Airspeed

The flight speed is the speed of an aircraft.

In general, all distinguished in aviation flight speeds in knots are given. For larger altitudes and speeds, the Mach number is (ie as a ratio to the speed of sound) of additional benefits.

Flights of civil aircraft at supersonic speeds (greater than Mach 1 ) is prohibited by law in Germany.

When airspeed two important reference systems must be distinguished:

Speed ​​relative to the air (travel, Airspeed )

Measurement methods

The speed is measured called by a Pitot tube, pitot tube also, in the German language area actually Prandtlsonde or Prandtl Pitot tube. It directly measures the pressure difference between the static pressure of the air that flows around the aircraft, and the total pressure of static pressure and dynamic pressure or back pressure. Because of this difference in pressure, the indicated airspeed (IAS ) is derived.

In principle, the measurement with the pitot tube in exactly the same degree of the ( decreasing upward ) air density is dependent on how well the lift on the wings, so that the influence of air density is taken into account automatically. Thus, the measurement method provides a good basis for the evaluation of flight performance.

In the early days of aviation aircraft attempts have been made at slow to use a vane for measuring the speed.

The measurement by pitot tube also contains a number of interferences. Therefore, several levels of correction be made, the intermediate results each with its own name:

Uncorrected airspeed (IAS )

The Uncorrected airspeed ( speed, devices or Indicated airspeed; . Engl indicated airspeed, IAS) is the speed of an aircraft relative to the surrounding air mass, which would result directly from the measured pressure difference.

The IAS has meaning only for small aircraft, since larger aircraft directly the CAS is displayed.

Calibrated airspeed (CAS )

When Corrected airspeed (English calibrated airspeed CAS) corrected the so-called static error, but not the dynamic error.

Characterized in that the fuselage moves the air to the side, a pressure wave is formed like the bow wave of a ship. This pressure wave leads to a falsification of the measurement result. The distortion can vary significantly depending on the aircraft models. Since the error depending on the installation location of the pitot tube also varied, one also speaks of the instrument and installation errors ( engl. static source error ).

If the measurement scale was not calibrated for the particular aircraft type, the correction is based on a table, possibly based on a formula specified in the technical manual of an airplane type. For small aircraft and flown with them low speeds, the error may remain so small that it can be disregarded.

Equivalent airspeed (EAS )

The equivalent airspeed (English equivalent airspeed EAS) additionally corrects the error due to the compressibility of air.

With increasing speed increases another Pitot tube measurement error considerably: the compression of the air. Characterized in that the air is compressed before the Pitot tube, the pitot tube provides a higher density of air, as it is. Since the error with the velocity first increases slowly, these dates vary the speed at which the error must be taken into account, significantly, in the range from 100 to more than 250 nodes. For takeoff and landing, this correction plays a minor role because the speeds are lower case.

At medium to high speeds, this speed is, however, essential, because the air forces depend from it (important for stability in flight ) on the aircraft. One could say that the EAS is the speed that "feels" the plane.

However: Approaching the speed of the speed of sound, instead, takes the Mach number in the foreground. (see section below)

The equivalent velocity is a fictitious velocity that are on the aerodynamic conditions information. Equivalent for the same speed of an aircraft always generates the same lift, regardless of the density of the surrounding air. At sea level in the standard atmosphere corresponding to the equivalent speed of the True speed. The thinner is the air, the higher must be the true air speed (TAS ), to the same equivalent speed, so to achieve the same aerodynamic effect. However, the equivalents speed not only makes statements about the boost to, but equally well on other forces and sizes, for example, forces acting on the landing and control flaps, or how much frictional heat is generated and the like.

For small aircraft, the EAS is consistent because of the low speeds and altitudes and in sufficient approximation with CAS or even IAS and not have to be determined.

True airspeed (TAS)

The True airspeed ( engl. true airspeed, TAS ) is the actual speed of an aircraft relative to the surrounding air. In the standard atmosphere at sea level and below 100 knots TAS and IAS are almost identical. If the air density decreases ( with increasing altitude or temperature) or the drive (increasing compression ), the IAS is lower than the TAS.

The calculation of TAS carried out from the EAS, even by the air density is included in the calculation. They can be estimated from air pressure and temperature, and indicates how much " mass attack " an airfoil is available and how much mass flows towards the sensors in the pitot system. The density of the air must not be confused with its pressure it: warmer air at the same pressure has a lower density than colder.

For commercial aircraft, the peak height to sweep off the runway a wide range of different air densities and speeds, this correction is very complex to calculate, since it must be taken into account in addition, that this also is heated by the compression of the air. The measured temperatures must be corrected accordingly.

Practically the TAS is calculated from the CAS, so that two processing steps are combined (CAS => EAS > TAS). For high speeds, the calculation is rather complex. If the calculation of the TAS is not performed by a computer-based display, they can be in an iterative calculation with the help of a flight Calculators make ( circular slide similar to a slide rule, with additional auxiliary scales and tables).

For small aircraft and their field of application in turn simplified formulas simplified display instruments can be used or are used in which, for example, the TAS is determined by the scale ring is rotated while the so-called pressure altitude is associated with the outdoor temperature to cover (see illustration of a sheath knife at the top). For high altitude and high speeds, however, such instruments would be completely inappropriate.

For pilots of small aircraft in a first approximation, the following rule of thumb applies:

For example, at an altitude of 5000 ft at an IAS of 100 kt display the true speed 5 * 2% = 10 % higher, ie 110 kt TAS.

Formula

In principle, the following dependence holds:

With:

However, this formula helps in practice only limited further because the air density can not be measured directly but must be estimated from air pressure and temperature.

Mach number

The Mach number is the ratio of TAS to the speed of sound in current air temperature.

At high speeds, the Mach number is often given instead of the TAS. There is a reason that in particular in the areas of aircraft nose and on top of the wing (especially at the wing leading edge and the transition from the wing into the fuselage ) flow velocities occur, which are significantly above the airspeed. Consequently, so come already well below the speed of sound on the aircraft flow velocities in the supersonic range. Since the loads in the range of the sound velocity increases almost by leaps and bounds, the speed limit is not an absolute quantity, but a relation to the speed of sound. Since the speed of sound decreases with decreasing temperature, so does the speed (TAS) with temperature, so that the Mach number indicates how strongly you approach the speed limit. The Mach number is thus unsuitable for flight planning, but serves the aerodynamic control. Especially with fast-flying aircraft from about Mach 0.75 is the maximum speed is not determined by the equivalent airspeed, but instead by the Mach number.

Units KIAS and KTAS

In the international civil aviation, an indication of the speed ( IAS / TAS ) is often (usually node ) combined with the unit concerned. The most common figures are therefore KIAS ( knots indicated airspeed ) and KTAS ( knots true airspeed ).

Speed ​​relative to the ground ( ground speed )

The speed over ground (English, groundspeed and GS ) denotes the order of the wind influence (ie, the movement of the air through which the air mass itself ) corrected true airspeed. It represents the speed of an aircraft relative to the earth's surface represents the knowledge of the ground speed is important for flight planning. Only she says, when individual waypoints or the destination is reached. In counter or tailwind, the ground speed can considerably from the aerodynamically important speed display in the cockpit (EAS, for small aircraft: IAS / CAS ) differ. So the flight time shortened the flight in a jet stream in the flow direction clearly because of the then much higher ground speed.

In special circumstances, a too low ground speed associated with too high a display in the cockpit to accidents such as the crash of the Star Dust over the Andes, lead.

One can calculate the ground speed by taking into account the wind by wind triangle based on the information received from Air Weather stations, but nowadays by modern on-board systems measure (eg, inertial navigation system, Doppler effect, the aeronautical radionavigation service).

To assist this also navigation methods such as GPS in question. However, the use of GPS for commercial aviation is not a legally approved method, as it is operated under military responsibility and control, and recognized by any organization of civil aviation. It is also prone to failure at high solar activity. Because of its simple usability, however, it plays a central role in the hobby flying.

Example

The different speeds are compared in the following table. Note how despite decreasing IAS ground speed increases with increasing height eventually.

Typical speeds

For small engine airplanes, the airspeed is in the range of about 50 kt (90 km / h), commercial aircraft with turboprop engine with up to 350 kt (650 km / h). For commercial aircraft with jet drive it is around 80 % to 85 % of the speed of sound, which - depending on the temperature - about 500 kt ( 930 km / h) corresponds. The speed is (eg ft below 10,000 / 3,048 m altitude more than 250 kt) Limited by the aircraft structure by rules and no later than at cruise altitude by the Mach number ( closer to the "sound barrier "). The intrinsic speed of a glider is between 50 and 270 km / h The IAS / CAS / TAS of a balloon, however, is usually close to zero, because (apart from inertia effects ) follows the air flow.

Airspeed and engine power

For the airspeed in level flight the following approximate formula:

Where:

• Horizontal velocity in m / s
• : Engine performance in hp
• : Efficiency Propeller
• : Drag coefficient of aircraft
• : Acceleration due to gravity 9.81 m/s2
• : Air density in kg/m3
• : Wing area in m2

For example, What travel speed has a small engine plane with N = 100 PS; ηL = 0.8; ρ = 1.2 kg/m3; Cw = 0.06; A = 15 m2 in low altitude?