forward center of gravity limit (aircraft weight and balance). The most forward location on an aircraft allowed for its loaded center of gravity.
forward current (semiconductor characteristic). Current that flows across the junction in a semiconductor device when it is forward-biased.
forward-fan engine (aircraft gas turbine engine). A turbofan engine with some of its low-pressure compressor blades extended. These extended blades act as a fan and force air around the outside of the gas-generator portion of the engine. Most modern turbofan engines are forward-fan engines.
forward voltage drop (semiconductor characteristic). The voltage drop across a semiconductor device when forward current is flowing through it.
forward wing. 14 CFR Part 1: “A forward lifting surface of a canard configuration or tandem-wing configuration airplane. The surface may be a fixed, movable, or variable geometry surface, with or without control surfaces.”
fouled spark plug (malfunctioning ignition system component). A spark plug that is unable to fire because of contamination in its firing-end cavity. Oil, soot, silicon deposits, or lead deposits can build up in the firing end until they provide a bridge that allows the voltage to leak off before it can build up high enough to jump across the gap between the electrodes.
four-stroke cycle. A constant-volume cycle of energy transformation that has separate strokes for intake, compression, power, and exhaust. See Otto cycle.
four-stroke-cycle reciprocating engine. A form of reciprocating engine which has an intake and an exhaust valve in each cylinder. Four strokes of the piston (two in and two out) are needed to change chemical energy in the fuel into mechanical energy at the rotating crankshaft.
During the intake stroke, the piston moves inward, with the intake valve open and the exhaust valve closed. A mixture of fuel and air is pulled into the cylinder through the carburetor.
On the compression stroke, the piston moves outward with both valves closed. This compresses the fuel-air mixture. Near the end of the compression stroke, the spark plug fires and ignites the compressed fuel-air mixture.
The burning fuel heats and expands the air inside the cylinder, and the expanding air forces the piston inward on the power stroke. Near the end of the power stroke, the exhaust valve opens.