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Drag

Drag. Lecture 6 Chapter 3. What is Drag?. What are different types of Drag?. Drag. Drag is the term used to denote resistance to airflow. Example: Hold your arm out of the window of a moving car. What happens? You are experiencing drag. Pressure Drag.

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Drag

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  1. Drag Lecture 6 Chapter 3

  2. What is Drag? What are different types of Drag?

  3. Drag • Drag is the term used to denote resistance to airflow. • Example: Hold your arm out of the window of a moving car. What happens? You are experiencing drag.

  4. Pressure Drag • Most of the drag experienced by holding your hand perpendicular to the ground outside a vehicle window to the airstream is pressure drag. • Pressure drag results from the difference in pressure between the fore and aft sides of the hand.

  5. Skin Friction Drag • A frictional force over a surface is called skin friction drag or viscous drag. • A flat plate parallel to the airflow experiences skin friction drag. • Figure 3-2 page 57.

  6. Parasite Drag • An object will have friction along the surface(skin friction drag) and some pressure drag because a wake is being formed behind it. • Pressure drag and skin friction drag are not entirely independent because the size of the wake is dependent on the point of separation of flow.

  7. Parasite Drag • The sum of pressure drag and skin friction drag. • Figure 3-2 page 57 shows an airfoil with both pressure drag and skin friction drag.

  8. Boundary Layer • Boundary layer is the free airstream above the surface and the point where the velocity builds up above the surface. • Figure 3-3 page 58 • Within the boundary layer the velocity takes on a profile an gradually reduces from the free stream to zero at the surface. • The reaction to the retardation of the flow within the boundary layer is the skin friction drag.

  9. Back to Skin Friction Drag • The boundary layer is the mechanism by which skin friction drag is created. • The extent of skin friction drag depends on the shape and thickness of the boundary layer.

  10. Laminar & Turbulent Boundary Layers • Laminar Flow is a smooth, layered fashion, in which the streamlines all remain in the same relative position with respect to each other. • Turbulent Flow is one in which the streamlines break up & become all intermingled, moving in random, irregular patterns.

  11. Transition from Laminar to Turbulent • Transition region is the area where a boundary layer changes from laminar to turbulent. • Examples: Cigarette Smoke • Figure 3-4, 3-5 page 59

  12. Reynolds Numbers • Scientist Osborne Reynolds discovered many of the principles of fluid viscosity and boundary layers. • Remember viscosity of a fluid is the stickiness of a fluid.(all fluids have some amount of viscosity) • A reynolds number is used to measure to viscous qualities of fluid.

  13. Reynolds Number • Re= V x d/ v • Re = Reynolds number • V= Fluid velocity • d= Distance downstream from leading edge • v= Kinematic viscosity of a fluid • A low reynolds number is laminar • A high reynolds number is turbulent

  14. Reynolds Numbers • The higher velocities or longer distances downstream tend to produce higher Reynolds numbers (greater potential for turbulent flow) • Reynolds numbers are different for different operating airspeeds.(length is average wing chord)

  15. Wakes & Pressure Drag • The imbalance of pressure between the forward face of the plate and that on the aft face(in the wake) results in drag force. • Larger wake produces more drag • Figure 3-7 page 62.

  16. Adverse Pressure Gradient • A flow moves along a surface it is creating friction from the boundary layer. • As the boundary layer becomes thicker more friction drag is created. • As the cross-sectional area of the body is getting smaller in the downstream direction, the velocity is less, pressure increases causing an adverse pressure gradient.(the pressure is working against the flow rather than with it)

  17. Skin Friction & Adverse Pressure Gradient • The combination of skin friction/adverse pressure gradient gang up on air flow an prevent it from traveling farther along the surface. • It is ideal to delay the separation. • The longer it remains attached, the smaller the resulting wake and pressure drag.

  18. Bluff Bodies • Bluff bodies are bodies with diameters that are fairly large in relation to their length, pressure drag is the greatest offender. • Fuselages,nacelles,landing gear wheels are bluff bodies. • Figure 3-9 page 64.

  19. Sphere/Cylinder • At a low Reynolds number,(low velocity/small diameter)the flow will remain entirely laminar and separate early forming a large wake. • At a high Reynolds number the flow will transition to a turbulent boundary layer before reaching the separation point.

  20. Laminar/Turbulent Boundary Layers • Turbulent boundary layers have more than laminar boundary layers, the flow is turbulent, the separation is delayed and a smaller wake is formed. • Why do golf balls have dimples?

  21. Why do golf balls have dimples? • Roughing the surface will promote early transition and have less drag.

  22. Drag Coefficient • Drag(like lift) is proportional to the dynamic pressure of the air and the area on which it acts. • CD = drag coefficient CD= Drag/q x A • * = density • V = velocity • A = area • q = dynamic pressure

  23. Drag Coefficient • The drag coefficient can also be thought of as the ratio of drag force to dynamic pressure force. • The drag is actually being generated by a three-dimensional body, yet the drag is proportional to only two dimensions of the body.(when using a drag coefficient)

  24. Projected Area viewed from all sides • Projected areas (fuselage, nacelle, landing gear) have different shapes when view from different directions. • Frontal, planform, side • The drag components are also different for each side. • Porsche vs. Van

  25. Induced Drag • Induce drag is the direct result from the production of lift. • Figure 3-13 p. 69 • The lift vector, being perpendicular to the actual airflow is tilted backward, resulting in a component of lift in the streamwise direction. (drag) • Production of induced drag by downwash pushing down the airstream vector resulting in tilted vector.

  26. Reducing Downwash • Longer wingspan places the wingtips farther apart. • Higher aspect ratio. • Reduce lift coefficient( because induced drag is proportional lift coefficient)

  27. Factors that increase Induce Drag • High Weight • Less efficient wing (than elliptical) • High Altitude • Low velocity • Low wingspan

  28. Quiz on Lecture 6Chapter 3 Please take out a sheet of paper Include today’s date and your name

  29. Quiz on lecture 6Chapter 3 • Compare and contrast parasite and induced drag. Include examples of each. • What is a reynolds number? • Compare and contrast the two types boundary layers discussed today.

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