= Airspeed
{title2=$V$}
\b[Airspeed] describes an <aircraft>'s motion relative to the surrounding air. When the kinematic magnitude is intended, it is the <true airspeed>,
$$
V=\lVert\mathbf v_{\rm aircraft/ground}-\mathbf v_{\rm air/ground}\rVert.
$$
It differs from <groundspeed> whenever wind changes the relative motion. A ground-station <velocity> alone cannot establish the aerodynamic <airspeed>.<Relative Velocity by NASA Glenn Research Center>[Airspeed and Wind Speed]{ref}
An airspeed instrument normally infers motion from <pressure>. At low <Mach number>, for an aligned <Pitot-static tube> in the regime of the <Bernoulli equation>,
$$
p_0-p=\frac12\rho V^2,\qquad V=\sqrt{\frac{2(p_0-p)}{\rho}}.
$$
Here $p_0$ is <stagnation pressure>, $p$ is <static pressure> and $\rho$ is air <mass density>. For a stipulated difference of $245\,\mathrm{Pa}$, $\rho=1.225\,\mathrm{kg/m^3}$ gives $V=20\,\mathrm{m/s}$; the same difference at $0.6125\,\mathrm{kg/m^3}$ gives $28.28\,\mathrm{m/s}$. The instrument senses a pressure difference, so interpreting its reading requires a density model and an <instrument calibration>.<Pitot – Static Tube – Speedometer by NASA Glenn Research Center>[Pressure connections, Solve for Velocity and Limitations]{ref}
<Indicated airspeed> is the displayed reading before instrument and installation corrections. <Calibrated airspeed> includes those corrections. <True airspeed> expresses actual motion through the air, rather than simply relabeling either reading. The FAA distinguishes these quantities in its instrument chapter.<Glider Flying Handbook by Federal Aviation Administration>[Chapter 4, printed pp. 4-4–4-6, Types of Airspeed and Pitot-static system]{ref}
The square-root calculation above assumes an appropriate low-speed stopping relation. At appreciable <Mach number>, <compressibility> changes that relation; a blocked passage or displaced <static pressure> source introduces a different measurement problem. Report which <airspeed> convention and corrections accompany a performance curve. Comparing speeds without that information can confound a change of air <mass density> with a change in the aircraft's loading. Functioning ports and known corrections are part of the inference, not extra measurements of <groundspeed>.
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