P-51D Fuel Vapor Return Line and Fuselage Tank Vent Not Simulated/Modeled

[Magic Zach]

TL;DR first: The P-51 does not have it's fuel vapor line simulated in DCS.  This could have a noticeable affect on how much fuel is recaptured into the fuel tank, as it could be as much as 10 gallons an hour.  In close conjunction, the fuselage tank vent line (that the vapor return line feeds into) does not appear to be modeled, quite literally.
Track:
P-51 Fuel Vapor Return Check.trk
 

I'm going to split this into two different parts.  One for the fuel return line, and one for the fuselage tank vent.

THE FUEL RETURN LINE:
1) THE ISSUE
The DCS P-51 currently does not simulate fuel being returned to the fuel tanks from the carburetor.  You can check this yourself by loading the P-51 with full fuel, and run on the left tank until it is empty.  Then run and drain the fuselage tank next, followed by the right wing tank.  You can check if any fuel was fed into the left or the fuselage tank by checking the gauges, and setting the fuel tank selector to either the left for auxillary tanks.  You'll notice that no fuel will feed, the tanks are still empty.  This is what's included in my track replay, linked above.

2) THE INFORMATION
The fuel vapor return passes excess fuel from the carbureator back to a fuel tank.
It is widely touted that this excess fuel was routed back to the left wing tank.  However, starting halfway through P-51D-15 production, this line was redirected to instead feed into the fuselage tank behind the pilot, using the same opening as the fuselage tank vent line.
Photos:
The following two photos were taken from the Pilot Training Manual, AAF Manual 51-127-5, dated 15 August 1945, from page 20 and 22 respectively:

Spoiler

Above: notice how there is a return line drawn between the carburetor and the fuselage tank.  This return line actually passes on the left side, not the right, but it's a simplified drawing for pilots, not maintainers.
Below: Notice how it specifies that this fuel return line was changed from the left wing tank to the fuselage tank on "later airplanes".  Also notice that it states as much as 10 gallons per hour may be recovered.  It also says that this may be lost overboard, through the overflow pipe.  This is referring to the fuel tank vents installed for all three internal tanks.  More on that later...

Now here we have a maintenance manual (AN 01-60JE-2, 13-Feb-1948 Section IV, Para 14-15) stating the same thing, with some more detail.  In addition, a representative drawing showing the placement and path of the fuel lines:

Spoiler

Above: wanted to point out here the revision date. This edition of the manual has pages as recent as 1948, but this particular section was only revised in 1945.

Above: Notice the yellow line.  This is the vapor return line, to the fuselage tank.  Also take notice here of the blue vent line out of the fuselage tank.

Now, how is this dated to the P-51D-15?
If you look at the schematics for the Mustang, you'll see assemblies for building the pipes to link the carbureator to the fuselage tank.

Spoiler

Here you'll see this fuel vapor return line, returning fuel to the fuselage cell vent.  On the bottom left, it reads used on P-51D Airplanes AAF 44-15253 & Subs[equent] also on AAF 44-11953 and Subs[equent].  The P-51D serial number is a D-15 airplane, halfway through their production run.
 

Spoiler

North American P-51D/K Mustang (joebaugher.com)

So it's clear that when referring to the Californian Mustangs, it's specifically pointing out the middle of the P-51D-15 variants, when the vapor line change was implemented.  However in this source, it seems that some Dallas-built Mustangs serial numbers are excluded or missed.  I think we can assume though that the change occured similarly as in Cali, on the D-15s.

3) THE FIX
At a rate at 10 gallons per hour or less, fuel should be fed back into the fuselage tank.  For simplicity sake, it could be a generalized 7 gallons per hour, as DCS pilots will constantly change their engine settings between economic settings and military power, or even WEP.

THE FUSELAGE TANK VENT LINE:

1) THE ISSUE
In DCS, this fuel vent line is not in the 3D model at all

Spoiler

https://www.mustangsmustangs.com/p-51/image_viewer/survivors/44-74230/131089819/5151/5

2) THE INFORMATION
This was a tube that vented the fuselage tank, and gave air a place to enter the tank to stabilize the pressure as fuel was consumed.  Additionally, with the fuel vapor return tube directly connected to it at the point that the joint where the vent tube met the tank, this would be the pathway that excess fuel would be leaked overboard and outside the aircraft, in the event the fuselage tank is full.

Schematic drawings:

Spoiler

The vent tube assembly from a top view (left is forward, right is aft):

Vent tube assembly from the left (left is forward, right is aft):

The adapter where the vent tube met the auxillary tank and the fuel vapor return.  The connection for the vent hose being the larger pipe, and the fuel vapor return being the smaller, forward-pointing one.  Also notice how the fuel vapor return line didn't merely connect to the vent tube, but was piped through the vent connection and directly spitting it into the fuel tank:

Here we have the opening, where the vent tube meets the outside world.  It's somewhat bowl-shaped, and is located on the "right side of the fuselage and just below the insignia":

Exerpts from the E&M Manual:

Spoiler

"The vent outlets on the right side of the fuselage, immediately below the insignia."

It bears reposting this image again, since it included both the vent tube and the fuel vapor return:

Close-up:
|
One of the images from instructions on removing the tank:

3) THE FIX
The vent line isn't even represented visually in the cockpit, but it's in a very hard to see location so it's not that pressing.  However, it is also not even seen from the outside, specifically the outlet port, that was located at the bottom of the USAAF insignia on the right side.  It is an external identifying mark of Mustangs with the fuselage tank installed.  And in addition, it is also where fuel will vent outside the aircraft, in the event the fuselage tank is already full, and getting fed by the fuel return line from the carburator.





 

Edited November 27, 2022 by Magic Zach

[grafspee]

@Magic Zach Agree, i did test this issue a lot of times, flying 1-2 hours then switching to fuselage tank not even single drop of fuel found in every test i did.

I'm not sure what affects vapor return rate, but i guess that higher power setting will generate higher vapor return rate. More fuel goes in to carb = more vapor

I must say this, I've brought this issue on this forum but with no response from team sadly.  

Edited November 27, 2022 by grafspee