XP-39 II - The Groundhog Day Thread

[P-39 Expert]

The P-40 always seems to have a higher critical altitude with equivalent engines.

How is that possible?

 

[fubar57]

They are here....Manual Index - American along with other countries here...Other Mechanical Systems Tech.

 

[pinsog]

How is that possible?

The air intake on a P40 was at the front of the plane and it benefited from ram at high speed. The air intake on a P39 was blocked by the canopy in a low pressure area so it didn't get the benefit of ram air like the P40 did.

 

[P-39 Expert]

They are here....Manual Index - American along with other countries here...Other Mechanical Systems Tech.

Thanks.

 

[GregP]

Thanks FlyboyJ.

As a retired electrical engineer, I have done a LOT of Excel, so doing weight and balances is right up my alley.

Cheers.

 

[P-39 Expert]

Yes, but better performing engines and better fuel was on the way. If there is no design capable of upgrading, it quickly becomes, not needed. However I do think the P-38 turbo was available, theoretically.

Right, the -85 engine with a greater critical altitude would be available in Nov 1942 for the P-39N, big performance increase.

Yes, I think the Japanese lost a lot of their hard to replace experienced airmen in the Guadalcanal and Solomon Sea battles facing these three American fighters and some Brit fighters. The young AAF pilots had to come up to speed quickly to live.

And the Japanese lost a lot of pilots at Midway too.

Possibly below 10k as the P-39 was only about 300 ft/min less than the Zero but above 10k I don't think that much weight could be removed. At 15k the Zero could climb about 1200 ft/min faster.

That's after the P-39 reduced power from combat power 3000rpm to normal power 2600rpm because of the 5 minute combat power limit. This limit was increased in June 1942 to 15 minutes. If combat power 3000rpm was used the difference in climb rate was a lot smaller. Reducing P-39 weight by about 300lbs reduces the difference further. Then in December 1943 the P-39N with the -85 engine substantially outclimbed the Zero.

I think below the fuselage, like the P-51coolent intake, would be a better place in higher pressure air and less turbulence.

Expand above.

 

[P-39 Expert]

The P-39 has a lot of structure to support the coolant tank and provide rigidity between the keel beams as shown in the photo Wuzak posted. In particular that bulkhead with the flanged lightening hole (visible behind the coolant tank support) between the two keel units adds a massive amount of rigidity to the structure as does that curved plate connecting both keel units.
View attachment 597569
The P-63 has a lot of fresh air and one single cross member in the same area in the forward fuselage and significantly less structure in the rear fuselage than the P-39 where the two fuselage halves join.

Front fuselage
View attachment 597572
Looking at this diagram below we again see that there is no bulkhead between the front and rear fuselage as you can see clear through into the forward fuselage. You will note that the oil return line (yellow) is totally hidden behind the metal part of the rear fuselage bulkhead (the edge being just to the right of the orange line. Have you ever wondered why they hid that pipe there when it would be so much more accessible for maintenance if it was inside the area that you say holds nothing but glider fuel. Now ask yourself why do they need to keep that area totally free of everything except glider fuel? Now tell my what your answer is.
Mine is the ASB partially occupies that area.
View attachment 597570


Maybe it is time to go back to the basics that any airframe driver (pilot) and spanner wrencher (aircraft maintenance technician) learns about day one. Everything contained within the single area bounded by the cowlings and aircraft structure/skin is the engine bay or engine compartment. On the P-63 that includes everything from the firewall behind the pilot on the P-39/63 to the next full bulkhead. On the P-39 that is the bulkhead that the oil tank protrudes through. On the P-63 that is the bulkhead behind the oil tank. So you are right - it only extends as far as the oil tank bulkhead at station 253. 253 is aft of the oil tank. How can that be part of the engine bay?
You will be pleased to know both aircraft are very questionable on these limits because there is no effective firewall at the back of the engine bay so one could also be technically accurate claiming the whole of the rear fuselage as engine bay. To say however that the engine bay ends at the fuselage joint line is a complete fallacy. The engine bay extends to the back of the oil tank. Call the engine bay what you wish, but most will agree that it ends at station 231.5 and the bulkhead at that point separates the engine bay from the oil tank.
To suggest that the designers went to all the trouble to move the tank as far back as they did, instead of just mounting it vertically behind the joint line bulkhead, for any reason except the engine installation needs that room for the back of the ASB and the associated plumbing, wiring and controls is equally fallacious. The ASB does not go past station 231.5. It can't or it would be in the oil tank.

Please expand above.

 

[Ivan1GFP]

Hello P-39 Expert,

There are lots of places online that people have posted aircraft manuals.
If you visit some of the flight simulator forums, especially those in which people get a bit argumentative over detail, you will often find that someone has linked to a manual or test report to try to back up their opinion.
I have also done pretty well lurking in some Russian language forums but of course most of the manuals there tend to be about Russian subjects though not always and not always about aircraft.
Scribd is another good place that has a few manuals and test reports but finding things is difficult and much is restricted unless you have a subscription which I do not.

- Ivan.

 

[P-39 Expert]

Hello P-39 Expert,

There are lots of places online that people have posted aircraft manuals.
If you visit some of the flight simulator forums, especially those in which people get a bit argumentative over detail, you will often find that someone has linked to a manual or test report to try to back up their opinion.
I have also done pretty well lurking in some Russian language forums but of course most of the manuals there tend to be about Russian subjects though not always and not always about aircraft.
Scribd is another good place that has a few manuals and test reports but finding things is difficult and much is restricted unless you have a subscription which I do not.

- Ivan.

Thanks.

 

[FLYBOYJ]

Where do you search onlne? Just asking, so I don't have to waste anymore of your time.

No waste of time. I searched P-39 flight manual and it gave me a few different models.

https://airandspace.si.edu/webimages/collections/full/Pilot's flight manual for P-39 Airacobra.pdf

P-39 Airacobra Pilots Flight Operating Instructions

http://www.wwiiaircraftperformance.org/P-39/P-39Q-1_Operating_Instructions.pdf

 

[Husky]

Ahhh, CG. Some aircraft with so limited CG range; mostly on the longitude axis, but as well the lateral axis. Can be a tough nut. My first thought; don't design an aircraft with such a limited CG range....leads to so much restrictions and growth. Unless that aircraft has a very quite specific purpose of operation and is intended to go no other.

 

[Ivan1GFP]

Ahhh, CG. Some aircraft with so limited CG range; mostly on the longitude axis, but as well the lateral axis. Can be a tough nut. My first thought; don't design an aircraft with such a limited CG range....leads to so much restrictions and growth. Unless that aircraft has a very quite specific purpose of operation and is intended to go no other.

Hello Husky,

I don't believe the problem was that the CoG range was particularly narrow.
23% - 31% MAC isn't that bad.
I believe there were two major problems. One was that very large disposable loads were very far from the longitudinal CoG.
The second problem was that the center of lift of the wing was too far forward in relation to the typical fixed loads in the aircraft which meant that although the aircraft could be brought within the allowable CoG range when fully loaded, it would get too close or even past the aft CoG limit when loads were expended.

- Ivan.

 

[P-39 Expert]

You guys are working this CG thing to death. A few more days of this and the plane won't fly at all.

 

[fubar57]

Some people make certain claims that would unbalance things..................

 

[FLYBOYJ]

Hello Husky,

I don't believe the problem was that the CoG range was particularly narrow.
23% - 31% MAC isn't that bad.
I believe there were two major problems. One was that very large disposable loads were very far from the longitudinal CoG.
The second problem was that the center of lift of the wing was too far forward in relation to the typical fixed loads in the aircraft which meant that although the aircraft could be brought within the allowable CoG range when fully loaded, it would get too close or even past the aft CoG limit when loads were expended.

- Ivan.

Hi Ivan - agree to a point but if you look at the loading charts most of the time it seems the aircraft was never forward than 28.8%, tail heavy through all operations, but I guess we're sought of saying the same thing.

You guys are working this CG thing to death. A few more days of this and the plane won't fly at all.

A few more days of this and maybe you'll learn something!

 

[ThomasP]

At the risk of further unbalance.

First, a video of a restored P-39 giving a good idea of the area/length taken up by the single-stage engine/supercharger:

""
at 29 & 40 sec

I ran across a reference to the engine fit for the P-63.

Apparently, the P-63 was originally intended to be powered by the Continental V-1430 engine (as was the P-39E). However, since the V-1430 did not become available, the Allison V-1710-93 was fitted instead.

Also, I ran a cross a reference that the P-39D-1 (as originally ordered by the British) was fitted with a different oil tank that was smaller, and was held vertically slightly further to the rear. This was in order to allow room for some sort of equipment in the lower-rear of the engine compartment (I do not know what equipment, but it may have been the obligatory 2x emergency landing flares). I can not guarantee the accuracy of this. I do not know if the P-400s were the same? Maybe someone can find an image of the P-39D-1/P-400 rear engine compartment, minus the engine but with additional equipment?

Then I looked at some photos of a production model P-63 being serviced and noticed these images of the auxiliary stage SC and engine compartment:

And then I looked around some more and found this image in a P-63 e&m manual:

The rectangular item with rounded corners in pics#2&3 is the protective cover for the auxiliary SC air intake.

I will leave you gentlemen to decide what, if anything, the above proves/disproves.

 

[MiTasol]

Where do you search onlne? Just asking, so I don't have to waste anymore of your time.

There are multiple sites that will sell you a download or a CD of P-39 and/or P-63 material.
I would avoid Mach-1 manuals and military-aircraft-photos and any other group that watermark their documents as the watermark often obscures the information you are looking for.

I can understand why they do that (the same reason I watermark all manuals I post to this site but the manuals I post are free and I want them to remain that way.

There are also outfits out there that will sell you a cd of aircraft blueprints and you will find that it only contains an index and a few blueprints - you then need to pay for several or many other CDs. I have no personal experience with these sites but a friend has just spent 90 pounds for what is advertised as a complete set of another types blueprints only to find that he must pay another 450 pounds to get the full set.

For a modest monthly fee WWII Era Military Blueprints & Manuals | AirCorps Library is good value though they do not have much on the P-63. They do have a set of P-39 blueprints and a selection of manuals that are a recent addition and so most drawings are only listed by part number. If you access a drawing that has no description please identify the content from the title box in the lower right hand corner. That makes it easier for you and others to search for the same thing later as you can then do a keyword search. For some drawings there are multiple pages and the title box is on the last page. They also have engine and prop manuals.

 

[MiTasol]

At the risk of further unbalance.

First, a video of a restored P-39 giving a good idea of the area/length taken up by the single-stage engine/supercharger:

""
at 29 & 40 sec

I ran across a reference to the engine fit for the P-63.

Apparently, the P-63 was originally intended to be powered by the Continental V-1430 engine (as was the P-39E). However, since the V-1430 did not become available, the Allison V-1710-93 was fitted instead.

Also, I ran a cross a reference that the P-39D-1 (as originally ordered by the British) was fitted with a different oil tank that was smaller, and was held vertically slightly further to the rear. This was in order to allow room for some sort of equipment in the lower-rear of the engine compartment (I do not know what equipment, but it may have been the obligatory 2x emergency landing flares). I can not guarantee the accuracy of this. I do not know if the P-400s were the same? Maybe someone can find an image of the P-39D-1/P-400 rear engine compartment, minus the engine but with additional equipment?

Then I looked at some photos of a production model P-63 being serviced and noticed these images of the auxiliary stage SC and engine compartment:
View attachment 597681View attachment 597682View attachment 597683

And then I looked around some more and found this image in a P-63 e&m manual:
View attachment 597684

The rectangular item with rounded corners in pics#2&3 is the protective cover for the auxiliary SC air intake.

I will leave you gentlemen to decide what, if anything, the above proves/disproves.

Excellent material, thank you Thomas.

I have to concede that it appears from those photos that the ASB is totally installed forward of the fuselage joint line making P39 expert correct. There only remains the question - why did Bell create all that waste space if not for the ASB. I shall continue looking and report further if I can solve that question. Maybe so the engine can be shifted rearwards during removal so that the stub shaft clears the roll over truss. Only a WAG as I know the P-39 engine can move rearwards, or be tilted, during removal.

 

[ThomasP]

Hey MiTasol,

I was wondering if the excess length was for the Continental V-1430 engine installation. I do not have any good references for the arrangement/dimensions/weights for this engine as it was planned for the P-39E/P-63. If it was a significantly lighter installation I suspect the center of the engine's mass would have to be farther to the rear to maintain CG . . . maybe that is why? Just a WAG.

 

[MiTasol]

That is certainly a reasonable assumption and may well be the total answer. I know someone on this forum wrote a book on that engine and that may complete the puzzle. Unfortunately that engine never did anything for me so I did not keep the author or book title in my tired old memory.