Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)

[Shortround6]

The radio/s were in the rear of the airplane.

The CG is right about the back of the door or just under the very back of the pilots seat/over the wing ammo boxes.

Fuel system. It is just a drawing and may not be accurate but the fuel cells appear to taper. They seem to be shorter and thinner the further out you go from the fuselage.
going further out into ever thinner parts of the wing is just making the volume to surface area ratio worse.

 

[Greyman]

Fighter Command looked into the issue of Airacobra armour 30 September, 1941:

It does not appear at present that armour can be deleted although this aircraft appears to be carrying a considerable amount.

One governing factor against addition or deletion of weight is the position of the Centre of Gravity. The front of the aircraft is heavily armoured, but removal of this would result in the necessity for returning the C.G. to safe limits by the use of ballast and, as the majority of the weight is already in the nose, the saving in all-up weight would be negligible. The reduction gear is heavily armoured, but it would appear unwise to reduce this protection since, apart from the above considerations, damage to the gear might well result in the engine extension shaft whipping violently, and causing the aircraft to break up. The two oxygen bottles in the nose have armour protection and these might be deleted if wire-wound bottles were used. Rear armour for the oil and coolant tanks is the minimum that can be accepted. No additional armour can be added to the rear without forward ballast to correct the CG position, with the resulting loss in performance.

It is not recommended that any alterations to the existing armour be made.

 

[Greg Boeser]

Can't comment on P-39 fuel tanks, but a comparison of B-26 bomb bay tanks yields 178 pounds for an aluminum, 354 lbs for a self sealing tank of the same capacity.
So that's about double the weight.

 

[Shortround6]

An awful lot has to do with the size and shape of the tank.

AHT says a P-47D with the 305 gallon set up had a 332lb fuel system
A P-47D-25 with the 370 gallon set up had a 422lb system.
A P-38J with a 400 gallon set up had a 506lb system.
A P-51B/C with the 180 gallon wing tanks had a 320 gallon fuel system
Putting in the 85 gallon rear tank added 255lbs, how much of that was brackets, piping and relocating other components I don't know.

All these weights include piping and valves, some may include pumps of some sort.

 

[Greyman]

Found some specific info on armour.

P-400 had 291.28 pounds of armour.

P-39D-2 differed in the following respects:

- no forward O2 bottle plate (-35.01 lb)​

- no aft O2 bottle plate (-27.60 lb)​

- different cabin armour plate (10.91 lb instead of 11.20 lb) -.29 lb​

- different forward armoured glass (21.72 lb instead of 15.66 lb) +6.06 lb​

- different bulkhead armour plate (same weight)​

- different turnover bulkhead plate (14.90 lb instead of 19.12 lb) -4.22 lb​

- different aft armoured glass (44.17 lb instead of 48.94 lb) -4.77 lb​

- body turnover armour plate added (+12.81 lb)​

- different main oil tank armour (same weight)​

​

Overall P-39D-2 drops 53.02 lb in armour.

 

[Shortround6]

Thank you,

So we may assume (with all that entails) that the British specified the armor around the oxygen tank/s ?

Was there a difference in the oxygen systems that the British felt that they needed such protection?
AHT says the US P-39s used a low pressure system with 1 to 4 bottles while the P-400 export version got a higher pressure system.

Was the British system (or high pressure system) particularly vulnerable to gun fire?
Bottles exploded when hit or ruptured violently?

 

[XBe02Drvr]

Thank you,

So we may assume (with all that entails) that the British specified the armor around the oxygen tank/s ?

Was there a difference in the oxygen systems that the British felt that they needed such protection?
AHT says the US P-39s used a low pressure system with 1 to 4 bottles while the P-400 export version got a higher pressure system.

Was the British system (or high pressure system) particularly vulnerable to gun fire?
Bottles exploded when hit or ruptured violently?

A low pressure system uses gaseous oxygen compressed, but doesn't have all that much total quantity in any one bottle.
A high pressure system is likely to be more vulnerable, as the stress on the bottle is much higher, making the likelihood of rupture greater in the case of a bullet or shrapnel strike, and the quantity of liberated gas is so much greater as to create a locally oxygen-rich environment with attendant combustion opportunities. Ever drop a handful of iron filings into a beaker of pure oxygen? Try it sometime.
Cheers,
Wes

 

[P-39 Expert]

Found some specific info on armour.

P-400 had 291.28 pounds of armour.

P-39D-2 differed in the following respects:

- no forward O2 bottle plate (-35.01 lb)​

- no aft O2 bottle plate (-27.60 lb)​

- different cabin armour plate (10.91 lb instead of 11.20 lb) -.29 lb​

- different forward armoured glass (21.72 lb instead of 15.66 lb) +6.06 lb​

- different bulkhead armour plate (same weight)​

- different turnover bulkhead plate (14.90 lb instead of 19.12 lb) -4.22 lb​

- different aft armoured glass (44.17 lb instead of 48.94 lb) -4.77 lb​

- body turnover armour plate added (+12.81 lb)​

- different main oil tank armour (same weight)​

​

Overall P-39D-2 drops 53.02 lb in armour.

So, the AAF thought the Brits had included too much armor, right? Now drop the nose armor to save another 100#.

 

[Ascent]

So, the AAF thought the Brits had included too much armor, right? Now drop the nose armor to save another 100#.

It's already been explained that you can't drop the armour in the nose without shifting the CoG dangerously unless you add ballast which totally negates the reason for dropping it.

By all accounts the P39 was marginal on CoG already.

 

[P-39 Expert]

It's already been explained that you can't drop the armour in the nose without shifting the CoG dangerously unless you add ballast which totally negates the reason for dropping it.

By all accounts the P39 was marginal on CoG already.

Nothing has been proven. Just move the radios up from the tail cone to behind the pilot/above the engine. Radios located here in lots of photos. Should actually improve handling since weight has been relocated closer to the CG.

 

[P-39 Expert]

And how are you going to compensate for the resulting rearward CG shift in a plane that's already borderline tail heavy? Move the battery and the radio boxes forward? Where you going to find room for them to gain back the moment arm you lost by deleting the armor? The farther the battery gets from the starter and generator, the fatter (and heavier) your cables are going to get and the more starting problems you're going to have. The farther from the CG your battery and radios are, the more G abuse they'll suffer in Air Combat Maneuvering. Tube radios located in the nose next to a 37 (or a 20) and two 50s are going to be subject to intense high frequency vibration stresses whenever you squeeze the trigger. There's more to it than just yanking the armor.
Cheers,
Wes

Just move the radios from the tail cone up above the engine right behind the pilot. Lots of original photos show the radios located here. Moves weight closer to the CG, should help maneuverability.

 

[Shortround6]

So, the AAF thought the Brits had included too much armor, right? Now drop the nose armor to save another 100#.

How do you get that?

The Americans used a different oxygen system that didn't require bullet proofing. The Americans removed 62.61 lbs of armor from the the oxygen set up and yet the total armor weight difference was only 53.02 lbs. so the Americans, with a bit of juggling the other pieces, were using 9.6lb more than the British once the armor for the oxygen tanks is taken out.


As to the radios, the radios changed between the different types/models and between countries. The British may have started fitting IFF gear before other countries. One diagram of a P-39 Q shows radios both in the original position and above the the engine but doesn't say what kind of radios they are. IFF gear tends to get lumped in with radios instead of being called a radar transponder.

From AHT just to show how confusing things are.

Category,,,,,,,,,......................,,,...P-39D..........P-39D-2............P-400
Elec system.................................225..................274.6.................278
communications........................62.....................80.2..................35
Misc. Eqpt (radio)......................----................129.2...............109.9

what piece of equipment got thrown into which catagory changed over time. I am not sure about the lack of radio weight for the P-39D. radio was not counted in basic weight? sample aircraft did not have radio fitted? I have no idea what "communications" covered if the radio weights are listed elsewhere.
The roughly 50lb increase in electrical weight is interesting. It cover quite a few things.
Voltage requirements for different pieces of equipment varied and things like this were used to convert it.

What catagory this goes in (electrical or radio) I have no idea.
people have already said that the old radios could be tempermental.
Some of us are old enough to remember even car radios being affected by poor ignition wires on car engines. Shielding the radios from static from the ignition system or other electric devices in the plane took some doing. They got a lot better at it fairly quickly but again, just because they could do something in 1943 doesn't mean they could do it in 1941.
Sometimes "the" radio was in one box, often you had a radio tray or box that contained several separate pieces of equipment or chassis. Sometimes descriptions, especially if translated are not real clear on exactly what was done.

Before you get too crazy about removing the nose armor please remember that when they went from the P-39C to the D/P-400 they also yanked about 50lbs of .30 cal guns and about 60lbs worth of ammo out of the nose. Plus accessories.

Larry Bell and company did build one really stripped aircraft which I have already mentioned but there doesn't seem to be record of it's CG location. And it was flown by how many pilots? a few company test pilots?
What is acceptable flight behavior with an aft CG location varies from country to country. Or even in the same country depending on how severe the combat situation is.
Just because the Russians thought an airplane was OK to fly doesn't mean the UK or the US would.

 

[Shortround6]

A low pressure system uses gaseous oxygen compressed, but doesn't have all that much total quantity in any one bottle.
A high pressure system is likely to be more vulnerable, as the stress on the bottle is much higher, making the likelihood of rupture greater in the case of a bullet or shrapnel strike, and the quantity of liberated gas is so much greater as to create a locally oxygen-rich environment with attendant combustion opportunities. Ever drop a handful of iron filings into a beaker of pure oxygen? Try it sometime.
Cheers,
Wes

I will take your word.

I was one of the SCBA bottle fillers on my shift at the fire dept. Granted this was quite a number of years after WW II but the bottles (1980 psi when I started) were ALWAYS filled when they were contained in a heavy steel cylinder (with lid) bolted to the floor. Later on we got a new compressor/filling station that held 3 bottles in heavy steel cylinders but the air valves wouldn't open until the heavy sheet metal doors were closed which rotated the cylinders to a vertical position. We never had an accident but highly compressed gases are nothing to fool with.

 

[XBe02Drvr]

We never had an accident but highly compressed gases are nothing to fool with.

Back in the '60s in upstate NY, a boat on the way to a dive site had a SCUBA tank, that was foolishly left standing up on a seat, fall over and bang it's old style tall thin valve assembly against another tank lying on the deck. The valve head snapped and the tank launched itself into the air and PENETRATED the vertical web of an I beam under a bridge the boat was approaching. The valve assembly ricocheted, bounced off the PFD of the boat's driver, and shot over the side. Nothing to fool with, for sure.
Back in the day dive shops used to fill tanks submerged in a water bath filled right to the overflow tube. A beaker under the overflow caught the water displaced by the tank's expansion as it took on 2,200 PSI of air. If the overflow exceeded the specified number of CCs, the tank had to be removed from service and sent off for hydrostat testing.
Cheers,
Wes

 

[Ivan1GFP]

Here is a pretty good reference for the weights and locations of the armour on various models of the Airacobra.

Shortround6, you made a comment that the CoG of the Airacobra was at the back of the pilot's seat and about even with the rear edge of the door.
Do you happen to know what fuselage station this would be?
I have tried working backwards from the CoG locations, loaded weights and moments from a P-39Q Loading Diagram from the manual but am quite convinced that this is not reflective of the CoG of the early Airacobra. I suspect that the CoG was at least a couple inches further back.

- Ivan.

 

[Shortround6]

First, thank you very much for this chart.

I don't have the frame number, a drawing for the CG of a P-39Q is in page 21 of this manual.

https://ww2aircraft.net/forum/attachments/p-39q-pdf.33643/

Hope this helps
it may be the one you have.

 

[P-39 Expert]

Regarding the nose armor, Daniel Whitney in "Vees for Victory" stated that the P-39M no longer needed the nose armor for ballast, which was it's primary purpose, and no personnel had ever seen a P-39 hit in that area. But, the nose armor was still included in the P-39M (and N/Q).

 

[Shortround6]

Now here we have an author's account conflicting with parts charts.
We also have Mr Whitley saying " Many Airacobras that had been shot down in combat had been lost because of one bullet penetrating the engine manifold, carburetor, or cam-shaft covers. The Objective was to provide maximum armor protection for these vital areas. In order to achieve a weight reduction a great number of more-or-less useful, but not necessary items were removed from the aircraft." he goes on to say that the gear box armor was removed and like you say the field reps had not seen any bullet strikes there.
Then " The first of these aircraft, a P-39M-1, became known at Bell as "old Ironsides""

Which hardly sounds like they reduced the armor, and according to the chart above the P-39M "old ironsides" unless it was a one off special, used exactly the same armor as the P-39K & L and the N & Q. No more and no less.

On many of these later P-39s (after the D and F) there were a variety of propellers (not all the same weight) different reduction gears, and the engines were not all the same weight. The Later ones got a bit heavier perhaps only 10-20 pounds. I don't know if there were differences in the accessories. Starters, generators and such (no hydraulic pump). The engine does not sit on the CG, the nose of the engine (where the reduction gear would be on a normal engine) sits on the CG with the engine actually behind the CG.

They may have intended to take out the nose armor but changed their minds?

On plane that is not supposed to be flown without ammo for nose guns unless it is carry ballast to compensate for the weight, taking 70-95lbs out of the plane 3-4 feet further forward seems like skating on a very thin edge.

 

[Glider]

I note that 601 Squadron immediately recognised six key issues that had to be addressed immediately, the fifth one people may find interesting in view of the comments about moving equipment within the aircraft:-

a) Introduction of a master valve so the oxygen could be switched on in the cockpit
b) Modification of the gun sight 'so the pilot could see out of the front of the aircraft'
c) Changes to the wing ammunition tanks as they quickly became distorted and jammed.
d) Modifications to the harness release mechanism
e) Removal of the IFF from behind the pilot as it badly obstructed the view aft
f) Introduction of a throttle control friction dampner

There were a host of other issues as they were crated stright off the production line without testing

 

[Shortround6]

OoooKaaay.
Fit one of the first "bubble' canopies on a fighter and then fill the back of the bubble with "stuff" so you can't see out of it