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This section addresses what was done to maintain the L39's inverted fuel system when installing the Garrett TFE731 motor. Please keep in mind that the 731 is not rated for negative G operation in its certified business jet configuration.
Inverted fuel - The original L39 inverted fuel uses a bladder that has a rubber diaphragm where fuel is pumped in on one side by the fuel boost pump and bleed air is supplied on the other by the last compressor stage via a regulator (same regulator that provides bleed air pressure to transfer fuel from the wing tip tanks). We wanted to limit bleed air access to fuel so we removed the bleed air source and replaced it with a light weight aluminum tank to act as an Air Spring. When the bladder is completely empty of fuel, pressure in the tank is set at four psi. When the boost pump fills the bladder (approximately 2 3/4 gallons), pressure in the tank climbs to just over 15 psi. The system is sealed and does not require service unless something is leaking. Checking spring tank pressure is a standard pre-flight procedure. Here are links to both bladder filling and draining.
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