# Aerodynamic Principles by 8A85d2

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```									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
Equilibrium

   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
Lift

   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
Airfoils

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

   Chord line

   Camber
Movement through the air

   Flight path

   Relative wind
Angle of Attack

   Angle between
–   Chord line
–   Relative wind
Question?

   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
chord
   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
Planform
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.

   Flaps
   Slats
   Slots
Weight

   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
Thrust

   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
Newton
Drag

   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
separate
Skin Friction Drag

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

   Combined effect of all parasite drag varies with
the speed of airplane
   Varies proportionately to the square of the
airspeed
–   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
ground
   Earth’s surface alters airflow patterns around
aircraft – reduces downward deflection of
airstream
   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
Stability
Stability

   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
Maneuverability

   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
Controllability

   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
directions
   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 –

   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
develops
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
force
   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
stability
Lateral Stability –

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

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

   Major contributor
to lateral stability

   High wing vs. low
wing
Sweepback

   Leading edges not at right angle to longitudinal
axis
   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 –

   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
Stalls

   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
–   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
airspeeds
   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
feeling
Stall Recovery

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

   Secondary stall
Spins

   Aggravated stall resulting in corkscrew path

   One wing more stalled than other

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

   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
directions

   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