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Simple question about drop tanks


gospadin

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For the C-Model there are 150-liter external fuel tanks available.

 

For the ZA-Model there are the larger 350-liter external tanks as well as the smaller 150-liter tanks available.

 

It is not possible to influence the fuel transfer sequence.


Edited by Lino_Germany

Kind regards,

 

Lino_Germany

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So per the flight manual, it says the following:

 

primary tanks = 1100L (825kg)

wingtip tanks = 200L (150kg additional, total of 975kg)

 

However, if I fuel myself to 50% on the ground, the gauge reads 525kg. This is neither half of 825 nor half of 975. Is this just a general inaccuracy in the gauge, or a bug?

 

75% reads 625kg, wingtip tanks empty (light green) ... off by a bunch, i'd expect gauge to read 7xx something.

50% reads 525kg, wingtip tanks empty (light green) ... off by 35kg?

25% reads 260kg, wingtip tanks empty (light green) ... off by 15kg

 

7492 pounds empty / 8032 pounds at 25% overall, is 540lbs of fuel (25%) ... 245kg, which should be my increment per 25% and is roughly calibrated to the 975kg capacity with wing tanks.

 

9652 - 7492 pounds = 2160lbs = 980kg from the loading screen.

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L-39C Technical Manual:

 

The system is set to provide indications as a function to a sample fuel with approximately 777 kg/m3 density (1,060 liters </= 0.777 = 824 kg= 1,816 lbs). The Jet A-1 fuel density of 15°C can vary from 775 to 840 kg/m3.

 

When the indicator reads zero, there is approximately 30 kg (70 lbs) of residual fuel in the fuselage tanks.

 

I don´t know if the fuel system is simulated in that depth, but you can get an idea that there is a great scope reading the fuel indicator.

Kind regards,

 

Lino_Germany

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Okay, did some more tests, and it behaves exactly as the manual says it should, i was just confused.

 

Below 61% or so fuel, all fuel is taken internally. Gauge is linear relationship to percentage full.

From 62% through about 83% or so, additional fuel goes into wing tanks due to weight balance limits.

84% and above goes into remaining space in internal tanks.

 

Gauge reads purely fuselage tanks. Drain order is in reverse, as long as wing pump circuit breaker is enabled.

 

If your fuel reads 625kg-630kg, and the wing tank light is off, then you're consuming from the wingtip tanks and you have no way to tell how much of them is left, except that you'll burn that 100L in 10-15 minutes.

 

Quick plot if anyone cares:

 

1CdiQvf.png

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If I understand that right...

When I put Drop Tanks on my Plane and fly the Drop Tanks will be drained first, then the Wing-Tip Tanks and then the Fuselage Tanks? Do I need the Wing-Tip Tank Pumps to be ON for the Drop Tanks to drain?

Modules: Well... all of 'em

 

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If I understand that right...

When I put Drop Tanks on my Plane and fly the Drop Tanks will be drained first, then the Wing-Tip Tanks and then the Fuselage Tanks? Do I need the Wing-Tip Tank Pumps to be ON for the Drop Tanks to drain?

 

None of my testing was with drop tanks.

 

Per the manual:

 

FUEL USE ORDER

Order of fuel use has to keep airplane’s center of gravity within specified operating limits. When

fully refueled (1300 liters) fuel is initially consumed from the fuselage’s tanks. When 575-625 kg

remains in the fuselage’s tanks, this can be monitored on fuel gauge; fuel is consumed from tip

tanks. It takes 15 minutes to use all fuel from tip tanks. Fuel gauge shows total remaining fuel in

kilograms in fuselage fuel tanks.

This gauge designed for measuring fuel amount and signalizing reserve amount fuel.

 

I believe that if you have drop tanks attached, they'll be used first, independent of wing pump state.

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There are several internal fuel bladders in the L-39... feed tank, saddle tanks, inverted tank. To fuel one involves much sloshing and glug-glug-glugging and waiting for fuel to find all the places to go.

 

There are no fuel pumps other than the main pump from the feed tank. Fuel from the drop tanks and tips are pushed to the feed tank via bleed air. At lower engine RPM there is not enough pressure to push external fuel in so on the ground you are always burning from internal tanks.

 

First the L-39 burns fuel from the feed tank, when some of that is gone it starts allowing fuel from the other tanks. This is done with a float in the feed tank. What you will see is the fuel gauge drop to ~700Kg and then it will stop dropping... what is happening is you are now on external fuel. It will start dropping again after all the external fuel has been pushed to the feed tank.

 

The system is ingeniously simple... push bleed air to the drop tanks, then the output of the drop tanks goes to the tip tanks, and back to the internal feed tank. So drop tank fuel goes out to the tips before coming back to the plane. When the drops are empty the air goes to the tips. Also when there are no drop tanks. If you take off with fuel in the drop tanks but not the tip tanks you can actually land with full tip tanks.

 

When you are on the ground with low RPM there is not enough pressure to activate the drop tank pressure sender and so it thinks the drops are empty. Low pressure = it thinks air is pushing on the sender not fuel. This is why you have a switch to turn off the drop tank light. You only turn it on when you have drop tanks.

 

PS. It's almost impossible to get this airplane out of CG. The front pilot can eject and the airplane will still be in CG. :-)


Edited by jonboede
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  • 1 year later...

As I didn't find any other thread related to the wing-tip tanks light (caution and advisory lights panel), I'd like to ask why the light is ON (label: БАКИ) with the fuel gauge showing 800+ kg?

 

9rN53p5.jpg

 

(tested in DCS v1.5.6, L-39C)

 

As I know the light may illuminate due to insufficient bleed air pressure, I put the wheel chocks and pushed the RPM above 100% during the tests.

The light only goes OFF if the fuel quantity is set above 83-84%.

 

Unless I didn't understand the sequence and behavior correctly, I'd expect:

 

- to see the needle stop around 625-680 kg for about 15 minutes while the fuel from the wing-tip tanks is fed to the engine and delivery tank

- then the light would show up with the needle still around 625-680 kg

- and then the fuel gauge would show the rest of the fuel being progressively consumed

 

I can't see if it's a bug or a feature. Please help!

What did I get wrong?


Edited by Bourrinopathe
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The light only goes OFF if the fuel quantity is set above 83-84%.

 

If you set the fuel to 84%, you fill the internal tanks while leaving the wing tips empty. In that case, you can turn off the warning on the right hand circuit breaker.

 

Importnant thing to realize is that the fuel gauge shows how much is in the internal tank, not the entire plane. So it'll show you 800kg of fuel even if you have 800kg internal and another 160kg in wing tips. And if you take away the fuel from wing tips, it's still 800kg internal.

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I didn't get any drop tank for the tests. ;)

 

If you set the fuel to 84%, you fill the internal tanks while leaving the wing tips empty. In that case, you can turn off the warning on the right hand circuit breaker.

 

Oh ok thanks.

 

Your comment made me understand what's happening. (I just ran a quick test in the sim for confirmation)

 

If the aircraft is spawned with the default configuration (84% fuel), the wing-tips tanks are empty.

 

I thought the wing-tip tanks would be at full capacity as long as you have more than 625-680 kg + the wing-tip tanks quantity (roughly 160 kg) (in order to match the fuel consumption sequence) (around 625-680 on the gauge but close to 785-840 kg in total).

 

(with 100% fuel, the wing-tip tanks light is OFF until the gauge shows around 600 kg (in my mission) for a few minutes - then the light comes on and the needle starts moving again)

 

I didn't read the whole manual yet, so maybe that's how the L-39 (and other aircraft?) is/are refueled in general (if the fuel quantity is inferior to the maximum capacity/value)?

Or is there some kind of discrepancy?

 

At least, it wasn't intuitive for me. :doh:

Thanks for your help!


Edited by Bourrinopathe

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  • 1 month later...

I thought I understood the wing tips tanks system but apparently I am missing something.

 

I set my fuel at 100% at the start (no external tanks).

I set the wingtip tanks switch to off. In that way I thought fuel will be burnt only from the main (fusolage) tank. And when I set the switch on, then it drains from the wingtips.

 

However in a couple of situtations when the fuel gauge was about 50%, when I switch the wing tips tanks, the light green goes up. Indicating they are empty.

 

Wasn't it supposed to be not consumed from the wing tips?

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The switch is there only to control the light. Fuel from wingtip tanks will always be consumed.

 

Mistery solved thank you.

Well I was hoping it was a bit more into "system management" rather than just switch the light off. But I guess that's how it was designed

| A-10C | MiG-21bis | Hawk T1.A | L-39 Albatros | F-5E | Ka-50 | Mi-8 | NTTR | CA | SU27 | M2000C | F-86F | AV-8B | F/A-18C | Mig 15 | Mig 19|

Specs

 

Intel i7-9700k

msi GTX 2060 Gaming Z

msi Z390 Gaming PLUS

16gb RAM

Hotas Warthog

 

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