Aircraft speed: Difference between revisions

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Aircraft speed (view source)

Revision as of 18:57, 26 October 2016

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Speed combines two factors, the ''distance'' travelled in a certain amount of ''time''. In aviation speed is most often expressed in '''knots''' (kt). One knot is one nautical mile per hour. In aircraft the speed is "measured" with a [[#Pitot tube|pitot tube]], the result is not the speed of the aircraft, it is the speed of the air flowing around the aircraft, the [http://en.wikipedia.org/wiki/Airspeed '''airspeed'''].
Speed combines two factors, the ''distance'' travelled in a certain amount of ''time''. In aviation speed is most often expressed in '''knots''' (kt). One knot is one nautical mile per hour. In aircraft the speed is "measured" with a [[#Pitot tube|pitot tube]], the result is not the speed of the aircraft, it is the speed of the air flowing around the aircraft, the [http://en.wikipedia.org/wiki/Airspeed '''airspeed'''].


The airspeed can be indicated in knots, km/h or even m/s. In general however, knots are used. But is some countries (like Russia and China, km/h is used).
In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometres per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in knot.  
In older planes, notably German WW II fighter planes, the airspeed is indicated in kilometres per hour (km/h), which is still used in present-day European glider planes. The conversion factor is 1.852, i.e. you can roughly divide a reading in km/h by two in order to get the value in knot.  


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* [http://en.wikipedia.org/wiki/Indicated_airspeed '''Indicated''' airspeed (IAS)] is the number displayed on the [http://en.wikipedia.org/wiki/Airspeed_indicator airspeed indicator].  
* [http://en.wikipedia.org/wiki/Indicated_airspeed '''Indicated''' airspeed (IAS)] is the number displayed on the [http://en.wikipedia.org/wiki/Airspeed_indicator airspeed indicator].  


Airspeed is usually measured with a [[#Pitot tube]] at the front of the aircraft. The IAS can be the CAS. The IAS is not the TAS since the pressure differs greatly with [[altitude]] (more specific the density of the air). The higher the [[altitude]] the lower the IAS while flying the same TAS.
The Indicated Airspeed is determined with Impact Pressure (measured with a [[#Pitot tube]]) and Static Pressure. Because of measurement faults IAS usually has a failure. Without this failure you get CAS. The IAS is not the TAS since the pressure differs greatly with [[altitude]] (more specific the density of the air). The higher the [[altitude]] the lower the IAS while flying the same TAS.


In spite of this dependence on [[altitude]], IAS is a very useful quantity in flight. Many aerodynamical properties, for example drag, lift, the stress on the airframe, stall speed or the forces on control surfaces depend on the dynamic pressure generated by the airstream, not on the actual aircraft speed. The stall speed of an aircraft at sea level is very different from the stall speed (in TAS) at 30.000 ft - but they correspond to the same IAS reading.
In spite of this dependence on [[altitude]], IAS is a very useful quantity in flight. Many aerodynamic properties, for example drag, lift, the stress on the airframe, stall speed or the forces on control surfaces depend on the dynamic pressure generated by the airstream, not on the actual aircraft speed. The stall speed of an aircraft at sea level is very different from the stall speed (in TAS) at 30.000 ft - but they correspond to the same IAS reading.


At sea level, a IAS of 400 knot roughly corresponds to 400 knot TAS. At 80.000 feet (the cruising altitude of a [[SR-71]]), the IAS of 400 knot corresponds to a TAS in excess of 1600 knot (..that corresponds with about Mach 3 at that altitude).
Per definition CAS is TAS in standard ISA conditions and sea level. At 80.000 feet (the cruising altitude of a [[SR-71]]), the IAS of 400 knot corresponds to a TAS in excess of 1600 knot (..that corresponds with about Mach 3 at that altitude).


=== Calibrated airspeed ===
=== Calibrated airspeed ===
* [http://en.wikipedia.org/wiki/Calibrated_airspeed '''Calibrated''' airspeed (CAS)] is calculated from the [[#Pitot tube]] measurement and correcting it for standard errors.
* [http://en.wikipedia.org/wiki/Calibrated_airspeed '''Calibrated''' airspeed (CAS)] is calculated from IAS and correcting it for measurement errors.


Modern equipment can most often can indicate the CAS. For navigation the CAS is the first step to calculate the GS.
Modern equipment can most often can indicate the CAS. For navigation the CAS is the first step to calculate the GS.


=== Equivalent airspeed ===
=== Equivalent airspeed ===
* [http://en.wikipedia.org/wiki/Equivalent_airspeed '''Equivalent''' airspeed (EAS)] takes into account another correction (above [[#Calibrated airspeed]], this time having to do with air properties rather than sensor errors.  
* [http://en.wikipedia.org/wiki/Equivalent_airspeed '''Equivalent''' airspeed (EAS)] takes into account another correction (above [[#Calibrated airspeed]], this time having to do with air properties rather than sensor errors. EAS at low altitude and low airspeeds is very close to CAS, but CAS incorporates compressibility effects, EAS assumes no compressibility.


At high altitude, the compressibility of air changes, so even CAS becomes more and more unreliable. For the [[SR-71]] Blackbird with a ceiling of 85.000 feet, the CAS becomes very unreliable and the plane has to be flown based on a EAS. For more conventional aircraft, EAS is not used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude. The EAS is the calculated result from the ram pressure (measured by the [[#Pitot tube]]) and the static pressure (measured by the [[altimeter]]).
At high altitude, the compressibility of air changes, so even CAS becomes more and more unreliable. For the [[SR-71]] Blackbird with a ceiling of 85.000 feet, the CAS becomes very unreliable and the plane has to be flown based on a EAS. For more conventional aircraft, EAS is not used. Thus, EAS is what a perfect dynamic pressure sensor would show when properly calibrated for the air compressibility at the current altitude.  


=== Mach number ===
=== Mach number ===
* The [http://en.wikipedia.org/wiki/Mach_number '''Mach number''' (M)] is the speed of the aircraft divided by the speed of sound (at that altitude). It is a calculated number without a unit.
* The [http://en.wikipedia.org/wiki/Mach_number '''Mach number''' (M)] is the speed of the aircraft divided by the speed of sound (at that temperature). It is usually calculated, but can also be directly determined with Impact and Static pressure. Mach has no dimension.


The aircraft's behaviour at Mach 1 at sea level is about the same as the behaviour of the aircraft at an altitude of 60000 feet.
The aircraft's behavior at Mach 1 at sea level is about the same as the behavior of the aircraft at an altitude of 60000 feet.
A Mach number below 1 means that the plane moves subsonic. A Mach number above 1 indicates supersonic flight. The Mach number is critical because a number of phenomena take place just around Mach 1 (transonic speed), for example a sudden increase in drag induced by shock-wave generation (sonic-boom). Aircraft that are not designed to fly supersonic will break up at Mach 1. The shape of the aircraft can cause parts of the aircraft being at or above Mach 1 while the fuselage is subsonic. Flying near Mach 1 can be quite dangerous, for most fast (but subsonic) aircraft Mach 0.83 is the limit. High flying aircraft, like passenger aircraft, can reach that limit easy while descending.
A Mach number below 1 means that the plane moves subsonic. A Mach number above 1 indicates supersonic flight. The Mach number is critical because a number of phenomena take place just around Mach 1 (transonic speed), for example a sudden increase in drag induced by shock-wave generation (sonic-boom). Aircraft that are not designed to fly supersonic will break up at Mach 1. The shape of the aircraft can cause parts of the aircraft being at or above Mach 1 while the fuselage is subsonic. Flying near Mach 1 can be quite dangerous, for most fast (but subsonic) aircraft Mach 0.83 is the limit. High flying aircraft, like passenger aircraft, can reach that limit easy while descending.


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| V<sub>3</sub> || Flap retraction speed.
| V<sub>3</sub> || Flap retraction speed.
|-
|-
| V<sub>A</sub>|| Design manoeuvring speed. Above this speed it is a bad idea to make sudden manoeuvres.  
| V<sub>A</sub>|| Design maneuvering speed. Above this speed it is a bad idea to make sudden manoeuvres.  
|-
|-
| V<sub>C</sub> || Design cruising speed, also known as the optimum cruise speed, is the most efficient speed in terms of distance, speed and fuel usage.
| V<sub>C</sub> || Design cruising speed is like V<sub>A</sub> related to the loads on the aircraft structure. For non-reciprocating aircraft it is the red line on the airspeed indicator.
|-
|-
| V<sub>D</sub> || Max dive speed (for certification only)
| V<sub>D</sub> || Max dive speed (for certification only). It is always higher as V<sub>C</sub> and it has a margin towards the speed where the aircraft is shown to be free from flutter.
|-
|-
| V<sub>FE</sub> || Maximum flap extended speed.
| V<sub>FE</sub> || Maximum flap extended speed.
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== Pitot tube ==
== Pitot tube ==
The [http://en.wikipedia.org/wiki/Pitot_tube pitot tube] is the tool to measure the airspeed. It is a tube directed forwards, exposed to the airstream. The air is being pushed inwards (rammed) by the motion of the aircraft and the (ram) pressure is measured. The measured pressure is corrected indicating the airspeed. The ram pressure is also called the dynamic pressure opposite the static pressure that us used to indicate [[altitude]]. Bigger aircraft have two pitot tubes and the indicator displays the average of the two. However, most often only one pitot tube is used to control the autopilot, even when the indicator is connected with two.  
The [http://en.wikipedia.org/wiki/Pitot_tube pitot tube] is the tool to measure the Impact Pressure. It is a tube directed forwards, exposed to the airstream. The air is being pushed inwards (rammed) by the motion of the aircraft and the (ram) pressure is measured. The measured pressure relative to Static pressure is used to determine the (indicated) airspeed. The Impact pressure is NOT the dynamic pressure as it incorporates compressibility effects. Bigger aircraft have two (or even three) pitot tubes.  


The pitot tube can be blocked easy, once blocked, or worse, partially blocked the IAS will have no relation with the speed of the aircraft. This situation is enhanced if the pitot tube controlling the autopilot is blocked.
The pitot tube can be blocked easy, once blocked, or worse, partially blocked the IAS will have no relation with the speed of the aircraft. This situation is enhanced if the pitot tube controlling the autopilot is blocked.
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