XP-39 II - The Groundhog Day Thread

[Peter Gunn]

As an aside, why do people find it so hard to believe that some designs gel so quickly? Why is it hard to believe that an engineering/aeronautical genius like Kelly Johnson and crew can't come up with the XP-80 in 140+/- days or that Edgar Schmued and company came up with the Mustang in 120+/- days. The argument is always that they had either "help" or they got design leads and plans from someone else because it's "impossible" to do it on their own. I've seen it in more places than here.

I find that highly dismissive and insulting to the men who designed and built some of the greatest aircraft ever to take wing. I really don't know why it's so hard to fathom that a talented group of engineers led by brilliant chief designers, working full time must take some inordinate amount of months to come up with a superlative design.

Hell, the formative design for the U.S.S. South Dakota class battleships was sketched on a cocktail napkin by the chief designer one evening, I'd have to dig out my Garsky and Dulin's to get the particulars and it really was just a silhouette but it's pretty damn close to the end result.

 

[XBe02Drvr]

As an aside, why do people find it so hard to believe that some designs gel so quickly?

I find that highly dismissive and insulting to the men who designed and built some of the greatest aircraft ever to take wing.

I think that in most cases these genius types have a lot of ideas up their sleeve they've been working on in the back of their minds for awhile. When somebody squeezes the trigger, they're spring loaded to the "Go!" position and just have to put their ideas on paper without a lot of head scratching and imagineering. Like Kelly Johnson and his "pre-engineered" P80 wing. And Edgar Schmued and his laminar flow wing and Meredith effect radiator. It helps if they have a team who can intuitively think on the same page and streamline the detail work. Assembling said team is part of the genius factor.

 

[windhund116]

I think that in most cases these genius types have a lot of ideas up their sleeve they've been working on in the back of their minds for awhile. When somebody squeezes the trigger, they're spring loaded to the "Go!" position and just have to put their ideas on paper without a lot of head scratching and imagineering. Like Kelly Johnson and his "pre-engineered" P80 wing. And Edgar Schmued and his laminar flow wing and Meredith effect radiator. It helps if they have a team who can intuitively think on the same page and streamline the detail work. Assembling said team is part of the genius factor.

"Necessity is the mother of invention." And "Form follows function."

 

[XBe02Drvr]

"Necessity is the mother of invention." And "Form follows function."

And genius is 10% inspiration and 90% perspiration.

 

[P-39 Expert]

And agree - to say something is "overweight" based on power to weight is only comparable to similar aircraft of the day. My point.

And my point is the early P-39 was overweight. As compared to other contemporary fighters. In relation to it's engine power.

They could not increase engine power quickly.

They could reduce weight quickly. And every pound saved increased climb rate by 1.2fpm.

 

[pinsog]

And my point is the early P-39 was overweight. As compared to other contemporary fighters. In relation to it's engine power.

They could not increase engine power quickly.

They could reduce weight quickly. And every pound saved increased climb rate by 1.2fpm.

I would go the other way, I would add 250 pounds of turbo charger and larger inter cooler (I know I know it wouldn't fit). It's climb would still be poor (although much better at altitude) but you fly it like an early P47, slow climb but fast level speed, slowly climb to altitude then use your level speed a zoom ability to do diving passes at G4M Betty bombers while using your superior speed to avoid Zeroes altogether.

 

[P-39 Expert]

I would go the other way, I would add 250 pounds of turbo charger and larger inter cooler (I know I know it wouldn't fit). It's climb would still be poor (although much better at altitude) but you fly it like an early P47, slow climb but fast level speed, slowly climb to altitude then use your level speed a zoom ability to do diving passes at G4M Betty bombers while using your superior speed to avoid Zeroes altogether.

Turbo wouldn't work, but the mechanical two stage -93 would have worked just fine.

Easier and quicker to just reduce the weight and increase the climb.

 

[GregP]

1 pound increased the rate of climb by 1.2 fpm? Maybe not.

From "Aerodynamic for Naval Aviators," rate of climb is: RC = 33,000 * (Pa - Pr)/(W), where: RC = rate of climb in feet per minute, Pa = power available (hp), Pr = power required for level flight, W = weight in lbs. I took a P-38Q aircraft. Rate of climb at 7,400 feet was 3,805 fpm. Power available was 1,200 hp. Weight I am taking at 7,550 lbs, which is about 20 lbs under normal takeoff weight. If you run that out in Excel, Pr for level flight is 329.46 hp.

If I change the weight by 1 pound, it weighs 7,549 pounds with everything else the same ... and rate of climb is 3,805.504 fpm, which is an improvement if 0.504 fpm, or about 1/2 fpm not 1.2 fpm. In reality, the weight might be slightly less, but it won't get to 1.2 fpm for a 1 pound difference.

I wasn't trying to bust your chops here, but since I am an engineer, I just got curious about the 1.2 fpm.

 

[FLYBOYJ]

1 pound increased the rate of climb by 1.2 fpm? Maybe not.

From "Aerodynamic for Naval Aviators," rate of climb is: RC = 33,000 * (Pa - Pr)/(W), where: RC = rate of climb in feet per minute, Pa = power available (hp), Pr = power required for level flight, W = weight in lbs. I took a P-38Q aircraft. Rate of climb at 7,400 feet was 3,805 fpm. Power available was 1,200 hp. Weight I am taking at 7,550 lbs, which is about 20 lbs under normal takeoff weight. If you run that out in Excel, Pr for level flight is 329.46 hp.

If I change the weight by 1 pound, it weighs 7,549 pounds with everything else the same ... and rate of climb is 3,805.504 fpm, which is an improvement if 0.504 fpm, or about 1/2 fpm not 1.2 fpm. In reality, the weight might be slightly less, but it won't get to 1.2 fpm for a 1 pound difference.

I wasn't trying to bust your chops here, but since I am an engineer, I just got curious about the 1.2 fpm.

You beat me to the punch Greg, I couldn't remember where I seen that formula ( "Aerodynamic for Naval Aviators" once sat on my coffee table). I tried to explain earlier (when all this weight and balance discussion started) that weight reduction in the matter explained by the OP is not linear!

 

[XBe02Drvr]

weight reduction in the matter explained by the OP is not linear!

There you guys go again; speaking in tongues and handling serpents at worship!

 

[windhund116]

How many variants of the basic P39 design was made throughout WW2? Were there any active combat units using it, by mid 1945?

Just curious. I don't remember seeing any US units using them on front lines by the end of the war.

Thanks!

 

[P-39 Expert]

1 pound increased the rate of climb by 1.2 fpm? Maybe not.

From "Aerodynamic for Naval Aviators," rate of climb is: RC = 33,000 * (Pa - Pr)/(W), where: RC = rate of climb in feet per minute, Pa = power available (hp), Pr = power required for level flight, W = weight in lbs. I took a P-38Q aircraft. Rate of climb at 7,400 feet was 3,805 fpm. Power available was 1,200 hp. Weight I am taking at 7,550 lbs, which is about 20 lbs under normal takeoff weight. If you run that out in Excel, Pr for level flight is 329.46 hp.

If I change the weight by 1 pound, it weighs 7,549 pounds with everything else the same ... and rate of climb is 3,805.504 fpm, which is an improvement if 0.504 fpm, or about 1/2 fpm not 1.2 fpm. In reality, the weight might be slightly less, but it won't get to 1.2 fpm for a 1 pound difference.

I wasn't trying to bust your chops here, but since I am an engineer, I just got curious about the 1.2 fpm.

My method is a little simpler, and specific to the early P-39. From wwiiaircraftperformance.org:

P-39C vs. P-39D I know C didn't have self sealing tanks and armor plate/glass, but we're comparing climb rate and weight only. Same engine and propeller. Same airplane.

P-39C weighed 6689lbs. Climbed at 3720fpm.
P-39D weighed 7525lbs. Climbed at 2720fpm.
Difference 836lbs. 1000fpm.

1000fpm divided by 836lbs = 1.2fpm at 10000' Same airplane, same engine, propeller, rpm (3000) and HP (1150). Only difference is weight.

For every pound lost climb is increased by 1.2fpm. Example: 300lbs lost, climb is increased by 360fpm.

 

[fubar57]

Simpler isn't always righter

 

[XBe02Drvr]

P-39C vs. P-39D I know C didn't have self sealing tanks and armor plate/glass, but we're comparing climb rate and weight only. Same engine and propeller. Same airplane.

Nothing in fluid dynamics is ever that simple. Two powerplants of the same model series and rated at the same horsepower and using the same model number propeller can give significantly different thrust values, depending on many variables such as atmospheric conditions, fuel compounds, prop reduction and supercharger gear ratios, propeller rig, etc, etc. And thrust is the significant value here. And do you KNOW the airframes are rigged and detailed identically? How do you KNOW that all of these conditions are IDENTICAL on two different aircraft on two different days, and possibly, locations? You don't. So what you have are approximate, not absolute numbers.
"Your mileage may be different."

 

[GregP]

That method is not sound, P-39 Expert. The C and D are different airframes, and no two engine, airframe, and prop combinations perform exactly the same. The atmosphere is also not the same between tests.

You could add 300 pounds to the lighter one and re-do the test, but you can't remove 300 lbs from the heavier one and do the same. I don't know of any test where they actually did that to check the rate of climb difference with weight change between variants. Usually is was only to one variant in a test, and climb charts at different weights for the same airframe are not numerous for WWII fighters.

Also, you have to make yourself be logical, even using the correct aerodynamics formulas. For instance, you could calculate that the rate of climb would be 2,052 fpm at a weight of 14,000 pounds! But, the landing gear would not handle 14,000 pounds and, if it could, the P-39 could not take off from any WWII runway at that weight. It would sink into the earth on a dry day or the tires would go flat!

The theoretical climb figure can only be achieved AFTER the airframe is airborne, up to height, and is at the best climb airspeed. The P-39 might well DO that at 14,000 pounds, but it could never GET THERE to run the test. The formula is best used within normal weight variation for the airframe.

Max takeoff weight for the P-39Q was 8,400 lbs and the rate of climb at that weight at 7,400 feet would be around 3,420 fpm. Empty weight was 6,516 lbs. If we allow at LEAST for 250 lbs of fuel and a 200 pound pilot, we get 6,966 lbs and, at that weight, it would achieve 4,124 fpm. But, without any ammunition for a fight, what would the point be?

The main reason why we see flight tests at "typical loadouts" is because the fighters in question were being used in combat at or near the test weights and loadouts. No other "weird" test weights were contemplated because they weren't wartime realistic loads.

 

[Shortround6]

Occasionally you will find tests of aircraft with and without drop tanks or a bomb (or bombs) but then we have two variables, the
Pr = power required for level flight already mentioned above.

The power needed maintain level flight changes due the change in drag of slightly greater incidence of the wing to generate the lift plus the drag of the bomb or drop tank (and the interference drag).

and then the weight.

In the case of the P-39C and D is the P-39C fully outfitted? Is there any changes between the two aircraft that would affect drag?
do four protruding gun barrels in the wings cause more or less drag than two guns in the cowl? Do the ejection ports for the fired cartridges cause any drag?
Anything in the canopy area or engine exhausts?

Do both airplanes fly the same speed at the same altitude at the same power? If not you have a difference in drag which is going to show up in the climb rate even if the weights are the same.

 

[GregP]

Hey windhund 116, the Soviet Union used them throughout the war ... VERY effectively, and they stayed in service until at least 1949. The last aerial victory of WWII was by A Soviet P-39 against an Fw 189 on 9 May 45. The last Luftwaffe victory was by an Me 262 against a Soviet P-39 on 8 Aug 45.

The P-39, in Soviet service achieve the highest number of victories of any U.S. type, in any conflict. Strange, considering the U.S.A. didn't like them or come up with successful tactics for employing them. That was left to the Soviet Union. The P-39 was always a formidable ground attack airplane, but the Soviet Union mostly used them in an air-to-air role, in support of ground forces ... though they DID fly all types of missions in Soviet service.

I am not sure exactly when it as retired from U.S. service, but it was after WWII as some were still operational in 1946.

 

[XBe02Drvr]

against a Soviet P-39 on 8 Aug 45.

Huh? August 1945??

 

[GregP]

Ya' got me there, XBe02Drvr ... 8 May 45. Must have been poor toilet training as a kid ... or a typo on my part, one or the other.

 

[XBe02Drvr]

Ya' got me there, XBe02Drvr ... 8 May 45. Must have been poor toilet training as a kid ... or a typo on my part, one or the other.

If you don't eat the right brainfood at the right time you're liable to experience brainfarts!