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

[SaparotRob]

Aaaaand…..it's Groundhog Day!

These are precisely the discussions that we had with the "expert."

He wanted to remove the nose armour because he thought it was of no use. Alas, he couldn't ever be persuaded to consider the CoG impacts.

That nervous tick has returned to my left cheek. I KNEW resurrecting this thread was a bad idea!

In times of strife, madness and great upheaval, I find solace in the Groundhog thread.

 

[ThomasP]

re

Spitfire's wings were even thinner, yet people shoved up 4 cannons per an A/C there - so I'd say that P-39's wing was thick enough. How much of the effort Bell's engineers and techincians wanted to invest into modifying the wing for the .50 to fit is another story.

and

The P-39D had a 37mm centerline, two cowl mounted .50 MGs and four wing mounted .30 MGs - this could have been modified to omit the two cowl MGs and replace the centerline cannon with a 20mm and replace the four .30 MGs with four .50 MGs

re fitting .50 cal Brownings in the wing (in roughly the same location as the .30 cal Brownings).

Based on the airfoil section of the P-39 (ie NACA 0015 - 23009) the maximum wing thickness at the ~50% span (about where the inner .30 cal gun is mounted) is only 8". Assuming the front of the .50 cal receiver butts up against the rear of the forward spar, the rear of the .50 cal receiver would have to penetrate the main spar. Not saying this could not be done but it might be a problem as the receiver for the M2 Aircraft Gun variant is 6.97" high not counting the mounting lugs.

The semi-elliptical planform of the Spitfire and its airfoil section (ie NACA 2213.2 - 2209.4) combined with the greater chord of the semi-elliptical wing at the ~35% span location of the inner gun (which is where they put the .50 cal in the 'E' wing arrangement) gives a maximum wing thickness of 11", and the single-spar structure allows the .50 cal receiver to fit behind the spar.

You could, of course, move the .50 cal gun locations inward spanwise on the P-39, but this would reduce the fuel load in the wings and/or displace the fuel tanks outward in the wing.

As to any weight & balance issues, we know the P-400, P-39D-1 and D-2 variants carried the 20mm HS404 and did not seem to have any unusual problems - so it appears to have been doable in a practical sense without any extraordinary effort.

(I think I did my measurements and calculations for the respective wing thicknesses correctly?)

 

[GregP]

How would you address the resulting CoG issues?
The centerline 37 mm cannon and cowl mounted .50 cal guns are all very far forward and all very heavy items. Unless you can replace all that weight with some other equipment, you will have a seriously tail heavy bird. Just expending the typical 400 rounds of .50 cal in the cowl seemed to be enough to make the aeroplane dangerously unstable.
The Wing guns are pretty near the aircraft CoG, so exchanging the .30 cal guns for .50 cal guns would not move the CoG much assuming they would actually fit into the existing spaces.

The obvious question would be, "What happens when the rather heavy 37 mm cannon gets replaced with a lighter 20 mm gun???"
On factory aircraft that were so equipped, the weight of armour in the nose was increased to maintain balance. Not sure what was done if the cannons were swapped in the field.

- Ivan.

That's siimple!

Add more nose armor!

Duhhhhh ...

Sorry, couldn't resist ...

 

[buffnut453]

Expending the 37 mm ammunition wasn't that critical. Expending all the .50 cal cowl gun ammunition was a lot worse. It was all a matter of the longer moment arms.

Hmmm...per post #4383, the 50cal ammo was further aft than the 37mm ammo (image repeated below to save scrolling). While I can accept that the 50cal ammo may have weighed more than the 37mm ammo (I don't know....but accepting that at face value for now), the moment arm for the 37mm ammo is smack in the middle between the 2 containers of 50cal ammo so I'm not sure about the "a lot worse" statement. Regardless of the weapon configurations, the P-39 CoG suffered from ammo expenditure...which isn't a good thing.

 

[MiTasol]

If we take into consideration aircraft technology and design of that point in time, Bell did a fantastic job of creating a fighter that was ahead of it's time.

Several innovations, like tricycle gear, buried engine, centerline cannon, protective armor and unitised construction were cutting edge.

And what is now called a 5 axis mill was designed to manufacture the spar caps. Revolutionary tech for the day.

 

[MiTasol]

Hmmm...per post #4383, the 50cal ammo was further aft than the 37mm ammo (image repeated below to save scrolling). While I can accept that the 50cal ammo may have weighed more than the 37mm ammo (I don't know....but accepting that at face value for now), the moment arm for the 37mm ammo is smack in the middle between the 2 containers of 50cal ammo so I'm not sure about the "a lot worse" statement. Regardless of the weapon configurations, the P-39 CoG suffered from ammo expenditure...which isn't a good thing.

View attachment 825285

Only a small number of P-39s had the two forward 30 cal ammo boxes. The vast majority did not have the 30 cal nose guns. The 50 cal ammo boxes come in from both sides on the same moment arm.

From memory there were only 30 37mm round so their affect on CoG is fairly minor but the 400 (again from memory) total 50 cal rounds weighed far more and were enough to make a significant change in CoG. Also the 50 cal round had belt links and the 37mm gun did not.

Our resident ammo experts can confirm the number of rounds and total weight of each load, including links, so people can do their own calculations. Combined the CoG shift was significant.

 

[GregP]

How about if we make the spinner and cannon blast tube from 1/4" armor plate?

That puts mass as far forward as it can possibly go, and doesn't change mass when the ammunition is fired.

 

[Ivan1GFP]

Hmmm...per post #4383, the 50cal ammo was further aft than the 37mm ammo (image repeated below to save scrolling). While I can accept that the 50cal ammo may have weighed more than the 37mm ammo (I don't know....but accepting that at face value for now), the moment arm for the 37mm ammo is smack in the middle between the 2 containers of 50cal ammo so I'm not sure about the "a lot worse" statement. Regardless of the weapon configurations, the P-39 CoG suffered from ammo expenditure...which isn't a good thing.

View attachment 825285

I am sorry, you are absolutely correct. The moment arm for the .50 cal ammunition is shorter but the ammunition is much heavier.
See attached Weight and Balance diagram. I believe it comes from the P-39Q but would not differ for the other models except that the weight of the .50 cal ammunition would be slightly different earlier in the war.

- Ivan.

 

[Ivan1GFP]

How about if we make the spinner and cannon blast tube from 1/4" armor plate?

That puts mass as far forward as it can possibly go, and doesn't change mass when the ammunition is fired.

We could also fill the tail with a bag containing hydrogen so it would weigh less.....

 

[Frog]

We could also fill the tail with a bag containing hydrogen so it would weigh less.....

It should be a fire resistant, armored bag, because the content is, well, hydrogen...

 

[SaparotRob]

Could we substitute Lowdrogen?

 

[Shortround6]

Legumrogen?
Bag is refilled in flight by tube from pilots posterior after a Legume rich diet.

 

[NevadaK]

You all are missing the most obvious solution. Let's get rid of the pilot. The plane's greatness should be enough on its own.

 

[Greg Boeser]

We could also fill the tail with a bag containing hydrogen so it would weigh less.....

This is America. We've got helium.

 

[Greg Boeser]

This is America. We've got helium.

Our helium embargo on Nazi Germany was the root cause of the Hindenburg disaster. The airship was designed to use helium.

 

[GregP]

It should be a fire resistant, armored bag, because the content is, well, hydrogen...

Just use the non-flammable hydrogen ... you can control the gas flamability with your cell phone and a suitable ap.

 

[yulzari]

Trained Walrusses? Or wild ones?

The Germans preferred Sealions.
I'll let myself out.

 

[yulzari]

So, that's where Buffalo wings come from.

When the Europeans arrived on the Plains they found them well wooded and the Buffalo's used to roost in the trees which made them hard to hunt and a falling hazard. Whilst the selective breeding programme slowly reduced the wings it was taking too long so the answer was to clear the forest of the Plains which are bare to this day. This aided the breeding programme by exposing the ungainly winged Buffalo's to more predation. So today the Plains are open and the Buffalo's have lost their wings but we recall their ancient spreading wings in our restaurants.

Always willing to educate and explain the world.

 

[ThomasP]

 

[GrauGeist]

When the Europeans arrived on the Plains they found them well wooded and the Buffalo's used to roost in the trees which made them hard to hunt and a falling hazard.

At least they weren't as troublesome as Australia's Drop Bears.