Aerodynamic Principles by 8A85d2


									Aerodynamic Principles

          Chapter 3
          Section A
          Four Forces of Flight
Four Forces of Flight

   Lift
    –   Upward force
   Weight
    –   Opposes lift
    –   Downward pull of gravity
   Thrust
    –   Forward force (engine power)
   Drag
    –   Opposes thrust – backward or retarding force

   Straight and level, unaccelerated flight

   Maintaining altitude, constant airspeed
Dig out the math and physics!

   Vectors

   Resultant force represented by arrow
    –   Length represents magnitude
    –   Arrow orientation represents direction

   Key aerodynamic force
Newton’s Three Laws of Motion

   First Law: body at rest tends to remain at rest,
    body in motion tends to stay in motion

   Second Law: a body acted upon by a force
    –   Resultant acceleration is:
            Inversely proportional to the mass of the body
            Directly proportional to the applied force

   Third Law: for every action there is an equal
    and opposite reaction
Bernoulli’s Principle

   As the velocity of a fluid (air) increases, its
    internal pressure decreases
   Example: venturi

   Any surface which provides aerodynamic force
    when it interacts with a moving stream of air.

   Chord line

   Camber
Movement through the air

   Flight path

   Relative wind
Angle of Attack

   Angle between
    –   Chord line
    –   Relative wind

   Is aircraft ATTITUDE the same as FLIGHT PATH?

   Explain
Bernoulli and Newton
Coefficient of Lift – CL
Stall – separation of airflow from
wing’s upper surface

   An airplane always stalls when the critical
    angle of attack is exceeded regardless of
    airspeed, flight attitude, or weight.

   Critical angle of attack
Angle of attack > CLmax
Indications of a stall

   Mushy feeling in flight controls
   Stall warning device – horn, buzzer, light
   Slight buffeting

   Recover
    –   Restore smooth airflow – decrease angle of attack
        to less than critical angle of attack
    Wing Design

   Camber
   Chord
   Aspect ratio – length of wing divided by average
   Wing area – surface area
   Planform – shape from above or below
    –   Elliptical
    –   Rectangular
    –   Tapering
    –   Sweptback
Aspect Ratio

   Determines lift/drag characteristics
    –   Higher aspect ratio – higher lifting efficiency of wing
Wing area

   Need sufficient area to support weight of
    aircraft, passengers, cargo, fuel

   Each square foot doesn’t produce a lot of lift
Angle of Incidence

   Angle between the wing chord line and a line
    parallel to the longitudinal axis of the airplane

   How the wing is mounted on the airplane
Stall characteristics

   Wing twist

   Stall strips
Controlling lift

   Aircraft design
   Pilot
    –   Change angle of attack
    –   Change airspeed
    –   Change shape of wing
Change the shape of the wings

   High-lift devices
    –   Flaps
Types of flaps
Flaps, lift and drag

   As flaps are extended
    –   Create relatively large amount of lift for small
        increase in drag

   Extended half way and more
    –   Significant increase in drag for relatively small
        increase in lift

   For take off usually limited to setting flaps at
    half or less.
Leading edge devices

   Flaps
   Slats
   Slots

   Force of gravity – opposes lift
   Acts through the center of the airplane toward
    center of the earth
   Varies with
    –   Equipment
    –   Passengers
    –   Cargo
    –   Fuel
   Changes in flight as fuel is consumed or other

   Forward acting force – opposes drag
   Force = mass x acceleration
   Propeller provides thrust – engine burns fuel
    and turns prop
   Propeller is a rotating airfoil – Bernoulli and

   Opposes thrust
   Limits the forward speed of airplane
   Classified as
    –   Parasite
    –   Induced
Parasite Drag

   Any aircraft surface that interferes with the
    smooth flow of air around airplane

   Three types:
    –   Form drag
    –   Interference drag
    –   Skin friction drag
Form drag

   From turbulent wake caused by airflow around
    a structure

   Depends on size and shape of structure
    (Pg. 3-14)
Interference Drag

   Interaction of varied currents that flow over an
    airplane and mix together
   Turbulence can be 50% to 200% greater than
Skin Friction Drag

   Roughness of airplane surfaces
   Rivet heads
   Irregularities
    –   Bugs / dirt
Parasite Drag

   Combined effect of all parasite drag varies with
    the speed of airplane
   Varies proportionately to the square of the
    –   Double airspeed = four times parasite drag
   Predominant at high airspeeds
    Induced Drag

   Drag created by the
    production of lift

   Inversely
    proportional to
    square of speed
Total drag = parasite + induced
Ground Effect

   Phenomenon of less induced drag close to
   Earth’s surface alters airflow patterns around
    aircraft – reduces downward deflection of
   At height equal to wing span and lower
   Effect increases closer to ground
   Takeoff and landing
   More noticeable on low wing aircraft
Aerodynamic Principles

          Chapter 3
          Section B

   Design characteristic that causes an aircraft in
    flight to return to equilibrium after disturbed.

   Initial tendency – static stability
    –   Positive static stability

   Tendency over time – dynamic stability
    –   Positive dynamic stability

   Characteristic that permits ease of
    maneuvering aircraft
   Aircraft can withstand the stress of maneuvers
   Design characteristic determined by
    –   Size, weight, flight control system, structural
        strength, thrust

   Capability of aircraft to respond to control
    inputs, e.g. size of ailerons or rudder
Three Axes of Flight
Center of gravity

   Common point for all three axes
   Where weight of airplane is concentrated
   Airplane moves about this point in all three
   Ailerons, elevator, rudder cause rotation about
    the three axes
Longitudinal Axis

                       Ailerons
                       Deflecting starts roll
                       Neutralizing stops roll
Lateral axis

   Elevators
Vertical axis

   Rudder – yaw
Longitudinal Stability –
about lateral axis

   Tendency to return to trimmed angle of attack
    after displacement.

   When stable
    –   Aircraft resists excessive nose-high or nose-low
        pitch attitudes

   When unstable
    –   Aircraft tends to climb or dive until stall or steep dive
Longitudinal Stability

   Balance between
    –   Center of gravity
    –   Center of pressure (lift)

   Best when center of lift is aft
    of center of gravity
    Center of gravity (CG)

   Approved range
   Too far forward
     –   Nose heavy
   Too far aft
     –   Tail heavy
   Out of range
     –   Might not have
         enough control
     –   Too far aft is worse
Horizontal Stabilizer
                           Provides tail-down
                           Downwash from
                            wings, prop
Power effects on
longitudinal stability

   Change in down wash on elevator
   Change in pitching moment
   High power, low airspeed = least longitudinal
Lateral Stability –
about longitudinal axis

   Tendency to return to wings level
   Affected by
    –   Weight distribution – you can control
    –   Dihedral – design
    –   Sweepback – design
    –   Keel effect – design

   Upward angle of
    wings from root to
    tip with respect to

   Major contributor
    to lateral stability

   High wing vs. low

   Leading edges not at right angle to longitudinal
   More pronounced in high performance aircraft
   Helps keep center of lift aft of CG
   Also aids directional stability
Keel Effect

   From vertical fin, side of fuselage
   Side force against these vertical areas tends to
    roll aircraft upright
Directional Stability –
about vertical axis

   Primary aid to directional stability is vertical tail

   Acts like a weather vane
Lateral and Directional Stability

   Dutch roll –
    –   Combo of rolling and yawing oscillations
    –   Reduced with a design that increases directional
        stability and reduces lateral stability

   Spiral instability
    –   Strong directional stability vs. lateral stability
    –   Roll moment increases

   Inherent stability important in recovery from
    stalls and spins
   Occur when maximum lift or critical angle of
    attack is exceeded
   Stall speed increases with
    –   Weight
    –   Forward CG
    –   Snow, ice, frost on wings – change shape, disrupt
        airflow, add weight
    –   Turbulence
Types of stalls

   Power-off stalls
    –   Simulates approach and landing conditions
   Power-on stalls
    –   Simulates takeoff, climb-out, go-around conditions
   Accelerated stalls
    –   To understand how stalls can occur at higher
   Crossed-control stalls
Stall Recognition

   First signs of impending stall
    –   Mushy feeling in flight controls and less control effect
    –   Loss of R.P.M in fixed pitch props
    –   Reduction in sound of air flow
    –   Buffeting, uncontrollable pitching or vibrations
    –   Kinesthetic sense of decreased speed or sinking
Stall Recovery

   Decrease angle of attack
   Smoothly apply max allowable power
   Adjust power as needed

   Secondary stall

   Aggravated stall resulting in corkscrew path

   One wing more stalled than other

   Usually happen when exceeding critical angle
    of attack while performing uncoordinated

   Direction of spin toward rudder being applied
Types of spins

   Erect spin – roll and yaw in same direction

   Inverted spin – roll and yaw in opposite

   Flat spin – yaw only
Weight & Balance considerations

   Heavier weights – slow initial spin rates but
    increases, longer recovery time
   Forward CG – more stall/spin resistant
   Aft CG – tend to flat spins
   Unbalanced or concentrated loads far from CG
    Spin Phases

   Incipient
   Fully developed
   Recovery
Spin Recovery

   Throttle idle
   Neutralize ailerons
   Determine direction of rotation (turn coordinator)
   Apply full opposite rudder
   Elevator forward – either relax back pressure or
    apply full forward pressure (depends on AC)
   As rotation stops – neutralize rudders
   Gradually apply aft elevator – return to level
Stall – spin accident prevention

   Page 3-41, top
   Page 3-43, last paragraph

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