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Relationships between Airspeeds

Indicated Airspeed: (IAS). The speed registered on the Airspeed Indicator.

Calibrated Airspeed: (CAS). An accurate measure of dynamic pressure when the aircraft is flying slowly. The position of the pitot tube(s) and static vent(s), together with the aircraft’s configuration (flaps, landing gear etc.) and attitude to the airflow (angle of attack and sideslip) will affect the pressures sensed, particularly the pressures sensed at the static vent(s).

Under the influence of the above conditions a false dynamic pressure (IAS) will be displayed. When IAS is corrected for this ‘position’ or ‘pressure’ error, as it is called, the resultant is Calibrated Airspeed. (The airspeed corrections to be applied may be displayed on a placard on the flight-deck, or in the Flight Manual, and will include any instrument error).

Equivalent Airspeed: (EAS). An accurate measure of dynamic pressure when the aircraft is flying fast. Air entering the pitot tube(s) is compressed, which gives a false dynamic pressure (IAS) reading, but only becomes significant at higher speeds.

At a given air density, the amount of compression depends on the speed of the aircraft through the air. When the IAS is corrected for ‘position’ AND ‘compressibility’ error, the resultant is Equivalent Airspeed.

True Airspeed: (TAS) or (V). The speed of the aircraft through the air. THE ONLY SPEED THERE IS - All the other, so called, speeds are pressures.

	TAS =	EAS	Where,	б is Relative Density
		√ б

The Airspeed Indicator is calibrated for ‘standard’ sea level density, so it will only read TAS if the density of the air through which the aircraft is flying is 1.225 kg/m3. Thus at 40 000 ft where the ‘standard’ density is one quarter of the sea-level value, to maintain the same EAS the aircraft will have to move through the air twice as fast!

The Speed of Sound: (a) Sound is ‘weak’ pressure waves which propagate spherically through the atmosphere from their source. The speed at which pressure waves propagate is proportional to the square root of the absolute temperature of the air. The lower the temperature, the lower the speed of propagation. On a ‘standard’ day at sea level the speed of sound is approximately 340 m/s (660 kt TAS).

At higher aircraft True Airspeeds (TAS) and/or higher altitudes, it is essential to know the speed of the aircraft in relation to the local speed of sound. This speed relationship is known as the

Mach Number (M).

	M =	TAS	Where (a) is the Local Speed of Sound
		(a)

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If the True Airspeed of the aircraft is four tenths the speed at which pressure waves propagate through the air mass surrounding the aircraft, the Mach meter will register M 0.4

Critical Mach Number: (MCRIT) The critical Mach number is the Mach number of the aircraft when the speed of the airflow over some part of the aircraft (usually the point of maximum

thickness on the aerofoil) first reaches the speed of sound.

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Airspeed

This information is to reinforce that contained in the preceding paragraphs.

The airspeed indicator is really a pressure gauge, the ‘needle’ of which responds to changes in dynamic pressure (½ ρ V2 ).

The Airspeed Indicator

is a Pressure Gauge

Calibration of the airspeed indicator is based on standard sea level density (1.225 kg/m3). The “airspeed” recorded will be different from the actual speed of the aircraft through the air unless operating under standard sea level conditions (unlikely). The actual speed of the aircraft relative to the free stream is called true airspeed (TAS), and denoted by (V). The ‘speed’ recorded by the airspeed indicator calibrated as above, if there are no other errors, is called equivalent airspeed (EAS).

It may seem to be a drawback that the instrument records equivalent rather than true airspeed, but the true airspeed may always be determined from it. Also, many of the handling characteristics of an aircraft depend mainly on the dynamic pressure, i.e. on the equivalent airspeed, so it is often more useful to have a direct reading of EAS than TAS.

Errors and Corrections

An airspeed indicator is, however, also subject to errors other than that due to the difference between the density of the air through which it is flying and standard sea level density.

•Instrument Error: This error may arise from the imperfections in the design and manufacture of the instrument, and varies from one instrument to another. Nowadays this type of error is usually very small and for all practical purposes can be disregarded. Where any instrument error does exist, it is incorporated in the calibrated airspeed correction chart for the particular aeroplane.

•Position Error (Pressure Error): This error is of two kinds, one relating to the static pressure measurement, the other to the pitot (total) pressure measurement. The pitot tube(s) and static port(s) may be mounted in a position on the aircraft where the flow is affected by the presence of the aircraft, changes in configuration (flaps and maybe gear) and proximity to the ground (ground effect). If so, the static pressure recorded will be the local and not the free stream value. The pitot pressure may be under-recorded because of incorrect alignment - the tube(s) may be inclined to the airstream instead of facing directly into it (changes in angle of attack, particularly at low speeds). The magnitude of the consequent errors will generally depend on the angle of attack and, hence, the speed of the aircraft.

•Compressibility Error: At high speeds, the dynamic pressure is not simply ½ ρV2, but exceeds it by a factor determined by Mach number. Thus the airspeed indicator will over-read.

Because of the errors listed, the ‘speed’ recorded on the airspeed indicator is generally not the equivalent airspeed. It is called instead the indicated airspeed. Corrections to rectify the instrument and position errors are determined experimentally. In flight, using special instruments, measurements are taken over the whole range of speeds and configurations, from which a calibration curve is obtained which gives the corrections appropriate to each indicated airspeed. The compressibility error correction may be obtained by calculation.

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The indicated airspeed, after correction for instrument, position (pressure) and compressibility errors, gives the equivalent airspeed ½ ρ V2.

V Speeds

These include: VS , V1 , VR , V2 , VMD , VMC , VYSE and many others - these are all Calibrated Airspeeds because they relate to aircraft operations at low speed. However, the appropriate

corrections are made and these speeds are supplied to the pilot in the Flight Manual as IAS.

VMO - The maximum operating IAS is, however, an EAS because it is a high speed, but again it is supplied to the pilot in the Flight Manual as an IAS.

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Summary

Dynamic pressure (Q) is affected by changes in air density.

Q = ½ ρ V2

Air density decreases if atmospheric pressure decreases.

Air density decreases if air temperature increases.

Air density decreases if humidity increases.

With the aircraft on the ground:

Taking off from an airfield with low atmospheric pressure and/or high air temperature and/or high humidity will require a higher TAS to achieve the same dynamic pressure (IAS).

For the purpose of general understanding:

A constant IAS will give constant dynamic pressure.

Increasing altitude decreases air density because of decreasing static pressure.

With the aircraft airborne:

As altitude increases, a higher TAS is required to maintain a constant dynamic pressure. Maintaining a constant IAS will compensate for changes in air density.

There is only one speed, the speed of the aircraft through the air, the TAS. All the other, so called, speeds are pressures.

The Airspeed Indicator is a pressure gauge.

Aircraft ‘V’ speeds are CAS, except VMO which is an EAS, but all are presented to the pilot in the Flight Manual as IAS.

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