XP-39 Wind Tunnel Tuft Tests

P-39 Expert · May 4, 2018

Shortround6 said:
One .50 weighs about 3 times what a .30 weighs.
.50 cal ammo weighs 30 lbs per hundred.
.30 cal ammo weighs 6-6.5 lbs per hundred.
If four .30s with 1200 rounds total ruined the P-39 then adding two .50s was hopeless.

Blaming 180 pounds of guns and ammo for ruining plane seems a bit extreme.

The 200# in the N with the uprated engines was not a big deal, but 400# in the earlier models with the 8.8 supercharger was a killer. Especially since they weren't very effective against other aircraft.

Zipper730 · May 4, 2018

P-39 Expert said:
The 4 x .30caliber wing guns weighed around 377# with 1000rpg (just guns & ammo, not including mounts, chargers, heaters and ammo boxes). Pull all that out and you are looking at 400# weight saved.

So that lowers the OEW to 5062, and 7100 for takeoff?

Fuel where the wing guns were is estimated at 50gal total, 25gal in each wing. Post war racers put 100gal in the same space but were using thin walled tanks instead of the leakproof rubber tanks used in WWII.

So that adds 301 pounds back into the plane in fuel, no idea how much the self sealing tank rubber weighs.

Agree with you completely regarding radial vs inline. For commercial applications radials were superior. Only purpose for an inline engine is for fighter planes IMO.

They were okay on some bombers (Mosquito in particular) too...

The small pieces of armor plate in the turnover bulkhead were to either side of the armor glass protecting the pilot's head. That piece of armor glass was large enough to protect the pilot without the small side pieces, so remove them and save the weight.

How much weight was saved?

Shortround6 said:
The guns were underneath, but the ammo was pretty much in the wing in the old .30 cal ammo storage area.

Could the guns and/or ammo fit inside the wing in their entirety?

P-39 Expert · May 4, 2018

Zipper730 said:
So that lowers the OEW to 5062, and 7100 for takeoff?
So that adds 301 pounds back into the plane in fuel, no idea how much the self sealing tank rubber weighs.
Initially with the YP-37, yes -- I'm not sure if it was as bad as the P-39 though.
They were okay on some bombers (Mosquito in particular) too...
How much weight was saved?

Could the guns and/or ammo fit inside the wing in their entirety?

Guns and ammo were actually included in useful load, not empty weight. 5462# empty, useful load 1700#, loaded 7162# or so. That would have been a hot rod even with the 8.8 engine.
Self sealing tanks weighed about 20# each, but those were for 10gal (avg) tanks, so maybe 50# per 25gal tank?
Removing the turnover armor saved 16#. Add that to the nose armor of $100# and save 116#, still have 115# of armor plate and glass protecting the pilot & oil tank.

Zipper730 · May 4, 2018

Shortround6 said:
If four .30s with 1200 rounds total ruined the P-39 then adding two .50s was hopeless.

Too much weight eh?

P-39 Expert said:
Guns and ammo were actually included in useful load, not empty weight.

The guns are factored into the OEW, the ammo isn't. Manufacturer empty omits some extra things as well, and I think the British called it Tare weight... still I see your point.

P-39 Expert · May 4, 2018

Zipper730 said:
Too much weight eh?

The guns are factored into the OEW, the ammo isn't. Manufacturer empty omits some extra things as well, and I think the British called it Tare weight... still I see your point.

P-39 Expert · May 4, 2018

See attached. Listed Empty Weight at the top, added guns, armor plate, pilot and oil for a subtotal Tactical Weight Empty, than added fuel, extra oil and ammo for total Gross Weight. Have heard Tactical Weight Empty referred to as Empty Equipped also.

Zipper730 · May 19, 2018

P-39 Expert said:
Regarding the P-37, it did result in a long nose. I think reliability of the turbo was the big problem at that stage of the game.

Yes, but if I recall, they were able to make it work, and it neatly fits under the engine.

The intercooler position isn't much different than on the P-39...

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