investigating 100 Gallons external tanks

[Rainbowgirl]

Hello! I have noticed that there is no difference in fuel quantity between 50 gal and 100 gal external tanks. Both types take same amount of time to deplete before outer tanks get into play.. I checked flight manual- Fuel System section, Cruise Flight and Limitations section. Nothing mentioned there about it. Am I doing anything wrong, such as flying above certain altitude or is it just pending bug? Thank you for help.

Edited February 23, 2022 by Rainbowgirl

[NineLine]

Hi, how are you checking, are you waiting till the outer tanks are dry, and you have to switch the fuel transfer cock on? And then are you timing the fuel usage based on steady flight?

[Rainbowgirl]

On 4/14/2022 at 11:24 PM, NineLine said:

Hi, how are you checking, are you waiting till the outer tanks are dry, and you have to switch the fuel transfer cock on? And then are you timing the fuel usage based on steady flight?

Hi, Nineline. Sorry. haven`t noticed your reply from April 2022. I used in past Mossie for multi-hours cruising around the maps at constant engine settings - 2600rpm,  6-8 psi ] and constant altitude [ around 5000m ]. Was curios how long Mossie can stay in the air comparing to P-47 and P-51. That was how I found that there was no difference in elapsed time between 50 and 100 gal externals before fuel transfer cock had to be switched. 50 extra gals per engine at cruising settings should add +45 mins of flight endurance at higher altitude or so, but the time in air remained the same and so I wrote a post here. Thank you for starting an investigation. Much appreciated.

Edited March 6, 2023 by Rainbowgirl

[Jel]

Out of curiosity: You are draining the outer tanks at least a bit first before switching to external tanks ?

I noticed that when externals are selected too early, their transferring flow into already full outer wing tanks leads to excessive spilling of fuel  Which is kind of a neat feature.

[J13 Serenity]

Hi

I would like to add to this topic

From my tests there is a difference between 50 and 100 gallon drop tanks

But

The amount of time your can fly with them is weird

I tested it like that: 
Editor, channel map, default weather
Without drop tanks on outer wing tanks at the altitude of 2000m +16/3000RPM I would consistently fly for 25 minutes. My rough maths is 116/25=4.64 gal per minute
With 50gal drop tank fuel is enough for 37 minutes, which gives about 172 gallons of fuel?
With 100gal drop tanks it runs for 47 minutes - 218 gal of fuel

I did read the manual, where they suggest to empty outer wing tanks, switch to inner and wait for the transfer, and then use outer again
Well, I tried that, and there was no difference
After emptying the outer tanks, with 100gal drop tanks it barely transfers 50 gallons to each side
With 50 gal drop tanks - about 30 gal or so

It seems like there is a bug, and 50 gal tanks are actually about 25-30 and 100 gal tanks are actually 50

Or, maybe, my testing is wrong?

 

[Rainbowgirl]

On 3/20/2023 at 10:31 AM, Jel said:

Out of curiosity: You are draining the outer tanks at least a bit first before switching to external tanks ?

I noticed that when externals are selected too early, their transferring flow into already full outer wing tanks leads to excessive spilling of fuel  Which is kind of a neat feature.

No. Did not know that there could be any excessively spilling fuel. It actually does not make logic as  fuel leaks is serious thing which does not sound like a standard allowable operational practice. Not going to argue about it, though, since I am not  aircraft engineer.

Edited September 6, 2023 by Rainbowgirl

[Jel]

vor 20 Stunden schrieb Rainbowgirl:

No. Did not know that there could be any excessively spilling fuel. It actually does not make logic as  fuel leaks is serious thing which does not sound like a standard allowable operational practice. Not going to argue about it, though, since I am not  aircraft engineer.

 

It actually IS a thing on many airframes. Fuel tanks have a mechanic to handle/compensate for pressure and temperature changes by venting. When your fuel tanks are filled too far (or are constantly re-filled from your external tanks to the point where they just can´t hold any more), some fuel may get "vented" also

I first didn´t realize they simulated this.

Edited September 7, 2023 by Jel

[Rainbowgirl]

On 9/7/2023 at 6:48 AM, Jel said:

It actually IS a thing on many airframes. Fuel tanks have a mechanic to handle/compensate for pressure and temperature changes by venting. When your fuel tanks are filled too far (or are constantly re-filled from your external tanks to the point where they just can´t hold any more), some fuel may get "vented" also

I first didn´t realize they simulated this.

 

Thank you. Will test it one day following your advise.

[Bozon]

At some point, de Havilland made a modification that allowed a regulated continuous transfer of fuel from the DT to the outer wing tanks without venting fuel. There is a comment about this in the pilot notes.

DCS FB.VI is modeled without this modification, so once the outer tanks are full, excess fuel that is pushed into these tanks will be vented.

Edited September 13, 2023 by Bozon

[Rainbowgirl]

On 4/14/2022 at 11:24 PM, NineLine said:

Hi, how are you checking, are you waiting till the outer tanks are dry, and you have to switch the fuel transfer cock on? And then are you timing the fuel usage based on steady flight?

Hi, Nineline. This week I performed 2 more long distance flights after a long time. Following yours and others` advice, plus studying Chuck`s Flight manual for DCS [  summer 2023 update ].

1. Loaded 100% internal fuel plus 2x 100 galls external tanks.

2. In flight I depleted outer tanks almost dry  [ less than 5 galls per tank following Chuck`s Flight manual ].

3. Switched external tanks fuel transfer cock to ON position. Did not switch tank pressurization cock ON as I flew at 6 km altitude. Fuel boiling was not an issue. 

4. Outer tanks got sloooowly filled...... peaking at 30 galls per tank.....then staying there for about 5 minutes and then beginning to get depleted all the way to  empty. I was able to run on externals duration comparable to only 50 galls per engine which was about 45 mins. 100 gallons per engine would definitely need to last way longer than this at 2400 rpm, 7psi.

5. After dropping externals I switched external tanks fuel transfer cock to close position and continued for the rest of the flight on inner tanks.

 

This confirms my past concern that there definitely is something going on with 100 gals external fuel tanks. They don`t last 2x as long in flight as 50 galls tanks.

 

Edited January 21 by Rainbowgirl