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and the aircraft assumes a constant forward velocity, the compass will return to it’s original East or West reading.
In like manner, deceleration of he aircraft causes an erroneous swing to a southerly reading of approximately 30
degrees at the same Latitude.. MEMORY AID: A N D S - Accelerate North, Decelerate South
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Magnetic Compass
http://www.uncletom2000.com/gs/compass.htm (4 of 4) [1/23/2003 11:18:52 AM]
The Engine
Engine
Most light aircraft use a conventional 4 stroke engine. It operates similarly to an automobile engine.
There may be 4, 6, or 8 pistons in the engine. The four strokes are:
1. Intake
2. Compression
3. Power
4. Exhaust
· Intake Stroke - The piston goes downward during the intake stroke. A valve called the Intake Valve is
open, such that an air/fuel mixture enters the cylinder through the carburetor. During this operation a
second valve, the Exhaust Valve closed.
· Compression Stroke - During this stroke, the piston is forced upward by the crankshaft. Both the Intake
and Exhaust valves are closed. Consequently the air/fuel mixture in the closed cylinder is compressed by
the upward movementof the piston.
·
Power Stroke - As the piston nears the top of the cylinder, the spark plugs fire under control of the
magnetos. There are 2 spark plugs in each cylinder, with a separate magneto supplying the electrical
spark current to each plug. The spark ignites the fuel/air mixture, causing an explosion to occur in the
cylinder. This forces the cylinder downward, and imparts power to the crankshaft. While one cylinder is
performing the power stroke, other cylinders are in some phase of the other three strokes. Therefore the
power stroke is the only one contributing to propulsion of the aircraft.
·
Exhaust Stroke - When the piston reaches the bottom of the power stroke, the Exhaust Valve opens. The
piston is pushed upward by the crankshaft, causing the burned fuel / air mixture to be purged from the
cylinder. The exhaust valve closes, and the piston is now in a position for another intake stroke.
The pistons connect to the crankshaft through connecting rods. They attach the piston, which has an updown
motion, to the crankshaft which turns in a rotary motion. The crankshaft is usually directly
connected to the propeller. In some aircraft, a gear arrangement connects the propeller to the crankshaft.
The crankshaft may also drive auxiliary devices such as the Magnetos, Vacuum Pumps, Alternator and
other devices. The connection may be directly or through lleys, belts and gears.
The ignition system is comprised of the magnetos and spark plugs, and is independent of the electrical
system. Even if the alternator and battery are inoperable, the ignition system continues to function. If
there is insufficient battery power to crank the engine, the engine can be started on most small aircraft by
http://www.uncletom2000.com/gs/engine.htm (1 of 2) [1/23/2003 11:18:53 AM]
“hand propping”. This is a procedure wherein the propeller is turned swiftly by hand to get the magneto
system to fire, and start the engine. It is similar to pulling the starter cord on a lawn mower.
EXTREME CAUTION MUST BE EXERCISED WHEN HAND PROPPING AN ENGINE.
A meter within the cockpit called the Tachometer indicates the engine Revolutions per Minute (RPM).
Monitoring devices such as the Oil Pressure and Oil Temperature gauge in the cockpit may be attached
to the engine.
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The Engine
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Fuel System
Fuel System
Most modern aircraft are equipped with 2 or more fuel tanks (or cells). In high wing aircraft, the cells are
housed in the wings. Since they are higher than the engine, the fuel flows down to the engine by the force
of gravity. A typical high wing system is shown at right.
On low wing aircraft fuel pumps are required. To initially get fuel to the engine for starting, an electrical
“boost pump” is turned on to pump fuel to the engine. After the engine is started, a mechanical fuel pump
driven by the engine feeds fuel to the engine. The electric boost pump can now be turned off.
Each fuel tank is equipped with a drain valve located at the lowest point in the tank. This drain allows the
pilot during preflight walk-around to check for and drain off any water which may have accumulated in
the fuel tank. There is usually another drain located at the lowest part of the fuel piping system. This
valve must also be drained during pre-flight to eliminate any water which may have accumulated in the
fuel lines. Associated with this drain is a fuel strainer which filters out foreign matter which may be in
　
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