Tag: Machmeter

Pitot Static System…Airspeed Calculation

 

A.  Airspeed Calculation:

Airspeed is calculated as a function of the difference between Pitot Pressure and Static Pressure as follows:

Calculated or Indicated airspeed is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.

Calibrated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows:

 

minus position and installation error correction.

 

Where

 is the calibrated airspeed,

 

is the impact pressure (inches Hg) sensed by the pitot tube,

 

is 29.92126 inches Hg; static air pressure at standard sea level,

 

is 661.4788 knots:, speed of sound at standard sea level

 

Units other than knots and inches of mercury can be used, if used consistently.

This expression is based on the form of Bernoulli’s equation applicable to a perfect, incompressible gas. The values forand   are consistent with the ISA i.e. the conditions under which airspeed indicators are calibrated.

Keep in mind that this is for your basic vanilla airspeed indicator and does not include calculations for TRUE Airspeed for which you must include the variables of True Temperature and True Altitude.

 

Stay tuned for upcoming Blogs

Pitot Static System ….Inside & Out

 

A. Pitot Pressure:
Pronounced: PEE-TOE, it is a French word

Pitot pressure is the measurement of the air forced into the Pitot Tube by the movement of the aircraft through the air. Pitot tubes are mounted on the aircraft facing forward so that air is forced into them. Most small aircraft have only one tube, larger aircraft have a redundant system and will have two tubes. The most common manufacturer of these tubes is Rosemont Corp. which is a division of BF Goodrich. Also on larger aircraft, those that fly at higher altitudes, the Pitot Tube is heated in order to prevent icing, smaller aircraft typically do not have this function.

The Pitot Tube is connected directly to the back of the airspeed indicator (the Pitot input) and if the aircraft is so equipped also to the Air Data Computer via a hose which is typically either plastic or rubber

 

B. Static Pressure:

Static pressure is the measurement of the ambient barometric pressure at the aircraft’s CURRENT location AND CURRENT Altitude.
The Static Port is located in a position on the aircraft that will not be affected by air flow as the aircraft moves through the air. This is typically on the side of the fuselage but can also be on the back side of the Pitot Tube or any other number of locations, it varies by the aircraft. Again smaller aircraft will typically have one Static Port, larger aircraft with redundant systems will have two.

The Static Port is connected directly to the following equipment, depending on aircraft configuration: The Airspeed Indicator (Static Input), the Altimeter, the Vertical Speed Indicator, the Altitude Encoder, the Air Data Computer. Again connection is typically made via a hose either rubber or plastic.

 

C.  Airspeed Calculation:

Airspeed is calculated as a function of the difference between Pitot Pressure and Static Pressure as follows:

 

Calculated or Indicated airspeed is indicated airspeed corrected for instrument errors, position error (due to incorrect pressure at the static port) and installation errors.

Calibrated airspeed values less than the speed of sound at standard sea level (661.4788 knots) are calculated as follows:

minus position and installation error correction.

 

Stay tuned for upcoming Blogs