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Aerodynamics PPt 3

Aerodynamics PPt 3. AVN-1020 Spring 2011. Aerodynamic Terms. Airfoil – is a structure or body which produces a useful reaction to air movement. Airplane wings, Helicopter rotors, etc .

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Aerodynamics PPt 3

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  1. AerodynamicsPPt 3 AVN-1020 Spring 2011

  2. Aerodynamic Terms • Airfoil – is a structure or body which produces a useful reaction to air movement. Airplane wings, Helicopter rotors, etc. • Chord line – an imaginary straight line from the leading edge of the wing to the trailing edge of the wing. (changing the shape of an airfoil will change the chord line, lowering the flaps.) • Relative Wind – is the wind that runs parallel and in opposite direction to the flight path of the airfoil. • Angle of Attack – the angle between the chord line and the relative wind. This can be changed by use of the controls (pitch). • Angle of Incidence – the angle at which the wing is attach to the fuselage, cannot be changed.

  3. Axes of Rotation & 4 Forces of Flight • Aircraft have 3 axis of rotation: Lateral, Longitudinal, and Vertical • Lateral Axis: an imaginary line from wingtip to wingtip. Rotation around the Lateral Axis is “pitch”. This controls the longitudinal stability of an aircraft. • Longitudinal Axis: an imaginary line from nose to tail. Rotation around the Longitudinal axis is “roll”. This controls the lateral stability of an aircraft. • Vertical Axis: an imaginary line extending through the intersection of the Lateral and Longitudinal axes. Rotation around the vertical axis is “yaw”. This controls the directional stability of the aircraft. • Center of Gravity (CG): the point at which an aircraft would balance if it were suspended at that point. The 3 axes intersect at the “CG”.

  4. Axes of Flight

  5. 4 Forces of Flight • Lift: is the result of a pressure difference between the top and bottom of the wing. Bernoulli’s Principle* • Weight: is the force with which gravity attracts all bodies vertically towards the center of the Earth…gravity sucks! • Thrust: is the force which is produced by the propeller acting as an airfoil to displace a large mass of air to the rear. • Drag: is a rear-ward force acting which resists the forward movement of an aircraft through the air. Drag may be classified into 2 categories: “Parasite” and “Induced” *Bernoulli’s Principle: Air traveling faster over the curved upper surface of the wing causes lower pressure on the top of the wing than on the bottom – causing the wing to rise up.

  6. Four Forces of Flight

  7. What a Drag! • 2 types of Drag: Induced and Parasite • Parasite or Parasitic: is the resistance of the air produced by any part of an aircraft that does not produce lift (mirror, antenna, etc.). • Induced: is a by-product of lift. As lift is generated, induced drag increases. Induced Drag will increase as airspeed decreases!

  8. Induced Drag Drag occurs at the trailing edge of the wing as lift is produced

  9. Weight and Lift

  10. Turns • To Maintain Level Flight: • Total lift vector increases • Higher AoA &induced drag • More thrust needed • Horizontal Forces: • Centripetal – from lift • Centrifugal – from mass of aircraft • When balanced, constant rate, level turn • “G Force” • Apparent weight you and the airplane feel Centrifugal G Force

  11. Limit Load Factor, “G Limits” • Limit Load Factor: the number of G’s an airplane can sustain without causing permanent damage • Aircraft are certified in 3 different categories: Normal -1.52g to +3.8g Utility -1.76g to +4.4g spins Acrobatic - 3.0g to +6.0g anything • Refer to airplane POH!

  12. Calculating Load Factor • To maintain constant altitude, G increases with bank angle • Corresponds to vertical component of lift vector • Increase of G results in increase of stall speed • Wing thinks that aircraft weight has increased!

  13. Flight Envelope Level Flight Aerodynamic Stall

  14. Stability of an Aircraft • Stability: the inherent ability of an airplane to return, to its original flight condition after being disturbed by an outside force, such as rough air. • Positive Static Stability: the initial tendency of an aircraft to return or not return to it’s original position. • Positive Dynamic Stability: is the tendency of an oscillating airplane (with positive static stability) to return to its original position relative to time. • The Center of Gravity (CG) with respect to the Center of Lift (CL) will determine the Longitudinal Stability of the aircraft. • An aircraft will be less stable at airspeeds if it is loaded to the most aft position. Stall recovery could be impossible!

  15. Stability (cont.) • An advantage of a stable aircraft is that it will take less effort to control. Stability = Ease of control. • Changes in Pitch can also be experienced with changes in power settings: Reduction in Power = nose pitches down Application of Power = nose pitches up

  16. Roll Stability • Low-wing airplanes have upward tilt of wing (dihedral) to provide roll stability • High-wing airplanes inherently roll stable due to CG being lower than wing

  17. Yaw Stability • Rudder provides centering force • Farther away rudder is from CG, the more effective it is

  18. Aircraft Pitch Stability • CG is forward of CoP (center of pressure) • Horizontal stab keeps tail down • Wing compensates for stab by creating more lift

  19. What is a Stall? • DEFINITION: When wing exceeds its critical AoA • Point where increase in AoA ≠ Increase in lift • When airflow separates from wing’s upper surface

  20. Center of Pressure • Center of Pressure of airfoil shifts with: • Camber (ie, flaps!) • Change of AoA • Result: • Control pressures and trim will change with AoA and flap extension

  21. Stall Facts • For a given airfoil, a stall always occurs at the same angle of attack • Airspeed, weight and attitude irrelevant • Stall characteristics vary with different airfoil shapes and wing planforms or “shapes” • Training airplanes usually have gradual stall patterns • Wing stalls at the wing root (where the wing meets the fuselage) and progresses outward to the wing tip • Recovery procedures same for all airplanes: • Maximum power • Reduce angle of attack • Roll wings level

  22. AoA – Flaps and No-Flaps FLAPS NO FLAPS

  23. Plain: entire flap is hinged Split: lower part is hinged Slotted: similar to plain flap Allows air from lower part of wing to pass through Fowler: extends on track & roller system Increases camber and size Types of Flaps

  24. Stall Mitigation: Flaps • Located on trailing edge • Can be combined with leading edge devices “Slats” • Effects: • Increases the camber of the wing • Increases amount lift for given AoA and airspeed • Decreases stall speed • Allows takeoff and landing at slower speeds

  25. Stall Mitigation: Wing Twist • Higher AoA at wing root verses wingtip • Wing root stalls first - ailerons keep working • Turbulent flow crosses horizontal stab – stall warning • One reason why flaps are put on inboard part of wing

  26. Stalls and Spins • As the Angle of Attack (AOA) is increased, (to increase lift), the air will no longer flow smoothly over the upper wing surface, but instead will become turbulent or “Burble” near the trailing edge of the wing. • At an angle of 18 – 20 degrees nose up, turbulence over the upper surface decreases lift so drastically that the lift is inefficient so much that the wing stalls. • Critical Angle of Attack: the angle at which the wing stalls. • Spin: an aircraft will spin only after the wing stalls. Rotation will continue as long as one wing (upper) is still producing some lift, more than the lower wing in a stalled condition.

  27. What is a Stall? • DEFINITION: When wing exceeds its critical AoA • Point where increase in AoA ≠ Increase in lift • When airflow separates from wing’s upper surface

  28. Spins • Essentials: • Stall • Yaw • Stages: • Entry • Incipient • Developed • Recovery • Recovery procedure breaks both stall and yaw

  29. Types of Spins

  30. Spin Aerodynamics • Both wings are stalled! • Outboard wing not stalled as badly • Creates more lift than inboard • Perpetuates rotation

  31. Ground Effect • Ground Effect: this occurs when flying within the distance of one wingspan length (or less) above the surface. • What is it? - Downwash from the trailing edge of the wing creates a “cushion of air” on which the aircraft “floats”. This is due to Induced Drag. • How can it effect you? - This can result by the aircraft becoming airborne beforethe aircraft reaches recommended flying speed. On landing it may result in “floating” during an approach to land. • An aircraft leaving ground effect will require an increase of angle of attack (AoA) to maintain the same lift as when out of ground effect. • More thrust will be required as well to counter the induced drag.

  32. Wake Turbulence • Source is wingtip vortices • High pressure air below curves around tip toward low pressure air on top of wing • Intensity of vortices proportional to amount of lift & induced drag being produced by a wing • Vortices worst when an airplane is: • Heavy • Clean • Slow • Vortices sink and go outward at around 5 knots

  33. Wake Turbulence Mitigation

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