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How to determine your true speed?


Braeden108
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So I know the indicated airspeed gauge is useless for determining your ground speed when you're at altitude. So I'm wondering what gauge I can use to know how fast I'm passing over the ground?

 

It's not useless, it will help you :)

Aircraft with INS or GPS can display ground speed (in HUD or on CDU or INS control panel display).

If no such equipment is available, you can compute using your gauges.

For this you need the pressure altitude, the temperature and the calibrated airspeed.

Then you can compute true airspeed using an E6B computer for example.

 

P. S. yet this is not how fast you're moving over the ground... For this you need to adjust the true airspeed with wind.


Edited by PiedDroit
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Awesome, I'll look into this. And I agree IAS isn't useless. I personally think its the most useful, because knowing your airspeed is mostly used for knowing how much lift you have so you can predict how the aircraft will behave.

 

So then what's the Mach gauge? Is that just a gauge who sources data from the pitot and runs on a different scale?

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I understand it now!

So indicated airspeed is read directly from the pitot tube, which is a pressure gauge. So the more pressure it reads the higher indicated speed you'll get.

So slow moving air at low altitude which is denser will produce as much pressure as fast moving air at high altitude which is less dense. Therefore the IAS gauge will read the same speed in these two cases

True airspeed measures the speed of air moving over the aircraft

So if air is moving over the aircraft at 300 knots the gauge will read 300 knots. Regardless of altitude and therefore air pressure.

Theoretically if there's a single gas molecule moving at 1000 knots across the aircraft the gauge will read 1000 knots (not really but)

Light the tires kick the fires!

 

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Mach speed only depends on air temperature (and a bit on humidity), it is used obviously to check how close you are from the speed of sound (avoid that loud bang and transsonic effects). But also useful as a reference for speed.

In civil aviation, below a certain altitude, the reference is calibrated airspeed, above it is mach (for cruise speed for example).

 

Yeah true air speed is within the air mass. With no wind, true airspeed = groundspeed.

Calibrated airspeed is indicated airspeed (= "pressure airspeed" if you will) plus an adjustment, I don't remember what.


Edited by PiedDroit
fixed true airspeed = groundspeed with no wind
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Calibrated airspeed is indicated airspeed (= "pressure airspeed" if you will) plus an adjustment, I don't remember what.

 

CAS is IAS corrected for a position/installation error.

This error is generated by the aircraft itself. The sheer presence of the aircraft "malforms" the streamlines of the air; it disrupts the airflow, thus you will get a slight error in your IAS reading, especially with an high AoA. It's usually neglible and only affects the static pressure. And this error not only affects IAS, but the (barometric or indicated) altitude indicator, too.

This is one of the reasons that many aircraft have their pitot tube(s) as far away from them as it is practical. See the A-10Cs tube at the wing tips or the MiG-21s tube extending far from the nose. :)

 

Details of the exact error and how to correct for this error are usually published in the flight manual of the aircraft in question.


Edited by Rakuzard
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Mach speed only depends on air temperature (and a bit on humidity), it is used obviously to check how close you are from the speed of sound (avoid that loud bang and transsonic effects). But also useful as a reference for speed.

In civil aviation, below a certain altitude, the reference is calibrated airspeed, above it is mach (for cruise speed for example).

 

Yeah true air speed is within the air mass. With no wind, true airspeed = airspeed.

Calibrated airspeed is indicated airspeed (= "pressure airspeed" if you will) plus an adjustment, I don't remember what.

 

This. Somewhat counter-intuitively the speed of sound in a gas is almost entirely dependent on the temperature of the gas and not the pressure, though for "non-ideal" gases the pressure does have a small but measurable effect.

 

Good Wikipedia article on the subject:

 

 

https://en.wikipedia.org/wiki/Speed_of_sound#Equations

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This. Somewhat counter-intuitively the speed of sound in a gas is almost entirely dependent on the temperature of the gas and not the pressure, though for "non-ideal" gases the pressure does have a small but measurable effect.

 

Good Wikipedia article on the subject:

 

 

https://en.wikipedia.org/wiki/Speed_of_sound#Equations

 

Cool détails

Oh damn there is a typo in my text I meant with no wind, true airspeed = groundspeed :D

 

p. s. yeah one would think it depends on density because mach increases with altitude but it's actually because of temperature.


Edited by PiedDroit
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Back to the original topic, some airspeed indicators have a second scale for TAS. You calibrate by manually matching your pressure altitude with the outside air temperature using a knob in the instrument.

 

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  • 2 weeks later...

TAS is speed at which you fly within a swiss cheese block of air. GS can be determined by TSD calculation with or without navaids (vfr)

 

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