XP-39 II - The Groundhog Day Thread

[GregP]

I'm inclined to desist.

I hope I can still be taught some new tricks since we're about the same age.

 

[XBe02Drvr]

I'm inclined to desist.

I hope I can still be taught some new tricks since we're about the same age.

Me too.

 

[BiffF15]

Yes the AAF tested different propellers on the P-40F. Obviously some were better than others. The P-39 had an Allison engine. The props tested on the Allison P-40E were all remarkably similar in performance, both in speed and climb. One would think that would be the better comparison in this case.

If you all are done patting yourselves on the back, can any of you divide 1000 by 836?

P39 Expert,

I think all of us can do division. The point you are ignoring is there is more to it than that. Something was different between those two tests, and it wasn't just the weight. Peter Gun published the German test results for the Mustang, and they recorded 1'48" to 10000, or a little better than 5k per/min initial climb rate. Should I use that going forward as the standard climb rate for the P51D? If not, why?

As for wing behavior at altitude, please understand that they are not linear and the results at one altitude will not be the same at another. The F16 can easily out turn an F15 at low altitude, however there is an altitude that they become even, and then it swings in favor of the F15. And at all altitudes the F15 has a thrust to weight deficit compared to the F16. Again, all things are not linear nor can they all be extrapolated based on one data point.

I don't know what type of car you drive, or if when you get on it you can tell the difference in performance between a hot humid day and a cool day. But in the aviation world, as Wes pointed out earlier, it makes a huge difference. I've taken off in an F15C on a cool day, full afterburner, and had 540 KIAS at the end of the runway. I've taken off on the same runway, in the same configuration, on a hot day, and had 100 KIAS less. Temp makes a huge difference.

Several guys have passed on reasons they suspect the test you are hanging your hat on is flawed. It doesn't fit. The reason there are some times barbs passed out in here is because you are being shown that 1+1+1=3, yet you have a piece of paper that says 1+1=3 and you won't budge. And those last three words is why this thread is 71 pages deep so far.

Cheers,
Biff

 

[Ivan1GFP]

As has been explained to you, the "service ceiling" of a standard P-39K was over 30000' BEFORE any weight reduction. They reduced the weight by 650lbs and increased the combat ceiling to 27000.

As has been explained to YOU, Those test results are under standard conditions or corrected to standard conditions.
Today happens to be a reasonably cool Fall day and Temperature is in the low 60's or not too far off standard temperature of 59 degrees Fahrenheit. Most of this Summer was way above that.
How many nice cool 59 degree days do you think you will find in the SWPA? What do you think happens when you run the same aeroplane in 110 degree temperatures? Do you think performance gets better?

If you are calling 27,000 feet the new combat ceiling, you are simply making things up.

This has been explained to you before. The difference in climb rate between the two virtually identical (except weight) planes was 1000fpm at 10000'. The difference in weight of the two planes was 836lbs. Divide 1000' by 836lbs and you get 1.2fpm per pound of weight. That means if you reduce the weight of the P-39D by 300lbs the climb rate improves by 360fpm. If you add 200lbs to the P-39C then you decrease the climb rate by 240fpm. That's at 10000'.

You have ABSOLUTELY NO EVIDENCE that the two planes were "virtually identical".
On the other hand, you have quite a lot of evidence that SOMETHING, most likely engine power or thrust was significantly different between the two aircraft. As GregP pointed out, the Power Required for the P-39D didn't even match well with what it was doing in its climb at higher altitudes.

Now how does your 1.2 fpm per pound rule translate to higher altitudes and improved service ceiling????

No amount of slightly different propellers (of the same diameter by the same manufacturer), an inch in length, fin fillet, symmetrical airfoil, adjustments to standard atmosphere or ANY OTHER FACTOR is going to make a difference of 1000fpm climb except almost a half ton of weight. The planes were virtually identical except for 836lbs weight.

Again, you make the assertion that the planes were virtually identical except you have no evidence that was true.
There certainly were external aerodynamic differences such as the .30 cal MG in the Cowl, the .30 Cal MG in the Wings.
I am not convinced that there was any extra length between the C and D models.
Most of the other factors you brought up are from a different discussion and not relevant here.

As for the Propeller, Your statement really tells us that you know nothing about propellers.
Take a look at the propellers on the Curtiss P-40. I believe the majority of them were made by Curtiss Electric and I believe all were 11 feet 0 inches in diameter. The blade designs are quite different and their performance is different.

You have been told that the engine outputs listed in the P-39D test do not match the climb performance and given evidence that you choose to ignore.
Any of these other factors alone would make the planes NOT identical.

- Ivan.

 

[Ivan1GFP]

Hello Ivan
Of course, 2 hours sleep and too much on 60-series Merlins lol!
Igor Kopilow in his article on La-5s in feeniks 3/2014 says that during 1944 there were improvements in the supercharger air intake.

Hello Juha3,

There were some pretty obvious improvements between the La 5F and La 5FN.
If these were improvements to the La 5FN, they do not seem to have translated to the later La 7 which shows the same critical altitude for its M-82FN engine.
The La 9 manual lists some slightly different statistics though.....

- Ivan.

 

[Juha3]

Hello Juha3,

There were some pretty obvious improvements between the La 5F and La 5FN.
If these were improvements to the La 5FN, they do not seem to have translated to the later La 7 which shows the same critical altitude for its M-82FN engine.
The La 9 manual lists some slightly different statistics though.....

- Ivan.

Hello Ivan
The sc air intake mods or more specifically mods in the sc air intake duct were inside -5FN production run. And because in La-7 the supercharger air intake was completely different, from the wing roots, so it was completely new design.

 

[Dash119]

 

[Peter Gunn]

Yes the AAF tested different propellers on the P-40F. Obviously some were better than others. The P-39 had an Allison engine. The props tested on the Allison P-40E were all remarkably similar in performance, both in speed and climb. One would think that would be the better comparison in this case.

If you all are done patting yourselves on the back, can any of you divide 1000 by 836?

 

[SaparotRob]

View attachment 600957

Sweet!

 

[XBe02Drvr]

"And the carousel goes round and round,
and the painted ponies go up and down,"*
all in the same flight path, all with identical rates of climb and descent, regardless of each one's gross weight. Doesn't that prove the point?
*Apologies to Joni Mitchell

 

[GregP]

Nice pic of the Planes of Fame F-86F.

 

[DerAdlerIstGelandet]

I think this thread has run its course everyone. There is no point. There is no getting through to people who have no desire to listen.

 

[Dash119]

Yes, but is has been entertaining...

 

[XBe02Drvr]

Yes, but is has been entertaining...

...and educational to everyone except the one most in need. Thanks, guys.

 

[fubar57]

Hey....let's start a new P-39 thread. It'll be swell

 

[ThomasP]

Hey GregP,

FWIW when I do the math it appears the power settings and ROC for P-39D S/N 41-6722 are correct.

However, the values for P-39C S/N 40-2988 cannot be correct - at least not for a sustained climb at the power setting and weight listed. My math says that the AC can do 3270 ft/min at 6680 lbs with 1160 BHP. (Don't know if the 3270 as opposed to 3720 is an indication of anything, could just be a coincidence.)

The only way I can get the sustained 3720 ft/min ROC is if the engine is putting out ~1300 BHP, or If you accelerate to Vmax at SL and go into a zoom climb at max power, then divide the height gained before you have to reduce power by the time it took to reach said height. I suspect the latter is what they did.


Hey P-39 Expert,

This is not an attack on the quality of the P-39. I too think that the P-39 got an unfairly poor reputation in common history.

 

[Peter Gunn]

Nice pic of the Planes of Fame F-86F.

Thanks, thought it'd be a nice tip of the hat to you while referencing my post #1409.

She is a beauty.

 

[Milosh]

Not all airplanes are the same. Two consecutive a/c off the assembly line will have different performances. They all must be within a certain performance tolerance, of say +/-5%. P-39 Expert, I will let you do the math.

 

[P-39 Expert]

You are quoting performance under standard conditions. As has been explained to you, there is no evidence these were "standard" conditions and the greatest likelihood is that the conditions were much less than optimal.
As for being "inclined to believe", you are welcome to believe what you want as you tend to anyway, but that does not change the contents of the document which states that 27,000 feet was a service ceiling.
General Harmon should have been pleased with this experiment; it actually worked! The other experiments did not work out nearly as well.



I don't believe anyone is disputing that reducing weight will usually increase climb rate and ceiling. The question is really how much effect they will have. We just don't believe your "calculations" have any basis in reality and that there were other more important factors that differed between the P-39C and P-39D tests.

Just to humour you for a second, what do your calculations tell you the increase in climb rate would be at 25,000 feet and please show your work. What do your calculations show the increase in service ceiling would be?

- Ivan.

Double post.

 

[P-39 Expert]

Not all airplanes are the same. Two consecutive a/c off the assembly line will have different performances. They all must be within a certain performance tolerance, of say +/-5%. P-39 Expert, I will let you do the math.

3720fpm x 5% = 186fpm maximum deviation. That's a long way from 1000fpm.