in my post #118. At 20000' the 190 climbed at about 2000feet per minute, about the same as the 1943 Hellcat, Corsair, Thunderbolt, Lightning F/G, Typhoon and Zero. All in the 1800-2200fpm range.
P-39 Expert
Non-Expert
That Merlin 46 graph is with ram air. Allison figures a without ram air.
The -93 engine as eventually installed in the P-63 (and XP-39E) didn't have an intercooler.
fastmongrel
1st Sergeant
I am not sure of the exact fuel used at the time. My point was that there was no British fuel the same as there was no US fuel it was as far as possible a universal Allied fuel.
P-39 Expert
Non-Expert
The AAF increased the time limit for military power from 5 minutes to 15 minutes for the Allison V-1710 (and P&W R-2800) in mid year 1942. About the same time they finally discarded the unnecessary and performance robbing backfire screens. Prior to that the 5 minute limit required reduction to normal power (2600rpm for the Allison, 2550 for P&W) during climb tests which greatly reduced the tested climb rate. The simple use of combat power for 15 minutes increased the tested climb rates dramatically, especially at the higher altitudes reached after 5 minutes. If you look at the tests, all of them reduced power after 5 minutes before 6/30/1942, and used full power at all altitudes after that date.
Rolls Royse (and the RAF) chose to rate the Merlins at 2850rpm for climb, which was barely under their 3000rpm combat rating. That's what they felt their engines could reliably bear under climb conditions. Different countries, different companies, slightly different ratings.
The N model had a few advantages that other P-39 models didn't. Tests of the D/F/K/L models were done before the 15 minute limit so their climb figures were lower after the 5 minute limit was reached and power was reduced to 2600rpm. The M/N/Q models had the benefit of 3000rpm for the full 15 minutes and the more powerful -85 engine. The M as tested had the 2:1 reduction gear and sway braces attached to the drop tank shackle. The N had the 2.36:1 reduction gear and no sway braces. And possibly the larger 11'7" propeller but we don't know that for sure. The Q had the obvious underwing .50calMG pods that cost 14mph in speed and who knows how much climb. All for an additional 130lbs in weight and an increase in firepower of about 2/3rds of one .50calMG.
Most all the AAF WWII fighters with Allison engines ran a little hot during combat power. Except the P-40.
Clearly they had never flown gliders.
Shelby Cobra and 427 Corvette Lose to Austin Cooper S ! (Old school Mini-Cooper bored out to 1.5L)
I saw it happen at le Circuit Mont Tremblant in 1968 at an SCCA regional. Admittedly, a rather tight course: the winning average speed at the Canadian GP later that year was barely 100 MPH. Saw that too. Monaco without the buildings and waterfront.
fastmongrel
1st Sergeant
Have a read about Stirling Moss winning at Monaco 1961 in a Lotus Climax which only had 145hp against the two Ferrari cars which had over 200hp.
I have Reardon's book on the Akutan Zero, but it's packed away in storage. Good excuse to dig it out and read it again. Fascinating read.
Pretty amazing what they were able to do with the old 1.5L F1 with no turbos allowed. But apparently the fans didn't find it "interesting enough" and there was agitation for more performance. Quantum leap when it went to 3L in the mid 60s.
Back to P39s. Do I detect the sickly aroma of an expired equine carcass? My toes are bruised and bleeding; how about yours?
Hello P-39 Expert,
For the most part, I agree with your statements above. The problem though is that I am finding that very often you have a tendency to cherry pick tests under one test and load condition and use them as a general conclusion under all conditions. The early P-39 with the -35 and -63 engines are a good example. They were cool little screamers at low altitude, but those engines didn't have enough performance at altitude. The climb test at each altitude against the Aleutian A6M2 proved that despite attempts by the pilot to play with boost settings, he couldn't make up the difference past about 12,500 feet even though the testing was arranged to make US fighters look as good as possible against a beat up A6M2.
The point is that the Airacobra isn't the horrible fighter that some people think. It had its strong points, but it also had some serious limitations inherent in its design that were impossible to correct without a serious re-design.
FWIW, the data from AHT isn't necessarily inaccurate. There are just plenty of typos and the data may not always be complete. The roll rates quoted for P-39 actually are in pretty good agreement with those in NACA testing.
Your ideas for armament reduction may have its merits but it obviously wasn't acceptable to US Army. I don't know that my idea of reverting back to P-39C armament would have been acceptable either.
Your charts are showing a different model FW 190. Take the same aeroplane and delete about 550 pounds of equipment from it and see how it behaves. This equipment deletion isn't a "custom" modification or speculation; it was simply not found on the ground attack FW 190A-5/U4. They were serious load lifters but also pretty respectable hotrods when the ordnance was gone. The problems though were a difference in engine tuning and also non-pressurized ignitions which is probably why they could not get the test aeroplane to fly above 34,500 feet.
- Ivan.
Hello XBe02Drvr,
Good book, but a bit lacking in technical details from the various test reports. My copy is on the shelf but for some reason it is disintegrating at the binding.
One point I forgot to mention about the repair work is that one prop blade needed straightened and reconditioned and it is not mentioned in the repair items.
One morning many years ago, I was at Fort Meade with a friend of mine very early in the morning. I watched the local Porsche club set up cones for an autocross event in a parking lot. When the competitions were run, I was a bit surprised when the 914s were winning everything. Some of the faster drivers simply laid on the gas and drifted through every turn. None of the other cars could do the same. One of the other cars, a 944 or 928 picked up a little too much speed and almost ran off the parking lot and down a rather long embankment.
- Ivan.
P-39 Expert
Non-Expert
The P-39D-1 was the heaviest of the early P-39s at 7850lbs. My whole point is that the weight could be easily reduced AT FORWARD BASES by removing some equipment that wasn't needed. The weight reduction was primarily to increase climb rate and ceiling AT ALL ALTITUDES. The P-39 was already 40mph faster at all altitudes than the 1942 Zero. Just using combat power in climb would make the early P-39s equal to the Zero, and the weight reduction would increase climb rate to well above the Zero. With climb AND speed greater than the Zero the air battles would have been very different.
Regarding the FW190A, look at the climb charts for each model -3 through -8. They are all about the same. Much lower than Faber's 190A-3 as tested by the British.
Last edited:
It's that mid-engine design with optimum weight distribution. Works a lot better for a Porsche 914 than a Bell P39.
Last edited:
- Status
Users who are viewing this thread
Similar threads
