XP-39 II - The Groundhog Day Thread

[Milosh]

So what happens in a climb, or dive, in a partially filled tank?

 

[MiTasol]

The P-39 oil tank is completely behind the bulkhead. It's base sat in the same type of cradle as the P-63, behind the bulkhead.

Yes it fits in a cradle similar to the P-63 but the tank is mounted at an angle and protrudes thru the bolted bulkhead at stn 228.5 as shown in the first diagram in post 293.
Seeing that you will not believe someone who has worked on these aircraft maybe you will believe the Bell Aircraft Company, though I am beginning to doubt even they can convince you.

As shown in post 293 the P-63 oil tank is mounted vertically some six to eight inches behind the bulkhead and the bulkhead has a far larger cutout.

1. The aux. stage blower was installed in the XP-39E.
2. The engine compartment was not enlarged to accommodate the aux. stage, it was the same length as the other P-39s and the P-63
3. as evidenced by the drawings in my post #236.

1. Yes, one out of three is correct

2. Yes and no.
Yes, the engine bay in the forward fuselage was not extended
No because the rear fuselage bulkhead was changed to allow the total engine bay to extend into the rear fuselage, unlike on all other P-39s, like on the P-63, and the angled oil tank was moved rearwards and positioned vertically, and the coolant tank relocated, to provide the space required to install the ASB.
The only way to fit the ASB in the P-39E was to extend the rear fuselage so that the ASB can extend through it like on the P-63 as shown above.

3. Station and skin panel drawings do not show any internals so are totally irrelevant to this discussion.

The E model was longer because the tail section was longer, not the engine section as reported by other sources.

Yes and no. See above. The rear fuselage was longer in part because the engine bay was extended into the rear fuselage and the oil tank was moved aft and mounted vertically, and coolant tank relocated, to provide room for the ASB.

 

[buffnut453]

Not if the fuel is on the center of gravity as it should be.

Or ammunition, or oil, or signal flares, or oxygen, or bombs...you can't put every consumable item "on the center of gravity". This is getting ludicrous.

 

[DerAdlerIstGelandet]

Not if the fuel is on the center of gravity as it should be.

CG shifts both longitudinally and laterally. Also, do you plan on moving the fuel tanks every time you remove components, equipment, etc., so that is always at the CG? What about oil? What about any other expendable item in the aircraft?

Read this...

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/faa-h-8083-1.pdf

 

[DerAdlerIstGelandet]

Or ammunition, or oil, or signal flares, or oxygen, or bombs...you can't put every consumable item "on the center of gravity". This is getting ludicrous.

Ding, ding, ding. Someone gets it!

 

[buffnut453]

Ding, ding, ding. Someone gets it!

I had a vision of a fuel tank that looked like a very tall, very slender test tube so that it could persistently sit directly on the CofG. Alas, that would only work for straight-and-level flight.

 

[DerAdlerIstGelandet]

There is a reason every time I go flying in my lil Piper Cherokee (Yeah I know its not a hot rod warbird, but weight & balance is still crucial for it too) that I do a W&B for before takeoff, and one for landing. I have to compute the amount if fuel and oil, how much each weighs, where my baggage (if any) are located, and how much it weighs, and the weight of my passengers. I have to do two because during the flight I will use up fuel. Therefore the CG shifts during the flight.

 

[P-39 Expert]

Yes it fits in a cradle similar to the P-63 but the tank is mounted at an angle and protrudes thru the bolted bulkhead at stn 228.5 as shown in the first diagram in post 293.
Seeing that you will not believe someone who has worked on these aircraft maybe you will believe the Bell Aircraft Company, though I am beginning to doubt even they can convince you.

View attachment 597411

As shown in post 293 the P-63 oil tank is mounted vertically some six to eight inches behind the bulkhead and the bulkhead has a far larger cutout.
View attachment 597414



1. Yes, one out of three is correct

2. Yes and no.
Yes, the engine bay in the forward fuselage was not extended
No because the rear fuselage bulkhead was changed to allow the total engine bay to extend into the rear fuselage, unlike on all other P-39s, like on the P-63, and the angled oil tank was moved rearwards and positioned vertically, and the coolant tank relocated, to provide the space required to install the ASB.
The only way to fit the ASB in the P-39E was to extend the rear fuselage so that the ASB can extend through it like on the P-63 as shown above.

3. Station and skin panel drawings do not show any internals so are totally irrelevant to this discussion.



Yes and no. See above. The rear fuselage was longer in part because the engine bay was extended into the rear fuselage and the oil tank was moved aft and mounted vertically, and coolant tank relocated, to provide room for the ASB.

I stand corrected, the oil tank did protrude slightly into the engine bay.

But I am correct in that the engine bay was not extended on the P-63, it was exactly the same size as the P-39, including the XP-39E. Please look at the drawings in my post #236 of the P-39 and the P-63. Those numbers above the fuselage are inches from the tip of the nose. On the P-39 the engine bay starts at station 138.25 and ends at station 228.5 meaning the length of the engine bay was 90.25". On the P-63 drawing the engine bay starts at station 141.25 and ends at station 231.50 for a difference of 90.25". The engine bay was 90.25" exactly for both the P-39 and P-63. The tail cone was lengthened but not the engine bay. The aux. stage supercharger would fit in the P-39 just like it would in the P-63.

Maybe the P-39 oil tank would need to be adjusted to stand vertically instead of at that angle, but the engine bay was not lengthened, it was the exact same size. Exactly.

 

[MiTasol]

I get the feeling that some people believe W&B and CG is a simple task, when there really is a science to it. You cannot simply take weight away from somewhere, and add some weight here. When I was on 2nd shift working as an A&P I had to work with QC quite a bit doing them after modifications had been performed. I always enjoyed it.

And then there are the structural modifications that must be made to move something.

At 9g a 150lb radio "weighs" 1350lb so the structure where it is being moved to needs to reflect that.

Further there is the structure that originally held the radio. Can all or part of it safely be removed? If it is left in will it produce other problems like too much rigidity in the area which will cause cracking nearby? This is not an insignificant issue as shown on the Lockheed L-188 which was too strong in one area. This resulted in the wings separating from the aircraft in flight and killed a lot of passengers and crew before the LEAP mods which changed the engine mounts to reduce a specific vibration, caused by whirl mode, and removed a lot of structure in the wing to fuselage joint area.

 

[DerAdlerIstGelandet]

And then there are the structural modifications that must be made to move something.

At 9g a 150lb radio "weighs" 1350lb so the structure where it is being moved to needs to reflect that.

Further there is the structure that originally held the radio. Can all or part of it safely be removed? If it is left in will it produce other problems like too much rigidity in the area which will cause cracking nearby? This is not an insignificant issue as shown on the Lockheed L-188 which was too strong in one area. This resulted in the wings separating from the aircraft in flight and killed a lot of passengers and crew before the LEAP mods which changed the engine mounts to reduce a specific vibration, caused by whirl mode, and removed a lot of structure in the wing to fuselage joint area.

That is something I deal with every day in my job, which deals with the engineering and design of retrofit and modifications. I am an aircraft mechanic by training, and not an engineer, but I basically work in the engineering support for this. I know longer turn wrenches. Structural testing and weight and balance are things we deal with every day.

 

[FLYBOYJ]

I also understand that the CG is also a fixed point on the plane

Right there you're wrong - the C/G will move (as shown) depending on weight, arm and moment

I also understand, as should you, that the manufacturer was able to maintain the CG within acceptable limits when items of different weights were substituted or removed.

Absolutely

Such as larger (heavier) propellers or different (lighter) cannons were substituted in the nose, or different radios were substituted or removed from the tail. Balance was maintained by the manufacturer through all these different equipment changes. Bell stated that the P-39M did not need the nose armor as ballast. And the Russians removed the IFF radio that weighed 120lbs from the tail cone of the plane yet did not remove anything from the nose to maintain balance. According to you all the Russian P-39s should not have been able to fly.

Of course different equipment will change weights and C/G limits but are you trying to say that Bell would pick out a "heavier" propeller or cannon of the same make/ model to adjust for weight and balance requirements on a specific production model? And again you're making the statement " According to you all the Russian P-39s should not have been able to fly." All I'm saying is "Show me the math" to prove your point! I actually think removing 120 lbs from the tail would have made the plane fly better, but that's just an opinion - I'd have to do the math

All of you are making much more of this than actually happened.

No - we're actually trying to get you to prove your point based on the data shown.

The P-39 was by all accounts easy to operate, safer than most and capable of performing all combat maneuvers. It had good stalling characteristics, acceptable spin characteristics (which could only happen after a stall) and would not tumble when ballasted properly. Most all other fighter planes had some unfortunate characteristics. Such is the nature of high performance airplanes.

Agree, providing the aircraft stays between 23 - 31% MAC (which is a little over 6 inches) during all flight regimes in it's factory condition.

Now do you want to do more math?

 

[MiTasol]

I don't think he's caught on yet that CG is not a fixed location. Most of my flying students were reluctant to wrap their heads around that idea, even after reading the W&B chapter in the student pilot manual.

Agreed so maybe he should study a basic introduction like https://skybrary.aero/bookshelf/books/879.pdf

 

[FLYBOYJ]

Not if the fuel is on the center of gravity as it should be.

Look - it's evident you never weighed a real aircraft. This has nothing to do with fuel being on the center of gravity!

 

[ThomasP]

Question - what IFF set are talking about here? The only Allied electronic IFF bare units used (before the very end of the war) topped out at around 36 lbs plus a few pounds for wiring and such (most of the installation weights I have seen were around 40 lbs total). Where does the 120-130 lbs value come from?

 

[MiTasol]

Yes the Russians removed the IFF radio that weighed about 120lbs from the tail section and didn't remove the 100lb nose armor. According to some folks on here the plane should have immediately fallen out of the sky.

Removing the IFF, but not the armour, as done in Russia, would prevent the CG moving as far aft once all the ammo was expended meaning the aircraft would be more stable and handle better with zero ammo and almost zero fuel.

Why would the IFF radio be important to the AAF and not the Russians? I don't know.

IFF means Identification Friend or Foe and the allied radar was able to interpret the IFF signals to tell if you were friendly and therefore the allied aircraft and antiaircraft guns would not shoot you down.

 

[MiTasol]

Question - what IFF set are talking about here? The only Allied electronic IFF bare units used (before the very end of the war) topped out at around 36 lbs plus a few pounds for wiring and such (most of the installation weights I have seen were around 40 lbs total). Where does the 120-130 lbs value come from?

You are right. The AAF SCR595 and USN ABK IFF units were both 32.5lb for the main box excluding mounting tray and a few pounds for the wiring and cockpit control box. I should have picked up on that earlier

 

[MiTasol]

Thank you for all the comments about what I understand and don't understand. I understand mean aerodynamic chord and CG being within certain limits of distance from the wing leading edge.

I also understand that the CG is also a fixed point on the plane

NO IT IS NOT

Go and read the posts about CG again and again and again until you get the message that the CG moves in flight with changes of fuel, oil and disposable load like ammunition and bombs.

On the P-40 and P-51 it moves considerably forward as fuselage tank fuel is burned and on the P-39 it moves considerably aft as the fuselage ammunition is used. On the P-39 it also moves slightly aft as the fuel and wing ammunition are expended. The only thing that moves the P-39 CG forward in flight is oil consumption and that is a minor change as so little is used relatively.

[My apologies to all the other forum members for shouting.]

 

[windhund116]

View attachment 597402

Didn't they call a P400, "A P39 with a Zero on its tail?"

 

[fubar57]

That's OK, I'm hard of hearing

 

[MiTasol]

Not if the fuel is on the center of gravity as it should be.

On most aircraft the wing fuel is NEAR the CG. On the P-39 it is slightly forward of the CG