Aircraft Speed

On board mechanical determination of aircraft speed relies on the dynamic pressure reading. This is the difference between the stagnation/pitot/total pressure and the static pressure reading using onboard instrumentation. This relation is obtained form the Bernoulli's equation. The Bernoulli's equation is used under the assumptions of steady flow, incompressible flow, and ideal/inviscid/no friction flow.

Applying the equation between points A and B on the same streamline, and A typically represents the free stream conditions. Usually the height differences does not matter.

A pitot tube, nose or wing mounted can be used to read the total pressure. The static port is usually located on the fuselage. Its location is very important for accurate reading. The speed is obtained from

Not so simple !! Note the density term in the equation. While actual pressures are measured, their differences are processed by a dial guage to read the speed through the Air Speed Indicator (ASI).

What value of density is used in the mechanical processing of the pressure difference ?

Standard sea-level value off course - instead of the true value of the density.

This requires that the reading must be adjusted.

Indicated airspeed (V_IAS): The air speed indicated by the mechanical ASI in the cockpit calibrated using standard density

Calibrated airspeed (V_CAS): The indicated airspeed corrected for static port location, attitude correction, etc.- provided by the manufacturee

Equivalent airspeed (V_EAS): The calibrated airspeed corrected forcompressibilityeffects - standard chart

True airspeed (V_TAS ) : The equivalent airspeed corrected for actual density value

Example: Consider the following example with the variation of MECE 348,the mini luxury design

The altimeter aboard the MECE 348 reads h_p = 5000 ft. The outside air temperature (OAT) is 505 ^oR. The pitot tube mounted at the wing tip reads 1818 lb/ft².

Find the True and Indicated airspeed.

Assumptions: Ideal Gas, No static and compressibility corrections

Solution: From Standard Table: p_static = 1761 lb/ft²

From ideal gas law the density ; 2.03 E-03 slug/ft³

V_TAS = 237 ft/s ( from formula above)

V_IAS = 219 ft/s (from formula above)