XP-39: pros cons

[Shortround6]

How much did the P-39 actually carry?

The 37mm gun weighed about 238lbs (or about what a pair of 20mm Hispanos did) while the ammo was about 900 grams per round so 30 rounds is just about 60lbs. 298lbs?
The .50 cal guns go about 140lbs a pair. 400 rounds of linked .50 cal is about 120lbs or a bit more. 280lbs.
Four .30s (a later requirement than the original) are about 94 lbs and 4000 rounds (a little over kill) of .30 cal is 260lbs. 355lbs for around 933lbs.
Please note that this for guns and ammo only, No ammo bins/boxes, no chutes/feedways, no gun heaters, charging systems, firing systems. Also no gun mounts. Mounting things like 37mm cannon and .50cal guns requires more than few L brackets pop-riveted to a bulkhead

Granted all this "stuff" would run up the weight of the fighters I mentioned earlier but things are usually in proportion although there are exceptions. Hispanos like substantial gun mounts as they were designed to be bolted to a V-12 engine block. Germans fastened their hub guns to the airplane and the gun barrel went through a tube in the engine. You could pull the engine and leave the gun behind.

Early P-39s had no wing guns but had two .30s in cowl outboard of the .50s. of course this model didn't have armor, bullet proof glass or self-sealing fuel tanks.

 

[Zipper730]

Shortround6,

1. Was the ammo specification dictated around the weight of the guns, the weight of the ammo, both?
2. Guns and ammo amounted to around 578 to 698 pounds prior to the 0.303's being added on...
3. How much would armor, bullet proof glass and self-sealing tanks add?

 

[Shortround6]

Ammo was often a legacy requirement at this time. ALL US Fighters during the 30s being capable of being fitted with a .50 cal gun and 200 rounds of ammo in place of one of the .30 cal guns. The .30s had 500 or 600rpg in old the old fighters. The 37mm got 30 rounds becasue that is what they could fit in the "magazine"/feedsystem. On the P-39 they may not have had the room to increase the ammo supply for the .50s.
The weight of Armour and bullet proof glass varied between the P-39D, the D-2 and the P-40 but 250-265lbs. the self-sealing tanks may have added over 100lbs, I don't have the weight of the unprotected tanks to compare to.

 

[Zipper730]

Ammo was often a legacy requirement at this time. ALL US Fighters during the 30s being capable of being fitted with a .50 cal gun and 200 rounds of ammo in place of one of the .30 cal guns. The .30s had 500 or 600rpg in old the old fighters. The 37mm got 30 rounds becasue that is what they could fit in the "magazine"/feedsystem.

I understand why the round numbers were lower... that's not complicated bigger bullets take up more space.

On the P-39 they may not have had the room to increase the ammo supply for the .50s

200 rounds per gun seems to meet the requirements though. I'm surprised that the requirement for 1000 pounds of ammo never was an issue

The weight of Armour and bullet proof glass varied between the P-39D, the D-2 and the P-40 but 250-265lbs. the self-sealing tanks may have added over 100lbs, I don't have the weight of the unprotected tanks to compare to.

That'll do.

So the plane would be 350-365 pounds more

 

[GregP]

Probably shouldn't say this, but Zipper, you really need to do a little research!

You're asking other people to fill you in on data all the time that they spent years accumulating. Probably need to Google it and check out books in libraries. Think about it and come back with discussion on the subject. Continuous requests for data that isn't that hard to find will grow old. You can probably find the answer to your last question in online P-39 specifications that are likely in a pdf file of the POH (Pilot's Operating Handbook).

Anyway, if you don't check it out, how do you know they're right? Most are, admittedly, but there are backup facts abounding for those that look.

People are happier to share data when you show some initiative yourself to go find things out, and then share it.

Just saying, a LOT of the things you ask about were covered in older threads. Search them for an answer with keywords. Most have been covered before. Cheers.

 

[tomo pauk]

...
While on it, how much ammo did the P-38 carry, and how much armor and self-sealing tank weight?

Several good sites offer reliable answers on questions like that (not that I'm against fellow forum members making straight answers):
- this forum, 'Techincal' section
- avialogs.com
- LiTOT: Content
- wwiiaircraftperformance.org
- kurfurst.org

Conten found on all of the web-sites listed would've lasted for a life time worth of reading.

 

[Zipper730]

Probably shouldn't say this, but Zipper, you really need to do a little research!

No biggie, I already found the information. The P-38 information isn't really all that important, the weight figures listed were fine.

Contract weight was 5,550lbs

I've been looking for that figure for some time.

 

[P-39 Expert]

Just my opinion, but that turbo in the XP-39 was never going into production. The biggest problem was the oil cooler and intercooler ducts were not adjustable. So basically the same volume of air was passing through both radiators whether the engine was operating at low power or full power. Too much cooling at low power and not enough at high power.

If the ducts could have been made adjustable with moveable trailing exits there were still too many problems. The turbo and exhaust were partially protruding into the airstream causing aerodynamic drag. The coolant radiator was in the leading edge of one wing and it's duct was not adjustable either. It needed to be enlarged and made adjustable.

All these problems were somewhat solved with the deletion of the turbo (and intercooler) and moving the coolant and oil radiators into the wing center section formerly occupied by the turbo. They now had adjustable exit flaps so the volume of cooling air could be adjusted.

 

[Zipper730]

Just my opinion, but that turbo in the XP-39 was never going into production.

It's just a hypothetical scenario

If the ducts could have been made adjustable with moveable trailing exits there were still too many problems.

Could you make a guess as to the size of the necessary radiators if such flaps existed?

 

[fubar57]

So long thread.

 

[P-39 Expert]

It's just a hypothetical scenario
Could you make a guess as to the size of the necessary radiators if such flaps existed?

I have no idea on the size of the radiators. And Bell did put adjustable exit flaps on the production planes, so it could have been done.

Better (to me) to delete the turbo and get the plane into production concentrating on keeping the weight as low as possible for good performance. And more powerful V-1710s were coming, one even had a mechanical second stage supercharger (went into the P-63).

 

[Zipper730]

I have no idea on the size of the radiators.

I've been told the typical radiator area for cooling is typically around 3 times the size of the carburetor intake. Would that be a good estimate point, to take the carburetor intake and triple the area?

And Bell did put adjustable exit flaps on the production planes, so it could have been done.

That's good to know!

Better (to me) to delete the turbo and get the plane into production concentrating on keeping the weight as low as possible for good performance.

Frankly, I'm honestly surprised they didn't just put a twin-staged supercharger on it more quickly (it would have been a simpler fix than a turbocharger).

Still, with the earlier turbocharger configuration: I'm curious why they mounted the turbocharger under the engine when there was enough room behind it, provided the cooling tank was moved ahead of the engine?

 

[P-39 Expert]

I've been told the typical radiator area for cooling is typically around 3 times the size of the carburetor intake. Would that be a good estimate point, to take the carburetor intake and triple the area?
That's good to know!
Frankly, I'm honestly surprised they didn't just put a twin-staged supercharger on it more quickly (it would have been a simpler fix than a turbocharger).

Still, with the earlier turbocharger configuration: I'm curious why they mounted the turbocharger under the engine when there was enough room behind it, provided the cooling tank was moved ahead of the engine?

That's the big question, why did it take so long to develop the mechanical two stage supercharger? Started in 1940, it finally made production in April '43. It's just a simple impeller in a diffuser driven from the engine by a jackshaft. Three full years to develop what should consist of off-the-shelf parts. High altitude performance without all the ducting of the turbo (14' per engine on the P-38 Lightning) and it didn't need an intercooler except for WEP. If you can figure out why this took so long to develop please tell me.

A turbo in the rear fuselage (instead of under the engine) like the P-47 would have caused (more) balance problems.

 

[wuzak]

Why would they be "off the shelf parts"?

Allison had a 9.5" supercharger for the V-1710 and a 10.0" supercharger for the V-3420. The auxiliary supercharger was 12.1875" - not something they had lying on a shelf.

As pointed out by SR on another thread, Allison was so busy being all things to all people that they didn't have the resources to develop a 2 stage engine as well as all the other crap.

 

[Shortround6]

That's the big question, why did it take so long to develop the mechanical two stage supercharger? Started in 1940, it finally made production in April '43. It's just a simple impeller in a diffuser driven from the engine by a jackshaft. Three full years to develop what should consist of off-the-shelf parts. High altitude performance without all the ducting of the turbo (14' per engine on the P-38 Lightning) and it didn't need an intercooler except for WEP. If you can figure out why this took so long to develop please tell me.

A turbo in the rear fuselage (instead of under the engine) like the P-47 would have caused (more) balance problems.

Oh boy , oh boy, oh boy.

Allison first toyed with idea of a 2 stage supercharger in 1938 or 1939 but quickly gave up as they didn't have enough engineering staff to work on it plus all the "stuff" the army wanted at the same time (Like pusher engines for the Airacuda).

first example (or more than the first) did use "off the shelf parts". The tried using the same impeller as the engine supercharger, I don't know about the diffuser. They soon learned that far from the optimal set up, very far.

When dealing with these superchargers you have several things going on. What is wanted for making power in the engine is mass flow of air, that is to say pounds per minute. But that is hard to measure but pressure isn't. So they use pressure gauges but they are only indicators. Since the weight of the air per cubic foot varies with the temperature even at the same pressure they have to make some assumptions in aircraft (they have a lot more instruments on a test bench/test cell). ANd as you go higher the density of air changes, by the time you get to 20,000ft air is just over 1/2 the weight it is at sea level per cubic ft. You need a much bigger intake system (inlet duct/s and supercharger inlet) to handle the low density air. Once you have used a 1st stage supercharger to squeeze the air down to something like sea level air your 2nd stage impeller-diffuser can be about what it was for the single stage engine.
RR got "lucky" in that they had a large supercharger already "in house". They just grabbed the impeller (for sure) and housing (maybe) from the Vulture engine as it flowed about the mass of air they wanted for for the hi 20,000 ft to 30,000ft area. This was for the first test rig, later prototypes and production engines used purpose designed impellers and diffusers.
Allison had no such large supercharger or impellers hanging around and no good place to get them.

You need intercoolers to make high power at altitude. Otherwise you are just adding weight, bulk and cost to make a slightly better low altitude engine. The supercharger adds considerable heat to the intake charge. For instance the supercharger on a Merlin XX added 148 degrees celsius (over 260 degrees F) to the temperature of the incoming air when in high gear. using more compression in the supercharger/s just adds more heat and at some point you reach the detonation point.

The higher temperature if using higher 'boost' without cooling means the air is less dense which means you get less power from the same 15lb of boost than if the air was "cooler" and at 15lbs boost. The heat load on the entire engine goes up. Raise the heat of the intake charge 100 degrees and peak temperature in the cylinder goes up 100 degrees and the heat of the exhaust goes up 100 degrees.
And you wind up having to use lower intake pressure because of the higher temperatures. Say your engine can handle 15lb of boost with an intake temperature of 300 degrees (hypothetical figures) with an intercooler. take out the intercooler and lets say the temperature goes to 400 degrees. Now lets say that extra 100 degrees pushes the mixture to a point where, as the piston rises and further compresses the mixture, it raises the temperature to the flash point and the mixture either ignites on it's own before the spark plugs fire or the when the spark plugs fire the added pressure of the flame front cause all the mixture to explode at once rather than burn in an advancing flame front across the cylinder as the cylinder goes past top dead center and starts to descend.

While using two stages gets you less temperature rise than using one stage for a given level of compression trying for high boost (and a Merlin 61 was compressing the air over 5 times in the superchargers at altitude) without using an intercooler runs into diminishing returns real quick.

 

[P-39 Expert]

Oh boy , oh boy, oh boy.

Allison first toyed with idea of a 2 stage supercharger in 1938 or 1939 but quickly gave up as they didn't have enough engineering staff to work on it plus all the "stuff" the army wanted at the same time (Like pusher engines for the Airacuda).

first example (or more than the first) did use "off the shelf parts". The tried using the same impeller as the engine supercharger, I don't know about the diffuser. They soon learned that far from the optimal set up, very far.

When dealing with these superchargers you have several things going on. What is wanted for making power in the engine is mass flow of air, that is to say pounds per minute. But that is hard to measure but pressure isn't. So they use pressure gauges but they are only indicators. Since the weight of the air per cubic foot varies with the temperature even at the same pressure they have to make some assumptions in aircraft (they have a lot more instruments on a test bench/test cell). ANd as you go higher the density of air changes, by the time you get to 20,000ft air is just over 1/2 the weight it is at sea level per cubic ft. You need a much bigger intake system (inlet duct/s and supercharger inlet) to handle the low density air. Once you have used a 1st stage supercharger to squeeze the air down to something like sea level air your 2nd stage impeller-diffuser can be about what it was for the single stage engine.
RR got "lucky" in that they had a large supercharger already "in house". They just grabbed the impeller (for sure) and housing (maybe) from the Vulture engine as it flowed about the mass of air they wanted for for the hi 20,000 ft to 30,000ft area. This was for the first test rig, later prototypes and production engines used purpose designed impellers and diffusers.
Allison had no such large supercharger or impellers hanging around and no good place to get them.

You need intercoolers to make high power at altitude. Otherwise you are just adding weight, bulk and cost to make a slightly better low altitude engine. The supercharger adds considerable heat to the intake charge. For instance the supercharger on a Merlin XX added 148 degrees celsius (over 260 degrees F) to the temperature of the incoming air when in high gear. using more compression in the supercharger/s just adds more heat and at some point you reach the detonation point.

The higher temperature if using higher 'boost' without cooling means the air is less dense which means you get less power from the same 15lb of boost than if the air was "cooler" and at 15lbs boost. The heat load on the entire engine goes up. Raise the heat of the intake charge 100 degrees and peak temperature in the cylinder goes up 100 degrees and the heat of the exhaust goes up 100 degrees.
And you wind up having to use lower intake pressure because of the higher temperatures. Say your engine can handle 15lb of boost with an intake temperature of 300 degrees (hypothetical figures) with an intercooler. take out the intercooler and lets say the temperature goes to 400 degrees. Now lets say that extra 100 degrees pushes the mixture to a point where, as the piston rises and further compresses the mixture, it raises the temperature to the flash point and the mixture either ignites on it's own before the spark plugs fire or the when the spark plugs fire the added pressure of the flame front cause all the mixture to explode at once rather than burn in an advancing flame front across the cylinder as the cylinder goes past top dead center and starts to descend.

While using two stages gets you less temperature rise than using one stage for a given level of compression trying for high boost (and a Merlin 61 was compressing the air over 5 times in the superchargers at altitude) without using an intercooler runs into diminishing returns real quick.

Still, it's just an impeller in a diffuser. All that time just to come up with the perfect size impeller?

Big question is, why did they put the carb on the second stage? Why take so long to move the carb back to the engine for more efficiency like the P-38 and P-47?

 

[Shortround6]

Early F4Fs had problems with mis-matched impellers or speeds, they got rumbling in the ducts which is an indication of break down of air flow, not just an annoying noise.
The combination has to work at a variety of altitudes and speeds/power outputs.
Allison first tried using a single speed drive to the auxiliary supercharger then replaced it with the variable hydraulic drive.

and you are ingoring the part about intercoolers. A number of the test rigs on the ground used intercoolers.

 

[P-39 Expert]

Early F4Fs had problems with mis-matched impellers or speeds, they got rumbling in the ducts which is an indication of break down of air flow, not just an annoying noise.
The combination has to work at a variety of altitudes and speeds/power outputs.
Allison first tried using a single speed drive to the auxiliary supercharger then replaced it with the variable hydraulic drive.

and you are ingoring the part about intercoolers. A number of the test rigs on the ground used intercoolers.

Sorry about ignoring the intercoolers. Do you view the water injection system employed on the P-63 as being inferior?

 

[Shortround6]

Of course. While intercoolers are large, heavy and bulky they are of unlimited duration.
And water injection is NOT light if it is to be used for long periods of time or as a substitute for intercoolers.
The F6F and F4Us used both but the water injection was to supplement the intercoolers, not replace them.
The P-47s used both and since the Turbo could supply more air than the mechanical drive superchargers it would up with a 30 gallon tank.
P-63s used a 25 gallon tank (actually two tanks) , 186.5 lbs of fluid. Compact but hardly light. This was good for 15 minutes at max boost.

Then we have the total engine cooling problem. The auxiliary stage supercharger is NOT free.

Our P-39 that can make 1150 hp at 15,000ft is making 1450-1500hp in the cylinders? (about 100hp for friction in the engine and 200-250hp to drive the supercharger).
The P-63 making 1150hp at 25,000ft (or lower?) needed to make another 250hp or so in the cylinders to power the auxiliary supercharger.
Granted it wasn't using quite as much power to drive the engine supercharger but the cooling load has gone up and bigger radiators are going to be needed. Not to mention bigger oil coolers to handle the heat generated by the hydraulic drive.

The auxiliary supercharger weighed about 125lbs and added about 21 in to the length of the engine. Two weights are given. 175lbs for the one ones with the carb on the auxiliary supercharger and 125lbs for the ones with the carb still on the engine. It could be that the difference is the weight of the carb, and that it is counted as part of the auxiliary supercharger when attached?

Superchargers got to be much more than just an impeller in a diffuser. Say you had two superchargers, both supplied the same amount of air at the same pressure but one was 65% efficient and the other was 70% efficient. Lets say that the first require 100hp to the input shaft to give you the required airflow and pressure. That means that only 65hp worth of work is actually being done and that 35hp is not only going to waste, it is going to waste as heat, unnecessarily heating up the intake charge.
The 2nd supercharger only needs 93hp driving the input shaft, leaving an additional 7 hp for the propeller, but has 7hp (5220watts) LESS heating up the intake charge. about a 20% reduction in waste heat. There is a considerable amount of heat generated in compressing air even at a 100% theoretical efficiency so practical difference is not as bad as these figures show. But supercharger designers were trying to make things as good as possible and not just slam a few parts together and then live with the consequences.

 

[P-39 Expert]

Of course. While intercoolers are large, heavy and bulky they are of unlimited duration.
And water injection is NOT light if it is to be used for long periods of time or as a substitute for intercoolers.
The F6F and F4Us used both but the water injection was to supplement the intercoolers, not replace them.
The P-47s used both and since the Turbo could supply more air than the mechanical drive superchargers it would up with a 30 gallon tank.
P-63s used a 25 gallon tank (actually two tanks) , 186.5 lbs of fluid. Compact but hardly light. This was good for 15 minutes at max boost.

Then we have the total engine cooling problem. The auxiliary stage supercharger is NOT free.

Our P-39 that can make 1150 hp at 15,000ft is making 1450-1500hp in the cylinders? (about 100hp for friction in the engine and 200-250hp to drive the supercharger).
The P-63 making 1150hp at 25,000ft (or lower?) needed to make another 250hp or so in the cylinders to power the auxiliary supercharger.
Granted it wasn't using quite as much power to drive the engine supercharger but the cooling load has gone up and bigger radiators are going to be needed. Not to mention bigger oil coolers to handle the heat generated by the hydraulic drive.

The auxiliary supercharger weighed about 125lbs and added about 21 in to the length of the engine. Two weights are given. 175lbs for the one ones with the carb on the auxiliary supercharger and 125lbs for the ones with the carb still on the engine. It could be that the difference is the weight of the carb, and that it is counted as part of the auxiliary supercharger when attached?

Superchargers got to be much more than just an impeller in a diffuser. Say you had two superchargers, both supplied the same amount of air at the same pressure but one was 65% efficient and the other was 70% efficient. Lets say that the first require 100hp to the input shaft to give you the required airflow and pressure. That means that only 65hp worth of work is actually being done and that 35hp is not only going to waste, it is going to waste as heat, unnecessarily heating up the intake charge.
The 2nd supercharger only needs 93hp driving the input shaft, leaving an additional 7 hp for the propeller, but has 7hp (5220watts) LESS heating up the intake charge. about a 20% reduction in waste heat. There is a considerable amount of heat generated in compressing air even at a 100% theoretical efficiency so practical difference is not as bad as these figures show. But supercharger designers were trying to make things as good as possible and not just slam a few parts together and then live with the consequences.

All P-39s had cooling problems on the ground, but this was a 1325HP engine and the cooling was okay on the V-1710-63 that was also 1325HP.