PITOT-STATIC SYSTEM The airspeed indicator, vertical-speed indicator, and altimeter are components of the Pitot-static system
The Pitot tube and static vent are mounted outside the airplane. The Pitot tube is positioned so that its front faces into the stream of air—the air rams into the opening. The static vent is usually flush-mounted on the side of the airplane so that there's no impact air measurement—in other words, the air doesn't rush into the vent.
EXAMPLE OF A PITOT TUBE MOUNTED ON THE WING OF A CESSNA 172
AIRSPEED INDICATOR The airspeed indicator shows the indicated airspeed of the air-plane in nautical miles per hour, commonly called knots. The airspeed indicator works by measuring the difference between the ram air pressure in the Pitot tube and the static air pressure in the vent.
Red radial line indicates the never exceed velocity Yellow arc indicates the cautionary range White arc indicates the permisible limit of flap operation 0 – 160 knots range for light aircraft 0 – 430 knots range for large and faster aircraft Another type for max. Allowable airspeed
MEAUSREMENT OF AIRSPEED When the airplane is parked on the ground, the Pitot tube senses the ambient air pressure (assuming no wind). Since the difference between the pressure in the Pitot tube and the pressure in the static vent is 0, the airspeed indicator indicates an airspeed of 0. When the airplane is in flight, the pressure in the Pitot tube is greater than the ambient pressure measured by the static vent. This pressure difference is indicated as airspeed on the airspeed indicator.
If the airplane flies at sea level on a standard day the indicated airspeed accurately reports the speed at which the airplane is moving through the air. When the airplane is operated at other altitudes or in nonstandard atmospheric conditions, the indicated airspeed doesn't accurately reflect the true airspeed. But true airspeed can always be calculated if temperature, pressure, and indicated airspeed are known.
MEASUREMENT OF TRUE AIRSPEED For Flight Simulator purposes, your true airspeed can be estimated by multiplying your indicated airspeed by 1 plus 1.5 percent for each 1000 feet above sea level that you're flying. For example, if you're flying at 5000 feet with an indicated airspeed of 100 knots, your true airspeed is approximately knots (100 * [1 + 5(.015)]).
TRUE AIRSPEED INDICATOR Consist of aneroid, differential pressure diaphragm and bulb temperature diaphragm responds respectively to changes in barometric pressure, impact pressure and free air temperature Indicates true airspeed from 1000ft. Below sea level and 50000ft. Above the sea level under free air temperature conditions from +40 to - 60 deg.Celcius
ALTIMETER The altimeter is the only instrument which shows how high the airplane is above some level. The altimeter has two hands like those of a clock and a small indicator that appears near the numbers on the outer ring of the gauge.
The large hand indicates hundreds of feet. The small hand shows thousands of feet. The small indicator indicates tens of thousands of feet. The altimeter is an aneroid barometer that displays pressure in feet above sea level (mean sea level), not above ground level. The altimeter cannot work accurately unless the pilot sets it to the current altimeter setting, which is the pressure at sea level under existing atmospheric conditions. Barometric dial gives pressure in inches of mercury
ERRORS Mechanical error – scale is not correctly oriented Hysteresis error – when aircraft is maintaining certain altitude for extended period of time, there is a sudden change in altimeter reading Installation error – change of alignment of static port
VERTICAL SPEED INDICATOR (RATE OF CLIMB INDICATOR) The vertical speed indicator, like the altimeter, is connected only to the static vent. The vertical speed indicator shows whether the airplane is flying at a constant altitude, climbing, or descending, and if climbing or descending, at what rate.
The face of the instrument is graduated in hundreds of feet per minute, with the top half showing climbs, and the bottom half, descents. WORKING Static pressure is fed into the diaphragm as well as in the case. While climbing, the pressure inside the diaphragm decreases, causes the contraction of the diaphragm. Due to this contraction, the needle deflect and give the indication as climb (positive value)
During level flight, the diaphragm comes to its initial position, thereby indicating level flight. During decent, the pressure inside the diaphragm increases, causes the diaphragm to expand. Due to this expansion, the needle deflect and give the indication as decent (negative value)
The next three instruments—the attitude indicator, turn coordinator, and heading indicator—are gyroscopic instruments. Each instrument uses a gyroscope to maintain its orientation relative to one or more of the axes of the airplane.
GYROSCOPE A gyroscope is a device for measuring or maintaining orientation, based on the principles of angular momentum Within mechanical systems or devices, a conventional gyroscope is a mechanism comprising a rotor journaled to spin about one axis, the journals of the rotor being mounted in an inner gimbal or ring, the inner gimbal being journaled for oscillation in an outer gimbal which in turn is journaled for oscillation relative to a support.
DEGREES OF FREEDOM In general there will be of six types of degrees of freedom. They are, Moving up and down (heaving);heaving Moving left and right (swaying);swaying Moving forward and backward (surging);surging Tilting forward and backward (pitching);pitching Turning left and right (yawing);yawing Tilting side to side (rolling).rolling
DEGREES OF FREEDOM FOR ROTOR The center of gravity of the rotor can be in a fixed position. The rotor simultaneously spins about one axis and is capable of oscillating about the two other axes. Thus the gyro wheel (rotor) possess three degrees of freedom.
GYRO WHEEL IN ACTION
GYROSCOPE IN OPERATION
PRECESSION ON A GYROSCOPE
ATTITUDE INDICATOR (ARTIFICIAL HORIZON)
An attitude indicator (ADI), also known as gyro horizon or artificial horizon, is an instrument used in an aircraft to inform the pilot of the orientation of the aircraft relative to earth. It indicates pitch (fore and aft tilt) and bank or roll (side to side tilt) Attitude indicators use a gyroscope (powered via vacuum pump or electrical motor) to establish an inertial platform. The gyroscope is geared to a display that has two dimensions of freedom, simultaneously displaying pitch and bank.
Marks around the top half of the instrument on some versions of Flight Simulator indicate angles of bank of 10°, 20°, 30°, 60°, and 90°. The essential components of the indicator are: The miniature wings emulate or represent the wings of the aircraft. The center horizon bar separates the two halves of the display, with the top half usually blue in color to represent sky and the bottom half usually dark to represent earth.
The degree marks represent the bank angle. They run along the rim of the dial. On a typical indicator, the first 3 marks on both sides of the center mark are 10 degrees apart. The next is 60 degrees and the mark in the middle of the dial is 90 degrees. a 45 degree turn is approximated by placing the indicator equidistant between the 30 and 60 degree marks. A standard rate turn is 15 degrees of bank.
On the attitude indicator are two white or yellow horizontal lines with a dot between them. The horizontal lines of this symbolic aircraft represent the wings and the dot represents its nose. If the symbolic aircraft dot is above the horizon line (blue background) the aircraft is nose up. If the symbolic aircraft dot is below the horizon line (brown background) the aircraft is nose down. When the dot and wings are on the horizon line, the aircraft is in level flight.
Vacuum pressure draws fast moving air along side the cupped edges of this gyroscope causing it to spin. This spinning gyroscope remains rigid in space, regardless of the pitch or bank attitude of the airplane. Note the horizon reference arm attached to the gyroscope. This reference arm, with its attached blue and black card representing the sky and ground respectively, remains upright relative to the actual horizon as the airplane climbs, descends, and banks.
The turn coordinator is actually two instruments in one. The airplane replica in the middle of the instrument rolls proportionally to the roll rate of the airplane. When the bank angle is maintained, the replica indicates the rate of turn. When the right or left wing of the replica is aligned with the lower mark, the airplane is turning at a rate of 3° per second (so a full 360° turn takes two minutes). This rate of turn is known as standard rate. i.e., 2min indicator.
The other instrument in the turn coordinator is called an inclinometer. The inclinometer shows whether or not use of rudder and aileron is coordinated. It consists of a curved glass tube with kerosene and a common steel ball bearing. If the ball moves to the outside of the turn, the airplane is skidding. If the ball moves to the inside of the turn, the airplane is slipping. Another type of inclinometer is 4min indicator. The aircraft is turning at 1.5°per second (so a full 360° turn takes four minutes). This is called as half standard rate.
The third gyroscopic instrument is the heading indicator. The heading indicator is used because a magnetic compass only works accurately when the airplane is flying straight and level in unaccelerated flight. Any time the airplane is banked, pitched, accelerated, or decelerated, the magnetic compass gives a wrong reading. The heading indicator solves this problem by using a gyroscope instead of a magnet.
In many advanced aircraft (including almost all jet aircraft), the heading indicator is replaced by a Horizontal Situation Indicator (HSI) which provides the same heading information, but also assists with navigation
The heading indicator operates on the same principle. An internal gyroscope, spun by a fast flow of air produced by the vacuum pump, remains rigid in space as the aircraft turns around its vertical axis.