rthrbeflying86
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Airspeeds And Mach Numbers

Mon Oct 13, 2003 11:58 am



I notice in this picture that the Mach number is 0.80, while it is only travelling at an indicated airspeed of 285 kts. Anyone know how these two are related? Does it have anything to do with the headwind component?
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rthrbeflying86
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RE: Airspeeds And Mach Numbers

Mon Oct 13, 2003 11:59 am


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goboeing
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RE: Airspeeds And Mach Numbers

Mon Oct 13, 2003 12:03 pm

As you go up higher, the air is less dense. Half of the atmosphere is below 18000' actually. So there is less air hitting the pitot tube which is measuring the ram air pressure. You have to correct for this error with altitude. The higher up you are, the larger the error. I think around 35,000 to 40,000, the indicated airspeed (IAS) will be approximately half of what the true airspeed (TAS) is.

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RE: Airspeeds And Mach Numbers

Mon Oct 13, 2003 2:08 pm

Does it have anything to do with the headwind component?

No it doesn't. IAS is not affected by headwind, only Groundspeed is. If anything a headwind will slightly increase your IAS. Let's say you are lined up on the runway with a 10 kt headwind- your IAS will be 10 kts but your groundspeed will be zero. IAS corrected for compression error is called EAS(Equivalent airspeed).
 
danialanwar
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RE: Airspeeds And Mach Numbers

Mon Oct 13, 2003 7:23 pm

As you go higher and, say, keep your Indicated AirSpeed (IAS) constant at, say, 290kts, you'll find that your Mach reading will increase.

Flying below 10,000 feet usually has a speedlimit of 250 kn IAS ... at cruising altitude of 31,000 feet, mach .85, the IAS barely increases to 290.
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jumbojettim
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 12:14 am

True airspeed increases approximately 2% for every 1000ft. For example if you are at 33000ft in the posted picture then take the 285kts IAS and multiply by 1.66% (2%*33+1), you'll get a TAS of approximately 473kts.

Then Mach is the ratio of the aircraft's TAS to the speed of sound. And this number changes with temperature.
 
flyboyaz
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 5:44 am

Even when I was in flight school, I could never figure out why they use IAS instead of TAS....anyone know? It always confused me.
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danialanwar
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 6:28 am

The tube (forgot name) that measures speed is actually measuring the impact of the air molecules onto it. And it is this that also defines the physics of staying in the air ... you need a certain mass (number of molecules) to go over & under the wing at different speeds to create lift. Thus it measures

IAS = mass * velocity / [kg]

(divide by kg to keep units clean). clearly velocity of TAS ... the actual relative speed of the molecules. in other words,

IAS = volume * density * TAS / [kg]

the volume is constant (the tube doesnt change size), but the density is less at altitude. So higher up your TAS has to go up to keep IAS in an acceptable range.
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CcrlR
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 6:53 am

I just had a test on this stuff today. Indicated Airspeed is what's read on your airspeed indicator, True Airspeed is the actual airspeed of the aircraft in the air.

With Mach numbers, anytime it's behind the poin(.) it's under the speed of sound(like a 737 going Mach .82/285 knots). If it's past that point(.), it's past the speed the sound(I.E. The concorde goes up to Mach 2.00)
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 10:42 am

Even when I was in flight school, I could never figure out why they use IAS instead of TAS....anyone know?

Because as far as the plane is concerned that's all it cares about. All the critical speeds that you need as a pilot are in IAS. The plane will stall at a certain IAS, you rotate at a certain IAS, you approach at a certain IAS etc. As Danial mentioned above IAS is a direct measure of the air molecules impacting against the wing/pitot tube which is what determines if the plane will stay in the air. The TAS merely changes to compensate for the IAS drop at high altitude.
 
flyboyaz
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 10:49 am

Thank you for the information! Makes more sense now....so I'm assuming that since as you ascend and the IAS doesn't go up very much.....Groundspeed is used to determine exactly how fast you are going? I don't mean to sound stupid....I just never had much exposure to flying at high altitudes.

EDIT:
I forgot......now if I'm correct, IAS becomes pretty much useless at a certain altitude, when they then switch to MACH speeds to determine how fast the plane is going. Mach speeds also vary by altitude/air pressure, as MACH .75 is not the same speed at 35K feet as it is at 20K feet? They switch to MACH speeds at 18K feet?

[Edited 2003-10-14 04:01:08]
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rthrbeflying86
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 10:49 am

This is all very interesting, thanks for the input.
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HeavyMX1
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 11:39 am

Mach is a measure of the speed of sound. Mach .80 is 80% of the speed of sound. But that 80% is not always the same speed because the speed of sound changes with temperature as Jumbojettim said. The density of the air does not affect sound speed. So the Machmeter compensates for temperature change. But get this it doesn't measure the temperature to compensate, it uses altitude. In goes on the bases that at a certain altitude it is a certain temperature. Sorry if this confused you but I tried my best.
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flyboyaz
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RE: Airspeeds And Mach Numbers

Tue Oct 14, 2003 11:56 am

Yeah....sure....I understand now..... Nuts

No, I'm kidding...that makes a lot of sense!

Thank you  Smile
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