# Units

The metric system is taken as fundamental, this being the educational basis in the UK. Imperial units are still used extensively, however, particularly in the USA but also by industry and other organizations in the UK. For dimensional examples in the text and diagrams, therefore, those units are used which it is felt have stood the test of time and may well continue to do so. Often units in both systems are quoted; in other cases reference may need to be made to the conversion tables set out below. In either system, units other than the basic one are sometimes used, depending on the context; this is particularly so for velocity, where for example aircraft flight speed is more conveniently expressed in kilometres/hour or in knots than in metres/second or in feet/second. The varieties used in the book are included in the table.

 Quantity Metric unit and symbol Imperial equivalent Primary quantities: Mass kilogram (kg) 0.0685 slug Weight newton (N) 0.2248 pound (lb) Length metre (m) 3.281 feet (ft) Time second (s) 1.0 s (sec) Temperature kelvin (K) Celsius (°C) Temp(K) = temp (°C) + 273.15 Derived quantities: Weight (force) kilogram force 9.807N (kg) 2.2046 lb Density kg/m3 0.00194 slug/ft3 Pressure N/m2 0.0209 lb/ft2 Velocity 0.1020 kg/m2 m/s 3.281 ft/sec 3.600 km/h 196.86 ft/min Acceleration m/s2 1.941 knots 3.281 ft/sec2 Accel. of gravity 9.807 m/s2 (g) 32.2ft/sec2 Power watt, N m/s (W) 0.7376 ft lb/sec Metric horsepower 75 kg m/s (mhp) 0.9863 HP English horsepower 76.04 kg m/s 550 ft lb/sec

Abbreviations

 ABC Advancing Blade Concept ADT Actuator disc theory Aero. J. Journal of the Royal Aeronautical Society AFCS Automatic flight control system AGARD Advisory Group for Aerospace Research and Development AIAA American Institute of Aeronautics and Astronautics ARC Aeronautical Research Council ASE Automatic stabilization equipment AUW All-up weight BERP British Experimental Rotor Programme[1] BET Blade element theory CFD Computational fluid dynamics CG Centre of gravity DOF Degree of freedom FAI Federation Aeronautique Internationale FOM Figure of merit FUMS Fatigue and Usage Monitoring System HUMS Health and Usage Monitoring System IFR Instrument Flight Rules IGE In ground effect ISA International standard atmosphere JAHS Journal of the American Helicopter Society NACA National Advisory Committee for Aeronautics (now NASA) NFA No-feathering axis NFP No-feathering plane NPL National Physical Laboratory OGE Out of ground effect PIV particle image velocimetry RAE Royal Aircraft Establishment RAF Royal Air Force
 RLD Really low drag R&M Reports and Memoranda of the ARC SAE Society of Automotive Engineers SA Shaft axis sfc Specific fuel consumption SHM Simple harmonic motion SL Sea level SNP Shaft normal plane TPA Tip-path axis TPP Tip-path plane TRLF Transmission loss factor UAV Unmanned aerial vehicle UK United Kingdom USA United States of America VRS Vortex ring state VTOL Vertical take-off and landing WHL Westland Helicopters Ltd