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Appendix 1
η (eta) 1 Generalized symbol for efficiency, with suffixes
indicating which, eg η a antenna aperture, η m mechanical,
η ο overall, η p propulsive, η r radiation, η T thermal, or
tailplane setting angle, η V volumetric.
2 Elevator angle.
3 Ratio of wing lifts (biplane).
4 Electrolytic polarization.
5 Dynamic viscosity [μ is more common].
6 Normalized incremental coefficient of backscatter.
7 First Townsend coefficient.
8 Fraction of semi-span 2y/b.
θ (theta) 1 Generalized symbol for bearing, azimuth
direction, angular distance, birdstrike impact angle, polar
co-ordinate angle, etc, denoted by suffixes a [azimuth], e
[elevation], etc.
2 Angle of pitch (all meanings, eg of fuselage attitude,
propeller or rotor blade, etc); subscripts many, eg bl blade
pitch angle, T, thrust-vector angle, 0 collective.
3 Angle of twist or torsional strain.
4 Phase displacement (also φ).
5 Temperature or temperature ratio.
6 Semi-vertex angle of Mach cone.
7 Spherical co-ordinate measures.
θ/R
Bearing and range.
Θ (capital theta) 1 Absolute temperature, or temperature
ratio.
2 Pitch attitude, usually in radians.
3 Often with subscript n, deflection of total gross
thrust vector from aircraft datum.
4 Subscript a, scan width in azimuth.
5 Subscript e, scan width in elevation.
6 Subscript F, deflection of forward nozzle(s).
7 Antenna beamwidth; subscript NN, null-to-null.
8 Subscript R, deflection of rear nozzles.
9 Subscript 3dB; 3dB antenna beamwidth.
10 Subscript XG, inclination of total gross thrust vector
to flight path.
ι (iota)
No common meanings.
κ (kappa) 1 Permittivity (dielectric constant) (also ).
2 Compressibility.
3 Conductivity, esp. in electrochemistry.
4 Helicopter induced power factor.
5 Radius of gyration of strut or other structural part.
Κ
(capital kappa) Equivalent spring rate or stiffness.
λ (lambda) 1 Wavelength.
2 Area ratio of wing S/b 2 ; also taper ratio, tip/root
chord.
3 Damping coefficient.
4 Scale ratio (model: full size).
5 Thermal conductivity [also k].
6 Longitude (terrestrial or celestial).
7 Solid rocket volumetric loading.
8 Eigenvalues, with suffixes such as ph = phugoid
mode, sp = short-period.
9 Aerodynamic pich of propeller.
10 Landing-gear reaction factor.
11 λ c = Compton wavelength.
Λ (capital lambda)
1 Angle of sweepback, measured at
0.25 chord; if leading or trailing edge, with subscripts LE,
TE.
2 Permeance.
3 Ionic/molar conductance.
µ (mu) 1 Generalized prefix, micro (× 10 –6 , one
millionth), thus µm = 10 –6 metre.
2 Without any other symbol, micrometre or micron.
3 Coefficient of friction.
4 Permeability; µ 0 , of vacuum.
5 Joule-Thomson coefficient.
6 Dynamic viscosity.
7 Aircraft velocity (non-dimensional).
8 Ratio of spans (biplane).
9 Amplification factor (radio).
10 µ 1 , relative density of aircraft (aerodyne); µ G is in
response to a gust.
11 µ´, specific fuel consumption of jet engine (normally
air-breathing). Not to be confused with fact that sfc of
shaft-drive engines is measured in µg/J (microgrammes
per joule).
12 Helicopter rotor-blade advance ratio or [in US] tipspeed
ratio.
13 Engine bypass ratio.
14 Subscript FORS, microfibreoptic rate sensors.
15 µs, microsecond[s].
16 Refractive index.
17 Crossfeed phasing parameter.
18 Diffusion coefficient of a stressed panel.
19 Mass per unit length.
20 In the US, Poisson’s ratio [elsewhere, ν].
ν (nu) 1 Kinematic viscosity; suffix t, effective
turbulent.
2 Wave, or phase, velocity.
3 Reluctance, reluctivity.
4 Number of molecules (stoichiometric); ν c , molecular
diffusivity.
5 Frequency in s –1 .
6 Poisson’s ratio.
7 ν¯, wave number (also ν).
ξ (xi) 1 Aileron angle.
2 Damping, for each mode; ξ . is average.
3 Shielding factor of spacecraft.
ο (omicron)
Too like o to be used separately.
Greek Symbols
π (pi) 1 Ratio of circumference of circle to diameter,
3.141592653.
2 Solar parallax.
3 Generalized multiplying factor in failure-rate
analysis.
Π (capital pi) 1 Generalized symbol for a product
(math).
2 Often italic, osmotic pressure.
ρ (rho) 1 Generalized symbol for density; ρ o air density
at SL.
2 Radius of curvature.
3 Resistivity.
4 Reflection factor, reflectance.
5 Electric charge density (MHD, C/m 3 ).
σ (sigma) 1 Relative density, eg to standard atmosphere,
ρ/ρ o .
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