5: Aircraft Systems

Magnetic Compass

Pitot-Static Instruments

Gyroscopic Instruments

Automation Management

The Electrical System

Oxygen Systems

Cold Weather Operation

The Powerplant

The Ignition System

Fuel Systems

Air-Fuel Mixture Control

Induction Systems

Detonation

Propeller Design

Propeller Forces

Critical Engine of a Multi-Engine Airplane

Constant-Speed Propellers

Rotorcraft Systems

Glider Instrumentation

Magnetic Compass

The most basic instrument required for flight is the magnetic compass. The compass consists of a float which is free to turn on a hardened steel pivot that rides in a glass bearing. There are two small bar magnets attached to the bottom of the float, and a calibrated card is mounted around the float. The float assembly rides in a bowl of compass fluid, which is a highly refined kerosene-type liquid. The calibrated card is visible to the pilot through the glass front of the bowl, and the direction the aircraft is headed is read on the card opposite the vertical lubber line just behind the glass. The magnetic compass is subject to several errors and limitations:

Variation—This is the error caused by the compass pointing toward the magnetic north pole, while the aeronautical charts are oriented to the geographic north pole. Variation is not affected by changes in heading, but it does change with the location on the earth’s surface. Aeronautical charts show the amount of variation to be applied.

Deviation—This is the error caused by local magnetic fields produced by certain metals and the electrical systems in the aircraft. Deviation error is corrected for by “swinging” the compass. The aircraft is aligned with the directional marks on a compass rose on the airport, and the small magnets inside the compass housing are rotated to minimize the error between the compass reading and the direction of the mark with which the aircraft is aligned. Corrections are made on the four cardinal headings, and the errors are read every 30°. A compass correction card is made and installed near the compass to show the pilot the compass heading to fly for each magnetic heading.

Magnetic dip error—This error is caused by the compass magnets pointing downward as they align with the earth’s magnetic field. This downward pointing is caused by the vertical component of the field, and is greatest near the magnetic poles.

Northerly turning error—This error is caused by the vertical component of the earth’s magnetic field. When flying in the northern hemisphere, on a northerly heading, and banking in either direction to start a turn, the vertical component of the magnetic field pulls on the north-seeking end of the magnets and rotates the compass card in the direction opposite that of the turn being started. When flying on a southerly heading, and banking in either direction to start a turn, the vertical component pulling on the magnets rotates the card in the same direction as the turn is being made. The card moves in the correct direction, but at a rate greater than the actual rate of turn.

Acceleration error—This error is caused by the center of gravity of the compass float being below its pivot. When the aircraft is flying in the northern hemisphere on an easterly or westerly heading and accelerates, the rear end of the float tips upward, and the magnetic pull on the compass magnets causes the card to rotate and indicate a turn toward the north. When the aircraft decelerates on an easterly or westerly heading, the rear end of the float dips down and the magnet is pulled in the direction that rotates the card to indicate a turn toward the south. A memory device to help remember the effect of acceleration error is the mnemonic ANDS: accelerate north, decelerate south. There is no acceleration error when accelerating or decelerating on a northerly or southerly heading.

When making turns by reference to the magnetic compass, these corrections should be made:

  1. If you are on a northerly heading and start a turn to the east or west, the indication of the compass lags or shows a turn in the opposite direction.
  2. If you are on a southerly heading and start a turn to the east or west, the indication of the compass leads the turn, showing a greater amount of turn than is actually being made.
  3. The amount of lead or lag depends upon the latitude at which you are flying. For all practical pur-poses, the lead or lag is equal to approximately 1° for each degree of latitude.
  4. When rolling out of a turn, using a coordinated bank, the rollout should be started before the desired heading is reached by an amount that is equal to approximately one-half of the bank angle being used.

Pitot-Static Instruments

The altimeter, airspeed indicator, and vertical speed indicator are important flight instruments that are operated by air pressure. All three of these instruments receive their static pressure from the static air ports located on the side of the fuselage or the vertical tail surface.

When flying at high angles of attack, such as when practicing power-off stalls with flaps down, the air flowing around the aircraft is disturbed to the extent that the air pressure at the static ports is no longer that of undisturbed air. The indicated airspeed at the moment of the stall will likely be lower than that indicated by the white arc on the airspeed indicator. This erroneous airspeed indication is caused by an installation error in the static system, called position error. An airspeed correction table is often included in the Pilot’s Operating Handbook to show the airspeed correction to be applied when flying with flaps down. Pitot-static system errors caused by position error are generally the greatest at low airspeed.

The airspeed indicator is a differential pressure indicator that measures the difference between the static air pressure and the ram air pressure caused by the movement through the air. The airspeed indicator is connected to both the static ports and the pitot tube which is an open-end tube that points directly into the airstream and picks up the ram pressure of the air. If both the ram-air input and the drain hole in the pitot tube become blocked, there will be no variations in the airspeed indication in level flight even if large power changes are made. There will, however, be changes in the airspeed indication if the aircraft changes altitude with the pitot pressure inlet blocked.

The airspeed indicator is color-coded to help the pilot immediately recognize the important airspeeds and ranges of airspeed. The color codes are:

White arc—Flap operating speed. The bottom of this arc is at the power-off stall speed with the gear and flaps down, VS0. Where the green arc meets the white arc (bottom of green arc), is the power-off stall speed with flaps and gear up, VS1. The top of the white arc is at VFE.

Green arc—Normal operating range. The top of the green arc is the maximum structural cruising speed, VNO.

Yellow arc—Caution range. The aircraft should not be flown in this speed range in rough air. This arc extends from the top of the green arc to the never-exceed red line.

Red radial line—This is the never-exceed speed, VNE.

Blue radial line—This is the best rate-of-climb speed for a twin-engine airplane with one engine inoperative.

The design maneuvering speed (VA) of an aircraft, which is the maximum speed at which the aircraft can be safely stalled, is not color-coded on the airspeed indicator. This maneuvering speed is noted in the Aircraft Flight Manual and is often marked on a placard on the instrument panel. If the VA for an aircraft is not known, a generally accepted rule of thumb for a safe maneuvering speed is 1.7 times the normal stalling speed. See Figure 5-1.

Figure 5-1. Airspeed indicator

The altimeter is an absolute pressure indicator that measures the static pressure of the air surrounding the aircraft. This pressure is referenced from the appropriate level and is indicated on the instrument dial in units of feet. If the barometric pressure in the window is set to standard sea level pressure of 29.92 inches of mercury or 1013.2 millibars, the altimeter will indicate pressure altitude. If the pressure in the window is set to the existing altimeter setting, the altimeter will indicate indicated altitude.

The altimeter is connected to the static port for its normal operation, and in the event the static port ices over, an alternate static air source valve may be opened to allow the altimeter to take its static pressure from inside the aircraft. If the alternate static air pressure valve in an unpressurized aircraft is opened in flight, the altimeter will likely indicate higher than that actually being flown. The air flowing over the fuselage speeds up and its pressure drops. This causes the pressure of the air inside the cabin to be lowered enough to give a slightly inaccurate indication on the altimeter. If the static system of a pressurized aircraft should leak and allow pressurized air to enter the system while flying at high altitude, the altimeter will read the cabin altitude, which is lower than the actual flight altitude.

The altimeter measures the absolute pressure of the air to indicate the altitude. Temperature affects the density and thus the pressure of the air, and correction must be made to the air pressure to find the true altitude. If you fly into an area where the temperature is colder than standard, the air will be denser than standard air, and the pressure levels will be closer together. This will cause the aircraft to be lower than the altimeter indicates. If, on the other hand, you fly into an area where the temperature is warmer than standard, the air will be less dense than standard air, and the pressure levels will be farther apart. The aircraft will be higher than the altimeter indicates. See Figure 5-2.

Figure 5-2. Temperature vs. altimeter indication

Gyroscopic Instruments

Gyroscopic flight instruments are divided into two categories; attitude indicators and rate indicators. The attitude indicators are the directional gyro, gyro horizon, and horizontal situation indicator. The rate indicators are the turn and slip indicator and the turn coordinator.

Gyroscopic instruments can be operated either electrically or pneumatically. As a safety feature, airplanes that have pneumatically operated attitude indicators often have electrically operated rate indicators. Pneumatically operated instruments can be connected to the airplane’s vacuum system or to the pressure air system. Instruments connected to the vacuum system are actually driven by air pressure flowing into the instrument to fill the case that has been evacuated by the vacuum pump.

Automation Management

Automation management is the demonstrated ability to control and navigate an aircraft by means of the automated systems installed in the aircraft. It is one of the components of single pilot resource management (SRM), which also includes aeronautical decision making (ADM), risk management, task management, situation awareness, and controlled flight into terrain (CFIT) awareness. In advanced avionics aircraft, proper automation management requires a thorough understanding of how the autopilot interacts with the other systems. An advanced avionics safety issue identified by the FAA concerns pilots who develop an unwarranted overreliance in their avionics and the aircraft, believing that the equipment compensates for pilot shortcomings. Advanced avionics should be used to increase safety, not risk.

The Electrical System

Most large aircraft use alternating current (AC) electrical systems as their main source of electrical power. But because of the ease of storing direct current (DC) electricity for starting the engine, almost all small airplanes use DC systems. When AC is needed for instruments, an inverter is used to change DC into AC.

A typical light airplane electrical system uses a battery to start the engine. It uses a generator or alternator to supply all of the electrical needs when the engine is running and to keep the battery charged.

Most older airplanes have generators, but almost all of the newer airplanes use alternators. One of the basic differences between a generator and an alternator is the number of magnetic poles used to produce the electricity. Generators normally have 2 or 4 poles, while alternators have between 8 and 14 poles. The larger number of poles allows an alternator to produce its electrical power at a lower engine RPM than a generator.

The ignition system in an aircraft engine is entirely separate from the aircraft electrical system. The engine can operate normally with the electrical system turned off. The only time the engine needs the electrical system is to turn the starter.

If an aircraft electrical system should fail in flight, the ignition system will not be affected, but the fuel quantity indicators, aircraft lighting system, avionics equipment, and all instruments that require outside electrical power would be inoperative.

Oxygen Systems

Modern aircraft that fly at high altitudes are equipped with oxygen systems to supply the occupants with supplemental oxygen when there is not enough outside air pressure to force the required oxygen into the lungs.

Most general aviation aircraft oxygen systems carry gaseous oxygen in either installed or portable high-pressure steel bottles. These systems must be serviced with “Aviators Breathing Oxygen” which meets Federal Specification BB-O-925A, Grade A or equivalent. Hospital or industrial oxygen must never be used.

The high pressure of the oxygen is reduced with a regulator and fed to the masks at the pressure needed to allow the oxygen to flow into the user’s lungs. The more complex oxygen systems use demand-type regulators that meter the oxygen to the user only during inhalation.

The simpler systems use a continuous flow of oxygen into a rebreather-type mask. In this mask, the air exhaled from the lungs flows into a flexible bag where it mixes with oxygen. When the wearer inhales, fresh oxygen and some of the exhaled air is taken into the lungs. This type of mask conserves oxygen.

Cold Weather Operation

When an aircraft has been exposed to extreme cold for any length of time, extra care should be taken when preparing the aircraft for flight. Check all drain valves, oil tank sumps, oil drains, fuel strainers, vent lines and all main and auxiliary control hinges and surfaces for the existence of ice or hard snow. Check the crankcase breather line to be sure that it is not plugged with ice.

When possible, the aircraft should be preheated, with heat applied to the following sections or parts of the aircraft: accessory section, nose section, Y-drain valve, all oil lines, oil tank sump, starters, instruments, tires, flight deck, and elevator trim tabs.

The Powerplant

Most of the smaller general aviation airplanes are powered by four-stroke reciprocating engines that operate on the following principle:

  1. On the intake stroke, the piston is moving inward, the intake valve is open, and the gaseous air-fuel mixture is pulled into the cylinder.
  2. On the compression stroke, the piston moves outward with both valves closed. The air-fuel mixture is compressed. Near the top of the compression stroke, the spark plugs ignite the mixture and it begins to burn.
  3. On the power stroke, the burning and expanding gases force the piston inward, doing work. Near the end of the power stroke, the exhaust valve opens so the burned gases can begin to leave the cylinder.
  4. On the exhaust stroke, the piston moves outward and forces the burned gases out of the cylinder.

See Figure 5-3.

Figure 5-3. Four-stroke cycle

An engine can be operated efficiently only when the pilot monitors all conditions and keeps them within the allowable ranges. Aircraft engines are highly susceptible to damage if the oil temperature and cylinder head temperature are allowed to exceed their allowable limits. Excessively high temperatures can cause a power loss, excessive oil consumption, and permanent damage to the engine. Damaging high temperatures can be caused by operating the engine at a power output that is too high, or with a air-fuel mixture that is too lean for the power being developed.

The Ignition System

The ignition system of an aircraft reciprocating engine is a dual, self-contained electrical system. Two magnetos produce high-voltage AC electricity and direct it to spark plugs in each cylinder so that a spark will ignite the air-fuel mixture inside the cylinders at the proper time.

Magnetos are electrical generators that produce AC electricity when they are rotated. To turn a magneto off, its output is directed to ground (the engine structure) through the ignition switch. If the ground wire between the magneto and the ignition switch becomes disconnected, the engine cannot be shut down by turning the switch to the OFF position.

Dual ignition provides a safety factor in case one system should fail, but more importantly, having two spark plugs in each cylinder provides better combustion. The air-fuel mixture inside the cylinder is ignited at two places, so that it requires only half the time to completely burn the charge inside the cylinder.

If one ignition system should fail, or if one spark plug becomes fouled, the mixture is ignited at only one point, and the time required for complete combustion allows the unburned mixture to become so hot it explodes, or detonates. Detonation produces enough heat and pressure to seriously damage the engine.

Spark plugs can become so fouled they cannot ignite the mixture, if the engine is operated with an excessively rich air-fuel mixture, or if the fuel used in the engine has a lead content greater than that recommended for the engine.

Fuel Systems

It is important that aircraft engines be adequately supplied with the proper grade of fuel, free from water or other contaminants. Aviation gasoline (100LL Avgas) is rated according to its ability to resist detonation, and it contains dye to identify its grade. It is extremely important that only the proper grade fuel be used in an aircraft engine. If too high a grade of fuel is used, there is a possibility that the excess lead will foul the spark plugs. If too low a grade of fuel is used, there is the extreme danger of detonation which will seriously damage the engine. Many light-sport aircraft are designed to run on auto gas. If the correct grade of fuel is not available, it is generally permissible to use the next higher grade of fuel until the proper grade can be obtained. No fuel should ever be used that is not approved for use in the engine.

Water in the fuel can cause engine failure, and any water that has collected in the tanks must be removed before flight. In order to purge all of the liquid water from the fuel system, the fuel strainer drain and the sumps in all of the tanks must be drained. Water condenses inside a partially full fuel tank when the air temperature drops. Some water is absorbed into the fuel and the rest collects in the bottom of the tank. The amount of water the fuel can absorb is determined by the temperature of the fuel. The warmer the fuel, the more water it can hold. When the fuel is cooled, the water condenses out.

Since many light-sport aircraft use auto fuel which contains alcohol, this can absorb water and run it through the system harmlessly. However, if Avgas is used for systems designed for auto gas, additional precautions must be taken since fuel drains may not be present and the additional lead in the Avgas requires maintenance. Consult the POH for details on fuel use.

Absorbed water is a greater problem with turbine-powered aircraft than it is with reciprocating engine-powered airplanes because of the lower atmospheric temperatures at which turbine engines operate: the water condenses out of the fuel and freezes on the fuel filters. To properly remove all water from an aircraft fuel system, fuel must be drained from every one of the fuel tank sumps and the main fuel strainer, until fuel that is free from any indication of water, flows from them.

Aircraft fuel systems normally consist of more than one tank, as well as the proper selector valves, pumps, and strainers needed to supply the correct volume of clean, contamination-free fuel to the carburetor or fuel injection system on the engine.

All fuel tanks have a vent that allows air to enter the tank and take the place of fuel as it is used. It is important that the vent be open, and it should be checked on the preflight inspection. If the vent becomes plugged, the air pressure above the fuel will become too low to force a steady flow of fuel to the fuel pump or carburetor.

Vapor lock is a condition in which fuel vapors form in the fuel line between the tank and the engine. The pressure of the vapor is high enough that it prevents liquid fuel from flowing to the engine, and the engine dies of fuel starvation. Vapor lock is most likely to form on hot days when the engine is operated for a long time on the ground. The danger of vapor lock can be minimized by using the fuel boost pump, if installed, to maintain a positive pressure on the fuel in the lines between the tank and the engine. Vapor can also be introduced into the fuel line by running one tank dry before selecting a full tank. This happens when the engine-driven fuel pump or electric boost pump draws air into the fuel lines, causing a vapor lock that prevents fuel reaching the carburetor or fuel injection system.

Static electricity, formed by the flow of fuel through the hose and nozzle, creates a fire hazard during refueling. To guard against the possibility of a spark igniting the fuel fumes, a ground wire should be attached to the aircraft before the cap is removed from the tank. The refueling nozzle should be grounded to the aircraft before refueling is begun and throughout the refueling process. The fuel truck should also be grounded to the aircraft and the ground.

Carburetion is the process by which the air entering the engine is measured and the correct amount of fuel is metered into it. The fuel is vaporized and evenly distributed to all cylinders. Aircraft reciprocating engines may be equipped with float-type carburetors, pressure carburetors, or fuel-injection systems. Float-type carburetors are used on most of the smaller engines because of their simplicity. Their operation is based on the pressure drop caused by the high velocity of the air flowing into the engine through the venturi. The pressure drop is proportional to the volume of air being pulled into the cylinders, and the amount of fuel metered into this air is determined by the pressure drop.

A float-type carburetor is an efficient refrigerator. Both the expansion of the air leaving the venturi and the vaporization of the fuel drop the temperature of the air flowing into the engine. This temperature drop causes moisture to condense out of the air and freeze, blocking the flow of air into the engine. The formation of carburetor ice does not require ambient air temperature to be below freezing, but it is most likely to form when the air temperature is between 20°F and 70°F, and there is visible moisture present or the humidity is high. The first indication of carburetor ice on an airplane equipped with a fixed-pitch propeller is a drop in RPM. On an airplane with a constant-speed propeller, this first indication is a drop in manifold pressure. The presence of carburetor ice can be verified on an airplane equipped with a fixed-pitch propeller by noting the RPM after carburetor heat has been applied. If the RPM decreases at first, then gradually increases, carburetor ice was present.

Two of the main disadvantages of a float-type carburetor, uneven mixture distribution and carburetor icing, are overcome by the fuel injection system. The air entering the engine is measured, and the correct amount of fuel for this air is metered and distributed to the intake valve, then mixed with the air as it enters the engine cylinder. This method of air-fuel mixing, inside the warm intake valve chamber, prevents the temperature drop which can occur in carburetor systems.

Air-Fuel Mixture Control

For an aircraft engine to develop its power, it must be supplied with the correct mixture of aviation gasoline and air. The ratio of the mixture must be maintained within close limits, and the pilot must be able to vary it to allow the engine to operate most efficiently. Some modern light-sport four-stroke engines have automatic mixture controls, while two-stroke carbureted engines require manual inputs for operations at higher altitude airports.

The air-fuel mixture ratio is the ratio of the weight of the fuel to the weight of the air entering the cylinders. This ratio is set so that it is correct for full-power operation in normal density conditions (such as on the ground) when the mixture control is in the FULL RICH position. When the aircraft climbs to altitude, the air is less dense and fewer pounds of air are taken into the engine, but the fuel metered into this less-dense air remains constant. The mixture becomes excessively rich and the engine loses power. To maintain efficient engine operation at altitude, the pilot must use the mixture control to lean the mixture. This reduces the amount of fuel entering the combustion chamber and keeps the air-fuel mixture ratio in the efficient operating range.

If the mixture is too rich, the spark plugs will be fouled, and if it is too lean, the engine is likely to detonate and overheat. Detonation is the condition in the engine cylinder in which excessive heat and pressure cause the air-fuel mixture to explode (burn instantaneously) rather than burning evenly as it should. Preignition could also occur, which is the uncontrolled combustion of the air-fuel mixture in advance of the normal ignition.

It is possible for the pilot to monitor the air-fuel mixture being delivered to the cylinders by observing the exhaust gas temperature (EGT) indicator. The temperature of the gases leaving the cylinders is an indicator of the air-fuel mixture being burned. The highest temperature is produced by the mixture of 15 parts of air to one part of fuel. But burning this mixture can be harmful to the engine. The engine manufacturer recommends the mixture ratio desirable for full power and for cruise.

The normal leaning procedure using an EGT consists of setting the engine up for cruise power, then leaning the mixture until the EGT peaks, then enriching the mixture until the EGT drops a specified number of degrees. This puts the mixture on the rich side of peak EGT.

For a two-stroke engine, the jets should be installed for the proper mixture at the lowest altitude airports. Climbing to altitude will cause the engine to run rich, which is better than running too lean. This could create too high an EGT and stop the engine.

Induction Systems

An aircraft engine develops its power by converting a given weight of fuel and air into heat. As the air-craft goes up in altitude, the density and thus, the weight of the air, decrease and the engine is no longer able to develop the power it produced at sea level.

Manifold pressure is an indirect indicator of the amount of power an engine is developing. When the engine is not operating, the manifold pressure is the same as the existing atmospheric pressure, 29 "Hg. When the engine is started and idling, the manifold pressure is low, and when the engine is developing full power, the manifold pressure approaches the pressure of the ambient air. The manifold pressure of a nonsupercharged engine can never be more than slightly greater than the ambient air pressure. However, both an internal supercharger or a turbocharger can increase the engine manifold pressure above that of the surrounding air.

To increase the power available at altitude, airplane engines may be fitted with turbochargers. These are air compressors driven by a turbine in the exhaust system. A turbocharger does cause a loss in power because the back pressure it produces restricts the flow of exhaust gases from the engine. But it produces more power than is lost by increasing the density of the air before it enters the engine cylinders.

The critical altitude of an aircraft engine is the maximum altitude, under standard atmospheric conditions, at which the engine will develop its rated horsepower.

When climbing in an aircraft equipped with a turbocharged engine, the manifold pressure will remain approximately constant until the critical altitude of the engine is reached. From this point on, the power will decrease as altitude is increased.

Detonation

Detonation is an instantaneous burning of the air-fuel mixture inside an engine cylinder that occurs when the air-fuel mixture reaches its critical pressure and temperature. Detonation is an explosion that can damage the engine.

Operating the engine with too lean a mixture, induction air that is too hot, cylinder head temperature that is too high, manifold pressure that is too high, or a fuel of too low a grade can cause the air-fuel mixture to detonate.

Propeller Design

A propeller is a rotating airfoil that produces thrust by an aerodynamic action. A low pressure is produced over the back of the propeller blade (from the perspective of outside the flight deck, looking at the front of the airplane), the portion that corresponds to the curved top of a wing, and a high pressure is produced at the flat face of the propeller that corresponds to the bottom of a wing. This pressure difference pulls air through the propeller, and this in turn pulls the airplane forward.

The efficiency of a propeller is a measure of the ratio of the thrust horsepower produced by the engine-propeller combination to the brake horsepower of the engine. Propeller efficiency varies between 50 and 87 percent, depending on how much the propeller slips.

The blade angle of a propeller is the angle between the chord line of the blade and the plane of propeller rotation. This angle is high near the blade root, because the radius of rotation is small and the speed is low. Near the tip, the blade speed is high and the blade angle is low. This change in blade angle, or geometric pitch, is called twist, and permits a relatively constant angle of attack along the length of the blade when the propeller is in cruising flight.

The geometric pitch of a propeller is the distance the propeller would advance in one revolution if it were turning in a solid. The effective pitch of a propeller is the distance the propeller actually advances in the air. Slip is the difference between the geometric and the effective pitch.

Propeller Forces

A rotating propeller has a characteristic known as gyroscopic precession. This causes a force applied to the blade at one point to be felt not at the point of application, but at a point 90° ahead, in the direction of rotation. If the tail of a tail-wheel-type airplane is suddenly raised for takeoff, a forward force is applied to the top of the propeller disc, and this force is felt on the right side of the disc. This forces the airplane, now balanced on its two main wheels and moving at a speed when the rudder is not very effective, to yaw to the left. In flight, gyroscopic precession on the propeller will cause any sudden yawing about the vertical axis to produce a pitching moment. See Figure 5-4.

Figure 5-4. Gyroscopic precession

The propellers on most single-engine airplanes built in the United States rotate in a clockwise direction as viewed from the flight deck. This rotation causes the air moved backward by the propeller to spiral around the fuselage and strike the bottom of the left horizontal stabilizer and the left side of the vertical fin. The spiraling slipstream increases the angle of attack and lift of the left horizontal stabilizer, which tends to rotate the airplane to the right about its longitudinal axis. It also increases the angle of attack of the vertical fin so that it causes the airplane to rotate to the left about its vertical axis. See Figure 5-5.

Figure 5-5. Spiraling slipstream

The P-factor causes the propeller of an airplane flying at a high angle of attack to produce a force which causes the airplane to yaw to the left about its vertical axis. When flying at a low angle of attack, the propeller blade moving upward and the one moving downward have the same angle of attack and their thrust is symmetrical. But when the airplane’s angle of attack is high, the angle of attack of the rising propeller blade on the left side of the airplane is decreased, and the angle of attack of the descending blade on the right side is increased. This difference in the angle of attack of the propeller blades on the opposite sides of the airplane produces an asymmetrical thrust, which tends to yaw the airplane to the left about its vertical axis. See Figure 5-6.

Figure 5-6. P-factor

Critical Engine of a Multi-Engine Airplane

When an airplane is flying at a high angle of attack, the descending propeller blade has a higher angle of attack than the ascending blade. The descending blade, the one normally on the right side of the engine, produces more thrust than the blade on the left side, and as a result, at high angles of attack, the asymmetrical thrust tends to pull the airplane to the left.

The critical engine of a light multi-engine airplane on which both engines rotate clockwise is the left engine. Both engines produce more thrust on their right side and try to rotate the airplane to the left.

The center of thrust of the right propeller is a greater distance from the vertical axis of the airplane, and it produces a greater left-yawing force than the left propeller.

If the left engine quits during a climbout, the longer moment arm of the right propeller thrust can create a potentially dangerous yawing action. See Figure 5-7.

Figure 5-7. Critical engine

Constant-Speed Propellers

A constant-speed propeller is an adjustable-pitch propeller with a blade pitch angle controlled by a governor. When the propeller pitch control is adjusted to a specific RPM, the blade angle automatically adjusts to maintain this RPM as the air load changes. When the air load decreases, the blade angle increases to restore it. When the air load increases, the blade angle decreases so that the engine can turn the propeller at the same speed. By constantly varying the blade pitch setting to maintain a constant propeller load, the engine is held at a constant RPM so it can operate most efficiently.

Maximum power and maximum thrust can be produced by an aircraft engine equipped with a constant-speed propeller when the propeller blade angle is adjusted to produce a small angle of attack, and the engine is allowed to develop high RPM.

Changes to the power setting of an engine equipped with a constant-speed propeller should be made in this way:

Rotorcraft Systems

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 seesaw 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 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.

Glider Instrumentation

Variometers used in sailplanes 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 sailplane is climbing in rising air currents.

[10-2024]