XP-39 II - The Groundhog Day Thread

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Obviously somebody cares or this thread would not be 1000 posts long.

Question is, does weight influence climb? Obviously it does. Then we should be able to quantify how much weight influences climb, all other factors being equal.
 
P-39 Expert said:
Obviously somebody cares or this thread would not be 1000 posts long.

Question is, does weight influence climb? Obviously it does. Then we should be able to quantify how much weight influences climb, all other factors being equal.
Click to expand...

I think its starting to wear thin on everyone...

Do you really think that only weight influences climb, and that all factors are equal? That when removing weight at one location, and adding weight at another won't have differing effects. How is your new CG going to effect performance. Oh wait who cares about CG right?

The point that all us "non experts" are trying to tell you is that simply dividing weight and horsepower (excuse me rate of climb) will not give you a rate of climb. But hey what does everyone know right? Working knowledge means shit all.
 
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Above planet earth there are two extremes to the atmosphere, one has air at sea level and the other has almost no air on the edge of space. An air breathing engined aeroplane is happy at sea level. Some like helicopters and the Fairey Rotodyne don't need fixed wings to take off vertically, the propellers/ rotors can do it all on their own. The higher you go the more problems you have developing power and lift. Eventually there is a point where the maximum speed and the minimum speed become the same. It cant make more power and it cant make more lift and it cant make less power or it wont have enough lift, it can only fly in a straight line because any input increases drag and causes a stall. You could put almost anything inside a 2000BHP fighter and it would make little difference to sea level performance but Spitfires tasked with intercepting recon planes above 40,000ft even took the radio of one plane out and flew in pairs. How much effect weight has depends on "stuff" lots of "stuff" and usually involves terms and calculations that give me a headache.
 
 

Even if you are within limits the shift in CG may still have an effect on performance. Everyone single person here has explained that to you already.

Dividing increase in climb rate by increase in weight has also been pointed out as a fallacy, but no need to repeat that, you don't care. Rate of climb is effected by so much more than just weight. So why would dividing weight and rate of climb be accurate?
 
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Hello P-39 Expert,

This is looking a lot like the movie Groundhog Day.
Hopefully you might learn something like Bill Murray did.

P-39 Expert said:
Just the pilot slightly varying the mixture control to get the best power based on feel and sound. Why wouldn't that be done in combat?
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You obviously don't know how the carburetors work in these aircraft.
The Auto Rich setting gives a slight margin for detonation under maximum throttle settings and the automatic mixture controls also compensate for altitude (air density) changes with varying flight conditions.
To achieve the extra 25 HP and 3 extra MPH, the pilot had to switch to manual mixture control and lean out the mixture.
This might work just fine if you are flying along straight and level with nothing else is going on, but in COMBAT, this is just plain STUPID. Without the automatic mixture control working, the pilot would need to compensate for any altitude changes immediately. Losing a few thousand feet of altitude and finding that your engine is now detonating or burning valves is a pretty lousy way to find you need to walk or swim home.

P-39 Expert said:
Highest speed listed in that report is 321mph at 18000'.
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You are correct. IIS 85 is not the correct reference.
You are also not reading the report correctly. It actually says 326 MPH.
It is interesting though that the actual information that was eventually distributed more widely was 332 MPH.

This is a better reference.
Memorandum 23 Oct 1942

Note that maximum speed in this report is 335 MPH @ 16,000 feet and the test aircraft is still achieving 331 MPH at 20,000 feet.

Do you remember my comment that if you didn't like a comparison of climb rate with the A6M5 at 15,000 feet, you would like the comparison with the A6M2 even less?
Note that the climb rate is 2480 feet/minute at 15,000 feet. How does that compare with the P-39D????

The most interesting information listed in this memorandum report is the note at the bottom which states that the manifold pressure was limited to 35 inches Hg. This is only +129 mm boost which isn't even Military Power.
Military Power would be +150 mm or 35.83 inches Hg
Emergency Power would be +250 mm or 39.76 inches Hg
I had missed this note in my earlier readings.......

This is why I am tending to believe now that Mr. Dunn's argument is probably correct considering that a rebuilt wreck can achieve 335 MPH without even using MILITARY power and with a carburetor that wasn't working correctly.

P-39 Expert said:
See Windhund's post #1020, 330mph at 15500'.
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First of all, the source of information that is listed in that table is a book from 1981.
Second, your conversion isn't entirely correct. Speed should be 331.5 MPH (288 Knots).
Third, you should read the footnote to that entry. The information that is listed there suggests that the author is a bit confused:

"The speed was increased after thickening the outer wing panel"? How does THAT improve speed?
Sounds like he was confusing level speed with maximum diving speed..... There is much more detail to this discussion about the authors errors that is beyond the scope of this discussion.
Note also that earlier speed was specified as 275 Knots. This was NEVER the maximum speed as we understand the term here.
275 Knots (316 MPH) was the maximum speed under "Normal Power" settings of +50 mm boost at 2350 RPM.

P-39 Expert said:
That was the D-1 model that weighed 7850lbs. Could have easily weighed 7250lbs after removing redundant or unnecessary equipment. Read the chart in my post #1006 and compare those climb numbers to IIS 85 test. That P-39 weighed 7650lbs.
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If you have information that can confirm that the P-39D-1 in the test was loaded to 7850 pounds please post it.
Considering that the pilot in the test was willing to try 70 inches Hg at Take-Off to prove he could beat the Zero off the line suggests that they were not above a few tricks.

In any case, 7250 pounds was not the loaded weight of operational P-39D-1 in service and your hypotheses are not proven and irrelevant.

- Ivan.
 
P-39 Expert said:
300lbs lighter at 1.2fpm per pound would mean adding 360fpm to the values on the chart in my post #1006.
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You have made a mistake with your units, the pounds are minus pounds or you could add "per pound reduction" this is how you start to argue increasing weight increases climb. Dont worry though, I have seen engineers do it on pipeline bundles discussing "negative boyancy".
 
Ivan1GFP said:
Hello P-39 Expert,

This is looking a lot like the movie Groundhog Day.
Hopefully you might learn something like Bill Murray did.

*SNIP*

This is a better reference.
Memorandum 23 Oct 1942

*SNIP*

- Ivan.
Click to expand...
What's interesting to me is how the A6M2 holds its speed at all altitudes (almost), the deviation from 12k to 20k doesn't fluctuate more than 15mph.
 
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