Richard Lancaster [email protected]
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Glider Instruments Richard Lancaster [email protected] ASK-21 glider outlines Copyright 1983 Alexander Schleicher GmbH & Co. All other content Copyright 2008 Richard Lancaster. The latest version of this document can be downloaded from: www.carrotworks.com [ Atmospheric pressure and altitude ] Atmospheric pressure is caused ➊ by the weight of the column of air above a given location. Space At sea level the overlying column of air exerts a force equivalent to 10 tonnes per square metre. ➋ The higher the altitude, the shorter the overlying column of air and 30,000ft hence the lower the weight of that 300mb column. Therefore: ➌ 18,000ft “Atmospheric pressure 505mb decreases with altitude.” 0ft At 18,000ft atmospheric pressure 1013mb is approximately half that at sea level. [ The altimeter ] [ Altimeter anatomy ] Linkages and gearing: Connect the aneroid capsule 0 to the display needle(s). Aneroid capsule: 9 1 A sealed copper and beryllium alloy capsule from which the air has 2 been removed. The capsule is springy Static pressure inlet and designed to compress as the 3 pressure around it increases and expand as it decreases. 6 4 5 Display needle(s) Enclosure: Airtight except for the static pressure inlet. Has a glass front through which display needle(s) can be viewed. [ Altimeter operation ] The altimeter's static 0 [ Sea level ] ➊ pressure inlet must be 9 1 Atmospheric pressure: exposed to air that is at local 1013mb atmospheric pressure. 2 Static pressure inlet The pressure of the air inside 3 ➋ the altimeter's casing will therefore equalise to local 6 4 atmospheric pressure via the 5 static pressure inlet. Atmospheric pressure 0 [ 3500ft ] ➌ 9 1 decreases with altitude. Atmospheric pressure: 890mb 2 As atmospheric pressure ➍ decreases the aneroid capsule expands, moving the 3 linkages and hence rotating Decrease in pressure the display needle(s). has caused aneroid 6 4 capsule to expand and 5 rotate the needle. [ Atmospheric pressure disturbances ] An aircraft moving through ➊ the atmosphere changes the pressure of the air surrounding it. This is how wings generate lift. Even the air inside the cockpit will not quite be at local atmospheric pressure. This means that care must ➋ be taken when trying to expose the altimeter's static pressure inlet to air that is at local atmospheric pressure. [ Static vents and altimeter pipework ] Static vents Altimeter When designing a glider, the engineers will try A pair of “static vent holes” ➊ to find a location on the fuselage where the ➋ are drilled into the fuselage at passing airflow exerts a pressure on the this location and connected to aircraft's surface that is close to local the altimeter's static pressure atmospheric. They do this using a combination inlet via a flexible plastic tube. of their experience, calculations and test This allows the air in the results. On a glider, such a point is often altimeter's casing to equalise located about half way down the tail boom. with local atmospheric. [ Barometric pressure subscale ] The adjustment knob on the ➊ front of an altimeter allows the pilot to dial in, on the Barometric pressure subscale barometric pressure subscale, the current atmospheric pressure at the altitude they wish to call 0ft. 0 9 1 For example they might dial in the current pressure at 8 2 sea level, or the current 985 pressure at ground level on a 990 particular airfield. 7 3 6 4 The knob adjusts the 5 ➋ linkages within the altimeter, such that when the atmospheric pressure around the aircraft matches Adjustment knob the setting on the subscale the altimeter will read 0ft. [ Air Speed Indicator (ASI) ] [ The Pitot tube ] Pitot tube: A tube, often mounted in the glider's nose, the open end of which is exposed directly to the oncoming airflow. [ Pitot tube characteristics ] When a glider is in flight, the Pitot tube ➊ oncoming air will try to flow into the [ Speed: 0kts ] open end of the Pitot tube. No oncoming airflow. Capsule pressure: 1013mb Connecting a capsule to the back E.g. sea level atmospheric ➋ end of the Pitot tube will mean that the air flowing in has nowhere to go. Capsule The pressure in the capsule will therefore rise until it is high enough to prevent any further air from [ Speed: 30kts ] entering. Capsule pressure: 1015mb Sea level atmospheric Increasing the airspeed of the glider + 2mb due to oncoming airflow ➌ will cause the force exerted by the oncoming air to rise. More air will therefore be able to push its way into the capsule and hence the pressure within the capsule will increase. [ Speed: 60kts ] Capsule pressure: 1019mb Sea level atmospheric The pressure inside the capsule + 6mb due to oncoming airflow ➍ will therefore increase as airspeed increases. [ ASI anatomy ] Display needle Linkages and gearing: Connect the diaphragm 50 capsule to the display needle. 40 60 70 Total (Pitot) Static pressure inlet pressure inlet 80 Diaphragm capsule: A capsule with elastic 10 90 properties that is 0 airtight except for its connection to the total pressure inlet. Like a balloon it will Enclosure: expand as the pressure inside it Airtight except for the static pressure increases above the inlet. Has a glass front through which pressure outside it. display needle can be viewed. [ ASI pipework ] Total pressure inlet Static pressure inlet Static vents ASI Pitot tube [ ASI operation ] In a stationary aircraft, the air 50 [ Speed 0kts ] ➊ throughout the ASI will have 40 60 equalised with local atmospheric pressure. The 70 diaphragm capsule will 1013mb 1013mb therefore be collapsed like a Atmospheric 80 Atmospheric deflated balloon and the pressure pressure needle will be displaying zero. 10 90 0 As airspeed increases, the ➋ pressure inside the 50 [ Speed 60kts ] diaphragm capsule will rise 40 60 above local atmospheric due to the force exerted down the Pitot tube by the oncoming 70 airflow. The capsule will 1019mb 1013mb therefore inflate like a 80 Atmospheric balloon, moving the linkages pressure and rotating the needle. 10 90 0 [ The effect of altitude on the ASI ] 50 Like pressure, air density 40 60 ➊ also decreases with altitude. 30 70 ASI of an aircraft flying at 60knots at sea level The ASI's diaphragm capsule 20 80 ➋ is calibrated to correctly 10 90 display airspeed when the air 0 that the aircraft is moving 50 through is of average sea 40 60 level density. 30 70 ASI of an aircraft flying at 10,000ft60knots atat 10,000ft60 knots Above sea level, due to the 20 80 ➌ lower air density, the build up 10 90 of pressure in the diaphragm 0 capsule will be lower than 50 the ASI expects it to be. The 40 60 ASI will therefore under read. 30 70 ASI of an aircraft flying at 60knots at 20,000ft The higher the altitude 20 80 ➍ above sea level the more 10 90 the ASI will under read. 0 [ TAS and IAS ] TAS: True Air Speed The true speed at which the aircraft is moving through the air. IAS: Indicated Air Speed The airspeed displayed on the ASI. At altitudes above sea level this will What is the point of an ASI that be lower than the true airspeed. isn't capable of displaying true airspeed at altitudes above sea level? We will attempt to answer this question over the next couple of slides... [ Flight dynamics, altitude and TAS ] The decrease in air density with ➊ altitude also affects a glider's flight dynamics. Altitude True airspeed (TAS) at which stall occurs in steady wings level flight For example, a glider that stalls ➋ at a true airspeed of 40knots in steady wings level flight at 1,000ft 40 knots 1,000ft, will stall at a true 10,000ft 46 knots airspeed of 54knots at 20,000ft. 20,000ft 54 knots Consider therefore the 30,000ft 64 knots ➌ inconvenience that this would cause if the ASI did actually 40,000ft 79 knots display true airspeed (TAS). The pilot would need to continuously Table 1: Illustration(+) showing how steady wings use quick reference cards to level flight stall speed is affected by altitude. look up the stall speed, best L/D (+) speed and min sink speed for Remember that different gliders stall at different speeds. This table is just an illustration. The figures for your glider the current altitude. Not a will be different, but the trend will be the same. particularly convenient thing to have to do while trying to fly. [ Flight dynamics, altitude and IAS ] Fortunately for the pilot, the ➊ amount by which the ASI under Altitude Indicated airspeed (IAS) at reads approximately cancels out which stall occurs in steady the air density related changes wings level flight to the glider's flight dynamics. 1,000ft 40 knots This means that if the indicated 10,000ft 40 knots * ➋ airspeed (IAS) at the point of stall at 1,000ft in steady wings 20,000ft 40 knots * level flight is 40knots. Then the indicated airspeed at 20,000ft at 30,000ft 40 knots * the point of the stall will also be 40knots. Despite the fact that 40,000ft 40 knots * the stall is actually occurring at a 14knot higher true airspeed. Table 2: Illustration(+) showing that the indicated airspeed (IAS) at which a glider stalls in steady wings level flight does not vary with altitude. The pilot therefore only needs to (+) ➌ remember one set of numbers Remember that different gliders stall at different speeds. This table is just an illustration. The IAS figure for your that will work at all altitudes. glider will be different. There is however one very * In reality, secondary effects will cause small variations as important exception... altitude increases. [ IAS and Vne ] The previous slide detailed how ➊ most of the flight characteristics Altitude Vne (IAS) of a glider (stall speed, best L/D speed etc.) occur at a particular indicated airspeed (IAS) 5,000ft 151 knots regardless of altitude.