XP-39 II - The Groundhog Day Thread

[FLYBOYJ]

 

[fubar57]

LMAO

 

[XBe02Drvr]

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?

 

[pbehn]

Not all airplanes are the same. Two On an aircraft a difference of 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.

When I was 16, myself and many of my friends had Yamaha mopeds. 50cc and all "new" as in less than a year old. The difference from fastest to slowest was 5MPH which is 10%. Yamaha did a race series where they provided the bikes and invited riders competed with riders just being given the keys. Despite all bikes being run on a dyno to be nominally the same output, that is only peak output on one day, come race day the difference between fastest and slowest bike was over a second per lap. On an aircraft a difference of 3MPH or 50 ft/min is no difference at all, a week or two later the difference could be reversed.

 

[GregP]

Reversed? Does that change the meaning?

Let's see: A man a plan a canal panama.

Go ahead and reverse it!

How about: racecar?

Hey! This is fun!

 

[pbehn]

Reversed? Does that chang the meaning?

Let's see: A man a plan a canal panama.

Go ahead and reverse it!

How about: racecar?

What I meant was in a week or two, one plane gets cleaned, has an overhaul, new paint job, filled all the tanks etc etc etc. 3MPH is less than 1%

 

[GregP]

Yah. Knew that, but tried to inject some dry palindrome humor ... and obviously failed.

But, I might succeed tomorrow, after a new paint job.

And supposedly "identical" racecars and fun ,too. The Sports Renault class was fun! All the engines were tuned to makle the exact same horsepower on a dyno and had the same brakes and tires and weight. Then you race! What it actually meant in real life was that the drivers tended to trade paint more often becasue they couldn't pull away from their competition. Still, someone usually won the race. But it wasn't usually the same someone every weekend.

 

[Peter Gunn]

I think all would be clear if we had a colour picture of a p-39 in flight.

See post #881 waaaaaaay back in the thread...

 

[Peter Gunn]

Absolutely nothing wrong with a symmetrical airfoil. The A6M2 will be gasping for air just like the P-39K at 27000'. Sure it has a much lower wing loading, and will be more maneuverable at 27000' just like it is at 10000'. But it won't climb any better and it sure won't dive any better.

Wait, are you actually trying to contradict and then instruct a real pilot with more hours behind the stick than Carter's got pills?

You know, if X XBe02Drvr or FLYBOYJ or GregP or BiffF15 or DerAdlerIstGelandet just to name five (5) guys that did or do fly ACTUAL airplanes, work in the industry and have backed up everything they've told you with actual data... I'd listen.

You've been trying to contradict I Ivan1GFP and nuuumannn as well, even though they too have shown you facts and figures to illuminate the error of your ways.

I now believe that you're just a time wasting troll with no desire to learn and even when you're wrong you won't admit it, if others want to continue a discourse (to no useful end) that's fine. Harsh words perhaps but I'll risk a "vacation" for speaking my mind on this, I mean, did you not learn anything when drgondog took you to task last year on this subject?

But please, go ahead an argue thy book knowledge with fellows that are in the industry, there's no popcorn shortage at my house.

 

[pbehn]

Wait, are you actually trying to contradict and then instruct a real pilot with more hours behind the stick than Carter's got pills?

You know, if X XBe02Drvr or FLYBOYJ or GregP or BiffF15 or DerAdlerIstGelandet just to name five (5) guys that did or do fly ACTUAL airplanes, work in the industry and have backed up everything they've told you with actual data... I'd listen.

You've been trying to contradict I Ivan1GFP and nuuumannn as well, even though they too have shown you facts and figures to illuminate the error of your ways.

I now believe that you're just a time wasting troll with no desire to learn and even when you're wrong you won't admit it, if others want to continue a discourse (to no useful end) that's fine. Harsh words perhaps but I'll risk a "vacation" for speaking my mind on this, I mean, did you not learn anything when drgondog took you to task last year on this subject?

But please, go ahead an argue thy book knowledge with fellows that are in the industry, there's no popcorn shortage at my house.

You miss the point, this is an ideas supermarket thread, "I don't buy it" is some sort of technical argument, I have two pounds, three shillings and four penny three farthings, so I am unsure of what technical information I can take from a discussion.

 

[fubar57]

You miss the point, this is an ideas supermarket thread, "I don't buy it" is some sort of technical argument, I have two pounds, three shillings and four penny three farthings, so I am unsure of what technical information I can take from a discussion.

I'm sure glad I went to Great Britain after the currency change-over

 

[special ed]

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).

 

[Snautzer01]

Your wish is my command.

View attachment 601126

It is in flight, fleeing the water, a task at which it seems moderately successful, much like it's success as an interceptor.

See post #881 waaaaaaay back in the thread...

I take it you do know the longest running joke on this forum? It is not post 881.

 

[pbehn]

I'm sure glad I went to Great Britain after the currency change-over

I just remember the old currency but my experience of "farthings" was we had them all over the place and they were worthless. There is something magnificently comic about it all, and I didn't even venture into "guineas".

 

[fubar57]

I was there in '78 and remember the ½ penny. I thought it was a great coin

 

[GregP]

Hi Snautzer01,

I forgot. That was back before there was mass hand sanitizer, wasn't it?

 

[ThomasP]

Hey Simon Thomas & GregP,

Interesting. I started out with the assumption that the test for the P-39D was accurate and the P-39C test was possibly/probably incorrect. The reason I chose the P-39D as the basis for my calculations is that the values for climb and speed are more in line with test results of other aircraft (ie Spitfire, Hurricane, P-40, etc) with similar weight, engine power, drag, etc. Plus the tests were at a later date for an airframe already in production, hopefully meaning that there was no reason to overhype the test results.

Would you guys do me a favor (seriously) and do your calculations again, but this time treat the values for the P-39D as if they are the correct ones? Then apply the appropriate values to the calculations for the P-39C. I would like to see if your results are then similar to mine.

PS. If it matters I used a Cd of .020 for the P-39C and .022 for the P-39D.
[EDIT: My apologies, I used a Cd of .021 for the P-39C]

 

[Simon Thomas]

Let's look at the P-39C. At 10,000 feet, the rate of climb is 3,720 fpm. An Allison V-1710-35 makes 1,150 hp at 12,000 feet and the airplane is at a weight of 6,689 lbs. It just so happens we have data for 10,000 feet. Rate of climb is 3,720 fpm. Calculating backwards, the power required for level flight at that altitude is 396 hp. Personally, I'd expect the power required for level flight to go up as the altitude goes up becasue there is less air density, so the wing has to "work harder." By work harder, I mean a slightly higher angle of attack, and that creates more induced drag, requiring more power.

You need to use THP not BHP. During climb, I got a prop eff of 0.81, which drops 1150 BHP to 931.5 THP.

 

[Simon Thomas]

Hi ThomasP. It will take me a day, but I will get to it. I like number crunching like this - although all my calcs come with a 30 second / 30 yard warranty.

 

[GregP]

If I don't misremember, the flat plate area of a P-39D is 4.60 square feet.

However, to get any better numbers, I'd want an L/D plot for the P-39D, and I'm not to sure where (or if) I can find that. It's not as if the P-39D was a popular and well-documented WWII fighter. I know there is an airspeed for L/D max and it varies as the square toot of the weights. so: V2 / V1 = sqrt (W2 / W1), where V = L/D max airspeed and W - wirhgts. So, if weight (W2) goes up, then V2 goes up, too.

Let me think about this.