Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (3 Viewers)

Shortround6 · May 18, 2021

Clayton Magnet · May 18, 2021

Shortround6 said:
Blanks are not going to cycle a gun like live ammo. In fact you may need a restrictor in the bore to get full automatic functioning?

On recoil operated gun having no bullet certainly affects the recoil. They can play games with the powder and other things.

I am assuming they are not M2's, but some sort of gas operated alternative. Or a very light and modified barrel, light enough to cycle the gun with only gases leaving the barrel

GrauGeist · May 18, 2021

Without the resistance of the bullet being driven down the barrel by the expanding gasses, the cyclic rate of the weapon will be affected.
The barrel restrictor helps to maintain a certain amount of gas pressure needed to properly work the MG's firing cycle.

*edit* just saw SR's post with the link - that should have better info than my post

ThomasP · May 18, 2021

From Airborne Radio Equipment Handbook 1943 edition SC3858A:

GrauGeist · May 18, 2021

Those Dynamotors are cool - GE and Motorola were still using them on automotive two-way radios as late as the late 80's.

FLYBOYJ · May 18, 2021

ThomasP said:
From Airborne Radio Equipment Handbook 1943 edition SC3858A:

View attachment 623864

Very cool, this answers a lot of my questions - but there still seems to be a later installation on the shelf behind the pilot, this is shown in the P-39Q flight manual.

Schweik · May 18, 2021

In this video, you can see the size of the shell casings. They look like .50s to me. I've shot M2s before.

Compare this to a pyrotechnic replica .50 doing simulated shooting via propane and O2

GrauGeist · May 19, 2021

Quite a few restored warbirds have original weapons installed and are functional (where allowed by law).
Their blanks will be actual casings with fully functional primers (electric in the case of German weapons) with a gunpowder load appropiate for blank firing topped off by a wad and are loaded into the belts just as if it were live ordnance.
The weapon functions the same as if it were live: the blank round is loaded from the belt, the blank discharges and the shell is ejected and the next round is loaded into the chamber from the belt during the cycle.

It looks realistic because it is - the only difference is that the aircraft isn't hurling several hundred rounds downrange. The "bang", smoke and cascade of casings is genuine.

P-39 Expert · May 19, 2021

Milosh said:
But, did Expert use it?

I had saved the chart for future use, but it would no longer let me make any adjustments. Said my excel subscription had expired, which I never had. Great chart and fun to play with, wish the radio could be included.

As I remember it, as the nose ammunition was expended the CG moved back but never got past the aft limit.

P-39 Expert · May 19, 2021

FLYBOYJ said:
Well after some research, I think I figured this out.

The SC-535 is the IFF radio. It was removed by the Soviets as you say. So we'll remove that from discussion for now...

The Radios in the P-39 - An SCR 274 or an SCR-522.

The SCR 274 was a 5 unit system, 3 receivers and 2 transmitters. It would be obvious you need to keep the transmitter away from RF interference, so most of the time you would have a transmitters mounted as far away as possible, so that's why it was initially shoved in the empennage with the control head in the cockpit. This system was sometimes contained in one metal box (US Installations) or had separate unit installations (RAF).

This is what the installation looked like in the P-51B. Although there is more equipment in this aircraft, the basic units are shown.

View attachment 623840

Now it seems the British and Australians used the same system but those units were not contained in one box and they seemed to fit in the area behind the pilot.

View attachment 623842

Why did the Aussies install their system this way? To move the C/G forward? It would seem you would have heat issues. I seen photos and cut-aways of the P-39 D with nothing in the deck above the engine but the "boxed" SCR-274 unit was firmly in the empennage.

View attachment 623853

Here's early P-39s with the RAF - no radio behind the pilot.

View attachment 623845

And a P-39N

View attachment 623846

Unless you use the same set-up was the Aussies and "unbox" the system, I don't think it was going to fit behind the cockpit.

The SCR 522 was also a 2 system unit but was eventually combined into one box. It replaced the SCR 274. It also came with a separate "dynamotor" or power supply. The SCR 522 was 16 x 12 x 10 and weighed 49 pounds. The dynamotor was 12 x 8 x 6 and weighed 37 pounds. The control head was about 2 pounds and was in the cockpit.

View attachment 623847

Now it seems on some P-39Qs part or all of this installation was squeezed into the deck behind the pilot.

View attachment 623849
This P-39Q was from the 4th Reconnaissance Squadron.

View attachment 623850

View attachment 623852

P-39Q of the 82d Reconnaissance Squadron (Fighter), 71st Reconnaissance Group, New Guinea 28 May 1944, they seemed to have this installation but was it all radios or photo equipment?

Now with along with this installation, was the IFF removed from US aircraft? I would say not if they were in theater.

So at the end of the day - are you going to be able to move the radios? If you have the "boxed" SCR 274 US set up - no.

If you use the RAAF SCR 274 set up - yes.

If you use the SCR-522? It looks like it was factory or field installed but were all the components installed on the deck or within the radio compartment in the empennage?

So I'll somewhat stand to be corrected.

Excellent work, and very thorough. Just a note, the radio location issue is only for the 1942 P-39-D/F/K/L models with the lower powered V-1710-35/63 engines to save weight. The 1943 N/Q models had good performance at normal gross weight, no reduction was needed. Although the Soviets did delete the wing guns and IFF radio on most of their P-39s.

FLYBOYJ · May 19, 2021

P-39 Expert said:
Excellent work, and very thorough. Just a note, the radio location issue is only for the 1942 P-39-D/F/K/L models with the lower powered V-1710-35/63 engines to save weight. The 1943 N/Q models had good performance at normal gross weight, no reduction was needed. Although the Soviets did delete the wing guns and IFF radio on most of their P-39s.

Thank you - A couple of things

If you get the station of where the radio compartment is located you can calculate the removal of the IFF unit. BTW, you were confusing me calling this area a "tail cone." It's actually an aft fuselage or empennage.

I believe on US aircraft, one of the reasons why you had all or part of the radio unit moved to the shelf behind the pilot was because of the IFF installation. There was only so much room in the radio compartment so the remainder of the comm system had to go somewhere. I believe the area behind the pilot was initially ignored for radio installations were because of RF and heat issues, but that's my opinion.

FLYBOYJ · May 19, 2021

P-39 Expert said:
I had saved the chart for future use, but it would no longer let me make any adjustments. Said my excel subscription had expired, which I never had. Great chart and fun to play with, wish the radio could be included.

As I remember it, as the nose ammunition was expended the CG moved back but never got past the aft limit.

In it's original configuration yes.

Remove the nose armor, expend all the nose ammo and allow fuel to go below 1/4 tank - No

P-39 Expert · May 19, 2021

FLYBOYJ said:
In it's original configuration yes.

Remove the nose armor, expend all the nose ammo and allow fuel to go below 1/4 tank - No

pbehn · May 19, 2021

If you take out the nose armour could you put some lead in to balance things? Or am I over thinking this?

P-39 Expert · May 19, 2021

I worked Greg's CG chart again and deleted the nose armor, nose cannon and MG ammunition, wing guns and wing ammunition and the CG was one half inch past the aft limit before any adjustment for the radio in the aft fuselage. Was not able to delete the fuel but it is located on the CG. For some reason was not able to attach the graph to this post.

P-39 Expert · May 19, 2021

pbehn said:
If you take out the nose armour could you put some lead in to balance things? Or am I over thinking this?

Purpose of deleting the nose armor is to save weight.

pbehn · May 19, 2021

P-39 Expert said:
Purpose of deleting the nose armor is to save weight.

I thought I was in too deep.

FLYBOYJ · May 19, 2021

pbehn said:
If you take out the nose armour could you put some lead in to balance things? Or am I over thinking this?

You are to a point - to make a difference you have to take weight out of the tail

GregP · May 19, 2021

Here's an accurate scale representation of the P-39Q and P-63A.

The P-39Q is 30" 2" long and the P-63A is 32" 8" long. In the above pic, I lined up the front of the exhaust manifolds since they are bolted to the engine. I went to the aft part of the air intake. Note the length added to the P-63 is aft of the windscreen.

As it happens, we have a P-63 in the final stages of work just before first engine run. We also have a P-39, complete with external engine and nose case. I volunteer every week on Tuesday, so I was there yesterday, but I was working on parts for another aircraft. Next week, I'll get measurements of the same areas shown above. Now, our P-63 is not using a 2-stage V-1710, but the engine compartment is stock. The engine we are using is a 100-series and makes about 1,600 hp at full MAP. Of course, it will very rarely see 1,600 hp and will likely be operate at the 1,200 - 1,400 hp max limit when it flies, and only at an airshow at that. Most of the time, it should see that limit only on takeoff and initial climbout.

Easy since we have the subjects right there. Talk with you then.

Cheers.

FLYBOYJ · May 19, 2021

P-39 Expert said:
I worked Greg's CG chart again and deleted the nose armor, nose cannon and MG ammunition, wing guns and wing ammunition and the CG was one half inch past the aft limit before any adjustment for the radio in the aft fuselage. Was not able to delete the fuel but it is located on the CG. For some reason was not able to attach the graph to this post.

22Gallons of fuel (1/4) tank puts it out of C/G

Now something to consider - This data was taken from the P-39Q flight manual. The manual did not show what radio (or IFF) was installed for weight and balance purposes, that would be specific to the aircraft and found in the maintenance records that follow the aircraft.

Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (3 Viewers)

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