4. SPEED

INTRODUCTION

SPEED = DISTANCE / TIME

Three types of speeds:

• AIRSPEED: Rate of change of position in relation to time, of air molecules as they are flowing though

a certain passage • GROUNDSPEED: Relative velocity an aircraft has

in relation to the ground. (Being the ground considered still)

• RELATIVE SPEED: Relative change of position between any two objects

INTRODUCTION

INTRODUCTION

INTRODUCTION

AIRSPEED INDICATOR READING (ASIR)

DEFINITION: The one measure by the pitot-static system without any corretions

INSTRUMENT ERROR: All inaccuracies shown on an airspeed indicator due to the inherent

imperfections of the instrument. (Frictions, readablity, timming, accuracy, scale reading,…).

Usually considered negligible

INDICATED AIRSPEED (IAS)DEFINITION: Airspeed shown on an airspeed indicator

whenever the actual incoming from the pitot-static system indication is corrected for INSTRUMENT ERROR.

The one actually shown on a usual ASI

POSITION ERROR: Missinterpretation of the real airspeed due to the fact of the pitot-static system’s incoming not being undisturbed. (Drifting, turbulence, water, maneuvreing,…). Taken into account for certain maneuvres and situations

CLIBRATED AIRSPEED (CAS)DEFINITION: Sometimes called Rectified airspeed (RAS) Stands for the IAS corrected for position error.

DENSITY ERROR: Due to the fact of the Pitot-Static system only being able to measure dynamic pressure the systems takes into account a constant value for the air density. This mistake increases as density decreases, and so, as altitude increases. ≈2% each 1000ft AMSL

EQUIVALENT AIRSPEED (EAS)DEFINITION: Calibrated Airspeed (CAS) of an airflow

taking into account compressibility of the air.

Must take into account that the air is taken as not compressible bellow ≈300Kts

DENSITY ERROR: Density varies in accordance with the Ideal Gases Formula P/ℓ = MRT. So temperature and pressure will affect the

airspeed indicator reading

If ∆T > 0 ∆ℓ < 0 ASI will read a lower speedIf ∆P > 0 ∆ℓ < 0 ASI will read a lower speed

TRUE AIRSPEED (TAS)DEFINITION: Airspeed relative to the airmass in which

the aircraft is actually flying.

The EAS corrected for density error.

½ ℓ0 EAS² = ½ ℓh TAS²

NOTE: Must understand that density thermodinamyc charateristics vary in a “known” way through the atmosphere

MACH NUMBERDEFINITION: Adimensional number that relates the

actual airspeed of a body to the speed that the sound would have inside the airmas (with its thermodinamyc charateristics) through wich the body is moving.

LSSTAS

M

)(º92.38 KTLSS )(º92.38 KT

TASM