XP-39 II - The Groundhog Day Thread

Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules

Status
Not open for further replies.

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
 
P-39 Expert said:
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.
Click to expand...

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.


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????


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.
 
Juha3 said:
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.
Click to expand...

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 said:
Click to expand...
"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
 
Last edited:
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.
 
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.
 
Double post.
 
Milosh said:
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.
Click to expand...
3720fpm x 5% = 186fpm maximum deviation. That's a long way from 1000fpm.
 
Status
Not open for further replies.

Users who are viewing this thread

Total: 1 (members: 0, guests: 1)

Similar threads

S
Day fighter Ta 154
Replies
0
Views
536
spicmart
S
M
Pinnacle of Piston fighter: XP-72 vs Spiteful Mk XVI?
3 4 5
Replies
83
Views
5K
Jugman
J
M
P-39 Turbo - Here We Go Again
Replies
13
Views
2K
GregP
1937-45: Doubling down on the 2-engined 'day fighters'
7 8 9
Replies
175
Views
8K
Admiral Beez
A
P-39-style fighter by other people?
2 3
Replies
50
Views
2K
GrauGeist
Menu
Log in
Register