The Basics & Derivation of Lift Flashcards
What is newton’s first law?
a body will continue in rest or motion unless acted on by an external force
What is newton’s second law?
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
what is newtons third law?
to every action there is an equal and opposite reaction
In a stationary body it is … that is a measure of its resistance to change
mass
In a moving body it is … that resists change of motion
momentum (mass x velocity)
work done is…
force times the distance moved in the direction of force
power is …
the rate of work done
Energy of a moving body (kinetic energy) is proportional to …
mass times velocity squared – 1/2MV(squared)
boyles law…
at fixed temp increasing pressure will decrease the volume and thus increase density
Charles’s law …
at fixed pressure increasing temp will increase volume and thus reduce density
Pressure law …
at fixed volume increasing temp or pressure will increase the other factor
Air density is roughly half the sea level value at … (653 g/m3) and a quarter the sea level value at …
20 000ft
40 000ft
formula for mass flow through a convergent duct =
area x speed x density
equation of continuity =
(flow through convergent duct)
K = A x V x Rho
K is a constant
Symbol for dynamic pressure?
Q
equation for dynamic pressure?
½ x rho x Vsquared
How does dynamic pressure vary with speed?
proportional to the square of the true airspeed
density decreses with increasing altitude so to maintain lift at altitude what must increase?
TAS
V in the lift equation must increase because density has reduced but all other factors in the equation remain equal
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
perpendicular
what is an indication of 1/2 rho Vsquared?
IAS
what is the lift formula?
CL x 1/2Rho x Vsquared x S
what is CL formula?
lift / 1/2Rho x Vsquared x S
Thickness of a wing is often defined as a …, usually expressed as …
thickness/chord ratio
a percentage of the chord
compared to symmetrical aerofoils, a cambered aerofoil will have a greater CLmax and critical alpha will …
decrease
as alpha increases on a cambered aerofoil, what happens to CP?
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
what is aerodynamic centre?
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
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?
25% chord
Unlike the CP its position does not change with changes in alpha, in subsonic flight
what pitching moment on a cambered aerofoil section when CL = 0
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
what is load factor formula?
lift / weight
LF or ‘g’ limits for large transport aircraft?
+2.5g, -1g and +2g with flaps extended
