F-15 Fuel tanks usage question.

[Ozmandi]

Hello,  

I have a question about fuel usage when external fuel tanks are attached.

Does the fuel in the external tanks get used before the internal tanks?  

And, do I have to manually tell the plane which tanks to use?

 

I could not find that information in the manuals I have read.

 

Thanks,

Oz

Edited December 14, 2020 by Ozmandi

[GGTharos]

To be pedantic, the fuel from internals is used first, but the externals feed them to keep them full.  (And as a consequence of the fuel system, the externals won't feed while the gear is down IIRC, and most tanks won't feed when your refueling door open...so make sure you have enough in the feed tanks to refuel).

 

There is no way to tell the aircraft which tanks to use.  You could sort of do that in the real aircraft but AFAIK it's just not something that's done, unlike say the F5 where you have switches specifically for this sort of action.

[Ozmandi]

OK.

I asked that question because I'm having some stability problems on long flights.

 

I created a long mission, 50-60 min flight time.  Nothing going on just wanted a long flight to get an idea of how much fuel is used at high altitude verses low altitude.

My problem is that the further I get into the flight the harder the plane is to control.

When I start, I can maintain a level pattern and easily engage the auto pilot. 

But the longer the flight goes the harder it gets to maintain that control.  It gets so bad that I can't engage autopilot because I can't maintain a level flight pattern.  

I thought the plane was getting off-balance because I  wasn't using the pylon tanks, so that's why I asked.

 

Thanks for your answer.

Oz

 

[GGTharos]

Is your aircraft getting too slow, or is the AP bugging out?

[henshao]

Based on my observations, the F-15 autopilot seems to not be respecting deadzones

 

also, the F-15 has functional instantaneous fuel flow gauges and also tank quantity display selector if you're really curious about what is going on in the fuel system

 

though I am very curious what kind of instability you are running into and why. with the refueling door open some strange fuel tank behavior is possible

[Ozmandi]

I'll fly it again today to find out if I'm going too slow.  I think I was maintaining a speed around 350-400 knots but will verify that today.
 

Don't think my refueling door was open but will check it today.

 

The deadzone on my stick is set to 4.  I just bought it brand new last May so it shouldn't be wore out yet.

 

I'll let you know how my next test goes.

 

Thanks for your input!

[Ozmandi]

Wanted to give an update on the TRIM problem I was having.

This details are in this thread: F-15C trim issues. - F-15C for DCS World - ED Forums (eagle.ru)

 

 

Just a quick follow up question about this.

On 12/14/2020 at 11:40 AM, GGTharos said:

To be pedantic, the fuel from internals is used first, but the externals feed them to keep them full.

 

So at some point the external fuel tanks would become empty and therefore, I could jettison them to same on the planes aerodynamics? 

If that's the case, then I should keep checking the fuel gauges to find out when those tanks are empty?

[GGTharos]

You can jettison them at any time you want, but if you want to wait for the tanks to be empty then you want to see your totalizer dip under 13450, or alternatively if you do indeed want to monitor the externals, you can use shift-d to switch between different tanks.

[Vekkinho]

On 12/19/2020 at 5:16 PM, Ozmandi said:

I'll fly it again today to find out if I'm going too slow.  I think I was maintaining a speed around 350-400 knots but will verify that today.
 

Don't think my refueling door was open but will check it today.

 

The deadzone on my stick is set to 4.  I just bought it brand new last May so it shouldn't be wore out yet.

 

I'll let you know how my next test goes.

 

Thanks for your input!

 

 

Do you program your joy X and Y deadzones thru DCS W axis assign menu or by an external app? Try using external app, DCS-World tends to mess up input profiles for no reason.

 

Also make sure to disable USB standby option located in USB Power management section of Your Device manager.

 

 

 

[-Vindicator-]

If I have 3 bags full of fuel, is there a totalizer number at which I can jettison the externals and know I have maximum fuel in the internals and the remaining bag?

[GGTharos]

13450.

[-Vindicator-]

IIRC 13450 is either fully fueled without externals or close to it.  Internals fully fueled plus only a full central bag is more.  Am I correct that if I start with internals full and three full bags, then fly around awhile, there is no totalizer number at which I can jettison the externals and know that I then have full internals and a full central bag?  Thanks!

[GGTharos]

Sorry, I didn't read the whole thing.  All bags drain pretty much simultaneously, so as long as you're over 13450, a third of the excess is what's in the centerline.

Edited January 30, 2021 by GGTharos

[-Vindicator-]

Thank you!