What Is Space Velocity?

Airspeed refers to the speed of the aircraft relative to the air. The unit is KM / H or NM / H. It is a necessary parameter for calculating the aerodynamics of the aircraft. It is also an important basis for estimating the range of the aircraft. Therefore, it is also a necessary parameter for the flight of the aircraft. According to the differences in measurement methods,

Airspeed refers to the speed of the aircraft relative to the air. The unit is KM / H or NM / H. It is a necessary parameter for calculating the aerodynamics of the aircraft. It is also an important basis for estimating the range of the aircraft. Therefore, it is also a necessary parameter for the flight of the aircraft. According to the differences in measurement methods,

Airspeed can be divided into indicated airspeed, calibrated airspeed, equivalent airspeed, and true airspeed. The measurement of various airspeeds includes both traditional methods based on fluid mechanics and modern methods based on Doppler frequency shift.

Chinese name

airspeed

Foreign name

Air Speed

Solid

Aircraft speed relative to air

Classification

Indicated airspeed, calibration airspeed, etc.

Real airspeed

True Air Speed, also known as vacuum speed. Represents the speed of the aircraft relative to the surrounding air during flight. Its English abbreviation is TAS, which is represented by the symbol VT. The flying vacuum speed of an aircraft is defined as the component of the aircraft's center of gravity relative to the velocity of the airflow projected into the plane of symmetry of the aircraft's longitudinal axis. The speed of the aircraft relative to the air can also be equivalently regarded as the aircraft not moving and the air flowing through the aircraft at the same velocity and in the opposite direction. According to the Bernoulli equation of the fluid, the flow velocity can be converted into pressure, that is, dynamic pressure (approximately equal to stamping). According to aerodynamics, shock waves are generated when the air velocity is equal to or greater than the speed of sound. State parameters such as air pressure and temperature before and after the shock wave will change drastically, which is very different from low-speed airflow. Therefore, in fluid mechanics, air velocity is generally divided into less than the speed of sound (subsonic speed) and greater than the speed of sound (supersonic speed). Two cases are used to discuss the constraint relationship between pressure, temperature, flow rate and other parameters. Because the vacuum speed dial is calibrated according to the standard atmospheric state at sea level, as the flight altitude changes, the air density changes accordingly. The speed indicated by the speedometer is different from the true airspeed of the aircraft relative to the air. The relationship between the two is:

among them:

Is the density of air at flight altitude

Is the density of sea level standard atmosphere

Is the vacuum speed at flight altitude

Is equivalent airspeed

Vacuum speed is used in aircraft performance calculations and pilot calculations.

Airspeed indication

Indicated Air Speed, also known as table speed, is based on the measured dynamic pressure and the relationship between air speed and dynamic pressure under standard atmospheric conditions at sea level (760 mm mercury column, air temperature above 15 degrees). The abbreviated form of the speed value (IAS) is represented by the symbol Vi. For flight safety, pilots operate the aircraft based on the indicated airspeed.

Dynamic pressure in flight has a very important relationship with flight safety. For example, the lift of an airplane is directly proportional to the dynamic pressure. In the term "dynamic pressure",

Pd = * V * V * 1/2, where: is the atmospheric density, and V is the flying speed (vacuum speed)

This is the case when air compressibility is not considered.

Under standard atmospheric conditions at sea level (760 mm mercury column, temperature above 15 degrees Celsius), its density is Pd, so,

Pd in the formula is dynamic pressure. The resulting speed is the indicated airspeed.

It actually represents the size of the aerodynamics of the aircraft and is one of the most important flight data related to flight safety. Therefore, there is a special instrument (or one of the combination instruments) on the aircraft to indicate it, so the indicated airspeed is also called gauge speed.

The figure below is a typical combined airspeed indicator. The thick pointer indicates the airspeed and the thin pointer indicates the vacuum speed.

Airspeed calibration

Calibrated air speed is also called Calibrated Air Speed. It indicates the air speed displayed on the airspeed meter after correcting the installation error and the instrument indication error. Its abbreviated form is (CAS), which is represented by the symbol Vc. .

Under high-speed and high-altitude conditions, the calibration airspeed also needs to correct errors due to air compressibility. At this time, the calibration airspeed is also called equivalent airspeed (EAS). When sailing at sea level and under international standard atmospheric conditions (15 ° C, 1013 hPa, humidity 0%), the calibrated airspeed, equivalent airspeed and true airspeed (TAS) are the same, if there is no wind at that time This speed is also the ground speed (GS). Under other conditions, the calibration airspeed may be different from vacuum or ground speed.

If the unit of calibration airspeed is knots, the abbreviation of KCAS is generally used, and the abbreviation of the indicated airspeed at this time is KIAS. In aviation, there are two main applications for calibrated airspeed:

(1) During navigation, if you want to know the real airspeed from the indicated airspeed, you need to get the calibrated airspeed first.

(2) In flight control, the calibrated airspeed (or equivalent airspeed) is the main reference signal because it describes the dynamic pressure acting on the surface of the aircraft. Equivalent airspeed is used for design when controlling the aircraft, but when the altitude changes, the simple airspeed indicator cannot display the value of equivalent airspeed. Because at sea level, the calibrated airspeed will be equivalent to the equivalent airspeed, and the calibration airspeed will be similar to the equivalent airspeed at higher altitudes. Therefore, the calibrated airspeed will be used as the reference for correcting the airspeed indicator.

Because of the use of global positioning systems and other advanced navigation systems, pilots can directly read ground speed (often true airspeed), so the importance of the first application mentioned above has rapidly decreased. However, the second application is still very important. For example, when the aircraft has the same load, the aircraft can roll and climb at any altitude, its calibrated airspeed is almost unchanged, but the ground speed and real airspeed are significantly different. .

The corrected airspeed is the airspeed reading after the indicated airspeed value is corrected by the position error. The position error is due to the direction of the airflow at the total and static pressure pipes installed at a certain position on the aircraft. It will change with the specific aircraft and the angle of attack. , Affect the accuracy of total and static pressure measurement, resulting in airspeed error. Corrected airspeed

) And indicated airspeed (

The relationship is:

Where

It is the position error correction value, which is obtained through test flight and given in the flight manual. It is related to the angle of attack, flap position, ground effect, wind direction, and other factors. Corrected airspeed is mostly used to indicate the speed of flight tests, such as stall speed and takeoff speed, but the takeoff speed in the flight manual is still indicated by the indicated speed. ^[1]

Airspeed equivalent airspeed

Equivalent Air Speed (Equivalent Air Speed) is the airspeed value of the corrected airspeed data after a specific altitude adiabatic compressibility correction. Its abbreviated form is (EAS), which is represented by the symbol VE.

Equivalent airspeed is not only a function of the total and static pressure difference, but also related to the pressure at flight altitude. This requires an equivalent speed dial corresponding to the total and static pressure difference to be created for each barometric altitude. This is obviously not the case. It is possible, so an equivalent speed dial based on the standard atmospheric state at sea level is usually used. The readings of this meter are accurate only under standard atmospheric conditions at sea level, and other heights need to be corrected. This correction is called Additional adiabatic compression flow correction. Equivalent airspeed

) And corrected airspeed (

The relationship is:

The correction value for adiabatic compression is obtained by formula calculation or look-up table. It has nothing to do with specific models, but only related to flight altitude and corrected airspeed. The equivalent speed is often used to indicate the speed of the load on the aircraft strength calculation. ^[1]