Simviation Forum - KIAS vs. Mach

Reply #15 - Sep 1^st, 2005 at 3:40pm

James Turbett Offline
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Mach 1 is slower at higher altitudes (At FL350, 290knts is about 0.82M and at around FL270, 290knts is somthing like 0.72M).
The figures may not be accurate but while knots is a constant speed no matter what hight you are, the speed of sound is slower the higher up you go.

(somthing to do with air density alowing vibrations to travel through the molecules quicker).

James Turbett&&jimmyturbo@hotmail.co.uk

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Reply #16 - Sep 1^st, 2005 at 3:42pm

James Turbett Offline
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Quote:

Mach 1 is slower at higher altitudes (At FL350, 290knts is about 0.82M and at around FL270, 290knts is somthing like 0.72M).
The figures may not be accurate but while knots is a constant speed no matter what hight you are, the speed of sound is slower the higher up you go.

(somthing to do with air density alowing vibrations to travel through the molecules quicker).

the denser the material the quicker the sound travels. However the density of the air doesn't affect the speed of sound as much as you describe.

Airspeed (and altitude) is determined by the pitot static system, it measures the air passing by the aircraft (I think, someone else will be able to describe it bettter)

Because the air is less dense, then as your ground speed stays the same (the speed you are relative to the ground), however your Indicated airspeed(IAS) changes - goes down as you go higher (this is what happens when you select mach speed as your autopilot mode and you change altitude.) (mach speed is about the same as ground speed)

conversely when you select IAS mode, and your IAS stays the same (say 270kts) then when you change altitude your ground speed changes, it gets slower as you go down.

Someone else will probably be able to give a much better explanation than I can.

James Turbett&&jimmyturbo@hotmail.co.uk

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Reply #17 - Sep 1^st, 2005 at 3:43pm

James Turbett Offline
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IAS is done via what the aircraft 'feels' its speed at. So its accurate at sea level but as you climb, the air becomes less dense and so the aircraft 'feels' like it is going slower. For example Concorde at Mach 2 and 60,000ft is only doing around 435 IAS, but 1,150kts true airspeed.

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Reply #18 - Sep 1^st, 2005 at 3:44pm

James Turbett Offline
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IAS is what manufacturers use as the basis for determining aircraft performance. IAS does not normally vary with alt or temp as they affect the aerodynamics of the airframe and ASI equally.

You need to know the types of airspeeds to get the idea of Mach. Most pilots use an acronym of ICETG. Indicated, Calibrated, Equivalent, True, Ground. Indicated is what you read on the ASI. Calibrated is Indicated corrected for installation errors...the pitot tube not pointing into the wind at high AOA. Equivalent is Calibrated corrected for adiabetic flow...compression...at airspeeds above 200 KIAS. True is the actual airspeed you fly through undisturbed air. Ground is ground speed...duh.

Mach is the ratio of the A/C TRUE airspeed to the speed of sound. Ex: .85 is 85% of the speed of sound at that temperature. If you want to compute the True airspeed from a conventional ASI, you have to think about air density which is affected by temp and altitude. With a mach indicator, the temperature determines the speed of sound, and provides a more useful index for aircraft.

Don't think about the speed of sound as being the speed that sound travels through a material of a certain density...it's more about what the temp is and what the speed of the A/C is. As you go higher, the air is cooler, therefore, the IAS is slower compared to GS.

Hope that makes it clear as mud.

James Turbett&&jimmyturbo@hotmail.co.uk

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Reply #19 - Sep 1^st, 2005 at 3:45pm

James Turbett Offline
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All aircraft have limiting airspeeds. With faster aircraft, the limiting mach # comes into play. At higher speeds the mach readout is more important than IAS, because IAS has no *direct* relationship(you can say the speed of sound is 700 knots, but you at the same time have to specify the altitude) with the speed of sound. Yes, sound travels slower in less dense air(less molecules for which the wave to move through of course), but the aircraft is flying in that less dense air too. The mach indication is therefore accurate, unlike the IAS which is based on a dynamic and static air pressure, and can be said to be inaccurate so long as the instrument is calibrated for sea level pressure, which it is. If you took off and climbed to say, FL300, at 250 kias, your ground speed would have been accellerating the whole time, instead of the constant airspeed you thought you were flying. Mach should have climbed the whole time.

James Turbett&&jimmyturbo@hotmail.co.uk

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