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3 Answers

TAS error with Temp

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Aircraft Systems

I understand how temperature (directly related to desnity, or number of molocules of air per volume) is related to Alitmeter errors, based on the shrinking and expanding of the atmosphere for changing temperatures. I also understnad that the ASI is based on how much air is being rammed into the pitot tube, more molocules is equal to more pressure, so a higher airspeeed is indicated. I have rote knowledge of 'with higher tmep comes higher true airseeed,' but that doesnt sit right with me. What I'm confused on, is HOW this all works. If the ASI is based on the differenctial pressures seen in the static and pitot, with a change in temperature, both densities are changing. The warmer air means less pressure in the pitot AS WELL AS the static, so I would think that they would both see the same change, and TAS versus IAS would not change?

 

What am i missing?

sorry for the poor wording.

 

Chris

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3 Answers



  1. Lucas on Apr 09, 2013

    Dear Chris

    The airspeed indicator as you said is connected to the pitot tube and the static source. This is done to eliminate any errors due to changes in pressure, but not in temperature.
    I like to explain this with the following formula (this is not a real formula, its just to make it easier to understand) V=(D + S) – S where V = Indicated Airspeed at a certain pressure S – the pressure S taken from the static port. So lets say that the speed is 100 Kt and the pressure is 27.92, if everything is working fine the indicated AS would be (100 + 27.92) – 27.92 = 100. If the static port blocked at this altitude and we climbed 2,000 feet then V= (100 + 25,92) – 27.92 = 98 and vice versa (again these numbers are not accurate). So as I said as long as the whole pitot static system works fine pressure errors are nonexistent.

    What you are talking about is density of the air. Yes a higher temperature would lower the pressure but as shown above that error is automatically fixed by the system itself.

    What is happening is that for example if you are flying in cold air there is (more) air pushing against the diaphragm of the ASI pushing it further and therefore showing a higher indication on the ASI (Indicated airspeed) than the one you are actually flying at (True Airspeed) and vice versa if you are flying in hot air there is less air pushing on the diaphragm so the ASI indicates a lower speed than the one you are flying at.

    So do not look at it under a pressure point of view (that, as we said, is not going to affect the ASI) but rather as an actual amount of air pushing mechanically on the diaphragm of the instrument.

    I made the following video a while back to explain how an altimeter works and although your question asks about ASIs take a peak at it anyway. It might shed some further light on your instrumentation understanding.

    http://www.youtube.com/watch?v=dLdsdQa6cmo&list=UU1AilFA8X2FR3FepaFvLBDw&index=6

    Cheers Lucas

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  2. Nathan Parker on Apr 09, 2013

    “The warmer air means less pressure in the pitot AS WELL AS the static, so I would think that they would both see the same change, and TAS versus IAS would not change?”

    The face of the pitot is also slamming into the air ahead of the aircraft, which the static port is not doing. So the pitot tube measures both this slamming pressure and the ambient static pressure, but the static port measures only the ambient static pressure. The airspeed indicator makes both ambient pressures cancel out, leaving only the slamming pressure.

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  3. Sam Dawson on Apr 12, 2013

    A picture is worth a thousand words:

    http://www.luizmonteiro.com/Learning_Pitot_Sim.aspx

    You can change the outside conditions for a lower than standard temperature and see what happens.

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