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Please someone describe the following sentence in simple words:

The Airspeed Indicator is calibrated for 'standard' sea level density, so it will only read TAS if the density of the air through which the aircraft is flying is 1.225 kg/m^3.

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Airspeed is basically measured by measuring the kinetic energy of the air molecules impacting dynamic pressure port (for example on a pitot tube).

The higher the velocity, the more energy, hence more pressure, and higher speed indication.

If the density of the air is changed while speed remains the same, the energy will change. If the air is denser, for example during high ambient pressure weather, there will be more molecule impacts, higher pressure, and the indicator will show higher than actual true airspeed reading.

If the air is less dense, like when flying higher, the opposite happens: while the true airspeed is the same, there are less molecule impacts, less energy, less pressure, and the indicator will read lower than normal.

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    $\begingroup$ I find extreme examples useful when trying to keep things like this straight in my head. Your air speed indicator will read too low in space and likely too high in water. $\endgroup$ May 23 at 14:23
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Remember that an airspeed indicator is a simple differential pressure gauge. It reacts to a difference between static pressure and ram (stagnation) air pressure.

Static air pressure is just the pressure exerted on air within our atmosphere at a certain altitude by the weight of the air above it. Gravity is constantly pulling all the molecules of our atmosphere toward the Earth, which compresses the gas against the Earth’s surface.

Static air pressure increases the lower in the atmosphere you are and decreases the higher up you are.

Ram air pressure (scientific term is stagnation pressure) is air pressure which is the sum of both the local static pressure as well as the pressure created at the boundary layer from a change in momentum of the air. Put simply, an object moving through the atmosphere is forcefully pushing the atmosphere out of its way and this exerts a component of pressure on the air as well.

Since an airspeed indicator works by simple manometry, it needs some means to correlate that differential pressure change to an analogous pressure it would experience if the gauge was sampling at a particular airspeed. Manufacturers typically calibrate an airspeed gauge to accurately display airspeed at sea level in SI standard atmospheric conditions. So if a pitot tube is sampling dry air at 101.5 kPa, 15° C, the indicated airspeed reflects the actual airspeed. But since the air gets thinner with altitude, temperature, moisture, etc the actual airspeed will be higher in those conditions than what is indicated by the airspeed indicator. This is the difference between indicated airspeed (IAS) and actual or true airspeed (TAS).

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The airspeed indicator is based on ram pressure for a very good reason: because the wings need to deflect a certain mass of air in a certain amount of time to generate lift to support the aircraft.

When the aircraft is higher, the air is thinner, but the aircraft mass is more or less constant$^1$. In order to deflect enough air to maintain lift, the aircraft must move faster.

The pitot tube works the same way. The plane must move faster when air is thinner to collide with enough air molecules to increase pressure in the tube equally to a slower speed in denser air.

This is why True Air Speed increases relative to Indicated Air Speed in air of lower density.

$^1$ the plane does get lighter with fuel consumption

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  • $\begingroup$ And not just the lift on the wings! Everything aerodynamic that happens to a body in motion in a fluid is dependant on the change in momemtum (the force), of the fluid particles impacting and deflecting from the surface of the body. $\endgroup$ May 23 at 13:33
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Ram air pressure is simply the airflow pushing against an object.

Imagine travelling down the freeway at 75 MPH. You open the window and stick out your hand. That pressure is ram air pressure.

The airspeed indicator takes that ram air pressure and subtracts out static air pressure or what you feel when the car is not moving. (You do feel something but may not realize it as it is always there).

The difference between ram air pressure and static air pressure is read on the airspeed indicator.

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ASI’s are calibrated so that IAS = TAS at sea level on an ISA standard day. As you increase altitude the air becomes less dense so to indicate the same IAS the TAS has to increase to produce the same dynamic pressure measured by the ASI.

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Please someone describe the following sentence in simple words

The higher you go, the thinner the air is.²
The Airspeed Indicator is calibrated for the 'thickness' of air at sea level. At higher altitudes, it will give 'wrong' readings, and not show your actual speed (TAS, True Air Speed)

² = Air being thinner means if you have a 10 liter box at sea level, it will contain "more air" than it will at 10,000ft altitude.

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