Aerodynamics

Question 1

Newton’s First Law

Answer 1

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.

Question 2

Newton’s Second Law

Answer 2

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

Question 3

Newton’s Third Law

Answer 3

action and reaction- For every action there is an equal and opposite reaction. (torque effect in single rotor helicopters)

Question 4

Increase in speed of airflow=

Answer 4

decrease in static pressure (therefore decrease in speed=increase in static pressure)

Question 5

when an airfoil is positioned at an angle to a flow of air…..

Answer 5

speedup of air and reduced pressure occurs above the airfoil and decrease of airflow causes increased pressure beneath the airfoil (lift)

Question 6

Airfoil

Answer 6

surfaced body or structure designed to produce lift or thrust force when subjected to airflow

Question 7

Leading edge

Answer 7

rounded portion that projects into the relative flow of air (relative wind)

Question 8

Trailing edge

Answer 8

tapered portion that trails from the relative flow of air

Question 9

Chord

Answer 9

length of the chord from leading edge to trailing edge; longitudinal dimension of the airfoil section

Question 10

chord line

Answer 10

straight line intersecting leading and trailing edges of the airfoil (extends beyond edges)

Question 11

camber

Answer 11

shape or curvature; upper, lower and mean

Question 12

mean camber line

Answer 12

line drawn halfway between the upper and lower surfaces (2 equal halves)

Question 13

span

Answer 13

length of the rotor blade from the point of rotation to the blade tip (hub to tip)

Question 14

center of pressure

Answer 14

point along the chord line through which all aerodynamic forces are considered to act ( lift, weight, thrust, drag)

Question 15

Aerodynamic center

Answer 15

point along the chord line where are changes to lift effectively take place (lift forces occur)

Question 16

Symmetrical airfoil

Answer 16

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

Question 17

Non-symmetrical airfoil

Answer 17

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)

Question 18

Rotational relative wind

Answer 18

flow of air parallel to and opposite the flight path of an airfoil

Question 19

airspeed component of relative wind

Answer 19

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)

Question 20

Induced flow (downwash)

Answer 20

a downward component of air (vertically through rotor system, reduced by forward flight). At max during stationary hover and no wind

Question 21

Resultant relative wind

Answer 21

airflow from rotation that is modified by induced flow (strikes rotor blade). Rotational relative wind modified with vertical induced flow

Question 22

Angle of incidence

Answer 22

(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)

Question 23

Angle of attack

Answer 23

(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)

Question 24

Critical angle

Answer 24

exceeding the maximum angle of attack and producing stall. Max AOA is 15 to 20 degress on most airfoils

Question 25

Dissymmetry of Lift

Answer 25

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

Question 26

Flapping

Answer 26

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.

Question 27

Blowback

Answer 27

when blade flapping has compensated for dissymmetry of lift, the rotor disk is tilted to the rear (blowback)

Question 28

Total aerodynamic force

Answer 28

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.

Question 29

Two components of TAF

Answer 29

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.

Question 30

Induced Drag

Answer 30

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

Question 31

profile drag

Answer 31

frictional resistance of the rotor blades passing through the air. relatively constant at low airspeed but increases slightly at higher airspeed ranges

Question 32

parasite drag

Answer 32

drag created by fuselage; strut, skin friction, interference (any nonlifting components). lowest at hover, increases with airspeed. Major source of drag at higher airspeeds

Question 33

total drag

Answer 33

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

Question 34

Unaccelerated flight

Answer 34

no change in speed or direction; all opposing forces are equal and opposite. lift=weight thrust=drag ( law of inertia)

Question 35

Accelerated Flight

Answer 35

any time opposing forces become unequal, acceleration will result in direction of the greater force (law of acceleration)

Question 36

L= Cl P/2 S V2 pilot can control....

Answer 36

Cl and V2- coefficient of lift (angle of attack) and velocity (square of velocity of relative wind is most dominant factor)

Question 37

Air density increases....

Answer 37

high pressure, low temp., low humidity (conditions good for flight)

Question 38

Semirigid rotor system

Answer 38

simple, two blade construction, tilting disc relative to mast, opposite unit works simultaneously (flapping)

Question 39

Articulated rotor system

Answer 39

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

Question 40

Centrifugal Force

Answer 40

outward force produced whenever a body moves in a curved path (most dominant force). Proportional to rotational velocity; provides rigidity to rotor system.

Question 41

Centripetal force

Answer 41

inward force directed toward the center of rotation

Question 42

Blade coning

Answer 42

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.

Question 43

Causes of excessive coning

Answer 43

Low RPM, High gross weight, High "G" manuevers, turbulent air

Question 44

adverse effects of excessive coning

Answer 44

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

Question 45

gyroscopic precession

Answer 45

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).

Question 46

gyroscopic stability

Answer 46

rigidity is space which is proportional to mass and speed of rotation

Question 47

Blade twisting

Answer 47

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

Question 48

Blade tracking in the rotor system

Answer 48

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

Question 49

Effects when blade area out of track

Answer 49

low frequency VERTICAL vibration 1:1 or 2:1 (depends on number of blades out of track), increased instability in the rotor system

Question 50

Pendular action

Answer 50

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

Question 51

Effects of torque

Answer 51

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)

Question 52

Control of torque (single rotor)

Answer 52

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)

Question 53

Pedal turns at a hover

Answer 53

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)

Question 54

Translating tendency (counterclockwise main rotor)

Answer 54

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

Question 55

overcome translating tendency

Answer 55

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)

Question 56

contribution of rotor blade- rotation

Answer 56

creates rotational relative wind

Question 57

contribution of rotor blade-flapping

Answer 57

overcomes dissymmetry of lift (semi rigid-relative to mast articulate-relative to hub; blades flap independently)

Question 58

contribution of rotor blade- feathering with cyclic

Answer 58

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

Question 59

contribution of rotor blade- collective feathering

Answer 59

equally and simultaneously changes pitch of all blades, adjust overall AOA, pilot controls total lift and vertical control

Question 60

contribution of rotor blade- hunting

Answer 60

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)

Question 61

Total Force to tip path plane

Answer 61

(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

Question 62

Total force

Answer 62

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.

Question 63

Total Force when tip path plane is tilted

Answer 63

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

Question 64

Ground effect

Answer 64

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

Question 65

Loss of ground effect

Answer 65

altitude greater than one rotor diameter, trees and brush, tall grass, uneven terrain (slopes), tilt tip path plane, wind

Question 66

Effective translational lift

Answer 66

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.

Question 67

efficient operating airspeed range during translational flight

Answer 67

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

Question 68

Transverse flow effect

Answer 68

drag differential between front and rear of rotor (greater in aft), 10-20kts., more induced drag aft/less forward, causes shudder or vibration

Question 69

Settling with power

Answer 69

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.

Question 70

conducive to settling with power

Answer 70

hover out of ground effect, formation flights, mask/remask ops, downwind approaches, steep approach with high rate of descent

Question 71

corrective action for settling with power

Answer 71

increase speeds with cyclic, reduced collective pitch as altitude permits, adjust rotor RPM to normal operating range

Question 72

angular unbalance in articulated rotor system

Answer 72

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

Question 73

Ground resonance

Answer 73

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)

Question 74

Recover from ground resonance

Answer 74

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

Question 75

Regions of main rotor during autorotation

Answer 75

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)

Question 76

Autorotative turns

Answer 76

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)

Question 77

3 conditions for dynamic rollover

Answer 77

Pivot point, rolling motion, exceed critical angle (static rollover angle/critical angle)

Question 78

3 types of dynamic rollover

Answer 78

level ground, down slope, upslope during takeoff

Question 79

factors for dynamic rollover

Answer 79

physical factors (TIMSGCC) and human factors (FLIII)

Question 80

dynamic rollover recovery

Answer 80

smooth, slow collective reduction

Question 81

characteristics of stalls

Answer 81

caused by an excessive angle of attack--15 to 20 degrees

Question 82

correcting stalls

Answer 82

reduce the angle of attack to something less than the critical angle

Question 83

retreating blade stall

Answer 83

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

Question 84

symptoms of retreating blade stall

Answer 84

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)

Question 85

corrective actions of retreating blade stall

Answer 85

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)