Powerplant ASA/Baker's School List of FAA Oral Questions

Powerplant ASA/Baker's School List of FAA Oral Questions What is the main advantage of a horizontally opposed engine over a radial engine for powering modern aircraft? - The horizontally opposed engine has a much smaller frontal area and is easier to streamline than a radial engine. How many throws are there in the crankshaft of a six-cylinder horizontally opposed engine? - six What kind of connecting rod arrangement is used in a radial engine? - A master rod connects the single throw of the crankshaft with a piston. All other pistons are connected to the master rod with link rods. Of what material are most piston rings made? - gray cast iron What is the reason for using hydraulic valve lifters in an aircraft engine? - Hydraulic lifters keep all the clearance out of the valve operating mechanism. This decreases the wear of the valve train components. At what speed does the camshaft turn in relation to the crankshaft? - one half the speed of the crankshaft What kind of main bearings are used in a horizontally opposed engine? - steel backed, lead- alloy inserts On what stroke is the piston when the intake valve begins to open? - exhaust stroke On what stroke is the piston when the exhaust valve begins to close? - power stroke Why are both hot and cold clearances given for radial engines? - The hot clearance is given for valve timing purposes. The timing is adjusted with the valves in the number one piston adjusted with the hot clearance. When the timing is set, all of the valves are adjusted to their cold clearance. What is meant by "cam-ground" piston? - A piston that is not perfectly round. When the piston reaches operating temperature, the metal in the piston pin boss expands enough that the piston becomes perfectly round. Where is the piston when the ignition spark occurs? - 30 degrees before TDC What is a "full-floating" wrist pin? - a wrist pin that is not clamped in either the piston or the connecting rod. soft metal plugs keep the floating pin from scoring the cylinder wall. Why do most engines use more than one spring on each valve? - Different springs with different diameter and pitch have different resonant frequencies which minimize valve float. Would excessive valve clearance cause the valves to open early or late? - open late and close early What is the purpose of valve overlap? (2) - Exhaust gas inertia leaving the cylinder helps the fresh induction charge flow into the cylinder leading to 1) better volumetric efficiency 2) lower cylinder temperatures. What types of piston rings are installed on the pistons? - compression rings, oil control rings, oil wiper rings Why are some exhaust valves partially filled with metallic sodium? - It melts at operating temperature. Sloshing back and forth, it picks up heat, transfers heat to the valve stem which in turn transfers it to the cylinder head. What causes detonation? - Excessive heat and pressure in the cylinder causes the fuel-air mixture to reach its critical pressure and temperature which causes an explosion rather than a controlled burn. Why is a compression check important in determining the condition of an engine? - The compression check determines the condition of the seal between the piston rings and the cylinder walls, and the seal between the valves and their seats. What is meant by compression ratio? - The compression ratio is the volume of the cylinder with the piston at the bottom of its stroke to the top of its stroke. What cylinder is engine-to magneto-timing based on? - number one cylinder What are the main causes of the cylinder having low compression? (4) - broken rings, rings that do not seat properly, burnt valves, poorly seating valves What should be done to a nosewheel plane before removing the engine? - support the tail How are conventional reciprocating engines classified? (2) - They are classified according to 1) cylinder arrangement: in-line, radial, v-type, opposed 2) method of cooling: liquid, air