An airspeed indicator measures the difference between the pressure in a forward-facing pitot tube and the still air pressure at a static port, which equals ½ × air density × speed², so speed is the square root of twice that pressure divided by density. At sea level that pressure is 170 Pa at 60 km/h and 473 Pa at 100 km/h, so the scale is crowded at the slow end where stalls happen, and the reading depends on air density and on where the tube sits in the airflow. It must be calibrated against a known speed, and no scale can be copied from a drawing.
Part of Aerodynamics and Flight
How a pitot-static system turns air pressure into a speed reading, its limits, and how to calibrate one.
The Physics Is Bernoulli’s Relation, Used Correctly
A pitot tube pointing into the airflow brings the air to rest at its mouth, so its pressure is the still-air (static) pressure plus ½ρV². A static port on the side of the fuselage, flush with the surface, senses the static pressure alone. The difference is dynamic pressure q = ½ρV² (Bernoulli’s Principle). Speed V = √(2q ÷ ρ). At sea level ρ = 1.225 kg/m³ is the value the corpus uses elsewhere (Airfoil Profile (Lift vs Drag)).
Worked Example: The Pressures You Must Measure
1 mm of water column is 9.81 Pa (1,000 kg/m³ × 9.81 m/s² × 0.001 m).
| Speed | In m/s | q = ½ × 1.225 × V² | In mm of water |
|---|---|---|---|
| 50 km/h | 13.89 | 118.2 Pa | 12.0 mm |
| 60 km/h | 16.67 | 170.1 Pa | 17.3 mm |
| 80 km/h | 22.22 | 302.5 Pa | 30.8 mm |
| 100 km/h | 27.78 | 472.6 Pa | 48.2 mm |
Rounded, that is about 170 Pa at 60 km/h and 473 Pa at 100 km/h. Doubling the speed quadruples the pressure. The pressures are small, tens of millimetres of water, so the instrument is a sensitive differential gauge: a water U-tube or a thin diaphragm or capsule driving a needle. The corpus’s Pressure Gauge covers a Bourdon tube, which suits boiler pressures and is far too stiff for these.
Why The Scale Is Not Linear
Because q rises with V², the scale spreads out at high speed and bunches at low speed. From 50 to 60 km/h the pressure changes by 170.1 − 118.2 = 51.9 Pa; from 90 km/h (382.8 Pa) to 100 km/h (472.6 Pa) it changes by 89.8 Pa, so each km/h at the fast end moves the needle about 1.7 times as much as at the slow end. The result is that the instrument is least readable exactly around stall speed, which is the speed that matters most (Stall Dynamics). Lift also depends on q, so the aircraft stalls at nearly the same indicated speed whatever the altitude, which is a use of the instrument as well as a limitation.
Calibrating: Do Not Invent A Scale
Step 1: Mount the pitot tube and a water manometer on a vehicle or a boom, in clean airflow ahead of the body, in calm air. Drive a measured course (500 m, say) at steady speeds.
Step 2: Time each run in both directions. The arithmetic mean of the ground speeds of the two runs is the airspeed, because a steady wind adds to one and subtracts from the other: for example a 500 m course covered in 30 s (16.7 m/s) one way and 24 s (20.8 m/s) the other averages to 18.75 m/s (67.5 km/h). The times are an illustration.
Step 3: Record the manometer reading at each speed and mark the scale. That is a calibration of the instrument.
Step 4: Remember what this does not calibrate. In the aircraft the tube sits in a different flow, near the wing, propeller or fuselage, and a static port reads pressure that varies with angle of attack (position error). Further checks are made in the flight-test sequence.
Faults
- Blocked pitot (insects, water, ice, a cover left on): the reading stays or drops, and in a climb it may rise.
- Blocked static: the reading is wrong in a climb or a descent.
- Leaks and kinks in the lines.
- Static port near a surface disturbance.
Common Mistakes
- Copying a scale in km/h from a drawing.
- Ignoring density — at height the indicated speed is lower than the true speed.
- Leaving the pitot cover on — remove it as a pre-flight item.
Safety
A wrong airspeed reading can hide a stall
Pilots have flown an aircraft into a stall because the indicator, blocked or miscalibrated, read a safe speed. Look into the pitot and static openings before every flight, check the lines for kinks, and confirm the needle moves on the take-off run; if it does not, abort. Know the attitude and sound of the aircraft near its stall speed as well.
What’s Next
- Learn next: Altimeter, the other pressure instrument
- Combine with: Ground Testing, where instruments are checked