The Basics & Derivation of Lift

Question 1

What is newton’s first law?

Answer 1

a body will continue in rest or motion unless acted on by an external force

Question 2

What is newton’s second law?

Answer 2

the rate of change of momentum of a body is proportional to the applied force, and takes place in the direction in which the force acts

Question 3

what is newtons third law?

Answer 3

to every action there is an equal and opposite reaction

Question 4

In a stationary body it is … that is a measure of its resistance to change

Answer 4

mass

Question 5

In a moving body it is … that resists change of motion

Answer 5

momentum (mass x velocity)

Question 6

work done is…

Answer 6

force times the distance moved in the direction of force

Question 7

power is …

Answer 7

the rate of work done

Question 8

Energy of a moving body (kinetic energy) is proportional to …

Answer 8

mass times velocity squared – 1/2MV(squared)

Question 9

boyles law…

Answer 9

at fixed temp increasing pressure will decrease the volume and thus increase density

Question 10

Charles’s law …

Answer 10

at fixed pressure increasing temp will increase volume and thus reduce density

Question 11

Pressure law …

Answer 11

at fixed volume increasing temp or pressure will increase the other factor

Question 12

Air density is roughly half the sea level value at … (653 g/m3) and a quarter the sea level value at …

Answer 12

20 000ft

40 000ft

Question 13

formula for mass flow through a convergent duct =

Answer 13

area x speed x density

Question 14

equation of continuity =

(flow through convergent duct)

Answer 14

K = A x V x Rho

K is a constant

Question 15

Symbol for dynamic pressure?

Answer 15

Q

Question 16

equation for dynamic pressure?

Answer 16

½ x rho x Vsquared

Question 17

How does dynamic pressure vary with speed?

Answer 17

proportional to the square of the true airspeed

Question 18

density decreses with increasing altitude so to maintain lift at altitude what must increase?

Answer 18

TAS

V in the lift equation must increase because density has reduced but all other factors in the equation remain equal

Question 19

Lift is defined as the component of the total reaction acting … to the relative airflow, and in level flight is the force supporting the weight

Answer 19

perpendicular

Question 20

what is an indication of 1/2 rho Vsquared?

Answer 20

IAS

Question 21

what is the lift formula?

Answer 21

CL x 1/2Rho x Vsquared x S

Question 22

what is CL formula?

Answer 22

lift / 1/2Rho x Vsquared x S

Question 23

Thickness of a wing is often defined as a …, usually expressed as …

Answer 23

thickness/chord ratio

a percentage of the chord

Question 24

compared to symmetrical aerofoils, a cambered aerofoil will have a greater CLmax and critical alpha will …

Answer 24

decrease

Question 25

as alpha increases on a cambered aerofoil, what happens to CP?

Answer 25

it moves forward

CP is well aft at zero lift, but it moves forward to about 15% of chord as alpha starts to increase and, stays here until critical alpha is reached, when it sharply moves back aft

Question 26

what is aerodynamic centre?

Answer 26

the point on the aerofoil where the pitching moment due to lift forces is zero

The lift created toward the back of the aerofoil will create aerodynamic moments tending to pitch the leading edge nose down. The lift created towards the leading edge will create aerodynamic moments tending to pitch the aerofoil nose up

In between, there will be a point where the lift creates no moments in either direction and this is called the aerodynamic centre

Question 27

what is the average position for the aerodynamic centre for most cambered aerofoils?

what happens to the position of the aerodynamic centre with an increase in alpha?

Answer 27

25% chord

Unlike the CP its position does not change with changes in alpha, in subsonic flight

Question 28

what pitching moment on a cambered aerofoil section when CL = 0

Answer 28

A cambered aerofoil at -4 degrees alpha will produce no lift because the pressure patterns around the aerofoil cancel each other out, so CL=0

However, the distribution is uneven with a large area of low pressure underneath the leading edge which causes nose-down aerodynamic pitching moments, even though no lift is being generated overall

Question 29

what is load factor formula?

Answer 29

lift / weight

Question 30

LF or ‘g’ limits for large transport aircraft?

Answer 30

+2.5g, -1g and +2g with flaps extended