Ground lesson 3 - Aerodynamics and stability

• 4 forces of flight
• Lift
• Force created by the effect of airflow as it passes over and under the wing
• Opposes weight in straight and level flight
• Bernoulli’s principle
• As velocity increases, pressure decreases 

• Airfoils
• Any surface that provides aerodynamic force when it interacts with a moving stream of air 
• Symmetrical airfoil
• Same camber (curvature) on both the upper and lower surfaces
• Very stable
• Achieved by keeping the center of pressure virtually unchanged as the AOA changes

• Asymmetrical airfoil
• Center of pressure changes with changes in AOA
  

• Blade twist
• Produce a more even amount of lift along its span
• This is necessary because rotational velocity increases toward the blade tip

• Leading edge
• Front of the airfoil

• Trailing edge
• Rear of the airfoil

• Chord line
• Straight line connecting leading and trailing edges

• Camber
• Upper and lower camber divided by the chord line

• Mean camber line
• Imaginary center line of the airfoil cutting upper and lower chambers into equal halves

• Angle of attack (AoA)
• Angle between chord line and relative wind
• Higher AoA, Higher lift production

• Stalls
• Caused by the separation of airflow from the wings upper surface
• Results in a rapid decrease of lift

• For a given airplane, the stall always occurs at the same angle, regardless of airspeed, flight attitude, or weight. This angle is the stalling, or critical angle of attack.
  
  
  

• Pilot control of lift
• You can change the AoA and the airspeed, or change the shape of the wing by lowering the flaps
• Anytime you do something to increase lift, drag also increases

• Weight
• Force of gravity which acts through the center of the airplane towards the center of the earth

• Thrust
• Forward acting force which opposes drag
• Provided when the engine turns the propeller

• Drag
• Force that resists the movement of aircraft through the air 
• Produced when thrust is developed
• Always acts parallel to the relative wind
• Total drag is composed of two types of drag
• Induced
• Parasite
• Form drag
• Skin friction
  
  
  


• Parasite drag 
• Present any time the aircraft is moving through the air
• Increases with airspeed
• Any loss of momentum by the air-stream

• Form drag
• Turbulent wake caused by the separation of airflow from the surface of a structure
• Related to both the size and shape of the structure that protrudes into the relative wind

• Skin friction 
• Caused by surface roughness

• Induced drag 
• Generated by the airflow circulation around the wings as it creates lift
• High-pressure area beneath the wing joins the low-pressure air above the wing at the trailing edge and at the wing tips
• Causes a spiral, or vortex, which trails behind each wing whenever lift is produced
• Vortices deflect the air-stream downward in the vicinity of the wingtip, Creating an increase in down-wash

• Total drag
• sum of all three drag forces
  
  
  
• Ground effect
• Associated with the reduction of induced drag 
• During takeoff and landing you are very close to the ground, this causes a reduction of wing tip vortices and a decrease in upwash and downwash
• With the reduction of induced drag, the amount of thrust required to produce lift is reduced
• Reach the speed for a normal climb before liftoff, to ensure the aircraft doesn’t descend again

• Stability
• Characteristic of an airplane in flight that causes it to return to a condition of equilibrium (steady flight), after it is disturbed

• Static stability (positive, negative, neutral)
• The initial tendency to return to the position from which it was displaced 

• Dynamic stability
•  “Since the aircraft doesn’t return to its immediate position, but does over a period of time and successively smaller oscillations, the aircraft displays “positive dynamic stability”

• Maneuverability
• Characteristics that permit you to maneuver the aircraft easily and allow it to withstand the stress resulting from the maneuvers

• Controllability
• Capability of the aircraft to respond to your control inputs

• Three axes of flight
• Longitudinal
• Deflecting the ailerons begins a turn, creating an immediate rolling moment

• Lateral
• Movement of the elevator stabilizer causes forward or aft pitch

• Vertical 
• Applying pressure to the rudder pedals yaws the airplane

• Longitudinal stability
• Involves the pitching motion of the aircraft to move about its lateral axis
• A longitudinal stable aircraft tends to go back to its trimmed angle of attack 
• An airplane that is longitudinally unstable will tend to climb or dive

• Center of gravity position
• Determined by the distribution of weight either by design or by the pilot
• Can affect the longitudinal stability of an airplane
• If CG is too far forward, it will be nose heavy, if it is too far aft, it will be tail heavy

• Most airplanes are designed to be slightly nose heavy
• CG too far forward
• Longer takeoff distance
• Higher stall speed

• CG too far aft
• Tail heavy and unstable in pitch
• As CG moves aft, stabilator(elevator) effectiveness decreases 
• If CG is beyond aft limit, stabilator may be ineffective for stall or spin recovery

• Horizontal stabilizer 
• Designed with a negative angle of attack
• When the airplane is properly loaded, the CG remains forward of the center of pressure
• Causes the airplane to be nose heavy

• Designed with a negative angle of attack to counteract the nose heaviness

• Lateral stability
• Stability displayed around the longitudinal axis of the airplane.
• If one wing is lower than the opposite wing, lateral stability helps return the wings to a level attitude

• Directional stability
• Stability of the aircraft about its vertical

• Stalls
• The inherent stability of an airplane is important as it relates to the aircraft's ability to recovery from stalls and spins
• A stall will always occur when the maximum lift, or critical angle of attack is exceeded
• If an airplane's speed is too low, the required angle of attack to maintain lift may be exceeded

• Stall speed can also be affected by other factors such as weight and environmental conditions
• Types of stalls 
• Power off - simulates landing
• Power on - simulates takeoff

• Recognition
• Mushy feeling in the flight controls
• Loss of rpms in power on conditions
• Reduction of sound

• Stall recovery
• Forward pitch, decreasing the AoA
• Full throttle
• Adjust flaps

• Spins
• May be defined as an aggravated stall which results in the airplane descending in a corkscrew path
• Primary cause
• A stalled aircraft is a prerequisite for a spin 
• The cause of an inadvertent spin is exceeding the critical angle of attack while performing an uncoordinated maneuver

• Three types of spin
• Erect - slight nose down, rolling and yawing motion in the same direction
• Inverted - upside down spinning, yaw and roll occurring in opposite directions
• Flat - yawing about its vertical axis with pitch approximately level with the horizon
• Most aircrafts are designed to prevent flat spins provided the load and CG are within approved limits

• Spin phases
• Incipient 
• From where the airplane stalls and rotation starts until the spin is fully developed

• Fully developed
• After the incipient stage when the angular rotation rate, airspeed, and vertical speed are stabilized from turn to turn and the flight path is close to vertical

• Recovery 
• Application of anti-spin forces result in a slowing and/or eventual cessation of rotation

• Recovery (erect spin)
• Move throttle to idle - eliminates thrust and minimizes loss of altitude 
• Neutralize the ailerons 
• Determine the direction of rotation
• Apply full opposite rudder
• Briskly apply elevator forward to approximately the neutral position 
• As rotation stops (stall has been broken), neutralize the rudder
• Gradually apply aft elevator to return to level flight
• Aerodynamics of maneuvering flight
• Climbing flight
• Aerodynamic forces in a stabilized climb are in equilibrium
• Transition from straight and level to a climb normally includes a change in pitch with an increase in power
• Excessive thrust, not excessive lift, is necessary for a sustained climb
• As the angle of climb steepens, thrust will oppose drag and increasingly replace lift
• Left-turning tendencies
• A combination of physical and aerodynamic forces can contribute to a left turning tendency 
• Torque 
• Newton's 3 law → for every action there is an equal and opposite reaction

• Propeller rotates clockwise, causing a counterclockwise torque effect
• Gyroscopic procession
• (tailwheel airplanes)

• Asymmetrical thrust
• When flying a propeller-driven airplane at a high angle of attack, the descending blade receives more air than the ascending blade. 
• Caused because of the higher angle of attack for the descending blade

• Spiraling slipstream
• As the propeller rotates, it produces a backwards flow of air, or slipstream, which wraps around the airplane
• Causes a change in airflow around the vertical stabilizer
• Due to the direction in propeller rotation, the resultant slipstream strikes the left side of the vertical fin, causes a left yaw
• Descending flight
• In stabilized descending flight, aerodynamic forces are in equilibrium with the forces of weight comprised in two forces
• Weight perpendicular to the flight path
• Weight forward along the flight path

• As the nose of the aircraft is lowered, the component of weight acting forward along the flight path increases. If the power remains the same, airspeed will increase
• Increase in airspeed results in an increase in parasite drag, which works to balance the force of weight
• If thrust is removed(idle), a larger component of weight must be allocated to counteract drag and maintain a constant airspeed, accomplished by lowering the nose
• Turning flight
• Airplane overcomes inertia of straight flight to begin a turn
• Ailerons create turning force to bank the aircraft
• Lift force is divided, acting vertically to counteract weight, and horizontally 
• To maintain altitude, you will need to increase lift by increasing back pressure, also increasing angle of attack until the vertical component of lift equals weight
• Horizontal component of lift creates a force directed inward towards the center of rotation, called centripetal force 
• This center seeking force causes the airplane to turn

• Inertia counteracts centripetal force, this force is called centrifugal force

• Adverse yaw
• Aileron on inside of turn is raised, while aileron on outside of the turn is lowered 
• Lowered aileron produces higher angle of attack, producing more lift

• Since induced drag is a by product of lift, outside wing produces more drag, causing a yawing tendency
• Overbanking tendency 
• Outer wing has higher AoA, producing more lift, causing aircraft to overbank
• Correct by using slight opposite aileron

• Load factor
• Ratio of the load supported by the airplane's wing to the actual weight of the aircraft
• In cruising flight, the airplane has a load factor of one(G force). Meaning the aircraft is supporting itself
• Stall speed increases in proportion to the square root of the load factor
• Limit load factor
• Published in the POH
• Amount of stress that an airplane can withstand before structural damage