Propellers

Question 1

26.22.2 Explain the reasons for fitting independent dual ignition systems to aero engines.

Answer 1

Safer in the event of failure of one ignition system. Results in smoother and more efficient combustion of the fuel-air charge

Question 2

26.22.4 List the essential components of an ignition system.

Answer 2

A magneto. A distributor. High tension (high voltage) leads. Spark plugs.

Question 3

26.22.6 Describe, in broad terms, the operation of the following: (a) an aircraft magneto;

Answer 3

consists of a magnet which is rotated near a conductor which has a coil of wire wound around it. The rotation of
the magnet induces an electrical current to flow through the coil.

Question 4

26.22.6 Describe, in broad terms, the operation of the following: (b) impulse couplings;

Answer 4

it provides a short and sudden acceleration of the rotating magnet within the magneto which generates the high
voltage. It effectively retards the ignition timing a low cranking RPM, and then, immediately after the start up, it allows the timing of the spark to return to its normal position.

Question 5

26.22.6 Describe, in broad terms, the operation of the following: (c) the distributor;

Answer 5

distributes the high tension current to each spark

plug at the correct moment just prior to the start of the power stroke.

Question 6

26.22.6 Describe, in broad terms, the operation of the following: (d) ignition harness (high tension leads);

Answer 6

leads run from each distributor terminal to the individual spark plugs in the cylinders.

Question 7

26.22.6 Describe, in broad terms, the operation of the following: (e) spark plugs;

Answer 7

providing an air gap in the secondary circuit which the

high tension current jumps to create a spark.

Question 8

26.22.8 Explain the purpose of fitting an impulse coupling to a magneto;

Answer 8

Magneto speed at start up is very slow, 60 RPM or even
less. To generate a spark of sufficiently high voltage to ignite the fuel air mixture requires a magneto speed of about 100 -200 RPM. it provides a short and sudden acceleration of the rotating magnet within the magneto which generates the high voltage. It effectively retards the ignition timing a low cranking RPM, and then, immediately after the start up, it allows the timing of the spark to return to its normal position.

Question 9

26.22.10 Explain the reason ignition is timed to occur before TDC

Answer 9

So peak pressure is developed in cylinder well before the connecting rod and crank web reach the 90 degree angle.

Question 10

26.22.12 Describe the operation of the ignition switch(es) in the cockpit

Answer 10

Enables the pilot to select the left system L, or right system R, or both. Also pilot can perform a magneto check so see if each side of the magneto is operating correctly.

Question 11

26.22.14 Outline the correct procedures to be followed during a magneto check.

Answer 11

(Dead cut check) Idle at 800-1000 rpm. Switch briefly from BOTH-L-R- and back to BOTH. Perform this process again at a rpm setting given in the pilots operating manual for the aircraft type. A typical rpm setting would be 1700 rpm.

Question 12

26.22.16 Describe the indications of various problems manifested during a magneto check.

Answer 12

During the dead cut check, if the engine cuts out as L or R is selected then the ignition system is serviceable and the aircraft must not be flown until the defect has been rectified.

Question 13

26.22.18 Explain the causes of spark plug fouling and the methods of clearing such fouling.

Answer 13

Lead deposits can form on the spark plug electrodes causing misfiring. Maintain the aircraft in proper engine operating temperatures.