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Ernest

Aircraft Airspeed

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Hi.I was wondering if anybody else has noticed this.When I'm flying with any aircraft, the airspeed that shows on the airspeed indicator differs from what is shown when I use shift+z. The difference can be anywhere from 20 knots to higher depending on what aircraft I fly.I keep the setting on show true airspeed.Is this normal? If not, is there a correction which will allow both (shift+z/airspeed indicator) to give the same airspeed.Another example is when I'm flying the 737-800 above 30 thousand feet, in order to keep 330 knots, I have to set airspeed above 400 knts on the autopilot.Thanks for reading this.RegardsErnest

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>I keep the setting on show true airspeed.You should have the sim set to display Indicated Airspeed, not True Airspeed.All of your Vref numbers are based on IAS, the most critical of which is your a/c's stall speed... ;)

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Thank you for both replies.I will change the setting and see what happens.Also, is it possible the fsx config file could be corrupted, and if so, would deleting it and allowing it to re-load as it were be any use?RegardsErnest

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BillThis is a basic question. Say you are flying with headwind of 30 knots and for example coming into land your IAS might show 80kias(normal landing speed) but your GS may show only 50 knots (which could be your stall speed. Do you still keep 80kias to land or do you increase that to 110kias so that you land at 80knots GS and perhaps do not stall. In other words do you always use the kias irrespective of any wind tail or head?Peter Hayes

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If I understand this correctly, the GPS shows what the ground speed is in relation to IAS? If that is correct, when I use shift+Z and get that nice red readout along the top left of my monitor, is that also showing what the groud speed is? I'm just trying to make sure that when my aircraft's IAS is showing one speed and gps/shift+Z is showing something else while on final, I don't want to think I'm going to slow or too fast for landing. Or maybe I should just forget about using shift+Z altogether as it may not be of any value.RegardsErnest

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>If I understand this correctly, the GPS shows what the ground>speed is in relation to IAS? If that is correct, when I use>shift+Z and get that nice red readout along the top left of my>monitor, is that also showing what the groud speed is? I'm>just trying to make sure that when my aircraft's IAS is>showing one speed and gps/shift+Z is showing something else>while on final, I don't want to think I'm going to slow or too>fast for landing. Or maybe I should just forget about using>shift+Z altogether as it may not be of any value.>I have the menu setting at "indicated airspeed", and use "shift Z" ALL the time, since it's much easier for quick glances. With indicated airspeed as the setting, shift Z should should be the same as the IAS gauge.L.Adamson

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>Bill>This is a basic question. Say you are flying with headwind of>30 knots and for example coming into land your IAS might show>80kias(normal landing speed) but your GS may show only 50>knots (which could be your stall speed. Do you still keep>80kias to land or do you increase that to 110kias so that you>land at 80knots GS and perhaps do not stall. In other words do>you always use the kias irrespective of any wind tail or>head?>Peter HayesYes, you always use KIAS...........the issue being that it's the speed the aircraft is moving through the air that affects its ability to fly.Paul

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>Bill>This is a basic question. Say you are flying with headwind of>30 knots and for example coming into land your IAS might show>80kias(normal landing speed) but your GS may show only 50>knots (which could be your stall speed. Do you still keep>80kias to land or do you increase that to 110kias so that you>land at 80knots GS and perhaps do not stall. In other words do>you always use the kias irrespective of any wind tail or>head?Yes, always use IAS, since it's the actual measurment of air flowing across the top and bottom of the wings. Tail and head winds will just effect the actual speed across the ground.Something to think about..... High altitude airports will usually have longer runways, because your groundspeed will be higher for the same indicated airspeed. You'll find that the takeoff distance is longer to get to the required rotation speed.L.Adamson

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IAS is realistic and what should be used. When your flying the heavies though, it doesn't read correctly as it starts to compress as you get higher. This is also realistic and real pilots deal with this. In an airliner IAS is only needed to judge approach and TO speeds. Once you start climbing and get into cruise use Mach (select it instead of speed on the autopilot). You'll also see your mach tape on the IAS gauge. Thats how fast your really going. The actual number gauge will read like 330 knots when your cruising at mach .80, but your really going more like 470 knots.

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Just so you dont get too confused here by the many well meaning answers, some of which arent quite accurate, don't feel alone because many don't understand the different speeds...There are basicly 3 you need to understand and how they apply:IAS is the airspeed on your gauge, also called CAS if it has been calibrated. It is your standard reference speed for aircraft performace and all numbers in the POH are referenced to this.TAS is True airspeed and that is the IAS adjusted for temperature, humidity, and altitude. The higher you fly, the thinner the air. The plane goes faster for the same indicated airpseed. For example, 100kts IAS at sea level will be 100kts TAS. 100kts at 10,000 ft will result in a TAS much higher(I dont have an exact number). The third speed is Ground speed. That is your TAS computed for any wind. At 100kts TAS with a 20kt headwind your GS will be about 80kts.A fourth number is Mach if you fly at very high flight levels. That is a percentage of the speed of sound, and varies with altitude. Its used generally above FL270. .80 mach at FL270 and .80 at FL350 will result in different TAS, as the speed of sound changes with altitude.This is a simplified explanation, I hope it helps.XP Pro SP2-FSX SP2AMD FX60-8800GTS-2 Gigs RAMFEX-GEX-UTUSA-FSGenesis-and a bunch of other stuffComputer optimized by www.fs-gs.com

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I believe TAS varies from IAS, as a rule of thumb, by 2% per 1000ft of altitude. So 100kts IAS at 10000ft would be 120kts TAS, and then figure winds into the equation. Perhaps someone with real knowledge could correct or expand upon my post, as I'm only going by memory of something I read. Thank you,KailFlightSimmer since 1987C2D E6850 3.0GHz 1333FSBXFX Nforce 680i LT SLI2x XFX 8800GT 512MB SLI'd4GB Crucial Ballistix PC6400 800MHzCreative SB Audigy2 ZSUltra Xfinity 600W SLI PSUSeagate 320GB SATA-3GB/S HD for O/SWD 160GB SATA 3GB/S HD for FSXWindows XP SP3 / FSX SP2 / FS9 SP1

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I thought that I might throw some wood on the fire. The simple Airspeed indicator in aircraft has two inputs one from a static source which measures the local undisturbed air pressure and one from a dynamic source (from a pitot tube) this measures the pressure exerted on the air due to forward speed through the air. The pitot pressure is fed into a closed but elastic container and the static pressure if fed into a closed area outside of the pitot can (or vice versa). As the dynamic pressure is greater than the static pressure the can changes its shape and this change is fed through a geared connecting mechanism to the instrument face. So an ASI is in reality a measure of dynamic pressure. The formula for dynamic pressure is: 0.5 * Air density * velocity squared, commonly referred to as 'Q'. So dynamic pressure and thus the reading on the ASI varies with aircraft velocity and density in the local atmosphere. So if a pilot is flying at a constant true speed and the local air density changes then the ASI will vary. If an aircraft gains altitude the local density (mainly due to falling temperature)will drop and the indicated air speed will change. BTW density is a function of Volume and Temperature whilst static pressure is a function of density. I'll let Geoffa drop in and discuss the difference between pressure and density altitudes. As previously stated, from the pilots viewpoint the only thing important to remember is that if the aircraft stalls when the ASI reads a certain indicated air speed it will always stall at that indicated air speed due to the mathmatical relationship between Q and the lift provide by the aircraft's lifting surfaces. The aircraft speed over the ground is totally irrelevant to aircraft pilotage, it is only relevant to navigation. The GPS measures the speed over the ground. I'm not sure what the Shift 'z' speed given is, I think it rather depends on how you have those windows set up but for the standard set up it is supposed to be air speed although I don't know how that figure is derived. I learned all this aerodynamics when I was in my teens and I'm now a proud sexy generian so I hope my memory serves me correctly.

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Thank you all for your replies and valuable imput.Since practice makes perfect, I'll just keep with the practice.Thank youErnest

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