Airspeed is the velocity of an aircraft relative to the air through which it flies. It is distinct from ground speed (velocity relative to the surface) because the air mass itself is usually in motion — wind. An aircraft flying north at 100 km/h airspeed into a 20 km/h headwind has a ground speed of 80 km/h. The aerodynamics do not know or care about the ground; all lift, drag, and stability depend on airspeed.
Indicated vs. true airspeed
Indicated airspeed (IAS) — what a pitot-static system measures: the dynamic pressure of the oncoming air, converted to speed assuming standard sea-level density. IAS is directly proportional to the aerodynamic forces on the aircraft. Stall speed, maneuvering speed, and maximum speed are specified as indicated airspeeds because the wing feels dynamic pressure, not true speed.
True airspeed (TAS) — the actual speed of the aircraft relative to the surrounding air. At sea level in standard conditions, TAS equals IAS. At altitude, where air density is lower, TAS is higher than IAS for the same aerodynamic condition. An aircraft indicating 30 m/s at 3,000 m altitude is actually moving at ~33 m/s through the air.
TAS = IAS × √(ρ₀ / ρ)
where ρ₀ is standard sea-level density and ρ is the actual air density. At 5,000 m, TAS is roughly 1.29× IAS.
Equivalent airspeed (EAS) — a compressibility-corrected version of IAS used for structural calculations and high-speed flight. At the speeds typical of small UAVs (M < 0.3), EAS and IAS are effectively identical.
Airspeed measurement in UAVs
Most fixed-wing UAVs measure airspeed with a pitot tube — a forward-facing tube that captures total (stagnation) pressure, compared against static pressure from a port on the fuselage side. The difference is dynamic pressure, from which IAS is calculated.
Airspeed is critical for fixed-wing UAV flight because:
- Below stall IAS, the wing cannot generate enough lift for flight.
- The flight controller needs airspeed to distinguish between a headwind gust (airspeed increases, groundspeed constant) and a tailwind gust (airspeed drops, groundspeed increases) — both look identical to GPS.
- Throttle and pitch management depend on airspeed for efficient cruise.
Multirotors do not use airspeed measurement because they can hover (zero airspeed) and their lift is generated by rotor thrust, not forward motion. Some larger multirotors add a pitot tube for wind estimation, but it is not required for flight.
Small UAVs with no pitot tube estimate airspeed from GPS groundspeed and an assumed wind model — a method that works acceptably in calm conditions but degrades in wind.
Related terms
- Dynamic Pressure — the quantity that airspeed measurement actually captures (½ρV²)
- Stall — the minimum-airspeed boundary of the flight envelope
- Mach Number — airspeed relative to the local speed of sound
- Atmosphere — the medium whose density determines the IAS-TAS relationship