Most reciprocating engines used to power small aircraft incorporate two separate magneto ignition systems. The primary advantages of the dual ignition system are increased safety and improved engine performance.
A magneto (“mag”) is a self-contained source of electrical energy, so even if an aircraft loses total electric power, the engine will continue to run. For electrical energy, magnetos depend upon a rotating magnet and a coil.
When checking for magneto operation prior to flight, the engine should run smoothly when operating with the magneto selector set on BOTH, and should experience a slight drop in revolutions per minute (RPM) when running on only one or the other magneto. The drop in RPM is caused by reduced efficiency of a single spark plug, as opposed to two.
If the ground wire between the magneto and the ignition switch becomes disconnected or broken, the engine cannot be shut down by turning off the ignition switch.
Carburetors are normally set to deliver the correct air-fuel mixture (air-fuel ratio) at sea level. This air-fuel ratio is the ratio of the weight of fuel to the weight of air entering the cylinder. The ratio is determined by the setting of the mixture control in both fuel injection and carburetor-equipped engines.
When climbing, the mixture control allows the pilot to decrease fuel flow as altitude increases (air density decreases), thus maintaining the correct air-fuel ratio. If fuel flow is allowed to remain constant by not leaning the mixture, the air-fuel ratio will become too rich as the density of air decreases with increased altitude, resulting in a loss of efficiency. Operating with an excessively rich mixture may cause fouling of spark plugs.
When descending, air density increases. Unless fuel flow is increased, the mixture may become excessively lean; i.e., the weight of fuel will be too low for the weight of air reaching the cylinders. This may result in the creation of high engine temperatures and pressures.
The best power mixture is the air-fuel ratio from which the most power can be obtained for any given throttle setting.
As air flows through a carburetor, it expands rapidly. At the same time, fuel entering the airstream is vaporized. Expansion of the air and vaporization of the fuel causes a sudden cooling of the mixture which may cause ice to form inside the carburetor. The possibility of icing should always be considered when operating in conditions where the outside air temperature is between 20°F and 70°F and the relative humidity is high.
Carburetor heat preheats the air before it enters the carburetor and either prevents carburetor ice from forming or melts any ice which may have formed. When carburetor heat is applied, the heated air that enters the carburetor is less dense. This causes the air-fuel mixture to become enriched, and this in turn decreases engine output (less engine horsepower) and increases engine operating temperatures.
During engine run-up, prior to departure from a high-altitude airport, the pilot may notice a slight engine roughness which is not affected by the magneto check but grows worse during the carburetor heat check. In this case the air-fuel mixture may be too rich due to the lower air density at the high altitude, and applying carburetor heat decreases the air density even more. A leaner setting of the mixture control may correct this problem.
In an airplane with a fixed-pitch propeller, the first indication of carburetor ice will likely be a decrease in RPM as the air supply is choked off. Application of carburetor heat will decrease air density, causing the RPM to drop even lower. Then, as the carburetor ice melts, the RPM will rise gradually.
Fuel injection systems, which do not utilize a carburetor, are generally considered to be less susceptible to icing than carburetor systems are.
Fuel does two things for the engine; it acts both as an agent for combustion and as an agent for cooling (based on the mixture setting of the engine).
Aviation fuel is available in several grades. The proper grade for a specific engine will be listed in the aircraft flight manual. If the proper grade of fuel is not available, it is possible to use the next higher grade. A lower grade of fuel should never be used.
The use of low-grade fuel or a too lean air-fuel mixture may cause detonation, which is the uncontrolled spontaneous explosion of the mixture in the cylinder, instead of burning progressively and evenly. Detonation produces extreme heat.
Preignition is the premature uncontrolled firing of the air-fuel mixture. It is caused by an incandescent area (such as a carbon or lead deposit heated to a red hot glow) serving as an ignitor in advance of normal ignition.
Fuel can be contaminated by water and/or dirt. The air inside the aircraft fuel tanks can cool at night, which allows formation of water droplets (through condensation) on the insides of the fuel tanks. These droplets then fall into the fuel. To avoid this problem, always fill the tanks completely when parking overnight.
Thoroughly drain all of the aircraft’s sumps, drains, and strainers before a flight to get rid of any water that may have collected.
Dirt can get into the fuel if refueling equipment is poorly maintained or if the refueling operation is sloppy. Use care when refueling an aircraft.
On aircraft equipped with fuel pumps, the practice of running a fuel tank dry before switching tanks is considered unwise because the engine-driven fuel pump or electric fuel boost pump may draw air into the fuel system and cause vapor lock.
Most light aircraft engines are cooled externally by air. For internal cooling and lubrication, an engine depends on circulating oil. Engine lubricating oil not only prevents direct metal-to-metal contact of moving parts, it also absorbs and dissipates part of the engine heat produced by internal combustion. If the engine oil level is too low, an abnormally high engine oil temperature indication may result.
On the ground or in the air, excessively high engine temperatures can cause excessive oil consumption, loss of power, and possible permanent internal engine damage.
If the engine oil temperature and cylinder head temperature gauges have exceeded their normal operating range, or if the pilot suspects that the engine (with a fixed-pitch propeller) is detonating during climb-out, the pilot may have been operating with either too much power and the mixture set too lean, using fuel of too low a grade, or operating the engine with not enough oil in it. Reducing the rate of climb and increasing airspeed, enriching the fuel mixture, or retarding the throttle will help cool an overheating engine. Also, rapid throttle operation can induce detonation, which may detune the crankshaft.
The most important rule to remember in the event of a power failure after becoming airborne is to maintain safe airspeed.
The propeller is a rotating airfoil which produces thrust by creating a positive dynamic pressure, usually on the engine side.
When a propeller rotates, the tips travel at a greater speed than the hub. To compensate for the greater speed at the tips, the blades are twisted slightly. The propeller blade angles decrease from the hub to the tips with the greatest angle of incidence, or highest pitch, at the hub and the smallest at the tip. This produces a relatively uniform angle of attack (uniform lift) along the blade’s length in cruise flight.
No propeller is 100% efficient. There is always some loss of power when converting engine output into thrust. This loss is primarily due to propeller slippage. A propeller’s efficiency is the ratio of thrust horsepower (propeller output) to brake horsepower (engine output). A fixed propeller will have a peak (best) efficiency at only one combination of airspeed and RPM.
A constant-speed (controllable-pitch) propeller allows the pilot to select the most efficient propeller blade angle for each phase of flight. In this system, the throttle controls the power output as registered on the manifold pressure gauge, and the propeller control regulates the engine RPM (propeller RPM). The pitch angle of the blades is changed by governor regulated oil pressure which keeps engine speed at a constant selected RPM. A constant-speed propeller allows the pilot to select a small propeller blade angle (flat pitch) and high RPM to develop maximum power and thrust for takeoff.
To reduce the engine output to climb power after takeoff, a pilot should decrease the manifold pressure. The RPM is decreased by increasing the propeller blade angle. When the throttle is advanced (increased) during cruise, the propeller pitch angle will automatically increase to allow engine RPM to remain the same. A pilot should avoid a high manifold pressure setting with low RPM on engines equipped with a constant-speed propeller to prevent placing undue stress on engine components. To avoid high manifold pressure combined with low RPM, the manifold pressure should be reduced before reducing RPM when decreasing power settings, and the RPM increased before increasing the manifold pressure when increasing power settings.
At low temperatures, changes occur in the viscosity of engine oil, batteries can lose a high percentage of their effectiveness, instruments can stick, and warning lights can stick in the pushed position when “pushed to test.” Therefore, preheating the engines, as well as the flight deck, before starting is advisable in low temperatures. The pilot should also be aware that at extremely low temperatures, the engine can develop more than rated takeoff power even though the manifold pressure (MAP) and RPM readings are normal.
Overpriming is a frequent cause of difficult starting in cold weather because oil is washed off the cylinder walls and poor compression results. The manufacturer’s instructions should be followed for starting an overprimed engine.
During cold weather preflight operations, be sure to check the oil breather lines. Vapors caused by combustion may condense and then freeze, clogging these lines.
Since most aircraft heaters work by using the engine to heat outside air, a pilot should frequently inspect a manifold type heating system to minimize the possibility of hazardous exhaust gases leaking into the flight deck.
The pitch angle of a helicopter rotor blade is the acute angle between the chord line of the blade and the plane of rotor rotation.
In a semirigid rotor system, the rotor blades are rigidly interconnected to the hub, but the hub is free to tilt and rock with respect to the rotor shaft. In this system in which only two-bladed rotors are used, the blades flap as a unit; that is, as one blade flaps up, the other blade flaps down an equal amount. The hinge which permits the flapping or see-saw effect is called a teetering hinge.
A rocking hinge, perpendicular to the teetering hinge and parallel to the rotor blades, allows the head to rock in response to tilting of the swash plate by the cyclic pitch control. This changes the pitch angle an equal amount on each blade—decreasing it on one blade and increasing it on the other.
In a fully articulated rotor system, each blade is attached to the hub by three hinges, oriented at approximately right angles to each other. A horizontal hinge, called the flapping hinge, allows the blades to move up and down independently. A vertical hinge, called a drag, or lag hinge, allows each blade to move back and forth in the plane of the rotor disc. This movement is called dragging, or hunting. The blades can also rotate about their spanwise axis to change their individual blade pitch angle, or feather.
Fully articulated helicopter rotor systems generally use three or more blades, and each blade can flap, drag, and feather independently of the other blades.
The freewheeling unit in a helicopter rotor system allows the engine to automatically disengage from the rotor when the engine stops or slows below the corresponding rotor RPM. This makes autorotation possible.
Because a helicopter rotor system weighs so much more than a propeller, a helicopter (except for those with free turbine engines) must have some way to disconnect the engine from the rotor to relieve the starter load. For this reason, it is necessary to have a clutch between the engine and the rotor. The clutch in a helicopter rotor system allows the engine to be started without the load, and when the engine is running properly, the rotor load can be gradually applied.
High-frequency vibrations are associated with the engine in most helicopters and are impossible to count, because of their high frequency. A high-frequency vibration that suddenly occurs during flight could be an indication of a transmission bearing failure. Such a failure will result in vibrations whose frequencies are directly related to the engine speed. Abnormal low-frequency vibrations in a helicopter are always associated with the main rotor.
Variometers used in gliders are so sensitive that they indicate climbs and descents as a result of changes in airspeed. A total energy compensator for a variometer reduces the climb and dive errors that are caused by airspeed changes and cancels out errors caused by “stick thermals” and changes in airspeed. The variometer shows only when the glider is climbing in rising air currents.
[10-2024]