XP-39 II - The Groundhog Day Thread

[wuzak]

No, the AAF just insisted on six .50calMGs AND a 37mm cannon, a much larger wing etc. until they got it up to 8900lbs.

Just put the damn thing into a regular P-39.

The XP-39E bore the same armament as the P-39D but featured a new wing with square-cut tips.

Bell XP-39E Airacobra.

The P-39D differed from the P-39C primarily in having a heavier armament. It had four wing-mounted 0.30-inch machine guns with 1000 rpg, two fuselage-mounted 0.50-inch machine guns with 200 rounds per gun, plus the 37-mm cannon (with increased ammunition capacity of 30 rounds).

Bell P-39D Airacobra

 

[Shortround6]

What is more than a bit strange about this saga of the P-39 holding the two stage engine is that the original XP-39 had a lot trouble with the intercooler (proper airflow for cooling).

Allison was planning on using an intercooler with the two stage engine, Perhaps Bell was too for the P-76 and P-63, but the subcontractor never delivered a suitable intercooler matrix forcing Bell to use large amount of water injection for the two stage engine. Did simplify the air scoop problem though.

Without a good intercooler high altitude performance is going to crap or short lived until the ADD fluid runs out.

 

[Ivan1GFP]

They put one in the XP-39E and flew it in April 1942. Unfortunately they also put a new wing and tail on it and in their own inimitable AAF style they made it weigh 8900lbs. It was decently fast up to 25000', but would barely outclimb a regular P-39D. It never got the four blade propeller it needed.

If they had just put that aux. stage into a regular P-39D it would have only weighed 7900lbs including 4 blade propeller. At 1000lbs less than the E it would have been a rocket.

Hello P-39 Expert,

ASSUMING that the Aux Stage Supercharger actually fit into the engine compartment, there are still a bunch of problems that need to be addressed.

First of all, regarding "it would have been a rocket", the benefit of the second stage is mostly increased altitude performance.
Low level performance would not have changed much especially with the increased weight.
The increased altitude capability also brings a greater requirement for cooling capacity which was somewhat lacking even with the engines that were already installed in the P-39.
The new ASB also sits at the aft end of the engine and would make a aft CoG problem even worse. Perhaps this could be counterbalanced by a significantly heavier 4 blade propeller, but would any steps be taken to bring the CoG in empty condition further forward to address handling problems?
The new P-39 Rocket would still have a relatively low internal fuel capacity. What mission would it be suited for?

- Ivan.

 

[ThomasP]

Hey Ivan1GFP,

Fuel problem solved:

P-39 with a 350 USgal LRFT.

 

[MiTasol]

With that fin area it will fly VERY straight after the tank is dropped.

 

[XBe02Drvr]

Hey Ivan1GFP,

Fuel problem solved:
View attachment 597820

P-39 with a 350 USgal LRFT.

Where's that midwife? Contractions have started already!

 

[P-39 Expert]

No, the six .50 cal guns were Larry Bell's idea when he realized the P-38E was never going to be an interceptor. He was trying to pitch it as a ground attack plane.

They were in the wooden mockups.

 

[P-39 Expert]

What is more than a bit strange about this saga of the P-39 holding the two stage engine is that the original XP-39 had a lot trouble with the intercooler (proper airflow for cooling).

Allison was planning on using an intercooler with the two stage engine, Perhaps Bell was too for the P-76 and P-63, but the subcontractor never delivered a suitable intercooler matrix forcing Bell to use large amount of water injection for the two stage engine. Did simplify the air scoop problem though.

Without a good intercooler high altitude performance is going to crap or short lived until the ADD fluid runs out.

They did plan on using an intercooler but eventually gave up on it. Should have discarded it earlier and moved the carburetor forward to the normal position on the engine stage supercharger for the additional critical altitude. Intercooler/water injection only needed for WEP. At military power the extra 400hp at 25000' would have been plenty.

 

[Shortround6]

They were in the wooden mockups.

got pictures?

They made a mock up of the Spitifre withe six 20mm cannon too, that didn't get far.

No photo of the actual P-39E shows any wing guns or even cowl guns.

 

[P-39 Expert]

Hello P-39 Expert,

ASSUMING that the Aux Stage Supercharger actually fit into the engine compartment, there are still a bunch of problems that need to be addressed. Apparently it did fit.

First of all, regarding "it would have been a rocket", the benefit of the second stage is mostly increased altitude performance. Yep.
Low level performance would not have changed much especially with the increased weight. Low level performance already pretty good.
The increased altitude capability also brings a greater requirement for cooling capacity which was somewhat lacking even with the engines that were already installed in the P-39. Only at military power, then only for the max 15 minutes. Tolerated just like on regular P-39s.
The new ASB also sits at the aft end of the engine and would make a aft CoG problem even worse. Perhaps this could be counterbalanced by a significantly heavier 4 blade propeller, but would any steps be taken to bring the CoG in empty condition further forward to address handling problems? Remember the aux. stage took the space that the coolant tank had occupied, so the small increase in weight in the rear would have been offset by the heavier 4 blade propeller. Balance maintained.
The new P-39 Rocket would still have a relatively low internal fuel capacity. What mission would it be suited for? Remove the .30s in the wing and put in fuel tanks.

- Ivan.

 

[P-39 Expert]

got pictures?

They made a mock up of the Spitifre withe six 20mm cannon too, that didn't get far.

No photo of the actual P-39E shows any wing guns or even cowl guns.

Photos in "Cobra" by Birch Matthews.

Performance test in Mike Williams' site for XP-39E said it had two .50s in each wing. Maybe just ballast instead?

 

[P-39 Expert]

Hey Ivan1GFP,

Fuel problem solved:
View attachment 597820

P-39 with a 350 USgal LRFT.

AAF wanted it to fly nonstop from San Francisco to Hawaii. Thank goodness they gave up on that.

 

[Ivan1GFP]

Hello P-39 Expert,

Only at military power, then only for the max 15 minutes. Tolerated just like on regular P-39s.

The problem here is that with the ASB and operating at higher altitudes, it ISN'T like on regular P-39s.
The power delivered to the propeller isn't the determining factor for cooling requirements. The extra power required by the ASB also requires extra cooling. The higher altitude and lower air density decreases the effectiveness of radiators as well.

Remember the aux. stage took the space that the coolant tank had occupied, so the small increase in weight in the rear would have been offset by the heavier 4 blade propeller. Balance maintained.

I would be very interested in seeing how much this "small increase" in weight actually affects that balance.
As was mentioned earlier, the P-39 had a tendency to be very near its aft CoG limit even without any changes.
To correct the handling problems, there needs to be more than just maintaining balance and I am not convinced that a propeller change would do it.

Remove the .30s in the wing and put in fuel tanks.

There are a couple problems with this idea.
First of all, the outboard sections of the wings where the guns are located are a rather small and flat volume.
Second, without the wing guns, the firepower becomes somewhat inadequate by US if not Soviet standards.

- Ivan.

 

[P-39 Expert]

Hello P-39 Expert,



The problem here is that with the ASB and operating at higher altitudes, it ISN'T like on regular P-39s.
The power delivered to the propeller isn't the determining factor for cooling requirements. The extra power required by the ASB also requires extra cooling. The higher altitude and lower air density decreases the effectiveness of radiators as well.

Don't know for sure until you try it.

I would be very interested in seeing how much this "small increase" in weight actually affects that balance.
As was mentioned earlier, the P-39 had a tendency to be very near its aft CoG limit even without any changes.
To correct the handling problems, there needs to be more than just maintaining balance and I am not convinced that a propeller change would do it.

Stop trying to invent CG problems. P-39 didn't have any real CG problems. Not like a P-51 with a full fuselage tank.

There are a couple problems with this idea.
First of all, the outboard sections of the wings where the guns are located are a rather small and flat volume.
Second, without the wing guns, the firepower becomes somewhat inadequate by US if not Soviet standards.

All that was needed was 30 extra gallons, 15 in each wing. Then it would have 150 gallons internal, just like the P-38 and P-40. The two outside tanks on the P-39 held over 8 gallons each. You couldn't fit the equivalent two more of those in each outer wing?

The plane had a 37mm cannon. It could not possibly be under armed.

You keep inventing imaginary problems for the P-39. It was in balance, heavily armed and a very good fighter plane with the -85 engine. Only real problem was excessive weight in the models with the -39 and -63 engines.

- Ivan.

Please expand above.

 

[fubar57]

Going into a fatal spin when nose ammo was expended wasn't a real CG problem?

 

[FLYBOYJ]

ARMY AIR FORCES
MATERIEL COMMAND
Wright Field, Dayton, Ohio
28 July 1944
MEMORANDUM REPORT ON
P-39Q Airplane, AAF No. 44-3455


Subject: Report of Spin Tests
Section: Flight
Serial No: ENG-47-1779-A

Conclusions
1. The P-39 should not be spun intentionally under any circumstances.
2. The P-39 should not be snap rolled as the roll usually ends in a spin.
3. The best spin recovery is to simultaneously apply opposite rudder and neutralize the stick.
4. Power should be cut immediately if a power on spin is entered.
5. Care must be excercised during the recovery to prevent an accelerated stall and re-enty into the spin.
6. The wing tip spin chute does not aid recovery of the P-39Q from a flat spin.

P-39 Performance Tests

Any aircraft that naturally sits at the most aft end of it's CG envelope will inherently have potential spin issues despite being "within balance" or not!

 

[FLYBOYJ]

"As the CG moves aft, a less stable condition occurs, which decreases the ability of the aircraft to right itself after maneuvering or turbulence."

Weight and balance 101 for those who don't know or understand (or refuse to understand) aircraft weight and balance!

Balance, Stability, and Center of Gravity – Effects of Adverse Balance

 

[FLYBOYJ]

Greg's most excellent Weight and Balance calculator.

 

[fubar57]

For those of us who don't run Excel

 

[Shortround6]

"The problem here is that with the ASB and operating at higher altitudes, it ISN'T like on regular P-39s.
The power delivered to the propeller isn't the determining factor for cooling requirements. The extra power required by the ASB also requires extra cooling. The higher altitude and lower air density decreases the effectiveness of radiators as well.

Don't know for sure until you try it."

Actually you have pretty good idea. That is one reason you have engineers. And ground tests. They KNEW how much power was needed to run a supercharger at a given rpm and airflow. They often used electric motors for such testing and could measure the current flow. Aircraft engines were often "turned over" by electric motors to measure friction from various components. They knew how much extra friction/drag stronger valve springs would cause. You think they were just going to bolt a more powerful engine into an airframe with existing radiators and oil coolers and fly it to see what would happen? Any taxi tests or flight testing was to confirm calculations and ground tests. Maybe they would hope things were more favorable but just hoping the existing cooling systems would handle around a 12% increase in cooling load seems like pretty poor planning. Most test reports of the P-39 mention over the limit cooling temperatures as it was. Over the limit does not mean overheating at that particular point.
Now consider that any standard text book was going to tell you that the air at 22,000ft was about 75-76% of the mass (density) of the air at 14,000ft and you can see the cooling problem before you even leave the ground (or indeed, when the plane is still on paper). and no, the cooler air temperature at 22,000ft is not going to make up the difference (s bit of it but certainly not all).

This book (or something like it)
Fraas. AIRCRAFT POWER PLANTS. 1st ed, 1943. | eBay

can provide a lot of knowledge about aircraft engines in general and what was known at the time (and not classified as secret) Like many things on Ebay, prices are all over the place.

Not my copy