Static and Dynamic Pressure
In fluid dynamics, two pressure types are distinguished. Both act on the aircraft and underlie lift, drag, and airspeed measurement.
Static pressure (p)
Definition: pressure exerted by a stationary parcel of air. Acts isotropically (equal in all directions).
- Unit: pascal (Pa), hectopascal (hPa), inHg.
- ISA MSL: p₀ = 1013.25 hPa = 101,325 Pa.
- Measured by a static port perpendicular to the flow.
- The altimeter measures this pressure and converts it to altitude.
Dynamic pressure (q)
Definition: pressure increment generated by moving air on impact with a surface. Acts only in the flow direction.
Formula:
q = ½ · ρ · v²
with:
-
ρ = air density (kg/m³)
-
v = flow velocity relative to the body (m/s)
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ISA MSL example (ρ = 1.225 kg/m³) at v = 100 kt ≈ 51.4 m/s: q = 0.5 × 1.225 × 51.4² ≈ 1,620 Pa ≈ 16.2 hPa.
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Scales with the square of velocity: doubling v → quadrupling q.
Total pressure (stagnation, pt)
Bernoulli: in incompressible, inviscid flow along a streamline:
pt = p + q = constant
- The pitot tube measures total pressure pt (flow is brought to rest).
- The static port measures static pressure p.
- Difference pt − p = q → this dynamic pressure is converted in the airspeed indicator (ASI) to IAS.
Effect of density
Density ρ changes with:
- Altitude: ρ decreases with altitude (ISA).
- Temperature: ρ decreases with rising temperature (p/ρT = R).
- Humidity: ρ decreases slightly with humidity (Subject 050 lesson "Density and Temperature").
→ At same IAS at higher altitude, the true airspeed (TAS) is greater because ρ is smaller and q the same.
Practical significance
- Lift: L = q × S × CL → grows with v².
- Drag: D = q × S × CD → analogously.
- ASI: measures dynamic pressure → indicates IAS (calibrated for ρ₀).
- Maximum structural speeds (Vno, Vne) are q-based → given in IAS.