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

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