Drag Flashcards
List the Types of Parasite Drag
Interference
Profile - made up of Form Drag and Skin Friction Drag
How is induced drag formed?
- Spanwise flow occurs due to air moving from high to lower pressure
- Wingtip vortices & trailing edge vortices formed
- Downwash created due to trailing vortices, pushing the RAF at an angle and reducing the effective AOA.
- Lift is therefore tilted rearwards, the rearward component of which in “induced drag”.
Why does Skin Friction Drag occur?
Skin friction drag is drag due to the viscosity of air. Shear stress in the boundary layer between sheets of airflow occur as airflow near the surface is retarded compared to the faster moving airflow closer to the edge of the boundary layer.
Factors affecting skin friction drag?
AOA - More AOA will result in more skin friction drag, as the transition point moves forward within the boundary layer, resulting in more turbulent flow and more skin friction drag.
Speed - Faster moving airflow will result in more skin friction drag, as the rate of velocity change (velocity profile) within the boundary layer is greater.
Shape - High speed and laminar airfoils are designed to delay transition to turbulent flow, thus minimising skin friction drag.
Surface Condition - contaminated wing will encourage more turbulent flow.
Size & Area - Scale effect (doubling the size of the aerofoil will more than double the drag) and wetted area will increase the likelihood of turbulent flow developing.
What is form drag?
Form drag is drag due to pressure differentials between leading and trailing edge of objects in motion.
What are the factors affecting form drag?
AOA - A higher AOA will increase the frontal surface area thus increasing form drag. Separation will also occur sooner, resulting in a greater pressure differential.
Speed - Form drag increases with square of the IAS.
Streamlining - This is making the curvature of a surface more gradual, delaying separation and reducing the pressure differential.
Size/area - A larger surface area will increase the pressure differential.
Factors affecting induced drag
AOA - Increases induced drag due to increase in lift vector
Speed - induced drag is higher at lower speeds
weight - increasing the weight will increase the requirement of lift, this a higher AOA is used for a given speed resulting in more induced drag.
Aspect ratio - higher aspect ratio reduces induced drag due to smaller vortices.
Design Features, wingtip, wing fences, elliptical wing, washout with taper
What is interference drag?
Interference drag is drag due to the converging of airflows at junctions of different components.
Fairings can be used to reduce interference drag, to smooth the angles of converging airflows.
What is Reynolds number?
Re is a number which will tell if a viscous flow is either laminar or turbulent. Turbulent flow will often have a Re of over 2000.
