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Advanced Aerodynamics

AF 202 – Chris Dimoulis. Advanced Aerodynamics. Objectives. A little bit of Theory A little bit of Physics A little bit of Math A whole lot of fun!. Definitions. Leading edge Trailing edge Chord Line. Definitions. Relative Wind Angle of Attack. Aerodynamic Forces. What is a force?

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Advanced Aerodynamics

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  1. AF 202 – Chris Dimoulis Advanced Aerodynamics

  2. Objectives A little bit of Theory A little bit of Physics A little bit of Math A whole lot of fun!

  3. Definitions Leading edge Trailing edge Chord Line

  4. Definitions Relative Wind Angle of Attack

  5. Aerodynamic Forces What is a force? Any influence to an object that causes a change in speed, direction, or shape. What is acceleration? A change in velocity (speed and direction) An unbalanced force is required for an acceleration. Force, acceleration, velocity are ‘vectors’

  6. Aerodynamic Forces The most known force formula: Force = Mass x Acceleration You can rewrite this to say that: Acceleration = Force/Mass

  7. Forces on Airplanes The 4 Airplane Forces are: Weight Lift Thrust Drag

  8. Level/Constant Speed Flight Remember what an acceleration is Change of speed and/or direction Requires and unbalanced force Opposing forces must be equal Thrust = Drag Lift = Weight

  9. Unbalanced Forces If weight and lift are not balanced… Weight > Lift = Airplane decends Lift > Weight = Airplane pitches up (NOT CLIMB!!!!) If thrust and drag are not balanced… Thrust > Drag = Aircraft speeds up Drag > Thrust = Aircraft slows down

  10. The Mathematical Way It is simply a matter of adding the vectors together… The resulting force is… This will produce an acceleration in the direction of that force. -90 100 10

  11. Weight Weight is a force The acceleration in F=ma is the pull of gravity (9.8 m/s2) The total weight of your plane is the downward force applied at the center of gravity (CG).

  12. Weight We feel weight It is equally opposed by the ground. Therefore there is no acceleration

  13. Lift Opposes weight Acts perpendicular to the flight path Occurs at the center of lift/pressure

  14. Lift Two principles are used to explain lift: Bernoulli’s Principle Newton’s 3rd law of motion

  15. Bernoulli’s Principle When the velocity increases the pressure decreases

  16. Bernoulli’s Principle High pressure always seeks a low pressure High pressure below the wing applies a force upward

  17. Newton’s 3rd Law For every action there is an equal and opposite reaction

  18. Lift What ways can lift be changed? Pilot Controlled: Change Speed Change Angle of Attack Non-Pilot Controlled Change Wing Surface Area Change Air Density

  19. The Lift Equation L=1/2 (CL V² Ρ A) CL= Coefficient of Lift V = Velocity (ft per sec) 1 knot = 6076 ft/hr = 1.68527 ft/sec P = Air Density A = Wing Surface Area (sq.ft.)

  20. The Coefficient of Lift The coefficient of Lift increase as Angle of Attack increases

  21. Lift Equation Velocity also has a great impact. Velocity increases – Lift Increases Air density and the design of the wing have an affect, but cannot be changed by the pilot

  22. Air Density Table Altitude Density Speed of Sound (Feet) (d) (Knots) 0 .002377 661.7 1,000 .002308 659.5 2,000 .002241 657.2 3,000 .002175 654.9 4,000 .002111 652.6 5,000 .002048 650.3

  23. Lift Equation Example Angle of Attack = 5 degrees Coefficient of lift = .4 Airspeed = 100 knots = 607600 ft/hr = 168.7 ft per sec Air density = .002175 (3000 feet) Wing surface area of 172 = 174 sq.ft. L = .5(.4 x 168.72 x .002175 x 174) Lift = 2154 lbs

  24. A little change… Increase the speed to say… 150 knots and let’s see what happens 150 knots = 253.167 ft/sec L = .5(.4 x 253.1672 x .002175 x 174) L = 4853 lbs THAT’S 2G’S AT FULL WEIGHT

  25. How great an equation!!! With just a little rearranging we can learn other wonderful things about lift Rearrange for velocity and we can see what angle of attack will do to it (assuming you remain level) Rearrange for Coefficient of Lift to see what Angle of Attack you need for a given Air speed

  26. Velocity vs Angle of Attack Keep L = 2154 lbs. for level flight V = SqRt(2L/ (CL Ρ A)) Make the Angle of Attack 10 degrees Coefficient of lift = .8 V = SqRt(4308/(.8 x .002175 x 174)) V = 119.2 ft/sec V = 70 knots

  27. Angle of Attack vs Velocity Once again keep lift at 2154 lbs. CL = 2L/(V² Ρ A) Slow airspeed from 100 to 80 knots 80 knots = 135 ft/sec CL= 4308/(1352 x .002175 x 174) CL = .62 Angle = 8 degrees

  28. So we can see a pattern ASSUMING LEVEL FLIGHT IS MAINTAINED Velocity Angle of Attack 100 5 degrees 90 6 degrees 80 8 degrees 70 10 degrees 60 13 degrees 50 16 degrees

  29. Velocity and Angle of Attack As Angle of Attack increases we decrease speed As speed decreases we need to increase angle of attack to maintain lift As speed increases we must decrease angle of attack.

  30. Critical Angle of Attack Airplane stalls at Critical Angle of Attack An airplane can stall at any airspeed Why then do we have Vso and Vs?

  31. Stalls The Airplane will stall at the critical angle of attack regardless of Speed Pitch Angle of Bank Below Vso and Vs you the required angle of attack to produce lift is beyond the critical angle of attack

  32. On the topic of stalls… Types of stalls Landing Takeoff Trim Cross-Control Secondary Accelerated

  33. Drag Drag is the rearward acting force opposing thrust Two types Parasite Induced

  34. Parasite Drag Three Types of Parasite Drag Form Drag Skin Friction Interference Increases when speedincreases (exponential)

  35. Induced Drag Inherent whenever a wing produces lift Increases when speed decreases WHY??????

  36. Induced Drag As speed decreases we need to increase angle of attack Induced drag increases as AOA increases

  37. Induced Drag Induced Drag is also caused by wingtip vortices High pressure below the wing is pulled toward the low pressure above the wing.

  38. Drag Formula D =1/2 (Cd V² Ρ A) Cd = Coefficient of Drag V = Velocity (ft per sec) P = Air Density A = Wing Area (Sq.Ft.)

  39. So let’s test it Speed = 100 knots = 168 ft/sec Air Density = .002175 Wing Area = 174 sq.ft. Angle of Attack = 2 degrees Coefficient of Drag= .03 D = .5(.03 x 1682 x 174 x .002175) D = 160 lbs of Drag

  40. Parasite vs. Induced Drag

  41. Region of Reverse Command After a certain speed drag increases and so the required thrust increases. Region of reverse command refers to the need for MORE power to fly SLOWER speeds This is reversed from normal (hopefully that is obvious to you)

  42. Ground Effect As one enters ground effect, much of the downwash, upwash, and wingtip vortices are reduced effectively increasing your Coefficient of Lift

  43. Thrust Forward force pulling/pushing plane through the air.

  44. Thrust Thrust is most easily described as lift in the horizontal direction The propeller aerodynamically functions similar to a wing By spinning it creates its own relative wind. Why does the propeller have a twist in it?

  45. Thrust Just like with lift, thrust on the propeller increases with angle of attack and with speed. The outside of the propeller spins faster thus requiring a smaller pitch

  46. Thrust

  47. Propeller Efficiency No Propeller is 100% Efficient Effecitve Pitch Geometric Pitch Slippage

  48. Turning Tendencies Asymmetrical Thrust (P-Factor) Gyroscopic Precession Spiraling Slipstream Torque from the engine

  49. Asymmetrical Thrust When pitching up, the angle on the DOWNWARD moving blade is greater than that on the upward moving blade. Causes a left yaw.

  50. Gyroscopic Precession A gyroscope is basically something that spins A force applied to a spinning object is felt 90 degrees in the direction of rotation. Pitch up – right yaw Pitch down – left yaw

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