Ch 7 - Introduction To Drag Flashcards
2 Types of Profile Drag
Form Drag
Skin Friction Drag
Neither are associated with the production of lift or 3D Flow
Drag Equation
D=qSCd
D=1/2 rho v^2 S Cd
D is proportional to IAS^2 S Cd
Coefficient of Drag
Cd = A measure of drag per unit area and unit dynamic pressure
Boundary Layer and the Velocity Profile
Air close to the surface that has been slowed by friction
Velocity Profile:
The area of airflow that is <1% slower than the free moving air
Two Type of Boundary
Laminar Flow Boundary
Turbulent Flow Boundary
Laminar Boundary
1-2mm Deep
Smooth layers
Up until transition point
Low energy
Causes drag between layers where VISCOUS FORCES
Speed change gradually up through the layers
Prone to separation
Decreased Skin Friction Drag
PREDOMINATE
More prone to stall
Flow parallel to the surface it is passing over
CANT EXIST PAST THE POINT OF MAX THICKNESS
Turbulent Boundary
Layers mix - INERTIAL FORCES PREDOMINATE
After the transition point
High energy
Increased skin friction
Increased Drag
Less prone to separation (less prone to stall)
1-2cm thick
Flow moves in many directions
Air mixes: High speed air closer to the surface mix
Draggy because of the depth and the greater mixing
Reynolds Number
The ratio of inertial to viscous forces
RN is proportional to the speed proportional to the distance from the leading edge
Low RN: - Laminar boundary layer where viscous forces predominate
- Close to the aerofoil leading edge
- Slower speeds
High RN: Turbulent boundary layer where inertial forces predominate
Adverse Pressure Gradient
Because air flows from high P. To Low P.
- Air from the underside of the wing where there is high pressure, wants to get on top of the wing where there is low static pressure
It does this by coming up from under the trailing edge as there is an adverse pressure gradient, and working its way up to the separation point
Separation Point
The point at which the airflow separates from the aerofoil
As AoA increases, SP moves forward
As AoA decreases, SP moves aft
AoA plays a big role in where the SP is
Laminar boundary layer
1-2mm
Parallel flow
Low RN as viscous forces predominate
Decreased amount of relative skin friction drag
Prone to separation
Turbulent Boundary
Relatively deep 1-2cm High RN as Inertial forces predominate Flow is in many directions Increased relative skin friction drag Less prone to separation
