Drag

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 todays 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.