Basic Flashcards
lift
perpendicular to relative wind regardless of attitude. Varies with airspeed, density, characteristics of airfoil, and AOA.
weight
pull of gravity acts through C.G. straight down toward center of earth. only changes with changes in aircraft gross weight
coning
helicopter blades rise above straight out position as they develop lift during takeoff and flight
airfoil lift
increased speed causes increased lift at a square of the increase in speed. 500 knots has four times lift of blade traveling at 250 knots
stall angle
point which airflow no longer follows the camber of the blade smoothly
point of impact
point at which the air separates to flow about the airfoil
where is the high pressure area or stagnation point formed in relation to the leading edge?
lower portion of the leading edge
types of drag
profile- from friction
induced- from lift production
parasite- from stuff hanging off that doesn’t fly
centrifugal vs centripetal
fugal- out from center
petal- counter fugal, keeps objects a certain radius from axis of rotation
symmetrical airfoil cons
identical upper and lower
less lift produced than asymmetric
undesirable stall characteristics
symmetrical airfoil pros
center of pressure remains constant
best lift drag ratios for full range of velocities from rotor blade root to tip
low cost / easy construction
non-symmetrical airfoil
increased lift drag ratios
more desirable stall characteristics
twisting force exerted about center of pressure
center of pressure moves with aoa changes
hover effect on blades
tip vortex reduces the effectiveness of outer blade portions and severely affect lift of the following blades
primary cause of high power requirements for hovering
continuous creation of new vortices and ingestion of existing vortices
translating tendency
lateral movement during hovering flight (tail rotor drift)
ground effect occurs where?
one rotor diameter over a surface (up to 48ft)
IGE v OGE power requirement
OGE is 30% > IGE
IGE hover factors
reduction of downward velocity of induced airflow (ground gets in the way. Result is less induced drag and more vertical lift vector)
reduction of rotor tip vortex
best IGE?
OVER SMOOTH PAVED SURFACES
transverse flow effect
downward flow at rear of rotor disk results in less lift production than at front
dissymmetry of lift
difference in lift between advancing half of rotor disk and the retreating half. advancing blade has more lift and wants to climb, retreating wants to drop
feathering
mechanical change in angle of incidence or pitch of blade
flapping
movement of rotor blades on an upward or downward path during rotation
what causes nose to pitch up?
phase lag. right side is producing more lift than left side
what does blade flapping eliminate?
dissymmetry of lift
retreating blade stall
limits the maximum airspeed of helicopter
translational lift
improved efficiency from directional flight; rotor becomes more efficient with airspeed
effective translational lift
the point at which the ac outruns its own downwash; 16-24 knots
phases of autorotation
entry, steady state descent, deceleration with touchdown
describe entry phase of ar
transition from powered to unpowered flight
describe steady-state descent
permits pilot to glide helicopter toward ground under controlled conditions
deceleration with touchdown phase
pilot must reduce airspeed and rate of descent just before touchdown by flaring.
most significant factor affecting rate of descent in an ar?
airspeed
autorotative regions
stall region, driving region, driven region
stall region
rotational relative wind is slow and resultant rw exceeds aoa. inner 25% of rotor disk.
driving region
total aerodynamic force (taf) is tilted forward, driving the blade during ar. 25%-70% middle section of rotor disk.
driven region
taf is tilted aft, producing lift but with more drag. 30% closest to tips.
how to achieve constant rotor rpm
balance driving region with driven and stall regions
what aerodynamic force causes forward flight?
thrust
what term describes upward sweep of main rotor blades caused by lift and centrifugal force?
coning
which force acts perpendicular to relative wind?
lift
in a no wind condition, the tip path plane of the blades remains…
horizontal
what two factors increase blade efficiency IGE?
reduction of velocity of induced airflow and reduction of rotor tip vortex
during ar what force is used to sustain rotor rpm?
relative wind during descent
