XP-39 II - The Groundhog Day Thread

[pbehn]

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.

In this discussion 50ft/minute is considered important, have you ever tried walking at 50ft/min? Slower than any bride or funeral cortege down a cathedral aisle. Without modern laser or other sensors I don't see how any 1940s aeroplane could be measured to such limits of accuracy.

 

[wuzak]

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.

I believe the engines had to be within +/- 5% of the rated power to be considered acceptable.

Not sure how much performance testing the factory acceptance tests went into. Certainly they checked for proper running and operation.

 

[wuzak]

They were as identical (except weight) as the manufacturer could make them. Same contract, same engine, same HP, same airframe. Different internal equipment.

The two planes were as identical as the manufacturer could make them, except for weight. The difference in climb wasn't from different propellers, atmospheric conditions, symmetrical airfoils, tail fillets, .30cal gun ports etc. It was because one was 836lba heavier than the other.

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).

Expand above.

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

 

[Simon Thomas]

In this discussion 50ft/minute is considered important, have you ever tried walking at 50ft/min? Slower than any bride or funeral cortege down a cathedral aisle. Without modern laser or other sensors I don't see how any 1940s aeroplane could be measured to such limits of accuracy.

Naturally it is only as accurate as the inputs. Based on reading numerous Air Corp reports, some of which have calibration info, I have a confidence the results are better than a WAG.
Fortunately the calc method I used for the P-39C test has effectively 3 unknowns, and there is far more test results than that, so there is some degree of error checking possible.

 

[wuzak]

According to the tests, the P-39N (available from Nov 1942) outclimbed the Griffon Spitfire and was about the same speed. The N would substantially outclimb the P-51A (same engine) and was very close in top speed. The N outclimbed the 1943 Thunderbolt, Corsair and Hellcat substantially at 20000', just like I said. The P-39N was equipped with full armament including the wonderful four .30s for the tests, no weight reduction needed.

Well, that depends on which engine and propeller the XII was using.

Of the two tests on Spitfire Mk XII Performance Testing the first was using an experimental propller:

DP.845 was the prototype Spitfire XII. It was fitted with a Griffon IIB engine and normal span wings. The report noted a Rotol "experimental" propeller was fitted, however, it was previously reported that this model propeller was "rather inferior" in climb and that maximum level speed did "not show any appreciable difference" to production propellers. AFDU Report No. 61 noted that speeds of a production Spitfire XII (EN.223) with Griffon III and clipped wings "were found to be almost identical" to DP.845. For comparison, Spitfire XII MB.878 with Griffon VI operating at +12 lbs/2750 RPM achieved 394 mph at 18,100 ft. (15th part of Report No. AAEE/692,o). The Spitfire XII Aircraft Data Sheet is in good agreement with these test results.

The same Spitfire XII tested with the Griffon VI (report mid 1943) had a peak climb rate of nearly 5,000fpm.

 

[ThomasP]

Hey GregP,

A Cd of .021 gives a flat plate of 4.5 ft2
A Cd of .022 gives a flat plate of 4.7 ft2

So I think a flat plate of 4.6 ft2 will do just fine.

 

[Milosh]

I believe the engines had to be within +/- 5% of the rated power to be considered acceptable.

Not sure how much performance testing the factory acceptance tests went into. Certainly they checked for proper running and operation.

For the Germans it was performance, like climb and speed.

 

[wuzak]

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).

The P-39D test also had the CoG at 25.8% MAC. The P-39C test does not mention this.

How much performance difference is there between a forward CoG (as the P-39D appears to be) and a rear CoG?

 

[Ivan1GFP]

I haven't made anything up, just quoting an AAF report. Why would a General quote a figure for climb at 100fpm?

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.

It translates to 10000'. It will decrease with altitude. Just like all climb rates.

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

Agree with you and it wouldn't make any difference anyway.

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

And you are an expert?
.....
Very slightly different performance. The propeller with the biggest difference was a British Rotol propeller. What are the odds of that propeller being used on an American plane? The propellers used on the Allison engine (like in the P-39) had very small differences.

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.
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.

The two planes were as identical as the manufacturer could make them, except for weight.

What evidence do you have for this assertion that you keep making?

- Ivan.

 

[Snautzer01]

Hi Snautzer01,

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

Indeed . That thread was in the time of tranquility. The age before this Age of Repetition.

 

[Ivan1GFP]

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".

Hello pbehn,

This sounds a bit like when I was in Thailand for a few days. I like to collect a few coins from various places I have been and was trying to do the same there. The problem was that I was never getting any 1 Baht coin.

After a couple days, I was running a bit short on cash and had to change more money. I casually made the comment about never seeing a 1 Baht coin to the fellow who was behind the counter. He just said "wait here" and came back a minute or so later with several of them. They were about the size of a typical thumb tack and looked brand new. He told me to just take them and when I asked what he wanted for them, he said "Nothing".
Those tiny little coins are probably still floating around in one of the places I keep souvenirs from trips.

- Ivan.

 

[GrauGeist]

But could the P-39 nuke Japan?

Wait...is this the wrong thread?

 

[buffnut453]

But could the P-39 nuke Japan?

Wait...is this the wrong thread?

Yes it could...but you'd have to strip off excess weight, fit bulged bomb-bay doors, and add a Merlin 86 engine.

 

[Dash119]

Yes it could...but you'd have to strip off excess weight, fit bulged bomb-bay doors, and add a Merlin 86 engine.

Don't forget, once you do all that you need to move the radio forward or the CG will be out of whack...

 

[XBe02Drvr]

for power required to maintain level flight at Vx, I'd expect the required power to go up as you go higher. The rate of climb test is not a top speed test and is run mostly at whatever best rate of climb airspeed is. I'd say it's somehwere around 1.4 * stall speed and slightly increases as you climb, and the power required to maintain level flight at Vx increases, too, as you climb.

Are you sure you mean Vx, best angle of climb speed? My creaky old memory is that Vy, best rate, is the low point on the power required curve, which would put Vx on the backside of the curve and would actually require more power to maintain level flight. If you're holding waiting for a landing clearance, your best endurance speed is at or about Vy, as long as icing is not a factor. If it is, you have to hold at a higher speed to keep from icing your under surfaces. Tail stalls are generally fatal. (AmEagle ATR at Roselawn, IN, Colgan Q400 at BUF).

 

[XBe02Drvr]

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]

I used a Cox .020 for my P39C, but had to step up to a Wen Mac .024 for my P39D because of the 836 grains of extra weight. The D snapped its "down" control line and went into a series of tight loops until it hit the ground.

 

[glennasher]

You folks are probably forgetting to factor in the pilot's breakfast choices before those flights, too. Did he have grits and bacon, or grits and sausage, or SOS? Did he have oatmeal and maybe a grapefruit?
If he was on Cactus Airstrip, did he have breakfast at all? Surely all the weight added by the well-fed pilot will alter the climbing rate of a P-39D more than a P-39C, right?

 

[XBe02Drvr]

Surely all the weight added by the well-fed pilot will alter the climbing rate of a P-39D more than a P-39C, right?

You forgot the buoyancy effect differences of the methane gas created by various dietary choices!

 

[glennasher]

You forgot the buoyancy effect differences of the methane gas created by various dietary choices!

My apologies, sir. There's only so much stuff I can process at one time, and that slipped by. Mea culpa, mea maxima culpa!

 

[Milosh]