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propeller angle of attack
propulsor). Similar screw for converting energy of slipstream
into shaft power is windmill or RAT (ram-air
turbine). Note : in 1930–50 UK usage resulted in a 1939
glossary having the entry ‘Propeller : colloquial term
meaning “airscrew”.’
propeller angle of attack Ang;e between axis of rotation
and free-stream airflow, usually close to zero, one symbol
α p .
propeller area Usually means total area of blades
obtained by integrating total of areas of elementary
chordwise slices, taking blades as having no thickness; ie
each slice is projected in plane of its local chord.
Essentially same as outline area of blades with twist
removed.
propeller balance stand Trestle having two horizontal
and parallel steel knife-edges (about 1–3 mm radius) on
which propeller can be balanced on short slave-shaft.
propeller bar Handtool used to loosen or tighten main
retaining nut on many lightplane propellers.
propeller blade Thrust-generating aerofoil of propeller.
propeller blade angle Except in feathered position (when
close to 90°) acute angle β between chord line of blade
measured at standard radius and plane of rotation, latter
being normal to axis of rotation.
propeller blade-width ratio Ratio of widest chord to
propeller diameter.
propeller brake Brake to stop rotation of propeller after
engine shut-down, either to speed passenger disembarkation
or, in case of free-turbine engine, to prevent
prolonged windmilling with aircraft parked.
propeller camber ratio Ratio of blade maximum thickness
to chord at any station.
propeller cavitation Generation of near-vacuum on
suction face near tip at high Mach numbers (not necessarily
at high flight speed).
propeller characteristic Fundamental curve of V/nD
(velocity of advance divided by rpm × diameter) plotted
against Cc (speed/power coefficient) (see Weick).
propeller disc Circular area swept out by propeller.
propeller efficiency Useful work expressed as thrust
imparted divided by power input; thrust hp/shp = thrust ×
slipstream velocity divided by 2π nQ where n is rpm and
Q is drive torque. Some authorities cite two values of **,
one called net (net thrust hp/shp) and the other propulsive
(propulsive thrust hp/shp).
propeller governor Usually simple centrifugal governor
which keeps shuttle valve oscillating about null position
feeding oil to increase or decrease rpm and thus hold rotational
speed constant irrespective of aircraft forward
speed.
propeller hub Central portion of propeller carried on
drive-shaft, usually made as separate unit into which
blades (with simple fixed-pitch propeller, whole propeller)
are inserted.
propeller interference Aerodynamic effects, mainly
drag, of bluff bodies immediately downstream, eg radiator
or cylinders.
propeller pitch The angular setting of the blades of a
propeller. The blade angle (θ) is the angle between the
chord at any element (station) on the blade and the axis
of rotation. The helix angle (φ), also called the angle of
advance, is the angle between the actual direction of
motion (velocity) at any element relative to Earth and the
actual direction of motion relative to the aeroplane; it is
propeller wash
equal to the blade angle minus the angle of attack (α). The
geometric pitch (p or P, rarely H) is a linear dimension
equal to the distance any blade element would move
forward in one revolution in absence of any slip ( = 2π tan
φ); it is likely to be almost constant from root to tip. This
is sometimes called aerodynamic pitch, represented by λ.
Experimental pitch, also called ideal pitch, is the distance
the propeller would move forward in one revolution when
giving neither thrust nor drag. Actual pitch, also called
effective pitch, or practical pitch or advance per revolution,
is the distance travelled forward relative to the atmosphere
(not to the slipstream). Standard pitch, also called
nominal pitch, is the pitch angle at standard radius. The
pitch ratio is the ratio of geometric pitch divided by the
circular distance travelled by the blade tip (circumference).
The effective pitch ratio is the ratio of effective pitch
divided by the tip circumference, often expressed as V/πnd
where V is TAS and n rotational speed in compatible
units. The ratio of effective pitch divided by ideal pitch,
expressed as a percentage, is called slip. Other characteristics
include power coefficient P/ρn 3 d 5 , where P is shaft
power and ρ air density, and thrust coefficient T/ρn 2 d 4 ,
where T is thrust. All the above apply to a fixed-pitch
propeller. When pitch is variable, fine pitch enables
maximum power to be achieved for takeoff, coarse pitch
enables high TAS to be achieved with fuel economy in
cruising flight, reverse pitch sets the blades at a negative
angle of attack to shorten the landing run, with the engine
delivering high power, and feathering pitch sets the blades
edge-on to the oncoming air so that, with the engine inoperative,
torque imparted by the air to the inner part of
each blade exactly neutralizes that over the outer part,
thus stopping rotation.
propeller rake See rake (3, 4).
propeller root On a simple fixed-pitch propeller, where a
blade joins the hub; on a propeller with separate inserted
blades, where the blade joins the shank.
propeller rotational speed Angular velocity, denoted by
n [usually rps] or ω[rad/s].
propeller shaft That on which propeller is mounted.
propeller shank The innermost part of each blade,
normally inside the spinner, gripped by the structure of
the hub and incorporating the drive for changing pitch.
propeller solidity See solidity.
propeller speed Propeller rotational speed, but often rpm.
propeller standard radius In US by custom, 75% of tip
radius; elsewhere, usually 66.6 . %, symbol P s .
propeller state Normal condition of helicopter main
rotor in which thrust is in opposite direction (upwards) to
flow both through and outside rotor disc.
propeller thrust That imparted by the aerodynamic foce
on the blades through the hub to propel the aircraft,
symbol T, normally = Pη/V ( 550 in Imperial measures),
where P is shaft power, η propeller efficiency and V true
airspeed.
propeller tipping Metal skin on tip and outer leading
edge of soft (eg wood) blade.
propeller torque Torque imparted by propeller drive
shaft and reacted by aerodynamic rolling moment of
aircraft (usually to some degree in-built) or asymmetric
load on left/right main gears on ground; symbol Q.
propeller turbine See turboprop.
propeller wash Slipstream, esp. on ground; also called
prop blast; see propwash.
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