## AIRCRAFT PERFORMANCE## Fixed Wing Aircraft |
## AIRSPEED MEASUREMENTThe speed of an aircraft relative to the surrounding air is a critical part of operation and performance analysis. No direct method is available for measurement of velocity relative to the air so instead the velocity is inferred from measurements of dynamic pressure. Dynamic pressure is defined as $q=1/2 ρV^2$ Various forms of dynamic pressure can be obtained depending on the instruments used to measure the pressure and the flight speed range of the vehicle. A simple airspeed indicator is shown in the section on Aircraft Instruments. All instruments rely on knowledge of Standard Atmosphere properties. The primary atmospheric properties can be expressed in the form of ratios compared to Sea Level conditions. The approximate dynamic pressure can be measured by a Pitot-Static system, This pressure difference is known as the where $q_i$ is defined as the
Applying the compressible flow Bernoulli equation gives the following expression for velocity, where $γ = 1.4$ is the ratio of specific heats for air. For incompressible flow this simplifies to As air density cannot be directly measured and static pressure measurements are subject to error, airspeed indicators use a calibrated ISA sea level pressure condition instead. The airspeed displayed on cockpit indicators is thus, or for incompressible flow, For transonic and supersonic vehicles additional compressibility corrections may need to be applied. If shock waves form in front of the pitot tube then total pressure ( $P_0$ ) will become inaccurate due to shock losses. There is thus a translation sequence between the observed speed ( $V_O$ ) on the aircraft's instruments and the actual airspeed of the vehicle ( $V$ ). For subsonic vehicles, $V_O$, Observed airspeed on
instrument + $ΔV_{IC}$ (instrument correction) For incompressible flows a rough estimate can be made that At high speed where compressibility correction becomes a large component then a Mach meter rather than an airspeed indicator may be used. Indicated Mach Number would thus be, or For supersonic flight when a shock forms in front of the pitot tube an additional compressibility correction is required. When then Based on these airspeed relationships, dynamic pressure can be formulated in several ways, |