Aerodynamics Flashcards
Newton’s First Law
Inertia-A body at rest will remain at rest and a body in motion will remain in motion at the same speed and the same direction until acted upon by some external force.
Newton’s Second Law
Acceleration- force required to produce a change in motion of body is directly proportional to its mass and rate of change in its velocity. Acceleration is directly proportional to force and inversely proportional to mass. (A=F/M, velocity=speed+direction) Greater force= greater acceleration Greater mass=less acceleration
Newton’s Third Law
action and reaction- For every action there is an equal and opposite reaction. (torque effect in single rotor helicopters)
Increase in speed of airflow=
decrease in static pressure (therefore decrease in speed=increase in static pressure)
when an airfoil is positioned at an angle to a flow of air…..
speedup of air and reduced pressure occurs above the airfoil and decrease of airflow causes increased pressure beneath the airfoil (lift)
Airfoil
surfaced body or structure designed to produce lift or thrust force when subjected to airflow
Leading edge
rounded portion that projects into the relative flow of air (relative wind)
Trailing edge
tapered portion that trails from the relative flow of air
Chord
length of the chord from leading edge to trailing edge; longitudinal dimension of the airfoil section
chord line
straight line intersecting leading and trailing edges of the airfoil (extends beyond edges)
camber
shape or curvature; upper, lower and mean
mean camber line
line drawn halfway between the upper and lower surfaces (2 equal halves)
span
length of the rotor blade from the point of rotation to the blade tip (hub to tip)
center of pressure
point along the chord line through which all aerodynamic forces are considered to act ( lift, weight, thrust, drag)
Aerodynamic center
point along the chord line where are changes to lift effectively take place (lift forces occur)
Symmetrical airfoil
equal camber on each side of chord, constant center of pressure, ease of construction and low cost. Disadvantage- less lift at given angle of attack and undesirable stall
Non-symmetrical airfoil
cambered upper surface, flattened lower surface, increased lift/drag ratios, better stall characteristics, more lift production at given AOA. Disadvantages- center of pressure movement causes twisting forces and stress, expensive. (more efficient and more aerodynamic)
Rotational relative wind
flow of air parallel to and opposite the flight path of an airfoil
airspeed component of relative wind
component of the total relative wind velocity created by directional flight velocity/airspeed (factor in airspeed to advancing and retreating blade-add to advancing, subtract from retreating)
Induced flow (downwash)
a downward component of air (vertically through rotor system, reduced by forward flight). At max during stationary hover and no wind
Resultant relative wind
airflow from rotation that is modified by induced flow (strikes rotor blade). Rotational relative wind modified with vertical induced flow
Angle of incidence
(pitch angle, mechanical angle) this is the acute angle between the chord of an airfoil and the plane of rotation (tip path plane). Changed on all blades simultaneously by collective pitch control. (change of individual blades is done by cyclic)
Angle of attack
(aerodynamic angle) the acute angle between the chord of an airfoil and the resultant relative wind (can change with no change in angle of incidence). Larger= more lift (less induced flow) Smaller= less lift (more induced flow)
Critical angle
exceeding the maximum angle of attack and producing stall. Max AOA is 15 to 20 degress on most airfoils
Dissymmetry of Lift
unequal lift between the advancing and retreating halves of the rotor disc caused by the different wind flow velocity across each half. Overcome by flapping
Flapping
advancing blade produces more lift- flaps up increasing angle of attack and loses lift. retreating blade loses lift- flaps down increasing angle of attack and gains lift.
Blowback
when blade flapping has compensated for dissymmetry of lift, the rotor disk is tilted to the rear (blowback)
Total aerodynamic force
pressure differentials between the upper and lower surfaces of the airfoil combined with the air resistance on the airfoil (constantly shifting). Acts as center of pressure and is normally inclined up and to the rear.
Two components of TAF
Lift-useful component, PERPENDICULAR to resultant relative wind, increases with AOA, decreases rapidly at stalling angle. Drag- non useful component, parallel and same direction of resultant relative wind, increases with AOA, increases sharply at stall angle.
Induced Drag
results from producing lift; retards forward motion caused by wing tip vortices (larger=more induced drag) and induced flow. Major source of drag at a hover, but decreases with forward airspeed
profile drag
frictional resistance of the rotor blades passing through the air. relatively constant at low airspeed but increases slightly at higher airspeed ranges
parasite drag
drag created by fuselage; strut, skin friction, interference (any nonlifting components). lowest at hover, increases with airspeed. Major source of drag at higher airspeeds
total drag
sum of induced, profile and parasite. Decreases with forward airspeed until best rate of climb speed is reached. speeds greater than best r/c will cause decrease in overall efficiency due to increasing parasite drag
Unaccelerated flight
no change in speed or direction; all opposing forces are equal and opposite. lift=weight thrust=drag ( law of inertia)
Accelerated Flight
any time opposing forces become unequal, acceleration will result in direction of the greater force (law of acceleration)
L= Cl P/2 S V2 pilot can control….
Cl and V2- coefficient of lift (angle of attack) and velocity (square of velocity of relative wind is most dominant factor)
Air density increases….
high pressure, low temp., low humidity (conditions good for flight)
Semirigid rotor system
simple, two blade construction, tilting disc relative to mast, opposite unit works simultaneously (flapping)
Articulated rotor system
multibladed, blades are hinged independently at hub and offset from the center of the mast (stationary hub), blades can flap up or down and lead or lag (each blade can flap at different angles), tilting of the disc is relative to the hub
Centrifugal Force
outward force produced whenever a body moves in a curved path (most dominant force). Proportional to rotational velocity; provides rigidity to rotor system.
Centripetal force
inward force directed toward the center of rotation
Blade coning
upward flexing of rotor blades (lift and centrifugal force determine. lift stronger= cone up centrifugal force stronger= reduces coning). Greatest coning angle is at the tip of the blade.
Causes of excessive coning
Low RPM, High gross weight, High “G” manuevers, turbulent air
adverse effects of excessive coning
loss of disc area, loss of total lift available, stress on blades (most critical), could lead to blade cracking or blade separation from the rotor system
gyroscopic precession
occurs in rotating bodies that manifest an applied force 90 degrees after an application in the direction of rotation (phase leg). Overcome by offsetting the linkage in the cyclic pitch control system to create an input of 90 degrees ahead of desired action (cyclic rigging).
gyroscopic stability
rigidity is space which is proportional to mass and speed of rotation
Blade twisting
necessary to better equalize lift along the span of the rotor blade. twisted- positive angle at hub (more lift) which decreases to negative angle at tip (less lift). Used on most production line helicopters
Blade tracking in the rotor system
refers to the track or path of each blade in the plane of rotation; each tip should pass through the exact same point in space as the preceding blade
Effects when blade area out of track
low frequency VERTICAL vibration 1:1 or 2:1 (depends on number of blades out of track), increased instability in the rotor system
Pendular action
CG below supporting elements, tilting rotor in one direction results in fuselage swinging in opposite direction, over control results in exaggerated pendular action, move cyclic at rate that main rotor and fuselage move as a unit
Effects of torque
newton’s 3rd law, combination of forces that tend to cause rotation, amount of yaw experienced is proportionate to engine power being delivered to rotor (greater power=yaw right decrease power=left yaw)
Control of torque (single rotor)
tail rotor powered from main rotor transmission system, provides right thrust (opposite of torque), amount of thrust controlled by pitch of tail rotor blade (right pedal=pitch decrease and less thrust left pedal=pitch increased more thrust)
Pedal turns at a hover
right pedal turn- turns with torque reaction and less power used (AOA decrease). left pedal turn-turn against torque reaction and more power is used (AOA increase)
Translating tendency (counterclockwise main rotor)
helicopter tends to drift laterally to the right and is caused by the thrust exerted to the right by the tail rotor to compensate for main rotor torque
overcome translating tendency
corrected by tilting main rotor to the left- rigging of the flight control systems, tilting mast slightly (built in left tilt), left cyclic (pilot induced)
contribution of rotor blade- rotation
creates rotational relative wind
contribution of rotor blade-flapping
overcomes dissymmetry of lift (semi rigid-relative to mast articulate-relative to hub; blades flap independently)
contribution of rotor blade- feathering with cyclic
increases angle on one half rotor and decrease angle in other half, pilot uses to tilt rotor disc for directional control and control dissymmetry of lift
contribution of rotor blade- collective feathering
equally and simultaneously changes pitch of all blades, adjust overall AOA, pilot controls total lift and vertical control
contribution of rotor blade- hunting
ARTICULATED rotor system- relieves stress forces on rotor blades created by flapping action Coriolis effect- blade flaps up, CG moves inboard, blade speeds up, drag hinge allows blade to lead, drag damper regulates amount/rate of lead (opposite sequence when blade flaps down)
Total Force to tip path plane
(plane of rotation) TF acts perpendicular to the tip path plane. If tip path plane is tilted by cyclic, the TF will incline in same direction of rotor tilt. *tilt does not change amount of TF- changes direction
Total force
multitude of total aerodynamic force act along the span of each blade can be summed together into a single force emitting from the center of the rotor disc.
Total Force when tip path plane is tilted
thrust is produced by tilting tip path plane- vertical component of TF opposes weight (acts as lift) and horizontal component opposes drag (acts as thrust) vertical lift+horizontal thrust= total force
Ground effect
improved performance encountered when hovering near a surface, one rotor diameter from rotor to ground, more pronounced over smooth, open surfaces with no wind. Reduction of downwash. Decreased induced drag-reduced tip vortices
Loss of ground effect
altitude greater than one rotor diameter, trees and brush, tall grass, uneven terrain (slopes), tilt tip path plane, wind
Effective translational lift
additional lift obtained because of increased efficiency of the rotor system with airspeed obtained either by horizontal flight or hovering into wind. Relative wind entering rotor become more horizontal. Occurs approx. 16-24kts. Flapping increases with forward airspeed and nose pitches up with left yaw.
efficient operating airspeed range during translational flight
ETL (outrun old vortices), induced drag/total drag reduced, increased efficiency continues UNTIL best rate of climb speed is reached. *airspeeds greater than best r/c will result in lower efficiency due to increased parasite drag
Transverse flow effect
drag differential between front and rear of rotor (greater in aft), 10-20kts., more induced drag aft/less forward, causes shudder or vibration
Settling with power
air upward flow through center of rotor. Conditions: vertical descent of 300 fpm, use of 20-100%, slow airspeed (less than ETL). Can develop vortex ring state and increase with increase of collective.
conducive to settling with power
hover out of ground effect, formation flights, mask/remask ops, downwind approaches, steep approach with high rate of descent
corrective action for settling with power
increase speeds with cyclic, reduced collective pitch as altitude permits, adjust rotor RPM to normal operating range
angular unbalance in articulated rotor system
unequal number of degrees between each blade, caused by unequal hunting, CG of rotor system is thrown off center and aggravated by centrifugal force, LATERAL vibration, leads to ground resonance
Ground resonance
articulated system only- can cause complete destruction of aircraft within seconds. 1) drag dampers allow excessive lead and lag, 2)defective landing gear/struts, 3)hard landings on one skid or wheel, 4)ground taxi over rough terrain, 5) hesitant or bouncing landings (80-90% airborne-light on the skids)
Recover from ground resonance
if operating RPM exists become airborne then land first suitable spot and check damage; if operating RPM does not exist, place firmly on ground by reducing power and normal shutdown procedures
Regions of main rotor during autorotation
stall- inboard, operates at max AOA, very little lift/considerable drag. Driving-middle portion, operates high AOA, TAF slightly forward of axis of rotation (creates lift), provides thrust. Driven- tip/end, less AOA, TAF is slightly aft of axis (creates drag)
Autorotative turns
rotor RPM will increase, disc tilted causes a loss of vertical lift, rate of descent increases, driving region increases due to change in increased upflow or relative wind, pilot must prevent rotor RPM overspeed with collective control (increase drag to slow rotor RPM- collective increase)
3 conditions for dynamic rollover
Pivot point, rolling motion, exceed critical angle (static rollover angle/critical angle)
3 types of dynamic rollover
level ground, down slope, upslope during takeoff
factors for dynamic rollover
physical factors (TIMSGCC) and human factors (FLIII)
dynamic rollover recovery
smooth, slow collective reduction
characteristics of stalls
caused by an excessive angle of attack–15 to 20 degrees
correcting stalls
reduce the angle of attack to something less than the critical angle
retreating blade stall
critical advanced stage of dissymmetry of lift;(contributing factors)- excessive airspeed (primary factor), high blade loading, low rotor RPM, high DA, high G manuevers, turbulence
symptoms of retreating blade stall
abnormal vibrations 2:1 (two bladed rotor), pitch-up of nose, roll toward stall side (left), loss of control (number of vibrations per one rotation of main rotor will be number of blades in main rotor-all blades)
corrective actions of retreating blade stall
reduce collective pitch (lower AOA), regain control of aircraft (cyclic as necessary-not forward, makes stall worse), reduce airspeed, increase rotor RPM to normal op range, minimize maneuvering, descend to lower altitude (high air density)
