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					                                     © Auld 2009-2011 – www.aerodynamics4students.com




                                        Aircraft Instruments
Mechanical Instruments
Primary flight instruments of many aircraft rely on direct measurement of aerodynamic pressure to predict the
altitude, airspeed and climb rate of the aircraft. Other primary instruments rely on the effects of gyroscopic
motion to measure rates of pitch, roll and yaw or aircraft attitude. These instruments use simple mechanical
means to display information to the pilot. Schematic diagrams and explanations of the working of these
instruments are shown below.
Altimeter
                                                 The altitude of an aircraft can be obtained by using a
                                                 measurement of the static pressure of the surrounding
                                                 atmosphere. A single pressure line from the Static Port is all
                                                 that is required as input for the instrument. Although the rate
                                                 of change of pressure with altitude is well known, based on
                                                 an International Standard Atmosphere model, the ambient
                                                 sea level pressure at any location of the earth's surface can
                                                 vary from day to day due to meteorological conditions. The
                                                 instrument will require zeroing before and sometimes during
                                                 the flight. This is done by setting the instrument to the
                                                 ambient pressure of the airport at the start of the flight and
                                                 resetting as required. It is therefore important to remember
                                                 that this instrument measures height above sea level and
                                                 not height above current ground location. For accuracy of
                                                 measurement the instrument usually has at least two
                                                 sometimes three indicator needles operating on different
                                                 scales. For the instrument shown the larger needle indicates
                                                 100's of feet and the smaller 1000's of feet. Ambient sea
level pressure pre-set value is shown in the rectangular box on the right (measured in HectoPascals). Click
on the Altimeter image to view a schematic diagram showing its internal operation.


Airspeed Indicator
                                                      The airspeed of an aircraft can be obtained from the
                                                      difference between atmosphere static pressure and the
                                                      measured total pressure from a pitot tube placed in the
                                                      airflow. The two pressures required come from lines to
                                                      the Static Port and an second to the Pitot Port. The
                                                      difference between these pressures is the dynamic
                                                      pressure created by the motion of the vehicle through
                                                      the air.
                                                                                       1    2
                                                                     Dynamic Pressure =  V
                                                                                       2
                                                      where V is the velocity of the aircraft and    is the
                                                      density of the surrounding air.

                                                       As the instrument does not have information on the
                                                       actual density of the air at the aircraft's altitude, an
                                                       assumed standard sea level density is used (1.225
                                                       Kg/m3). The true airspeed may be significantly higher as
the ambient density at the altitude will be lower than the sea level value. A correction will need to be made to
calculate true airspeed.
At high speed, where the surrounding air starts to be compressed by the flight of the vehicle, the assumed
sea level standard density will again be inaccurate and a further correction for compressibility made to the
measurement reading.
            IAS (indicated airspeed) → EAS (equivalent airspeed) : correction for compressibility
                             EAS → TAS (true airspeed) : correction for altitude.

                                             TAS= EAS×
                                                               ssl
                                                                 
The instrument only measures speed relative to the surrounding air so actual ground speed will need to be
calculated based on prevailing wind conditions.

The instrument shown gives a reading of indicated airspeed in knots (nautical miles per hour). Click in the
image of this instrument to see a schematic diagram showing its operation.


Vertical Speed (Climb/Descent Rate)


                                                  The vertical speed indicator shows the current rate of climb
                                                  or descent of the vehicle in feet per minute. It is feed from
                                                  a pressure line from the Static port in a similar fashion to
                                                  the altimeter. However this instrument does not measure
                                                  the absolute pressure but the rate at which the surrounding
                                                  static pressure is changing. Its internal metering system is
                                                  calibrated to give the rate of change of altitude equivalent
                                                  to the measured rate of change of pressure.
                                                  Click on the image of this instrument to see a schematic
                                                  diagram showing its internal operation.




Static Port




The pressure of the surrounding atmosphere is obtained through a flush mounted static port. This is usually
located on the side of the fuselage in a position which will have a local surface pressure which is closely
matching the stream ambient atmospheric air pressure. The position must be calibrated and not where there
is significant pressure change due to the pressure field of the wings, flow separations from joins or
protrusions on the fuselage, propeller slipstream or jet wake effects.
Pitot Port




A pitot tube is used to measure the kinetic energy of the airflow due to the motion of the aircraft. The tube
protrudes into the airstream and is aligned with the flow. The airstream impacting on the open end of the
tube is brought to rest. The pressure at this opening will thus be the sum of the static pressure of the stream
and its dynamic pressure. This is the total pressure of the flow field.


Pitot-Static Probe
In some cases, especially fro wind tunnel laboratory work, the pitot and static ports are merged into a single
unit, the pitot-static probe. The two tubes one inside the other can supply both the stream static pressure
and the stream stagnation pressure to a measurement system.
Turn and Bank Indicator
In order to determine rate of turn and correct balance for the bank angle required for a turn this instrument
combines two separate measurements.
The first is a simple curved tube filled with liquid, in which sits a ball with reasonable mass. The ball will
                                                    position itself in the tube depending on the resultant local
                                                    acceleration vector. In steady level flight this will be just due
                                                    to the effect of gravity and the ball will sit in the centre. For a
                                                    balanced turn the resultant of the gravity vector and the
                                                    angular acceleration vector will still keep the ball in the
                                                    center. If the ball is off centre then the accelerations are not
                                                    aligned correctly with the aircraft fuselage and a sliding or
                                                    slipping turn will result.
                                                   The rate of turn is measured by a gyroscope inside the
                                                   instrument. Click on the Turn and Bank Indicator image to
                                                   see a schematic view of the gyroscope and its operation.




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