Aerodynamics 3

1
Lecture 6

• Aerodynamics (3)
• Stability (Contd)
• Maneuvering
• Chapter 3 (B,C), Jeppesen Sanderson
• Chapters 3 and 4, Kroes and Rardon

2
Stability

• Stalls
• Definition and causes
• Factors affecting its development
• Recognition
• Recovery

3
Stalls Definition causes

• A stall is a loss of lift such that the lift
  becomes unable to counterbalance the weight.
• When the angle of attack (AoA) becomes too high
  (exceeding the critical AoA) the airflow over the
  upper surface of the wing breaks up and becomes
  no longer attached to the wing.
• As a result the lift due to Bernoullis principle
  drops dramatically. (Fig 3-8)

4
Breaking up of air current over wings upper
surface (3-8)
5
Causes for stall and stall speed (1)

• As shown in Fig 3-7 earlier in the notes for
  Lecture 3, the lift coefficient CL increases as
  AoA increases until it reaches the critical AoA,
  then suddenly CL drops very fast.
• From the lift equation shown above, if CL drops,
  the lift will also drop and becomes unable to
  counterbalance the weight. In such case, a stall
  occurs.

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Causes for stall and stall speed (2)

• The lift equation also indicates that the speed V
  affects the lift too.
• The speed at which the lift becomes unable to
  counterbalance the weight (i.e., the speed at
  which a stall occurs) is called the stall speed.

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Factors affecting stall development (1)

• Several factors can contribute to the development
  of a stall
• Airplanes speed gets too slow (needs to increase
  AoA to maintain enough lift)
• Too much weight (needs more AoA to overcome the
  heavy weight)
• CG too forward (need to press down the tail with
  the elevator, resulting in the need of more lift)

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Factors affecting stall development (2)

• (Continue from last slide)
• Modification of wing surface due to natural
  factors like snow or ice can change the shape of
  the wing, disrupting the airflow, and can
  increase weight and drag. All these will
  increase stall speed (stall easier).
• Turbulence that abruptly increase the AoA.
• Thus take-off and landing in bad weather are
  usually done with higher speed, in order to
  maintain a large margin above stall.

9
Stall Recognition

• Recovery from stalls are easier if stalls are
  recognized earlier.
• How does a pilot detect a stall at the early
  stage?
• A mushy (not solid) feeling in the flight
  controls, and loss of effective control as the
  speed decreases.
• Reduction of the sound of air flow along the
  fuselage.
• Buffeting (slipping from side to side),
  uncontrollable pitching, or vibrating.
• A sinking feeling.

10
Stall Recovery

• If you do not recover promptly, a secondary stall
  or a spin might occur.
• The following are proper stall recovery
  procedures
• Decrease the angle of attack.
• Smoothly applying maximum allowable power to
  minimize altitude loss and increase airspeed.
• As the plane recovers adjust the power to resume
  coordinated flight.

11
Stability

• Spins
• Definition and causes
• Types of spins
• Weight and balance considerations
• Spin recovery

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Spin definition and description (1)

• A spin is defined as a worsened stall which
  results in an airplane descending in a helical,
  or corkscrew path.
• To go into a spin an airplane must go into a
  stall first.
• Although both wings are in a stalled condition,
  one wing is stalled more than the other.

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Spin definition and description (2)

• The wing that is more completely stalled will
  often drop before the other.
• The low wing has less lift and more drag than the
  high wing.
• The nose will yaw in the direction of the low
  wing. This yawing, plus the dropping of the
  plane, results in the spiraling drop.

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Spin Causes (1)

• Typically the cause of a spin is exceeding the
  critical AoA while performing an uncoordinated
  maneuver (will go to this later).
• The uncoordinated maneuver usually comes from too
  much or not enough rudder control for the amount
  of aileron applied during a turn (such maneuver
  is called a crossed-control).
• If you do not initiate a stall recovery
  immediately, the plane will likely enter a full
  stall which may develop into a spin.

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Spin Causes (2)

• Pilots normally can maintain coordinated
  controls. However this ability often falls when
  a distraction occurs and their attention is
  divided.
• Distractions occurs when the pilot is working to
  avoid other aircraft, or clear obstacles during
  takeoffs, climbs, or landing, and suddenly has to
  make a turn.

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Types of Spin

• There are three main types of spin (Fig 3-46)
• Erect spin top of the plane remains on top,
  while plane rolls and spiraling down. Nose
  points slightly downwards.
• Inverted spin plane is upside down and
  spiraling down. Nose also point slightly down.
• Flat spin top of plane remains upwards and the
  plane does not roll but remains flat, just
  spiraling down.
• Flat spin is the most deadly because it is
  difficult or impossible to recover.

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Types of Spin (3-46)
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Weight and Balance on Spin (1)

• Heavier airplanes spins slower than a lighter
  plane at the beginning, but spins faster as the
  spin develops. It takes longer to recover.
• A plane with CG relatively forward is less likely
  to get into stalls or spins.
• Spins in planes with CGs relatively aft are more
  likely to become a flat spin.

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Weight and Balance on Spin (2)

• In a small plane (training plane) the addition of
  a single passenger in the back seat or a single
  baggage in the aft compartment can affect the CG
  enough to change the spin characteristics.
• Any concentration of weight that is far away from
  the CG is undesirable.
• An example is when a plane with tip tanks and
  unbalanced fuel usage in those tanks. The worst
  situation is when the tank on the outside of the
  spin spiral is full and that on the inside of the
  spiral is empty.

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Spin Recovery (1)

• Every plane spins differently. Recovery has to
  follow the POH. In general, recovery involves
  the following
• Move throttle to idle minimize loss of altitude
  as the plane goes down
• Neutralize the ailerons
• Determine the direction of the rotation (with the
  help of the turn coordinator).
• Apply full rudder to the opposite direction of
  the rotation. (continued on next slide)

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Spin Recovery (2)

• Quickly apply the elevator control forward to
  about neutral position.
• As rotation stops (indicating that the stall has
  been broken) neutralize the rudder. If you dont
  neutralize rudder when rotation stops you may
  enter a spin in the opposite direction.
• Gradually apply aft elevator (pull control wheel
  backwards) to return to level flight. Applying
  too quickly may initiate a secondary stall, and
  possibly another spin.

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Spin Prevention

• Pay attention to loading. Aft CG should be
  avoided.
• Do not take off with snow, ice, or frost on the
  wing.
• If forced landing is required soon after take
  off, dont attempt to return to the runway.
  Select a possible landing site straight ahead
  (avoid sharp turns).
• Maintain coordinated flight as much as possible.
  Especially, avoid skidding turns near the ground.
• Use a somewhat higher airspeed in takeoff and
  landings in gusty winds.
• Always concentrate on flying the aircraft and
  avoid prolonged distraction.

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• Aerodynamics of
• Maneuvering Flight
• Climbing
• Left-turning tendencies
• Descending (and gliding)
• Turning
• Load factor

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Climbing

• During a climb, weight is not perpendicular to
  the flight path. There is a backward component.
• This component has to be countered by the thrust
  instead of by the lift.
• In the extreme case when the airplane goes
  vertically up (attitude 90 degrees, planes like
  F-16 can do it), thrust alone takes care of both
  the weight and the drag. There is no lift. (The
  thrust-to-weight ratio for a Boeing 747 is about
  0.26 to 1, for the F-16, it is 1.1 to 1.) (Fig
  P.3-47)

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Straight up flight of F-16
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Left-turning Tendencies (1)

• By design, an airplane has some unavoidable
  tendencies to turn to the left which the pilot
  has to be aware of
• Torque the clockwise rotation (as viewed by the
  pilot) of the propeller induces a roll tendency
  in the anti-clockwise direction that results in a
  left-turning tendency.
• Spiraling wind by the propeller the rotating
  propeller produces a whirl wind that spirals in
  the clockwise direction, thus hitting the left
  side of the vertical stabilizer and turns the
  tail to the right, and thus the nose to the left.
  (Fig 3-52)

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Spiraling wind around plane hits right fin (3-52)
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Left-turning Tendencies (2)

• (Continue form last slide)
• Asymmetric Thrust when the AoA is large, the
  airspeed has a small component going from below
  to the top as seen by the propeller. As a
  result, downward rotation of the propeller blades
  on the right side has a larger angle of attack
  than the upward rotation of the blade on the left
  side, resulting in the thrust being stronger on
  the right then the left, and tends to turn the
  plane to the left. (Fig 3-51)
• The pilot has to compensate for these
  left-turning tendencies.

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AoA for propeller blades larger on the right
(3-51)