Aerodynamic Force

Question 1

3 characteristics of Airflow when aerodynamic forces produced

Answer 1

Airflow turned downwards.
Static pressure above thin body is low. High static pressure under the body.
Greater airflow acceleration above the thin body(wing)

Question 2

Aerodynamic Forces

Answer 2

Forces acting on aerofoils in motion relative to the air.

Question 3

What are Velocity Fields and Pressure Fields

Answer 3

Thin body affects the speed and direction of the airflow. Affected volume is velocity field.
Presence of thin body cause a change in pressure above and below the body. Total volume affected is pressure field.

Question 4

Unbalanced force

Answer 4

Produced by the difference in static pressure above and below the thin body.

Question 5

How are unbalanced forces generated

Answer 5

Object has to be curved, has a curved leading edge, sharp trailing edge and set at relatively low angle to airflow. Differential pressure created.

Question 6

Relationship between static pressures and velocity

Answer 6

As velocity increases static pressure will decrease. If the airflow is faster then static pressure will be lower.

Question 7

How is low pressure created above the wing.

Answer 7

Initially forms because of the bending of the airflow. Bending is acceleration and according to Bernoulli’s principle energy in a flow must remain constant.
If the flow accelerates then its kinetic energy (dynamic) increases and its pressure energy (static) decreases.

Question 8

Total reaction

Answer 8

Difference in surface pressure around aerofoil create unbalanced forces at every point. The sum of these unbalanced forces over the whole area of aerofoil results in a net unbalanced force known as the total reaction.
Incline slightly rear wards.

Question 9

Aerofoil

Answer 9

Shaped structure designed to produce a significant amount of force when a stream of air moves across it very efficient at creating relatively large for to lift while minimising drag.

Question 10

What is the chord line

Answer 10

Imaginary S.line between the leading edge and trailing edge. Used as reference point for angles.

Question 11

What is the chord

Answer 11

The chord is the distance from the leading to trailing edge.

Question 12

What is angle of attack

Answer 12

AOA is the angle between the chord line and the direction of the flow. Abbreviated as Alpha (‘a’). Critical to production of low pressure.

Question 13

Relative airflow

Answer 13

Direction of the air flow.

Question 14

Mean camber line

Answer 14

Imaginary line Half-way between upper and lower surfaces of the aerofoil.

Question 15

Camber

Answer 15

Difference between mean camber line and chord line. Highly cambered aerofoil has a greater maximum distance.

Question 16

Symmetric Aerofoil

Answer 16

Identical curved upper and lower surface. Has no camber as chord line and camber line coincide.

Question 17

Positive/Negative camber line

Answer 17

+camber line is when camber line is above chord line.

Question 18

Why does air flow faster over the top of the wing.

Answer 18

Streamlines above top of wing are closer together due to curvature of upper surface and reduction in cross-sectional area. Must flow faster to preserve mass flow rate. Closer streamlines means greater velocity. Velocity is inverse to pressure. So low pressure area above wing.

Question 19

Upwash/downwash

Answer 19

Distinctive upward flow of air before leading edge is upwash.
Downward flow of air leaving trailing edge is downwash.

Question 20

Stagnation Point

Answer 20

Point at which proportion of airflow stops. Dynamic pressure is zero and static pressure increases greatly.

Question 21

What is drag

Answer 21

Resistance to flow

Question 22

Skin Friction drag

Answer 22

Friction created between the skin of the aerofoil and the air.

Question 23

Form drag

Answer 23

Form drag is produced by a differential between pressure distribution over a surface of a body.
E.g rear cylinder because of viscosity of air streamline flow breaks down into wake of turbulent flow resulting in a net unbalanced forces. PG124-129

Question 24

Boundary layer

Answer 24

Volume of air around the surface which flows slower than the free steam velocity. Caused by airs viscosity. Layer of air closest to surface impacted (slowed) by imperfections which then impacts the layer above and so on up to a certain point.

Question 25

What is wake

Answer 25

Area behind an aerofoil often turbulent air.

Question 26

Thickness chord ratio

Answer 26

Ratio of aerofoils maximum thickness to the length of its chord.

Question 27

Lift is always…to airflow
Drag is always…to airflow
Centre of Pressure signifies

Answer 27

Parallel
Perpendicular
CP shows start point of total reaction.

Question 28

Maximum Thickness occurs

Answer 28

1/3 of the chord line.

Question 29

Centre of Pressure

Answer 29

Average point from which aerodynamic forces act.

Question 30

Region of lowest static pressure

Answer 30

Upper part of aerofoil within 1/3 of the wing chord. Highest velocity occurs.

Question 31

What is a cambered aerofoil

Answer 31

Different curves on upper and lower surfaces. Asymmetric.