XP-39 II - The Groundhog Day Thread

[MiTasol]

Table 26 has a line item "Miscellaneous Equipment (Radio)" 129.2lb for the P-39D-2 and 109.9lb for the P-400. It doesn't mention IFF.

Under the electrical system section in the table, "Communication" lists 62.0lb, 80.2lb and 35.0lb for the P-39D, P-39D-2 and P-400 respectively.

In the systems description section for communication, navigation and identification systems three radio systems are described as being typical equipment:

SCR-535A radio recognition set with detonator and crash inertia switch (that's be the IFF).

SCR-522A command radio set

SCR-274N radio.

Thanks Wuzak
I do not have an SCR-535 manual so I cannot provide a weight breakdown for that. If my memory is correct, and I avoided learning about boat anchors (radios) as much as I could so may well be wrong, all the IFF boxes fitted on the same racks and weighed about the same. Certainly this applied to the SCR-595 and SCR-695.

However the claim that the IFF alone weighed over 100 lb is now shot to hell by both the SCR-595 manual and your sources.

 

[MiTasol]

Go back and READ our calculations (Post 368). In our example using numbers from the P-39Q loading chart, IT PUTS THE AIRCRAFT OUT OF CG!! It's probably the same for the P-39M as well.

Does anyone have charts for the M model. The Q had the external 50 cals and I think they actually bought the CG forward a tad meaning the M would be affected more by the removal of armour as it probably had the heavier early armour. Vs for Victory is a great book but it is not infallible. I have never seen an error in it but then again I have never seen the quoted Bell statement that the M could safely delete the nose armour.

Some sources (such as Instructions of Red Army Air Force Commission No. 4 on Winterized P-39M Airplanes, No. 6, 29-Jan-1943) say the P-39M was fitted with BFGoodrich deicer boots on the wings and tail. That would make the aircraft basic weight greater which may be a factor although I would have expected the tail boots total moment to be vastly greater than the counterbalancing wing boots. The Russians removed said deicer boots.

 

[FLYBOYJ]

Does anyone have charts for the M model. The Q had the external 50 cals and I think they actually bought the CG forward a tad meaning the M would be affected more by the removal of armour as it probably had the heavier early armour. Vs for Victory is a great book but it is not infallible. I have never seen an error in it but then again I have never seen the quoted Bell statement that the M could safely delete the nose armour.

Some sources (such as Instructions of Red Army Air Force Commission No. 4 on Winterized P-39M Airplanes, No. 6, 29-Jan-1943) say the P-39M was fitted with BFGoodrich deicer boots on the wings and tail. That would make the aircraft basic weight greater which may be a factor although I would have expected the tail boots total moment to be vastly greater than the counterbalancing wing boots. The Russians removed said deicer boots.

That's the issue - that one loading chart that shows the horizontal and vertical GC locations in inches was in the P-39Q manual. If ANYONE comes up with a similar chart or data from a different model, I'll be glad to run the numbers (and show the math). However, I don't think there's going to be much of a difference.

 

[DerAdlerIstGelandet]

This is a very informative thread, and yet it is quite amusing.

 

[Ivan1GFP]

How many times do I need to say this: Bell stated that the nose armor was not needed for ballast on the P-39M. Quoted from Vees for Victory, which most of us recognize as the best reference for the Allison V-1710.

How does that work in your CG calculations?

Hello P-39 Expert,

IF the fixed equipment were all the same as the prior P-39L, there is still a matter of a propeller that is now 11 feet 1 inch diameter on the M while prior propellers were only 10 feet 4 1/2 inches. That would count for a bit of extra weight in the nose. The Gear Box Armour was 71 pounds in the L and M, so perhaps the difference in propeller weight made up enough of the difference to keep the CoG within proper range under most conditions.
This is not to say that under certain load conditions that the CoG will NOT be in the safe range on the M without its Gear Box Armour.
Please note however that the armour was still retained for the type which meant that the Army didn't buy the idea.

- Ivan.

 

[P-39 Expert]

From P-63 manual - notice the lack of coolant tank support structure.
View attachment 597520

Please expand the above drawing. This drawing explains exactly what I have been saying. The engine bay is 90.25" long. It extends from the aft edge of the cabin assembly to the bulkhead (with the hole in it) just in front of the oil tank. The entire engine and aux stage supercharger including carburetor are contained in that engine bay. It does not extend past the oil tank bulkhead.

The engine bay is exactly the same length on the P-39 as it is on this P-63. The aux. stage would fit in the P-39 engine bay after the coolant tank had been moved up to the top front edge of the engine as it is in this diagram. The XP39-E did not have a lengthened engine bay (as was often stated in reference books), it had a lengthened tail cone aft of the engine bay.

 

[P-39 Expert]

Table 26 has a line item "Miscellaneous Equipment (Radio)" 129.2lb for the P-39D-2 and 109.9lb for the P-400. It doesn't mention IFF.

Under the electrical system section in the table, "Communication" lists 62.0lb, 80.2lb and 35.0lb for the P-39D, P-39D-2 and P-400 respectively.

In the systems description section for communication, navigation and identification systems three radio systems are described as being typical equipment:

SCR-535A radio recognition set with detonator and crash inertia switch (that's be the IFF).

SCR-522A command radio set

SCR-274N radio.

The 522A and the 274N would not be used together, they perform the same function. P-39s after the D/F models had either the 522 or the 274, but not both. And the 535A.

 

[P-39 Expert]

OK - dinner is down along with a couple of beers!

So I recalculated and removed the gear box armor (70.7 pounds) and "Armor Plate FUME" (27 pounds) which appears to be over the nose 50s. I came up with a weight of 6841.3 with all ammo expended, half fuel left and a moment of 975294.7 which equates to a CG arm of 142.55, way out of the the earlier calculated envelope.

"Better start removing some weight from the tail"!

Exactly as I have said all along, radios from the tail would need to be moved forward above the engine just aft of the cockpit. Or removed altogether. Bell managed to keep the plane in balance with a variety of propeller, armor, armament, radio etc variations.

 

[P-39 Expert]

Here's one chart for a P-39N

View attachment 597407

Did you get that from a pilot's manual? If so, could you post that also? Thanks.

 

[GregP]

Does anybody know the moment arm for external centerline tanks?

 

[davparlr]

A few comments on P-39.
The P-39 appears to be optimized for the technology of the moment, with limited growth space, ala F-16, which had 2 sqft growth space when I did avionic installation work for the Nothrop AMRAAM proposal. With limited growth bound by size and critical Cg requirements, building a really capable fighter was difficult. They probably should have started off with a longer fuselage ala P-63, increasing the nose length and moving the pilot/engine forward moving the natural Cg forward allowing more flexibility in stability and equipment layout. This would have allowed the room for turbo or fuel or maybe both. Of course, slightly more weight, bigger wings needed?, more power?, on and on. It was quite fast compared to the P-40, F4F,and the Zero, so it had some airspeed to trade off against a bigger wing. The turbo would certainly improve performance at higher altitude. I kinda guess that they should have started off with a lightened P-63ish design.

Second, a few months ago, I read The First Team and the Guadalcanal Campaign and there was not too much derogatory comments about the P-39 except the limited high altitude ability. I it did provide quite capable close air support for the Marines. Also, General Kenney did not complain much about the plane and wanted more of them, of course, he was desperate for aircraft and really wanted P-38s.

Finally, degrading the P-39 performance in the Pacific was possibly the capability or lack thereof of the pilots. The army had a 100 new pilots fresh out of flight training enroute to the Philippines when war broke out and these were then diverted to Australia. I suspect these very inexperienced pilots had a tough time against Japans best.

I was much confused by the data on the P-39. While it has been ragged on as a second class fighter the flight test data in comparison with contemporary fighters, the P-40E and the F4F-4, and the Zero, the P-39 was significantly faster than the P-40E up to about 19k, significantly faster than the F4F-4 up to 25k (50 mph at 15k), and faster than the Zero up to 25k. In climb, the P-39 is equal to or better than both the P-40E and the F4F-4 up to 25k. The Zero is much better than the American planes in climb rate at all altitudes. So, below 15k the P-39 is quite a bit faster than both American fighters and the Zero, and below 10k, the Zero has only about 300 ft/min advantage in climb over the P-39. So, it is a low altitude fighter, but could certainly be formidable there in the hands of a capable pilot.

 

[FLYBOYJ]

Exactly as I have said all along, radios from the tail would need to be moved forward above the engine just aft of the cockpit. Or removed altogether. Bell managed to keep the plane in balance with a variety of propeller, armor, armament, radio etc variations.

The point is HOW MUCH do you move it and where and this becomes more critical if you're attempting to do this in the field. And again, are these W&B "variations" for specific aircraft or for an entire production run of a specific model or configuration?

Did you get that from a pilot's manual? If so, could you post that also? Thanks.

 

[fubar57]

Thanks Joe

 

[FLYBOYJ]

A few comments on P-39.
The P-39 appears to be optimized for the technology of the moment, with limited growth space, ala F-16, which had 2 sqft growth space when I did avionic installation work for the Nothrop AMRAAM proposal. With limited growth bound by size and critical Cg requirements, building a really capable fighter was difficult. They probably should have started off with a longer fuselage ala P-63, increasing the nose length and moving the pilot/engine forward moving the natural Cg forward allowing more flexibility in stability and equipment layout. This would have allowed the room for turbo or fuel or maybe both. Of course, slightly more weight, bigger wings needed?, more power?, on and on. It was quite fast compared to the P-40, F4F,and the Zero, so it had some airspeed to trade off against a bigger wing. The turbo would certainly improve performance at higher altitude. I kinda guess that they should have started off with a lightened P-63ish design.

Second, a few months ago, I read The First Team and the Guadalcanal Campaign and there was not too much derogatory comments about the P-39 except the limited high altitude ability. I it did provide quite capable close air support for the Marines. Also, General Kenney did not complain much about the plane and wanted more of them, of course, he was desperate for aircraft and really wanted P-38s.

Finally, degrading the P-39 performance in the Pacific was possibly the capability or lack thereof of the pilots. The army had a 100 new pilots fresh out of flight training enroute to the Philippines when war broke out and these were then diverted to Australia. I suspect these very inexperienced pilots had a tough time against Japans best.

I was much confused by the data on the P-39. While it has been ragged on as a second class fighter the flight test data in comparison with contemporary fighters, the P-40E and the F4F-4, and the Zero, the P-39 was significantly faster than the P-40E up to about 19k, significantly faster than the F4F-4 up to 25k (50 mph at 15k), and faster than the Zero up to 25k. In climb, the P-39 is equal to or better than both the P-40E and the F4F-4 up to 25k. The Zero is much better than the American planes in climb rate at all altitudes. So, below 15k the P-39 is quite a bit faster than both American fighters and the Zero, and below 10k, the Zero has only about 300 ft/min advantage in climb over the P-39. So, it is a low altitude fighter, but could certainly be formidable there in the hands of a capable pilot.

Dave - a lot of good points! We keep hearing what a dog the P-39 was but it did hold it's own in the SWP, personally I think much of the "P-39 bad press" was due to the P-38 coming on board and the results such units like the 39th FS had after they transitioned. Just my opinion.

 

[P-39 Expert]

The point is HOW MUCH do you move it and where and this becomes more critical if you're attempting to do this in the field. And again, are these W&B "variations" for specific aircraft or for an entire production run of a specific model or configuration?

Many thanks.

 

[P-39 Expert]

A few comments on P-39.
The P-39 appears to be optimized for the technology of the moment, with limited growth space, ala F-16, which had 2 sqft growth space when I did avionic installation work for the Nothrop AMRAAM proposal. With limited growth bound by size and critical Cg requirements, building a really capable fighter was difficult. They probably should have started off with a longer fuselage ala P-63, increasing the nose length and moving the pilot/engine forward moving the natural Cg forward allowing more flexibility in stability and equipment layout. This would have allowed the room for turbo or fuel or maybe both. Of course, slightly more weight, bigger wings needed?, more power?, on and on. It was quite fast compared to the P-40, F4F,and the Zero, so it had some airspeed to trade off against a bigger wing. The turbo would certainly improve performance at higher altitude. I kinda guess that they should have started off with a lightened P-63ish design.

Please remember the Allison only had 1150hp. Uprated engine or auxiliary stage supercharger wasn't available until 1943. Had to keep it small to have any performance at all.

Second, a few months ago, I read The First Team and the Guadalcanal Campaign and there was not too much derogatory comments about the P-39 except the limited high altitude ability. I it did provide quite capable close air support for the Marines. Also, General Kenney did not complain much about the plane and wanted more of them, of course, he was desperate for aircraft and really wanted P-38s.

Finally, degrading the P-39 performance in the Pacific was possibly the capability or lack thereof of the pilots. The army had a 100 new pilots fresh out of flight training enroute to the Philippines when war broke out and these were then diverted to Australia. I suspect these very inexperienced pilots had a tough time against Japans best.

This was huge. Best pilots in the world vs 100 guys fresh out of flight school. Hellcat (and even Lightning) were great planes but they never faced the Japanese first team.

I was much confused by the data on the P-39. While it has been ragged on as a second class fighter the flight test data in comparison with contemporary fighters, the P-40E and the F4F-4, and the Zero, the P-39 was significantly faster than the P-40E up to about 19k, significantly faster than the F4F-4 up to 25k (50 mph at 15k), and faster than the Zero up to 25k. In climb, the P-39 is equal to or better than both the P-40E and the F4F-4 up to 25k. The Zero is much better than the American planes in climb rate at all altitudes. So, below 15k the P-39 is quite a bit faster than both American fighters and the Zero, and below 10k, the Zero has only about 300 ft/min advantage in climb over the P-39. So, it is a low altitude fighter, but could certainly be formidable there in the hands of a capable pilot.

1942 P-39 data has always been biased. 750lbs lighter than the P-40E with the same engine and propeller it had to be faster and climb much better at all altitudes. Reduce the weight like the Russians did and it would climb with a Zero.

Good analysis, thanks for posting. Please expand above for more comments.

 

[SaparotRob]

Just wondering if a small nitrous oxide system could have been installed. It worked on my friend's old Mercury Cougar.

 

[davparlr]

Just wondering if a small nitrous oxide system could have been installed. It worked on my friend's old Mercury Cougar.

Good question. I know that the Germans used it in some of their advanced fighters in late '44. Haven't heard it pop up any where else. I suspect there are plenty of people on board that can answer your questions.

 

[Milosh]

Iirc some Mosquitoes had NOX installed.

 

[ThomasP]

Hey P-39 Expert,

As far as I can find, the standard electronics fit - TR.1133 (71 lbs) or TR.1143 (84 lbs), plus R.3003 IFF (30 lbs) - of the mid-war UK aircraft, or about 120-130 lbs including the other removable bits and pieces. I think this would have been the total weight of electronics in the P-400 as installed for the UK. If carrying the R.1147 DF receiver it would have another 19-23 lbs. As far as I know the USAF used the American made equivalents at the time (as mentioned by wuzak above).