The Four Forces Acting On An Aircraft

Question 1

Where does the weight force on the aircraft act through ?

Answer 1

Centre of gravity

Question 2

Where does thrust vector act along?

Answer 2

Force parallel to the longitudinal axis of the aircraft.

Question 3

Where does lift force act through?

Answer 3

Centre of pressure, perpendicular to the relative airflow.

Question 4

What is the total reaction?

Answer 4

Sum of the lift and drag forces that act through the centre of pressure on the wing.

Question 5

What direction does drag act through?

Answer 5

• Parallel to the relative airflow.

- Acts through the centre of pressure

Question 6

What is the lift equation?

Answer 6

L = 0.5 x rho x V^2 x Cl x S

Question 7

Within the lift formula , what can the pilot change?

Answer 7

• Altering the Cl by changing the AOA or configuration
• Altering velocity
• Altering wing area by using Fowler flaps

Question 8

What is the boundary layer?

Answer 8

Layer of air closest to the aero foil that is affected by friction from the wing.

Question 9

What is laminar flow?

Answer 9

Smooth flow over the wing

Question 10

What is the adverse pressure gradient? Where does it start on the wing? What is this point called?

Answer 10

• As flow starts to slow down it will encounter an adverse pressure gradient
• This starts after the transition point
• This occurs at the point of maximum thickness/chamber of the aerofoil.

Question 11

What is the separation point?

Answer 11

Point where flow will not be able to overcome the adverse pressure gradient and will separate from the surface of the aerofoil

Question 12

How do Cp, transition point and separation point move with increasing angle of attack?

Answer 12

Move closer to the leading edge of the wing.

Question 13

Where are the two stagnation points on the aerofoil? Are these greater than static freestream velocity?

Answer 13

• Leading edge and trailing edge

- yes

Question 14

Which angle of attack is the point of maximum lift achieved on the aerofoil?

Answer 14

Just before stall angle.

Question 15

How does lift dependent drag vary with the speed?

Answer 15

Decreases with the square of the speed.

Question 16

Does lowering the flaps increase or decrease the critical angle of attack?

Answer 16

Decrease.

Question 17

What occurs to the critical angle of attack with slats it leading edge devices?

Answer 17

Increases the critical angle of attack.

Question 18

What occurs to critical angle of attack when wing trailing edge and leading edge flaps/slats are deployed?

Answer 18

Increases critical angle of attack

Question 19

What is the drag equation?

Answer 19

Drag = Cd x 0.5 x rho x V^2 x A

Question 20

What are the two main drag effects?

Answer 20

• Lift dependent drag

- Zero lift drag

Question 21

What are the three components to zero lift drag?

Answer 21

1) Skin friction
2) Form drag
3) Interference drag

Question 22

What causes wingtip vortices?

Answer 22

Spanwise flow on top and bottom of wing that collided at trailing edge and forms vortices.

Question 23

What is the cause of lift dependent drag?

Answer 23

Wingtip vortices.

Question 24

What part of the wig are wingtip vortices the strongest ?

Answer 24

Wingtip

Question 25

What does the downwash do to the angle of attack?

Answer 25

Reduces it

Question 26

What does the wingtip vortices, downwash do to total reaction on the wing?

Answer 26

Tilts it rearwards

Question 27

What are some methods to reduced wingtip vortices?

Answer 27

1) Wing fence
2) Modified wingtip
3) Winglet
4) Wingtip tank

Question 28

What is ground effect?

Answer 28

Reduction in drag when close to the ground (half a wingspan)

Question 29

What is the ratio between lift to drag called? At what angle is this greatest normally?

Answer 29

4 deg

Question 30

When does the aerofoil have the greatest lifting capability?

Answer 30

At the critical angle of attack - around 16 degrees for an average wing

Question 31

What angle of attack does min drag occur normally?

Answer 31

0 deg

Question 32

What is the angle of incidence?

Answer 32

The angle at which the wing root chord line is set relative to the fuselage reference line.

Question 33

What modern design features can reduce zero-lift drag?

Answer 33

• Streamlined canopy = less form drag
• retractable undercarriage = less firm drag
• Polishes surfaces = less friction drag
• Less interference drag = smoothed fairings

Question 34

When for minimum total drag speed occur?

Answer 34

Max L/D at bottom of drag curve

Question 35

What speed does min power speed occur?

Answer 35

Smallest area contained by two lines to the total drag curve.

Question 36

What is best range speed?

Answer 36

Line drawn tangent to drag curve.

Question 37

What will increased weight and density altitude do to the drag curve?

Answer 37

Move it up and to the right

Question 38

What is wake turbulence ?

Answer 38

Turbulence caused by the wingtip vortices of another aircraft.