Download Ch3A(4 forces of flight) - Engineering the Future Workshop

Private Pilot Theory
Chapter 3, Section A
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Forces of Flight
• Four Forces of Flight
– Lift
• Upward force created by air flowing over & under the wings
• Supports the plane in flight
– Weight
• Opposes lift
• Caused by downward pull of gravity
– Thrust
• Forward force propelling the plane through the air
• Varies with amount of engine power
– Drag
• Backward force limiting the speed of the plane
– Equilibrium
• All four forces are in equilibrium during straight-and-level,
unaccelerated flight
• Unaccelerated flight: plane is maintaining a constant speed
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Forces of Flight
• Newton’s Three Laws of Motion
– First Law
• A body at rest tends to remain at rest, and a body in motion
tends to remain moving at the same speed and in the same
direction
– Second Law
• When a body is acted upon by a constant force, its resulting
acceleration is inversely proportional to the mass of the body
& is directly proportional to the applied force (Force = mass x
acceleration)
– Third Law
• For every action there is an equal & opposite reaction
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Forces of Flight
• Bernoulli
– Swiss mathematician
– Expanded on Newton’s ideas
– Bernoulli’s Principle
• As the velocity of a fluid (air) increases, its internal
pressure decreases
• Helps explain how air flows around an airfoil (wing)
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Forces of Flight
• Airfoils
– Any surface (wing) that provides aerodynamic force when it
interacts with a moving stream of air
– Airflow affects the pressure on an airfoil
– Wing shape
• Designed to take advantage of Newton’s & Bernoulli’s laws
• Greater curvature on upper portion of the wing
• Leading Edge
– Part that meets the air first
• Upwash
– Deflection of airflow up & over the wing
• Trailing Edge
– Where airflow from upper surface meets airflow from lower surface
• Downwash
– Deflection of airflow downward behind the wing
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Forces of Flight
• Airfoils
– Camber
• Curvature of an airfoil’s upper & lower surfaces
– Relative Wind
• Airflow parallel to, & opposite of, the airplane’s flight path
– Chord Line
• Imaginary line from leading edge to trailing edge of an airfoil
– Angle of Attack
• Angle between chord line & relative wind
– Coefficient of Lift (CL)
• Measures lift as it relates to angle of attack
• Greater AOA = greater CL...to stall point
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Forces of Flight
• Stall
– Definition
• A rapid decrease in lift caused by the separation of airflow
from the wing’s surface brought on by exceeding the critical
angle of attack
– Critical Angle of Attack
• The angle at which a stall will always occur, in a given
airplane, regardless of airspeed, flight attitude, or weight
• Warning signs of a stall
– Less effective flight controls, stall warning horn, slight buffeting
of aircraft or controls
• Stall Recovery
– Decrease the AOA to a point below the Critical AOA (pitch
down)
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Forces of Flight
• Wing Designs
– Factors
• Anticipated use of the airplane, cost, etc.
– Aspect Ratio
• Relationship between length & width of the wing
• Helps determine lift/drag information
• Higher aspect ratio = less drag per amount of lift & AOA
– Wing Area
• Total surface area of the wings...must be enough to support
the weight of the airplane
• Generally: wings produce about 10.5 lbs lift per square foot
of wing
• Example: 2100-lb plane needs a wing area of 200 square
feet
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Forces of Flight
• Wing Designs
– Planform
• Shape of wings when viewed from above or below
• Elliptical
– Good for slow flying: has less drag
– Difficult to build; not very good stall characteristics
• Rectangular
– Provides better stall characteristics (stalls at wing root first)
– Not as efficient as elliptical wing
• Tapered
– Decrease in drag, increase in lift...better at higher speeds
– Tends to stall first closer to the wingtip
– Combination of rectangular & tapered is best
• Sweptback
– Very efficient at high speeds...allows the plane to fly faster without the
wing experiencing supersonic airflow
– Not very good performance at slow speeds
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Forces of Flight
• Wing Designs
– Angle of Incidence
• Angle between chord line of the wing & longitudinal axis of
the airplane
• Washout
– Slight twist of the wing
– Gives wingtip a lower angle of incidence (& AOA) than the wing
root
– Helps prevent stalling at wingtips first (aileron effectiveness)
– Stall Strips
• Metal strips attached to leading edge of each wing near
fuselage
• Disrupt airflow at high angles of attack...wing area behind the
strips stall first, before wingtips stall
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Forces of Flight
• Controlling Lift
– Angle of Attack
• Pilot controls AOA
• Change in pitch = change in AOA & CL
• Increase AOA = increase in lift
– Airspeed
• Faster airspeed = greater lift
• Lift is proportional to the square of the plane’s speed
– Plane at 200 knots has 4 times the lift of a plane at 100 knots
– If speed is cut in half, lift is ¼ of what it was before
– Total Lift
• Depends on both airspeed and AOA
• Decrease in speed needs an increase in AOA to keep the same
amount of lift
• Increase in speed needs decrease in AOA
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Forces of Flight
– High-Lift Devices
• Purpose: to increase efficiency of airflow at low speeds
• Most common: flaps
• Flaps: increase lifting efficiency of wings, decrease stall speed
(changes chord line, increasing AOA)
– Configuration: refers to position of landing gear & flaps
» Clean: gear & flaps up
» Dirty: gear & flaps down
– Plain: attached to wing by a hinge; increases camber & changes chord
line
– Split: hinged to lower part of wing; produces more drag
– Slotted: similar to plain flaps; allows air under the wing to flow through a
slot – increases speed of airflow over the flap & gives more lift
– Fowler: moves both back & down; increases total wing area, camber &
chord line
– Considerations for Flaps: when using more than half, more drag than
lift is produced, reducing efficiency
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Forces of Flight
• Weight
– Gravity
• Acts vertically through the center of the plane (CG) toward center of earth
• Varies with equipment, passengers, fuel, etc.
• Thrust
– Forward-acting force
• Propels plane, opposes drag
• Thrust = Drag, when in straight-and-level, unaccelerated flight
• Drag
– Limits forward speed of an airplane
– Parasite: caused by any aircraft surface that interferes with smooth
airflow around the plane; amount of parasite drag is proportional to the
square of the airspeed (double airspeed = quadruple parasite drag)
• Form Drag: caused by separation of airflow from the surface of an object;
related to size & shape of object
• Interference Drag: when currents of airflow meet & interact (wing & gear
struts)
• Skin Friction Drag: roughness of plane’s surfaces; thin layers of air cling to
rough surfaces
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Forces of Flight
• Drag
– Induced: from airflowing around the wing as it creates lift; high pressure
under the wing spirals around to join low pressure above the wing
• Wingtip Vortices: deflect airstream downward from the wing (downwash)
• Induced drag is inversely proportional to the square of airspeed (if speed
decreases by ½, induced drag increases by 4)
– Total
• Sum of parasite & induced drag
• Drag Curve: parasite & induced drag vs. airspeed
• L/Dmax: greatest lift/drag ratio (best performance for least drag)
• Ground Effect
– a decrease of induced drag & increase in lift.
• The ground interferes with the air flowing around the wingtips (vortices) &
creates a “buffer” under the wings.
• This only happens within 1 wingspan of the ground. Can be both good &
bad (make you think it wants to fly before it’s actually ready)
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