external Fuel Tanks usage

[BitMaster]

Hey ;)

 

is it normal or a bug that when I fit my Mustang with ext. fuel tanks one of them seems to be empty ( engine quits ) when I switch from left to right or right to left ext. drop tank.

 

Usually, I fuel up the Mustang to 100% and get 2 drop tanks as well. It has always been that when I take of with int. left tank ( to get some volume for the back-flow ) and then switch to either one of the drop tanks after like 5-10 min climb out it then won't let me switch to the other drop tank as the engines quits when I do that. I have tried both drop tanks as 1. tank to empty and the other then seems to be empty when I switch over. AM I doing something wrong, is this normal and I don't just understand how they work or is this a bug ?

 

I usually try to switch between drop tanks every 15min to somehow keep the plane balanced but as described, that results in a stopping engine due to lack of fuel pressure ( empty tank ?).

 

Any info welcome

 

Bit

[BitMaster]

Did some testing:

 

When I empty all internals and fit only 2 drop tanks it works. Any other combination fails when you switch from one drop tank to the other. Fuel pressure goes to zero a minute after switching and the Merlin stops.

 

What am I doing wrong ?

 

Bit

[julian265]

Just checking - is your fuel booster pump on before you switch tanks?

[BitMaster]

Just checking - is your fuel booster pump on before you switch tanks?

 

 

Yes, I turn it on before I start the V12 and leave it on througout the flight.

 

Bit

[BitMaster]

It is so damn time consuming to really test this the proper way since the VA server restarts sooner or crashes before my fuel is empty :)

 

Still testing this issue..

 

Bit

[Merlin-27]

I think you may have found a bug with this. I tried it a few different ways. It appears that the first drop tank you use will act as empty if you switch to another drop tank and then back to the first. The second drop tank used can be used again after switching to any other fuel tank.

 

Also, as an FYI, the drop tanks are not affected by the booster pumps so its state will not matter with them. There are three booster pumps, one for each wing tank and the fuselage tank.

[PLP]

Out of curiosity, what kind of manifold pressure did you fly on the fuel tanks? I had the same problems, but just did a two-hour cruise (on 10x speed) on ext. tanks (with L-R internal tanks full, but main empty) while keeping manifold pressure in the green range.

[BitMaster]

Hey Merlin,

 

yes, this is exactly what I have found out. You can choose any of the drop tanks and it will work but refrain from switching them as you will get a dead stick scenario ;( Switching back to the initial drop tank works if you "hear" your engine stopping soon enough.

 

Funny thing, try starting your engine with ZERO internal fuel but 2 drop tanks and the effect is somehow gone but not completely. You can switch between the drop tanks then and it will work for quite some time but sooner or later, say ½ consumed in each drop tank the effect seems to come back.

 

You might still need to switch to internal left wing to get the Merlin roaring despite the gauge tells you ZERO fuel in it. I could only manage to start the Merlin once with the drop tanks but I think I had it flown a round before, landed and had the crew empty the internals. I could not reproduce a drop tank start-up with a fresh spawned AC last night and had to use left-wing tank for a few seconds to get it firing.

 

From what I read in the DCS P-51 manual, the Booster Pumps are only needed to avoid bubbling of the fuel in low air pressure at high altitude afair. They don't seem to be needed to actually feed the carb.

 

 

Anyway, at least on VA server I was never able to fly a fully fueled P-51 down to BINGO. The server usually reloads or crashed before but later on, when we accompany B-17's on their way to

Germany it is a must to use the drop tanks first(?).

 

 

Would actual combat pilots use up the rear fuselage tank first, or half way down, to have they plane in a more stable condition or would they switch to drop tanks soon after taking off.

This should be answered so we can handle it the proper way. I got used to fly the Stang fully fueled despite you really have to be gentle on the stick to not overdo it and end in a high speed stall. Trimming it is a pain when the rear fuselage tank is more than say 30-50% filled also.

 

Bit

[PLP]

from what I read they would bring the rear tank down to 25 gallons as soon as possible, then switch to ext.. The mustang was such a slouch with that rear tank full that no pilot wanted to risk an encounter in that situation.

[BitMaster]

PLP,

 

as soon as I have some time again I will test your MP advice and state the outcome here.

 

For the order of tanks to use, that is what I did too on my last flights:

 

Take off with left-wing tank, keep it emptying for about 5minutes, then switch to center fuselage tank until it is like down to 25-30% and then tried to use ext. tanks.

 

I think I have to fire up my Dell Desktop and host a mission that runs long enough so I can go through this all the way without being disturbed by server issues ;)

 

 

Bit

[PLP]

Yeah I just did two more test flights in singleplayer, and here are my results:

 

Takeoff left tank, main downto 25gals on the climb (full throttle), then cruise (~FL280) at no more than 40" manifold pressure -> fine.

 

Inflight start with L/R tanks full, center empty, cruise at no more than 40" manifold pressure -> fine.

 

Climb @ about 50" MP, & 50'MP maintained during cruise (fl~200) (and therefore tank switches) -> right ext tank no more fuel pressure

 

Level @ about 50-55" MP , maintained during cruise (low alt) -> no more pressure in from right ext. tank

 

 

I'm suspecting some kind of vapour lock is simulated if you ask too much pressure while you switch tanks.

 

My next test will be 50-55" MP, but reduced throttle for the fuel switch.

 

 

EDIT:

So this really seems to be it: keep Manifold pressure low enough when switching ext. tanks.

 

I did a flight were I cruised at high altidude, MP between 50-60", exept for the fuel switches, where I momentarily took MP down to the green range, and I was fine (was able to empty both tanks, as indicated by the fuel slider in alt ' ).

 

I did another flight where I just flew balls to the wall and switched ext. tanks, and sure enough after a couple of switches, no more fuel pressure from the right ext. tank

Edited March 8, 2014 by PLP

[PLP]

Bump.

Just came across this when reading a magazine, from a P51 crew chief:

[...]

Even though the tanks were pressurized, it was necessary to coax the fuel into the system during preflight. After the selector had been moved to the drop-tank position, the engine tended to die, and the selector was quickly put back to "main"; after a few of these switchbacks, the tanks would feed properly. The tanks were never brought home; [...]

 

Interesting, is this modelled? I doesn't seem to be exactly our case, but I'm not really sure.

[Merlin-27]

I did notice that the drop tanks don't require the fuel booster, which is accurate. I never noticed that specific issue quoted though.

[BitMaster]

Did a test yesterday, tho the server crashed 10min after switching to external tanks it didn't quit on me as before. This is how I did it, may be reproducible:

 

Take-Off left wing tank, fueled 100% + 2 drop tanks.

Use left wing tank for 5 min.

Switch to fuselage tank for climb out to 8500m ( what a pain to control the plane )

Stay high and use up all the fuselage gas at around 40-45 MP

Switch from Fuselage to left drop tank at 8500m and 40MP = engine stays alive

Kept on flying at 8500m +/- a few meters to have some fun until the server crashed 10min on the left drop tank.

 

VA Server, flying along the coast from far south to up north ;)

 

 

Bit