Aircraft Speed

Aircraft speed is a vital parameter in computing performance. It is measured using the difference between the total pressure, pt, and the static pressure, ps, expressed as (pt – ps). Static pressure is the ambient pressure in which an aircraft is flying. The value of (pt – ps) gives the dynamic head, which depends on the ambient air density, p. Unlike the ground speed of an automobile that is measured directly, an aircraft ground speed must be computed from (pt – ps); a pilot reads the gauge that is converted from (pt – ps). Following are various forms of aircraft air speed that engineers and pilots use. As shown, some computations are required – currently, onboard computers perform all computations:

Vi: The gauge reading as a pilot sees it in the flight deck; this is flight speed,

which is not the same as ground speed. The instrument includes stan­dard adiabatic compressible-flow corrections for high-subsonic flights at the sea-level standard day; however, it still requires other corrections.

VI: This is the indicated air speed (IAS). Manufactured instruments have

some built-in instrumental errors, Д Vi (typically minor but important considerations when an aircraft is close to stall speed). Manufacturers supply the error chart for each instrument. The instrument is calibrated to read the correct ground speed at the sea-level standard day with compressibility corrections. When corrected, the instrument reads the IAS as

Vi = IAS = Vi + Д Vi

VC: This is the calibrated air speed (CAS). Instrument manufacturers cali­

brate an uninstalled, bare instrument for sea-level conditions. Once it is installed on an aircraft and depending on where it is installed, the air­craft flow field distorts the instrument readings. Therefore, it requires position-error (Д Vp) corrections by the aircraft manufacturers:

Vc = CAS = Vi + Д Vp = Vi + ДVi + Д Vp

VeAS: This is the equivalent air speed (EAS). Air density p changes with alti­tude – it decreases because atmospheric pressure decreases with a gain in altitude. Therefore, at the same ground speed (also known as true air speed [TAS]), the IAS reads lower values at higher altitudes. The mathematical relationship between the TAS and the EAS, reflecting the density changes with altitude, can be derived as TAS = EAS/^/ст, where a is the density ratio (p/po) in terms of the sea-level value, p0. The constant EAS has a dynamic head invariant. For high-subsonic flights, it requires adiabatic compressibility corrections ^Vc) for the altitude changes:

Veas = EAS = Vc + ДVc. = Vi + Д Vi + Д Vp + Д Vc = TAS^a

TEAS: TAS is the aircraft ground speed. Compressibility corrections for posi­

tion errors are available; however, at this stage of design, the details can

be omitted without any loss of conceptual design work undertaken in this book. Supersonic flight requires further adjustments.