XP-39 II - The Groundhog Day Thread

you guys are getting way too bogged down by details !

all you got to do is paint black crosses on a P-39 and bingo !
the greatest fighter ever built, even better than those P-39's with red stars on them, Easy peasy lemon squeezy

Even I should have remembered that aerodynamic fix.

XBe02Drvr said:

P39 Expert said:
The average daily temperature on Guadalcanal is 81 degrees.
Whoa! Average daily temperatures are 24 hour averages. The P39 isn't a night fighter. You were closer to the mark when you quoted average daily high as 88°F, as that's likely much closer to the actual SL temperature by the time the Japanese arrive on their long trek from Rabaul.
I endured four years on Boca Chica Key, a coral island with a climate very similar to Guadalcanal. Our official summer daytime highs (measured at the civilian airport downtown) ranged 87°-90°F, but when you went out to your plane on the ramp, the OAT gauge would read 93°-95°. Now which air mass is my airplane going to perform in, the "official" 89° measured under the palm trees behind the NWS station downtown, or the 94° over the runway at the airbase?

Click to expand...

And the high temperature is 88 degrees. Use whatever temperature you want. The weather is not going to make up 1000fpm climb.

special ed said:

If I may submit a scenario with no intent to demean, I put forth a what if in a thread already scattered with what ifs. P-39 Expert is transported by time machine to mid 1942 and is flying alone in his P-39 customized to his specs ( no nose armor, wing guns or radio in the tail). He sees a lone A6M2 flying at his level and he engages. He is turning tightly about to pull lead.... Later that evening, as Saburo Sakai is painting another P-39 silhouette on his fuselage, He remarks to his crew chief, " He was very good. He ALMOST turned inside me. He did not know I have no armor, radio or wing guns).

Click to expand...

Actually the P-39 pilot would not attempt to turn with the A6M2, that had been drummed into him since he arrived in theater. In his lighter P-39 he would have the choice of outclimbing the A6M2 or diving away as he had previously done.

wuzak said:

Clearly the aircraft were not "as identical as the manufacturer could make them, except for weight", considering that the propeller was different and there were small changes to the airframe.

How much do these differences make? We will never know unless we test the differences independent of other factors (such as weight).

As I said above, the engine performance can vary from one to the next, and since they did not have torquemeters installed we will never know if the the P-39C and P-39D engines varied from the published performance ratings (ie we don't know if the engine was producing more or less power than the standard rating).




There has been a lot of expanding in this thread, just maybe not knowledge or opinion.

Click to expand...

None of these differences matter. No minor propeller change, minute changes to the airframe (moving the .30s from the nose to the wings), a fin fillet, an extra inch of length, a symmetrical airfoil or the weather will make up 1000fpm climb. Only 836lbs of weight will do that.

glennasher said:

I think you're typing to a brick wall.

He either is unwilling or unable to process the information so aptly put to him. He keeps up with a circular argument that has been refuted too many times to think it's not intentional.

I have learned a lot from this place, but I certainly don't claim to be an "Expert" at anydangedthing. I've often thought that self-declared "experts" are anything but that. I kinda think that he's just trolling you all. I'm moving on to other things.

Click to expand...

I pegged it back on page 28 of this thread. This thread has run it's course. It oughta be stomped into a mudhole and left for dead.

Ivan1GFP said:

You are NOT quoting from the report. The report says "Service Ceiling". Anything else is your (wishful) interpretation.
FWIW, the "General" did not quote any figures. The report simply stated that he was pleased with the result.

You know this is wrong. Why would the commanding general be pleased with a lower ceiling AFTER the weight reduction?

In other words, you don't really have a clue as to how much things change.

One of us is definitely clueless.

Length changes MAY make a difference though in this case it is not relevant.



I do not claim to be an expert on the subject of propellers though I have done a substantial amount of reading on the various theories of how they work and measurement of performance. You obviously have not and are making generalizations you cannot substantiate.
I find this to be an interesting reply when confronted with an example to contradict your assertion about similarity based on manufacturer, and diameter of propellers.
.....
You have no basis for assuming that propellers used on Allison engines (like in the P-39) had very small differences.
If we are discussing different models of P-39, there were vast differences as shown by the over 1 foot increase in diameter from the early to late models.
I already gave you an example of the P-40 which used quite different propellers with the Allison engine during its service life. Look at the Allison P-40 tests in post #1489. Discard the "impregnated wood" propeller (#2) and the maximum difference in climb was 40fpm at any of the altitudes tested. And the best performing propeller was the standard production propeller. 40fpm is a long, long way from 1000fpm.
Even with propellers of the same diameter, there were often obvious differences such as pitch range.
I suggest you calculate the Propeller Power Coefficient to compare a few of the propellers used. I suggest you stop this charade and admit that the C climbed better than the D because it was 836lbs lighter.



What evidence do you have for this assertion that you keep making? The official Wright Field performance tests. It wasn't a slightly different (if any) propeller, phantom CG problems, an inch in length (if any), a symmetrical airfoil, a fin fillet, slight aerodynamic differences due to the positioning of the .30s or the weather. And the tests weren't flawed as some here maintain. They were official Wright Field tests. You guys are all working really hard to refute the fact that the C climbed better than the virtually identical D because it was 836lbs lighter.

- Ivan.

Click to expand...

Expand above.

P-39 Expert said:

Expand above.

Click to expand...

Ahh...

So my post #1,469 on Page 74 was correct then.

Peter Gunn said:

Ahh...

So my post #1,469 on Page 74 was correct then.

Click to expand...

Ayup!

I like this thread. Things come up that I learn from. The explanation of tumbling by putting weights on either end of a paper airplane made it clear to me. No math. I don't remember who posted it and I'm not going to start scrolling back.

P-39 Expert said:

You know this is wrong. Why would the commanding general be pleased with a lower ceiling AFTER the weight reduction?
.....
One of us is definitely clueless.

Click to expand...

Read what the report actually says, not what you WANT it to say.
.....
I think folks have already concluded a few things and you are most definitely clueless.

P-39 Expert said:

Look at the Allison P-40 tests in post #1489. Discard the "impregnated wood" propeller (#2) and the maximum difference in climb was 40fpm at any of the altitudes tested. And the best performing propeller was the standard production propeller. 40fpm is a long, long way from 1000fpm.

Click to expand...

Good try on switching the subject. You commented that the same manufacturer and same diameter of propellers meant that the propellers had to be nearly the same. I gave you a simple example where that was not the case.
A specific test of different propellers proves nothing.

P-39 Expert said:

I suggest you stop this charade and admit that the C climbed better than the D because it was 836lbs lighter.

Click to expand...

I suggest you stop being so dogmatic and get a clue. There are plenty of knowledgeable people here who have vastly more experience in the industry than you do. Take a moment and try to understand what they are patiently trying to explain to you.
Your calculation of taking the climb rate and dividing by the weight difference is something most of us figured out by around the 5th Grade was too simplistic an explanation to be useful for anything except perhaps sounding smart to a 4th Grader.
If Aeronautical Engineering were so simple as you seem to imply, people wouldn't have to go to school to learn the subject.

P-39 Expert said:

The official Wright Field performance tests. It wasn't a slightly different (if any) propeller, phantom CG problems, an inch in length (if any), a symmetrical airfoil, a fin fillet, slight aerodynamic differences due to the positioning of the .30s or the weather. And the tests weren't flawed as some here maintain. They were official Wright Field tests. You guys are all working really hard to refute the fact that the C climbed better than the virtually identical D because it was 836lbs lighter.

Click to expand...

That bit of redirection won't fool anyone. I asked you how you know the P-39C and P-39D were "virtually identical" except for weight and this is not an answer. It IS starting to sound a lot like a discussion on religion though.
No one is telling you that the weight difference didn't mean something. We just don't believe it explains the entire difference in climb rate and that there were other important factors involved which you keep denying without evidence.

This thread so far has been very useful. It has established that any impression of credibility that you may have had was unjustified.

- Ivan.

SaparotRob said:

I like this thread. Things come up that I learn from. The explanation of tumbling by putting weights on either end of a paper airplane made it clear to me. No math. I don't remember who posted it and I'm not going to start scrolling back.

Click to expand...

Obviously! You've got 76 pages worth of company in that opinion. On what other forum would a troll be allowed to lead such a long, entertaining, and merry chase? I haven't run out of popcorn yet.

SaparotRob said:

I like this thread. Things come up that I learn from. The explanation of tumbling by putting weights on either end of a paper airplane made it clear to me. No math. I don't remember who posted it and I'm not going to start scrolling back.

Click to expand...

Shortround6. Page 19. Post #365. I forgot that one has to do research to stay here.

Hi XBe02Drvr.

Vy it is, not Vx. Must be the absolute annoyance of having to come up with ANOTHER explanation that gets ignored. Or I simply wasn't playing attention very well. Take your pick ...

I am just tired of "1,000 fpm and 836 lbs" without ANY acknowledgement of the fact that the tests as shown are not very well documented for analysis. But they are well documented like wartime production tests would be ... a typical combat airplane at typical combat loadings and typical combat power settings. There is not enough information to go back and dissect the reports since the weight and balance charts are not shown, nor are all the little "nice to have" details given, like finish quality, hours on the airframe, condition of the propeller, MAP readings along with the rpm settings, etc.

I did notice the rate of climb in both reports does not seem to be the indicated ROC at the current altitude, but rather the altitude divided by the time to height, or average rate of climb, which seems strange. We also have the climb airspeeds, which run from 153 mph at S.L. to 200 mph at 20,000 feet for the P-39C, and from 157 mph at S.L. to 203 mph at 20,000 feet for the P-39D. So we have Vy for selected altitudes.

We know the P-39C was basically the same as the YP-39 except for the V-1710-35 engine. I see the fuel consumption was noted as "excessive" and instructions for correcting that were forthcoming from the Power Plant Laboratory (!). If the carburetor needed adjustment to the point where it was noted in the test report, I wonder how much different the actual power delivered was from the Tech Order power table that was used for the report. This airplane seems like sort of a "test mule" type since only 20 were built. I am not surprised it was a lot lighter since it lacked armor and self-sealing fuel tanks. It was VERY similar to the YP-39. We do not know the %MAC for this test but it was tested at 6,689 lbs takeoff weight. The P-39C had the 37 mm cannon (but only 15 rounds!), two 50-cal on top of the nose, and two 30-cal on the bottom of the nose, and no wing guns. I cannot find a definite good set of specs for the P-39C, but the empty weight of the airplane was very much lighter than the P-39D.

There were 60 P-39D-BE's and 336 P-39D-1's. It had the 37 mm cannon, two 50-cals in the upper nose, four 30-cals in the wings, and no 30-cals in the bottom of the nose. They went to a V-1710-63 engine with 1,325 hp for the P-39D-2. The test airplane had the V-1710-35 engine, so it was either a P-39D-BE or P-39D-1. With wheels up, it was at 25.8% MAC, so it definitely has some ammunition in the nose. The P-39D had 30 cannon rounds, not 15 like the P-39C.

So we know the P-39D had armor that the P-39C lacked, 15 more cannon rounds, 2 more 30-cal MG and ammunition for them, with all of the 30-cals mounted in the wings, and self-sealing fuel tanks. That may well and likely does account for the extra 836 pounds. But 836 pounds won't supbract 1,000 fpm from climb. In fact, if we add the 836 pounds to the P-39C, the rate of climb should drop to 3,443 fpm, right about where the P-39D SHOULD have been if it had been developing full power.

I was using the formula from Aerodynamics for Naval Aviators agian: RC = 33,000 * [(Pa - Pr)/W].

That's why I think the P-39D test was run at normal rated power, not military power.

Okay, let's get this settled. One of you guys must know someone with a P-39. I'm looking at you, "Not Greg". Let's all kick in a couple hundred grand each and modify it to -D and -C and get out the stopwatches.

Maybe a little more if government officials need bribing. The Walmart near me is out of General Motors 37mm cannon and rounds.

SaparotRob said:

Shortround6. Page 19. Post #365. I forgot that one has to do research to stay here.

Click to expand...

To respond (belatedly) to Shortround's question in that prehistoric post: in conventionally configured planes, handling differences throughout the approved CG range are relatively minor. The limits are determined by where they start to become problematic.
There are caveats. Exceeding the forward moment limit, either by exceeding forward CG limit or by exceeding max gross with the CG at or near its forward limit can cause loss of elevator authority in the flare, or in transition to slow flight. This can come as a surprise, as there is no warning like the buffet preceeding a conventional stall, and can be deadly at low altitude. This is rare, as most planes are difficult to load to this condition in normal use. My first "stretch body" plane was a Cherokee 6, which my instructor insisted I experience in this condition. 100 pounds in forward baggage, two big guys in the front seats, and nothing aft. It lifted off at nearly 10 knots higher speed than normal with me maintaining heavy back pressure to make it climb. Trimming out the back pressure took trim nearly to the limits of its travel. At 6500 feet we attempted some power off and power on stalls. It wouldn't stall, just pitched down with no buffet and no stall warning light. My instructor said to note the speed where that happened and think about the landing. We made a no flaps high speed landing at a nearby former military base with a long runway, switched our "ballast" to aft baggage, and were rewarded with a sweet flying airplane once again. We both violated FARs in conducting this exercise, but my young impressionable brain was imprinted with a lesson that many "pilot error" accident victims never learned. I asked my instructor if there was an aft CG exercise as well, and he said: "Too risky, we won't go there, but aft CG is a known and constant issue with this airplane, and won't sneak up on you unsuspecting as forward will. Calculate always."
Aft CG limits are set with the idea of maintaining a reasonable stick force gradient, to keep pitch control from becoming too sensitive. This generally works well for planes of conventional layout, as pitch response is sensitive to polar moment of large forward masses as well as displacement of CG forward of CL. Unorthodox layouts such as pushers (Ascender), pylon mounts (Lake amphibian) and mid engines (P39) tend to behave a little outside the accumulated body of aeronautical experience and expectations.
Wait, why am I telling YOU guys all this stuff you already know? Dang! Caught preaching to the choir...again!

Fighter Rebuilders built up a P-39 or two before and Steve Hinton tested them. The thing is, they don't try to see if it meets factory specs after a warbird restoration. They run the engine in to seat the rings and put some "slow time" on it and fly it just enough to verify everything works. Then, they hand over a good-running, reliable warbird to the owner.

Steve Hinton flew the restored Fw 190 for Paul Allen and spoke to us volunteers about it very briefly, but noted the test card was not a performance verification, but rather gradually creeping up on ensuring a reliable airplane is delivered. He said none of the planes they build are tested to maximum performance unless so specified by the owners, and none of them DO that. Aircraft are restored, made airworthy, broken in, delivered, and then are flown by the owners when they so desire. Occasioanlly, they herd them around the Unlimited race course in Reno, as we can see in YouTube.

GregP said:

But they are well documented like wartime production tests would be ... a typical combat airplane at typical combat loadings and typical combat power settings. There is not enough information to go back and dissect the reports since the weight and balance charts are not shown, nor are all the little "nice to have" details given, like finish quality, hours on the airframe, condition of the propeller, MAP readings along with the rpm settings, etc.

Click to expand...

Bet they had no idea they were creating a historical document that "historians" 78 years hence would be scratching their heads over.

I'm done head-scratching about the P-39 unless something interesting comes up.

It was an overall mediocre fighter at best, given the requirements in Europe, but was pretty good down low if the mission was short. It could hit hard, but the big gun was very prone to jamming and had few rounds to boot. Still, several 37 mm hits on small boats were heavy damage.