SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
Never thought that technobabble would interest me. But I've been following this thread and now it's become a mystery! Far out!
XP-39 II - The Groundhog Day Thread (1 Viewer)
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wuzak
Captain
Hi ThomasP.
I pretty much came to the same conclusion. There is something hokey about the P-39C test, but I don't know what.
If I assume you are correct and the P-39C ROC should be 3270 fpm, then the Pr rises to 478 hp, which about in line with the P-39D and M. Then, if I vary the hp to get 3720 fpm, the hp turns out to be 1241 hp, not 1150. Here's the thing, we do NOT have the MAP readings for climb. When I was working at an Allison shop for a few years, we had retired General Davey Allison come by the shop for a visit. He told us they used to fight the P-40s in the AVG at 70" MAP. They had early Allisons, similar to the P-39C engine, but it had a prop shaft on the front in the P-40 and was probably a C-series engine ... likely a V-1710-33 in it. He did not know the horsepower, but it was definitely above what the stock MAP reading gave.
The early C-series nose cases were not too strong, but the -35 in the P-39C/D didn't HAVE the early nose case; they were early E-series engines and connected to a driveshaft, so they didn't needs a nose case.
If the P-39C pilot for the flight test we are looking at ran higher than normal MAP, he could well have been climbing at more than 1,150 hp, and may well have seen 3,720 fpm at the elevated hp and MAP. The thing is, we don't have any way to find out. But it is definitely different from the P-39D and M and other tests on the P-39.
As you said, interesting. Isn't precise analysis fun?
We know what we don't know, but we aren't sure what we know ... there's a joke in there somewhere. Either they ran the P-39D test at lower-than-reported hp or they ran the P-39C test at higher-than-reported hp ... or somebody misreported something ... or they held their tongue wrong during flight ... or ... a pretty girl walked past when they were recording the numbers.
At this point, I am leaning toward the P-39C test being run at higher-than-reported MAP and hp, combined maybe with a propeller that was better for climbing. But, the weight difference alone won't give you 1,000 fpm difference in climb. Some other factor was at work here. It may be forever a mystery unless we can find other P-39C tests. They only made 20 of them. How many tests were conducted? I surely don't know.
Poor toilet training as a kid on the part of the report writer might be in there somewhere, too.
Don't forget the engine rating acceptance tolerance of +/- 5%.
That equates to +/- 57.5hp from the rated 1150hp. Or a total of 115hp difference.
Could that be a contributing factor to the performance difference?
ThomasP
Senior Master Sergeant
Hey wuzak,
Yes, that much HP could definitely make a difference.
The problem in this case is that the climb rates for all of the types we have info on (except the C) generate values that are close to the power curve stated in the tests, ie the calculated values vary only a small +/- from the stated BHP. For the variance in engine power to explain the test C's 3720 ft/min ROC, all (I think) of the other tests would have to have been run with underpowered engines, and have engines that are close to the same in underperformance. The maximum speeds on some of the other tests may indicate slight variances in engine power, but without detailed drag indexes for the differences between the models I (we?) can not really factor that in meaningfully.
GregP got 1241 BHP needed (I assume upto about 12,000-13,000 ft), and I got 1275-1300 BHP+ needed upto about 10,000 ft, if the P-39C is going to have a 3720 ft/min ROC. Plus I had to use a Cd of .020-.021 (depending on the BHP used) for the C, as opposed to .022 for the others (I am not sure what value of Cd GregP used, but I would bet it is closer to .020 for the 3720 ft/min ROC).
PS. As mentioned up-thread, significant variance in propeller efficiency could have a significant effect also. But again, to match the test results for the other models, all of them would have to be using props of about the same efficiency, and all of the props would have to be significantly less efficient than the one used on the C model.
Yes, that much HP could definitely make a difference.
The problem in this case is that the climb rates for all of the types we have info on (except the C) generate values that are close to the power curve stated in the tests, ie the calculated values vary only a small +/- from the stated BHP. For the variance in engine power to explain the test C's 3720 ft/min ROC, all (I think) of the other tests would have to have been run with underpowered engines, and have engines that are close to the same in underperformance. The maximum speeds on some of the other tests may indicate slight variances in engine power, but without detailed drag indexes for the differences between the models I (we?) can not really factor that in meaningfully.
GregP got 1241 BHP needed (I assume upto about 12,000-13,000 ft), and I got 1275-1300 BHP+ needed upto about 10,000 ft, if the P-39C is going to have a 3720 ft/min ROC. Plus I had to use a Cd of .020-.021 (depending on the BHP used) for the C, as opposed to .022 for the others (I am not sure what value of Cd GregP used, but I would bet it is closer to .020 for the 3720 ft/min ROC).
PS. As mentioned up-thread, significant variance in propeller efficiency could have a significant effect also. But again, to match the test results for the other models, all of them would have to be using props of about the same efficiency, and all of the props would have to be significantly less efficient than the one used on the C model.
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fubar57
General
Found this on the site, listed as a contemporary of the P-39. Only one was made...Piaggio P.119 - Wikipedia
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Always thought the P.119 was interesting in the fact that they used a radial instead of a "V", which would have been a pain in the a** in regards to cooling issues.
I wasn't sure I really wanted to reply to your post because I don't think our back and forth posts ever really accomplish anything.
I will warn you in advance: My basic premise is that the NACA L-602 Report was generally correct and that when the CoG reached about 30.2% MAC, handling became dangerous even though this was still ahead of the "aft CoG Limit".
The primary goal for most of these changes is to ensure that the CoG of the Airacobra never goes beyond the range of 23% MAC as a forward limit and 28.5% MAC which appears to be a Safe aft limit. If you do not agree that this is a reasonable goal, then you will probably not agree with the changes.
I don't believe you are correct that the .30 cal MG were the reason for only 15 rounds of 37 mm ammunition in the P-39C.
Please see the attached diagram. The magazine for the 37 mm was simply different on the P-39C. The .30 cal MGs do not appear to be in the space that would be taken by the 30 round endless belt magazine for the 37 mm that was installed in P-39D and later models.
As for punch from the .30 cals, note that they are comparable to or even slightly more powerful than the .303 Vickers MG (Japanese Type 97) that were installed in the A6M2 through A6M5. With the low capacity of the 20 mm in the wings, many folks believe that the majority of kills by the A6M early in the war AGAINST Allied aircraft were with those synchronized MG.
As for weight, I believe these guns and their ammunition and associated equipment could substitute for the Gear Box armour.
The intention is NOT to reduce weight up in the nose because it would need to be made up with additional equipment, or ballast in the form of armour such as the pieces of cheek armour in the British Airacobra. For this reason, I believe 250 rounds would be a very reasonable ammunition load.
Without the 37 mm endless belt magazine or the 20 mm Hispano 60 round drum, there should be enough room above the cannon and between the .50 cal MG to mount an oil tank. There is a fairly small drive mechanism for the belt above the 20 mm Hispano, but it doesn't anywhere near the amount of room that the drum did.
I don't believe the 30 cal in the nose should be deleted because they would be a substitute for the ballast up in the nose that the Gear Box armour represented.
The idea was not really to reduce weight but to shift the CoG as far forward as possible
Other potential spaces for the oil tank might be in the wing center section or fuselage just ahead of the radiators and oil coolers.
-Ivan.
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Adding more weight to the wings will affect roll-rate, so be careful of that adventure.
Peter Gunn
Master Sergeant
I dunno, from some of the claims made through this thread apparently the Air Force brass were a bunch of chuckle heads that were out to "get" Larry Bell. I mean this plane obviously out performs the Mustang, Thunderbolt and Lightning combined and the Navy was stupid for not buying the Airbonita. I mean, its performance above 20,000 ft is simply stunning, it out climbs anything in the sky and is faster than anything else in the inventory, Allied or Axis.
P39 Expert,
And based on one test that is different from all others. You just aren't assimilating that there is something wrong with this test. I think Greg has probably guessed correctly that the guy did a zoom climb after accelerating to a speed well above climb speed. Also realize that "test pilots" have variations among group as well.
Cheers,
Biff
Huh?
I wouldn't count on "climbing away" from the Zero. That would only reliably work if the P39 started out in a high energy state and the Zero at cruise or other low energy state. Even IF (big IF) the P39 can outclimb the Zero, it'll be only by a small margin (don't trust climb numbers from allied tests of reconstructed Zeroes), and with its SIGNIFICANTLY heavier weight and its higher wing loading, will NOT have enough advantage in the acceleration and climb transition to get out of range unperforated unless it starts out with an energy advantage.
BINGO!
buffnut453
Captain
And that's another reason to disengage from the fight.
Did you see what I did there?
Ok...I'll get my coat!
US senator George Aiken after the Tet offensive in Vietnam: "Just declare victory and get the heck out of there!"Did you see what I did there?
Ok...I'll get my coat!
Really? I thought they had to be perfect clones of Steve Canyon before they'd let them out of Test Pilot School. My faith is shattered!
Hello GrauGeist,
With this "idealized" armament, there wouldn't be any wing guns at all.
I was thinking:
20 mm motor cannon - 250 Rounds.
2 x .50 cal synchronized MG - 270 Rounds per Gun
2 x .30 cal synchronized MG - 300 Rounds per Gun
All the guns have pretty similar ballistics. All the guns except the .30 cals have a pretty good duration of fire.
With this much armament and ammunition in the nose, the CoG will still shift aft pretty seriously when the ammunition is expended but hopefully the moving the oil tank from the tail to the nose will address much of that problem as would deleting the oil tank armour at the tail of the aircraft.
There is also the possibility of shifting radios a bit forward when the oil tank and armour are no longer there.
- Ivan.
"Bridge, Port Forward Lookout. Visual surface contact, twenty degrees port bow, four or five miles. Can't quite make out what it is, Sir."
"Conn, come left two zero degrees, course three one zero, all ahead two thirds. Port Forward, I need an ID, get with it, son! All hands, stand by General Quarters!"
"Bridge, Port Forward, I can make out a triangular shape through the mist, perhaps a small sailboat? Way out here?"
"C'mon, son, I need that ID! Forward Mounts, man up and stand by!"
"OMIGOD! It's the biggest pig's ear I've ever seen, floating upright in the water!...Sir."
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
To quote that brilliant American naval hero, Ensign Charles W. Parker;
"Gee, I LOVE that kind of talk".
"Gee, I LOVE that kind of talk".
P-39 Expert
Non-Expert
Almost forgot, the coolant tank could be moved up right behind the pilot very near the CG like on the P-63 and XP-39E. The oil tank could then be moved up to the space formerly occupied by the coolant tank. That may be the ultimate fix for any CG issues. Just a thought.
P-39 Expert
Non-Expert
My whole purpose of continuing this thread is that the climb of the early P-39D/F/K/L of 1942 could be improved by reducing the weight (.30s and nose armor). A P-39D/F or P-400 without those items would weigh about 7200lbs. At this weight it would outclimb the A6M2 and still have a significant speed advantage. In that case air combat in NG in 1942 could have been much different.
P-39N didn't need those modifications since it had the more powerful -85 engine and had an excellent climb rate.
There were chuckleheads all over. The Spitfire MkII was slower than the Mk I despite having a more powerful engine and a better propeller. It had a better climb performance and ceiling and carried more armour, this improvement was noted by LW pilots which means it was worth it.. The chuckleheads were concerned with getting a better military machine, not winning some arbitrary speed record. As with the P-51D which was marginally down on speed against the P-51B but was an all around better fighting machine.
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Dash119
Senior Airman
Okay, now I'm really confused... Is it chuckle heads or chuckleheads?
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