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Velocities of an Aircraft: Airspeed and Groundspeed

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Velocities of an Aircraft: Airspeed and Groundspeed Velocities of an aircraft : Airspeed and Groundspeed Pitot- static system and instruments 2 Measuring air speed •The air speed of an aircraft is the speed at which it progresses through the air mass in which it is flying. The simplest method of measuring air speed is to measure the pressure of air which is exerted against the nose of the aircraft by the air mass ahead of it. This principle is used in practice and the device used to measure the pressure is called the Pitot Tube and the pressure of the air that it samples is Pitot Pressure . The pressure so given is directly proportional to the air speed. The amount of pressure will not only be a function of air speed but it will also be a function of altitude and air temperature. Air normally becomes less dense with an increase in height so that a direct measurement of this nature can be misleading. Never the less it can be used, and provided account is taken of the density of the surrounding air, readings taken in this fashion can be converted to give the true air speed. 3 Pitot tube •Pitot pressure is composed of two elements : •a. Static Pressure: The natural pressure that is exerted by the atmosphere. •b .Dynamic Pressure : The pressure generated by the aircraft's movement. Pitot Pressure = Static Pressure + Dynamic Pressure or Dynamic Pressure = Pitot Pressure - Static Pressure 4 Airspeed Indicator A simple mechanical device attempting to “solve” a very complex equation 5 Airspeed indicator 6 Airspeed Indicator (ASI) • The ASI is a sensitive, differential pressure gauge which measures and promptly indicates the difference between Pitot (impact/dynamic pressure) and static pressure. These two pressures are equal when the aircraft is parked on the ground in calm air. When the aircraft moves through the air, the pressure on the pitot line becomes greater than the pressure in the static lines. This difference in pressure is registered by the airspeed pointer on the face of the instrument, which is calibrated in kilometers per hour, miles per hour, knots (nautical miles per hour), or both. • The ASI introduces the static pressure into the airspeed case while the Pitot ressure (dynamic) is introduced into the diaphragm. The dynamic pressure expands or contracts one side of the diaphragm, which is attached to an indicating system. The system drives the mechanical linkage and the airspeed needle. 7 8 9 10 11 КУС-1000/1200 12 13 14 15 Single-engine airspeed indicator • White arc —commonly referred to as the flap operating range since its lower limit represents the full flap stall speed and its upper limit provides the maximum flap speed. Approaches and landings are usually flown at speeds within the white arc. Lower limit of white arc (V S0 ) - the stalling speed or the minimum steady flight speed in the landing configuration. In small aircraft, this is the power-off stall speed at the maximum landing weight in the landing configuration (gear and flaps down). Upper limit of the white arc (V FE ) - the maximum speed with the flaps extended. • Green arc —the normal operating range of the aircraft. Most flying occurs within this range. Lower limit of green arc (V S1 ) - the stalling speed or the minimum steady flight speed obtained in a specified configuration. For most aircraft, this is the power-off stall speed at the maximum takeoff weight in the clean configuration (gear up, if retractable, and flaps up). Upper limit of green arc (V NO ) - the maximum structural cruising speed. Do not exceed this speed except in smooth air. • Yellow arc —caution range. Fly within this range only in smooth air, and then, only with caution. • Red line (V NE )—never exceed speed. Operating above this speed is prohibited since it may result in damage or structural failure. 16 Air Speed Indicator Errors • a. Instrument Error: Due to small manufacturing imperfections and the need to generate a large needle movement from a small capsule movement. Any error is noted when the instrument is tested and calibrated prior to use and an error card is mounted in the aircraft cockpit near the indicator. • b. Pressure Error : -Position Error - Caused by poor sitting of the Pitot Tube. Modern design has virtually eliminated this problem. -Manoeuvre Error - As the aircraft manoeuvres to the right or to the left the Pitot tube may temporarily not be facing directly into the free-stream air and will give false readings. c. Density Error : An air speed indicator is calibrated for the International Standard Atmosphere and will therefore be in error if there is any deviation from ISA conditions. Atmospheric density decreases rapidly with height an this causes the air speed indicator to under read by significant amounts. • d. Compressibility Error : At speeds above 300 knots the air is compresse and temporarily brought to a halt in front of the pitot tube. A false pressure enters the pitot tube and causes the air speed indicator to over read. 17 Compressibility Correction Chart 18 Compressibility correction factors 19 Types of airspeeds • Indicated airspeed (IAS) —the direct instrument reading obtained from the ASI, uncorrected for variations in atmospheric density, installation error, or instrument error. Manufacturers use this airspeed as the basis for determining aircraft performance. Takeoff, landing, and stall speeds listed in the AFM/POH (Aircraft Flight Manual/Pilots Operating Handbook) are IAS and do not normally vary with altitude or temperature. 20 Calibrated airspeed (CAS) • CAS — IAS corrected for installation error and instrument error. Although manufacturers attempt to keep airspeed errors to a minimum, it is not possible to eliminate all errors throughout the airspeed operating range. At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds. In the cruising and higher airspeed ranges, IAS and CAS are approximately the same. Refer to the airspeed calibration chart to correct for possible airspeed errors. 21 Equivalent air speed (EAS) • EAS is CAS corrected for compression of air in the Pitot tube (same as CAS in standard atmosphere at sea level; less than CAS at higher altitudes and faster airspeeds). EAS is an airspeed that has taken into account errors within the system due to the compression within the pitot tube. It's an airspeed that is never used in every day aviation. 22 True airspeed • TAS – is calibrated airspeed corrected for air density (altitude and nonstandard temperature) and is the speed of the aircraft through the air mass. Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure. Therefore, for a given CAS, TAS increases as altitude increases; or for a given TAS, CAS decreases as altitude increases. 23 Calculation True Air Speed TAS can be found by two methods: • The most accurate method is to use a flight computer. With this method, the CAS is corrected for temperature and pressure variation by using the airspeed correction scale on the computer. Extremely accurate electronic flight computers are also available. Just enter the CAS, pressure altitude, and temperature, and the computer calculates the TAS. • A second method , which is a rule of thumb, provides the approximate TAS. Simply add 2 percent to the CAS for each 1,000 feet of altitude. The TAS is the speed which is used for flight planning and is used when filing a flight plan. i) TAS = CAS in standard atmosphere at sea level ii) Some true airspeed indicators have temperature-compensated aneroid bellows iii) Others have a knob requiring pilot input to align outside air temperature (OAT) with pressure altitude so that TAS is then indicated 24 True air speed equation 25 TAS Calculation • TAS = EAS · 171233 ·[ (288±VAR)– 0.006496H] 0.5 : (288-0.006496H)2.628 here VAR – temperature deviation from ISA, 0C; H- altitude, m. • TAS = EAS · 171233 ·[ (288±VAR)– 0.00198H] 0.5 : (288-0.00198H)2.628 here VAR – temperature deviation from ISA, 0C; H- altitude,ft. 26 Calculation True Airspeed throw Conversion factors TAS = kCF ·EAS 27 Ground speed • Groundspeed is the aircraft’s actual velocity with respect to the surface of the Earth. It is true air speed corrected for wind or found from a vector sum VG=V+VWIND 28 Memory aids ICE Tea – Pretty Cool Drink • Instrument – Position error • Calibrated – Compressibility error • Equivalent – Density error 29 Airspeed Measurement ICE-T 30 Aircraft’s Speed Summary 31.
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