XP-39 II - The Groundhog Day Thread

[GregP]

Without that armor, the CG would be farther aft, and aft CG was already a problem. It would take some thinking that I'm pretty sure they DID.

 

[P-39 Expert]

Without that armor, the CG would be farther aft, and aft CG was already a problem. It would take some thinking that I'm pretty sure they DID.

We've spent pages on this already, nose armor can be deleted by making other internal adjustments such as moving the radio up from the tail cone to above the engine. Bell stated that nose armor was not needed. They were able to reballast for different nose cannons that differed in weight by 140lbs. More heresay beating a dead horse.

 

[GrauGeist]

We've spent pages on this already, nose armor can be deleted by making other internal adjustments such as moving the radio up from the tail cone to above the engine. Bell stated that nose armor was not needed. They were able to reballast for different nose cannons that differed in weight by 140lbs. More heresay beating a dead horse.

So, 75 years after the fact, we need to ask why they didn't do it?
Were they just complete dumbasses or what?

 

[wuzak]

They could have been a lot more judicious about what they put into the plane in the early models D/F/K/L/P-400. The later models with the uprated engine were fine as they were. The basic airframe was light enough. They didn't need the .30cal wing guns, after all they had a 37mm cannon and two .50cal MGs. And the 100lb nose armor plate was redundant, no other planes had their nose reduction gear armored. Those items total around 300-500lbs, depending on the amount of .30cal ammunition carried. A fully equipped P-39D without those items weighed about 7150lbs versus up to 7850lbs normal gross weight. Russians deleted the wing guns and the IFF radio and did really well with it.

Déjà vu

 

[P-39 Expert]

So, 75 years after the fact, we need to ask why they didn't do it?
Were they just complete dumbasses or what?

I don't know why they didn't eliminate those unnecessary/redundant items. All the other countries were very careful to control the weight of their planes, but the AAF and Navy seemed to try and cram as much crap into their planes as they could.

Now in the case of the P-39, only the early models were too heavy, the later M/N/Q models with the uprated engines were okay. But the P-40 weighed 8400lbs with a 1150hp engine (7.3lbs/HP) and the F4F Wildcat weighed 7975lbs with a 1200hp engine (6.6lbs/HP). Both suffered performance penalties because of the excess weight. Later model Hellcats and Corsairs were in the 6lbs/HP range and had much better performance. But 1942 was fought with P-39s, P-40s and F4Fs and they paid the weight penalty. Defense of Port Moresby NG began about April and Coral Sea was in May, so the the AAF and USN had until December/Feb before the P-38 and Corsair got into combat. The AAF did lighten some P-39Ks at Guadalcanal in September and increased rate of climb substantially but little is written about that. We have benefit of hindsight but the last half of '42 was a very busy time for the AAF and USN.

 

[wuzak]

I don't know why they didn't eliminate those unnecessary/redundant items. All the other countries were very careful to control the weight of their planes, but the AAF and Navy seemed to try and cram as much crap into their planes as they could.

Unnecessary items such as IFF?

 

[P-39 Expert]

Unnecessary items such as IFF?

IFF is redundant if there is no radar at your base. In 1942 Port Moresby didn't have radar until October and Guadalcanal until September. PM had Australian radar but it was too far away to do any good. IFF is also redundant in every plane on a mission. A 16 plane squadron mission didn't need every plane in the squadron to have an IFF set.

The Russians had American lend lease radar from May 1943 but removed the IFF set (along with the .30cal wing guns) as standard procedure anyway.

USN planes probably need a set on every plane given their unique situation. Each aircraft carrier had radar and needed to account for each plane sent out.

 

[jmcalli2]

The full size wind tunnel at Langley Field was limited to 125 mph, so those are estimates.
Plus was EVERY modification suggested by those wind tunnels test ever applied to a real P-39 ?

I find it interesting the Kelly Johnson pretty much refused the NACA recommendations for the P-38.

As far as I can tell, the NACA changes were all applied. One I question is placing the induction air intake behind the cockpit canopy. Again, compare to the P-38 where it is placed in clean air. I suspect being so close to the canopy negated ram effect; the P-63D with the bubble canopy pushed the intake significantly further back (to give the canopy room to slide) and that was significantly faster than P-63Es with the same wing & engine. That makes more sense to me now with the 125 limit of the wind tunnel. Thanks for that info!

The turbo in the XP-39 was located where the radiator was placed in the production models. The installation problems went to intercooler intakes. The only operational experience the AAF had with turbos was the P-30/PB-2. They didn't fly them high very often because of crew discomfort and expense; I believe that reinforces the idea that the turbo was killed because making it work would have taken too long.

Bottom line is I don't know of ANY combat equipped aircraft that could hit the 400+ mph at 15,000 ft prediction the NACA gave for the P-39 with their modifications with a single 1,050hp engine. 1,050hp at 20,000 ft, maybe. There is that much less air resistance up there.

This, - CRGIS review - used to be a good NACA wind tunnel reference site, but it is currently down for 'review.'

I also remember a NACA report on the drag of various single engine WWII planes, P-39, P-40, P-43, P-35, F4F, SBD I think, all in one report. I can't find it now; it most likely was in the NACA link above.

 

[P-39 Expert]

P-39 had the least drag of any American WW2 fighter except the P-51 according to AHT.

 

[Milosh]

he only operational experience the AAF had with turbos was the P-30/PB-2.

Ah, B-17 iirc.

 

[jmcalli2]

Ah, B-17 iirc.

The first B-17 with turbos, the YB-17A, first flew with turbos on 29 April 1938.
The XP-39 made it's first flight on 6 April 1938.

 

[GrauGeist]

The Bell YFM-1 had turbosupercharged Allison V-1710-9 engines - first flew in '37.

 

[Glider]

IFF is redundant if there is no radar at your base. In 1942 Port Moresby didn't have radar until October and Guadalcanal until September. PM had Australian radar but it was too far away to do any good. IFF is also redundant in every plane on a mission. A 16 plane squadron mission didn't need every plane in the squadron to have an IFF set.

The Russians had American lend lease radar from May 1943 but removed the IFF set (along with the .30cal wing guns) as standard procedure anyway.

USN planes probably need a set on every plane given their unique situation. Each aircraft carrier had radar and needed to account for each plane sent out.

I admit to finding the idea that not every aircraft in a squadron needed an IFF set somewhat bemusing. Almost every time a squadron went into combat they got split up. How many times have you heard of an aircraft getting a bearing home by using it's IFF set. I can imagine them drawing lots before take off because everyone would want the one with IFF.

 

[Glider]

Bell stated that nose armor was not needed.

Clearly the people who went into combat decided they did need the armour. In fact, all the airforces of all nations, who went to war decided that they needed armour.

They were able to reballast for different nose cannons that differed in weight by 140lbs.

If they had such flexibility why didn't they make changes to make the aircraft safer or at least more flexible from a COG position.

More heresay beating a dead horse.

More practical experience beating a theoretical argument.

 

[Reluctant Poster]

The Bell YFM-1 had turbosupercharged Allison V-1710-9 engines - first flew in '37.

As did the XP-37. The P-30 first flew in 1934. 60 of them were built.

 

[Milosh]

The first B-17 with turbos, the YB-17A, first flew with turbos on 29 April 1938.
The XP-39 made it's first flight on 6 April 1938.

A 14th Y1B-17 (37-369), originally constructed for ground testing of the airframe's strength, was upgraded and fitted with exhaust-driven General Electric turbochargers. Scheduled to fly in 1937, it encountered problems with the turbochargers, and its first flight was delayed until 29 April 1938.

 

[BiffF15]

I have what must be a morbid fascination with this thread. It's like I can't look away from a traffic accident as it unfolds.

 

[MiTasol]

Russians deleted the wing guns and the IFF radio and did really well with it.

But they kept the nose armor plate because they knew removing that would make the aircraft highly unstable once all or most of the ammunition was expended

 

[jmcalli2]

The Bell YFM-1 had turbosupercharged Allison V-1710-9 engines - first flew in '37.

True, and I considered including that aircraft with the P-30 in my post.

The XFM-1 first flew in September 1937, but the contract for 13 YFM-1s wasn't issued until May 1938. The first YFM-1 flew in September 1939 and was delivered to the AAF in February 1940.
The XP-39B flew in November 1939.

An interesting side note is that the USAAC dealt with underperforming prototypes in exactly the opposite way: for the B-17 they added turbos, but for the P-39 they removed the turbo.
Ain't bureaucracy grand.

 

[jmcalli2]

A 14th Y1B-17 (37-369), originally constructed for ground testing of the airframe's strength, was upgraded and fitted with exhaust-driven General Electric turbochargers. Scheduled to fly in 1937, it encountered problems with the turbochargers, and its first flight was delayed until 29 April 1938.

Not much experience if it's not flying.