F100 Control System
The original F100-PW-100 control system consisted of the hydromechanical Unified Fuel Control (UFC) and the supervisory Electronic Engine Control (EEC). The UFC controlled all of the basic control functions for the engine including receiving the mechanical throttle input from the pilot, controlling fuel flow to the main combustor, opening and closing the core compressor start bleed, controlling the angle of the Rear Compressor Variable Vanes (RCVV) on the core compressor, controlling the fuel flow and zone sequencing of the 5 zone augmentor, and the base positioning of the exhaust nozzle convergent area (Aj). The EEC scheduled the vane angle of the fan Compressor Inlet Variable Vanes (CIVV), trimming fuel flow to the main combustor to meet core rotor speed (N2) and Fan Turbine Inlet Temperature (FTIT) targets, enabled the UFC Idle Area Reset schedule to open the exhaust nozzle at Idle when the landing gear handle was in the down position, and trimmed nozzle position at Mil power and above to control low rotor speed (N1) to a scheduled rpm.
Part 1 - Unified Fuel Control
The UFC throttle input was called Power Lever Angle (PLA) - 0 degrees was cutoff, 15 was Idle, 86 was Mil power, 91 was minimum zone 1 AB, and 130 degrees was zone 5 maximum AB. The UFC had an internal rate limited throttle shaft called Power Lever Angle Prime (PLAP). Regardless of PLA angle, PLAP was limited to 15 degrees until the starting engine had achieved Idle speed. Once at Idle, PLAP would follow the PLA request up to Mil power. When AB was selected, PLAP remained at Mil until N2 speed reached scheduled Mil speed, then would be released to 91 degrees to initiate AB and then sequence up to zone 5 as requested.
Inputs to the UFC were throttle position, a hydromechanical flyweight N2 speed sensor mounted on the main fuel pump, a helium filled capillary tube hydromechanical sensor that measured Fan discharge temperature (Tt2.5), main combustor burner pressure (Pb), and push – pull cable feedback for the RCVV angle and nozzle position. Main combustor fuel flow for starting and Idle to Mil power was scheduled based on N2 and Pb, biased by TT2.5. Start bleed closure was based on N2 reaching near Idle speeds, RCVV vane angle was scheduled by N2 and Tt2.5 which are the parameters needed to calculate corrected compressor rotor speed. When trimming the engine, there were adjustment screws on the bottom of the UFC to set EEC Off Idle N2 speed, Mil power N2 speed, and RCVV position, all based on lookup tables in the engine Technical Order (T.O.) manual.
The F100 had 5 zones of afterburner. The sprayrings are oval in cross section, with many holes pointing inward across the exhaust flow, with a pointed rod secured to the opposite side of the tube and protruding thru the hole. This pintel rod mostly closed off the opening in the sprayring. When fuel pressure increased inside the sprayring, the oval tube would flex toward round, drawing the pintel rod away from the hole, creating a variable area spray port, small at low flow and bigger at high flow, maintaining a good spray pattern over a wide range of fuel flows. Each pintel rod was threaded at its base, and the spray patterns of each pintel a sprayring was manually adjusted at depot and then brazed in place to maintain that flow. Zone 1 sprayring discharges into the circular gutter area of the flameholder in the core airflow discharge and keeping this area burning is needed to keep all of the other zones flame lit. Zone 2 was just outboard of Zone 1, still in the core flow. Zone 3 was the first sprayring in the fan duct flow, then Zone 4 (three rings) in the inner area of the core flow, and Zone 5 was the outermost ring in the fan duct flow.
The UFC had just one AB fuel flow metering valve, with each zone having an On-Off valve, and there was a splitter valve that changed the balance of flow going to the Core sprayrings (Zones 1, 2, & 4) and Fan duct sprayring (Zone 3 & 5), biased by Tt2.5. The volume of the sprayrings dictated the use of a Quick Fill system that would fill each sprayring with fuel, and then based on increasing fuel back pressure, would then connect the sprayring to the metered flow. If this Quick Fill system did not work correctly, all of the metered flow would divert to the empty sprayring and starve Zone 1, putting the AB flame out. When PLA was advanced into AB, PLAP would advance to 91 degrees and hold at minimum zone 1, Quick Fill and metered fuel flow would enter the sprayring and AB ignition would be turned on for approximately 1-2 seconds. With the AB assumed to be lit, PLAP would ramp up to Maximum Zone 1 and hold, then Zone 2 Quick Fill turned on. When Zone 2 indicated full, Quick Fill is turned off and the metered AB flow would then be divided between Zone 1 and 2. This clockwork process repeated with each Zone requested until Max Zone 5 fuel flow was achieved. The AB fuel flow was scheduled by the UFC metering valve based on Pb and Tt2.5 inputs, and there was a trim screw that adjusted the Max AB fuel flow based on T.O. lookup tables.