Air in Motion/Boundary Layer

Question 1

Define subsonic flow

Answer 1

Air flowing at speeds less than at which sound travels. The air is considered to be incompressible at these speeds

Question 2

Describe the conservation of mass flow equation

Answer 2

As air passes through a Venturi, its velocity will increase to a maximum at the throat of the Venturi, where the cross-sectional area is the smallest.

At the narrowest part, the velocity will be the highest

Question 3

How do you calculate total/stagnation pressure?

Answer 3

Static Pressure + Dynamic Pressure = Total/Stagnation Pressure

Question 4

Briefly describe Bernoulli’s Principle

Answer 4

A rise in velocity will be accompanied by a fall in static pressure

Question 5

What is lift proportional to?

Answer 5

Lift is proportional to the angle of attack and to speed²

Question 6

Define ‘lift’ in regards to the total reaction

Answer 6

Lift is the component of the Total Reaction which acts at right angles to the relative airflow

Question 7

Define ‘drag’ with respect to the total reaction

Answer 7

Drag is the component of Total Reaction which acts parallel to the relative airflow

Question 8

If the angle of attack increases what happens to total reaction, lift and drag?

Answer 8

They will all increase

Question 9

What will happen to the centre of pressure as the angle of attack increases?

Answer 9

The CoP moves forward along the aerofoil towards the leading edge

Question 10

Within the normal range of the AoA, how much does the CoP move?

Answer 10

20% of the chord length aft of the leading edge - 30% aft of the leading edge

Question 11

How is the boundary layer formed?

Answer 11

Layers of air that are slowed down due to air viscosity and surface friction

Question 12

What can change the thickness of the boundary layer?

Answer 12

The roughness of the surface

Question 13

What is the transition point?

Answer 13

The point at which smooth, laminar flow air becomes turbulent

Question 14

What are the typical boundary layer thicknesses for laminar and turbulent flow?

Answer 14

Laminar: 0.1-0.2mm
Turbulent: 5mm+

Question 15

What happens to the transition and separation points as the AoA is increased?

Answer 15

They move forward on the aerofoil towards the leading edge

Question 16

Define a stall

Answer 16

When an increase in AoA does not produce an increase in lift, for an average aerofoil the stalling angle is 15°-16°.

Question 17

What is the boundary layer?

Answer 17

The thin layer of air next to the aerofoil surface has been slowed down by surface friction and viscosity

Question 18

What is the ‘separation point’?

Answer 18

The point where the air separates from the wing surface, no lift is generated only large amounts of drag

Question 19

What happens to the TP as AoA increases?

Answer 19

The TP moves forward, decreasing the size of the laminar area and therefore increasing drag

Question 20

What two factors affect a stall?

Answer 20

The angle of attack and the airspeed