Recently there were some comments (not unkind) about the fuel system on 750TL Lancair's first turbine powered LIV.and I hope that a little explanation about the belly tank might be interesting.

First a minor correction; the wings carry about 55 gals. each ( one inch below cap ) and the belly tank about 10 gals. A larger belly tank may be possible but probably not  as much as 40 gals.

 Our reasoning with regard the fuel system went like this;

From the perspective of function a belly tank was very desirable, a sump lower than the main fuel supply, easy to control  and plumb and simple in design.

The fact that it is on the bottom of the plane is a characteristic it shares with the wings although lower and under the passenger compartment.

Some past accidents involving fire show that no matter where fuel is carried in the airframe, it can be a hazard in an accident.

So if the wings will not hold any more, and more is desirable, where to put it.

Is a tank inside the fuselage better than a belly tank, we didn't think so.

External wing tip tanks or pods under the wings aside from the building complexity, come with a whole array of direct and indirect consequences.

We decided that a properly designed belly tank was the best of the alternatives.

So what is "properly designed".

At first we considered a quick dump valve, and then questioned what "quick" and "dump" should mean. Quick probably would not be all that fast and dump would not really be empty, given the shape of the tank. Also consider that when you dump belly tank fuel, the engine stops. In the most likely scenario, a wheels up landing, are you going to remember to dump fuel and forget to put the gear down?

There just didn't seem to be a logical reason for a dump valve.

In the end we designed a tank that materially reinforced the fuselage floor and is crush resistant. If you should unfortunately  land on the tank ( no wheels) the tank construction is designed not to rupture, thereby spreading fuel over a wide area, and if the plane skids for some distance, the tank is made so that it probably won't wear though, and if it does it might do so only in small  areas.

Another area concerns the vent system for this tank.

Because of the relative locations of the fuel pick-up pipe in the belly tank and the wing fuel ports it is desirable to not have the tank vented to atmosphere, when there is very low fuel and the plane is at a high angle of attack, but it is necessary that the tank be vented during refueling. If it is not, fuel will not displace the air and the tank will not fill correctly. There is more than one way of doing this, the way we chose was a normally closed solenoid valve. The only requirement was that the switch had to be "on" during refueling. This proved to be a problem, ( remembering to turn it "on").

We now have a normally open solenoid which closes when the master is turned on with an over-ride switch to open the valve with master on. 

 We believe we have a safe system, remembering, that "safe" is a relative term, and that is nearly impossible to design anything that will protect against all of the "what if" scenarios.

Having said that we are always open to better ideas.     

Ross Connell