> So, this morning, I tried doing some range testing, and found that the MD11
> prototype can now fly a distance of about 4500 miles. This may seem a bit
> short, but I discovered that the (aeromatic generated) JSBSim configuration
> file has four fuel tanks, but the engines draw fuel from only three tanks.
> Is there a way to fix this, so that the engines draw fuel from all four
> tanks?

Aeromatic by default creates one more tank than there are engines, so a 
3-engine airplane gets four tanks.  You can always edit the propulsion 
section to add or remove tanks, if you wish.  Any idea how many tanks a real 
MD-11 has?  My guess would be 3 or 5.

The best fix for the MD-11 config would be to remove one of the tanks, then 
distribute the remaining 3 symmetrically (one in the middle and one in each 
wing).  Then increase the capacities of the 3 tanks so that they have the 
same amount of fuel as the 4 formerly had.

In any case, if you want to set an engine up to feed from more than one tank 
just add another FEED item below the current FEED item.  The 737 config does 
this, and it causes both engines to feed from their respective tanks and the 
center tank simultaneously.  When the center runs dry they will feed only 
from their respective tanks.  This isn't quite how it works in the 737, but 
it's pretty close.  To model the fuel schedule exactly we need to set tank 
"priorities" for each engine, which is something we've talked about at JSBSim 
but haven't gotten around to yet.  

> Also, I was wondering whether or not I can  modify the TSCF and bypass
> ratio values in the JSBSim turbine engine files, without introducing any
> unwanted side effects.

The TSFC value will only effect fuel economy in the turbine model, and won't 
bother anything else. 

Bypass ratio is only used as a substitute for the rotational inertia, so that 
the acceleration rate of the engine can be estimated from it.  The fuel 
efficiency gains due to bypass are already contained in the TSFC value, so 
there was no need to consider an additional bypass effect on efficiency. 

> If I can get the sim to draw fuel from the fourth tank, we get a range of
> about 6000-6500 miles. Interestingly enough, that's pretty close to the
> published figure of about 6460 nautical miles that the original
> (non-improved) GE powered MD11's could do. :-)

Aeromatic got lucky.  

David Culp

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