Drag

Question 1

What is drag

Answer 1

Resistance to Movement/ opposition to motion. Requires energy to overcome (engine)

Question 2

What are the two types of drag

Answer 2

Surface Friction Drag.
Form Drag.
Known together as profile drag.

Question 3

What is surface friction drag

Answer 3

At molecular level surface of wing is rough. Air particles at surface get trapped by wing roughness slowing down and in turn affect air above and so on up to a certain point.

Question 4

What is Form drag

Answer 4

Form drag is caused by asymmetry in the areas of high (static) pressure in front of and behind of wing. Greater pressure at front of wing so net force is rearward.

Question 5

What is the boundary layer

Answer 5

The area extending from aircrafts surface out to the point where the speed of the airflow is 99% of the free stream flow.
All effects of skin friction drag take place in boundary layer

Question 6

Equation for Drag

Answer 6

0.5x rho xV*2 xS xCd

Where S is surface area and Cd is coefficient drag.

Question 7

What is drag coefficient

Answer 7

Dimensionless number that gives a measure of how much drag a wing produces.

Question 8

What are the two types of boundary layer

Answer 8

Laminar

Turbulent

Question 9

Laminar boundary layer

Answer 9

Thin,slippery,low energy
A few mm deep. Flow approximately parallel to surface. Gradual increase in speed from surface upwards. Known as shallow velocity gradient.
Comparatively less skin friction drag.

Question 10

Turbulent boundary layer

Answer 10

Thick,draggy, high energy.
Deeper boundary layer. Chaotic tumbling flow moving in several directions with different energy/speed levels. Mixing between and within layers adds energy to lower layers. Higher flow speeds at surface. Steeper velocity gradient.
Greater kinetic energy helps resist separation. Layer is more thicker because of mixing. B.layer causes more skin friction drag and layer is more deeper resulting in more air being affected.

Question 11

Separation

Answer 11

Flow detaches from surface of aerofoil due to less kinetic energy in lower part of boundary. More prone with laminar boundary but also occurs with turbulent.

Question 12

Transition Point

Answer 12

Point at which boundary layer changes from laminar to turbulent flow. Boundary layer thickens rapidly.

Question 13

Factors determining transition point

Answer 13

Speed of flow and Viscosity. Laminar flow occurs when viscous forces dominant.
Turbulent flow occurs when high inertial forces dominant
Surface quality.
Magnitude/strength of adverse pressure gradient.

Question 14

What is Reynolds number

Answer 14

Unit-less number able to predict when flow will go from laminar to turbulent. Higher number means more turbulent

Question 15

Surface Roughness and effect on Transition point

Answer 15

Rougher surface creates more friction reducing energy of boundary layer more quickly. So earlier change from laminar to turbulent

Question 16

Adverse pressure gradient effect on transition point

Answer 16

Pressure gradient opposing relative flow. Effect is to subtract from flow speed. Less energetic laminar flow easily disrupted by adverse PG.

Question 17

What factors affect where the transition point is

Answer 17

Thickness of aerofoil. So if maximum thickness is further back then the laminar flow will last longer. Also speed of flow increases more gently because of greater distance between leading edge and point of maximum thickness. Both factors delay turbulent flow.
HOWEVER TRANSITION POINT CAN NEVER BE BEHIND POINT OF MAXIMUM THICKNESS.

Question 18

Separation Point

Answer 18

Point at which boundary layer separates from upper surface. Kinetic energy of boundary layer can no longer overcome adverse pressure gradient.
Turbulent layer resists better than laminar due to more kinetic energy in lower layers.
Moves forward with AOA increase as adverse PG
gets stronger.