01 Aerodynamics & Flight Mechanics

What is the lift equation, and what does it tell you operationally?

L = ½ρV²SCL. Lift depends on air density, speed squared, wing area and lift coefficient; if one changes, another must compensate to maintain the required lift.

What is angle of attack (AoA), and why is it more fundamental than airspeed for stall awareness?

AoA is the angle between the wing chord and relative airflow. A wing stalls when critical AoA is exceeded; the corresponding airspeed varies with weight, load factor and configuration.

How does CL normally change with AoA?

It increases approximately linearly with AoA until near CLmax; beyond critical AoA, flow separation causes lift to decrease.

Can an aircraft stall at any airspeed or pitch attitude?

Yes. A stall is caused by exceeding critical AoA, not by a particular airspeed or nose attitude.

What is the correct aerodynamic priority in stall recovery?

Reduce AoA below the critical value first; then restore the desired flight path while managing thrust and configuration.

Why is adding thrust alone not a stall recovery?

Because the wing remains stalled until AoA is reduced below critical.

How does stall speed vary with load factor?

VS, new = VS × √n.

How does stall speed vary with weight?

It varies approximately with the square root of the weight ratio.

What happens aerodynamically in a coordinated level turn as bank angle increases?

Required lift and load factor increase, which increases AoA for a given speed and raises stall speed.

What is the load factor in a coordinated level turn?

n = 1 / cos(bank angle); approximately 1.4 g at 45° and 2 g at 60°.

What is an accelerated stall?

A stall occurring at a load factor greater than 1 g, so it occurs at a higher airspeed than the 1 g stall.

How do speed and bank angle affect turn radius and turn rate?

More bank decreases radius and increases turn rate; more speed increases radius and decreases turn rate for the same bank.

Why is a skidding turn near the stall particularly dangerous?

Yaw and asymmetric AoA can cause one wing to stall more deeply, promoting spin entry.

What two aerodynamic conditions are required for a spin?

A stalled wing and sustained yaw.

How do parasite and induced drag vary with speed?

Parasite drag rises roughly with V²; induced drag rises strongly as speed decreases because higher CL is required.

Under the simple drag-polar model, when is total drag minimum?

Approximately when parasite drag equals induced drag.

What does maximum L/D represent operationally?

Maximum aerodynamic efficiency and, in still air, the shallowest glide angle / greatest still-air glide distance per unit altitude.

Does aircraft weight change maximum L/D?

Approximately no for a given configuration, but a heavier aircraft reaches the same L/D at a higher speed and higher sink rate.

How does aspect ratio affect induced drag?

Higher aspect ratio reduces induced drag for a given lift.

Why do wingtip vortices form?

High pressure below the wing flows around the tip toward lower pressure above, producing trailing vortices and induced drag.

When are wake vortices generally strongest?

When the generating aircraft is heavy, slow and clean.

What is the main aerodynamic effect of ground effect?

It reduces downwash and induced drag, improving lift-to-drag performance close to the surface.

What are the main aerodynamic effects of trailing-edge flaps?

They increase camber and CLmax, reducing stall speed, but also increase drag.

What are the main aerodynamic effects of leading-edge slats?

They delay upper-surface flow separation, increasing CLmax and usually increasing critical AoA.

What is the aerodynamic purpose of spoilers?

To reduce lift and increase drag; on landing they also transfer aircraft weight onto the wheels and improve braking effectiveness.

What is the difference between static and dynamic stability?

Static stability describes the initial response to a disturbance; dynamic stability describes how the resulting motion develops with time.

How does CG position affect longitudinal stability and efficiency?

A forward CG generally increases stability but requires more tail force and trim drag; an aft CG reduces stability and trim drag but reduces recovery/control margins.

Why can an excessively aft CG make stall recovery more difficult?

Natural nose-down stability and available nose-down control margin are reduced.

What is the dihedral effect?

A sideslip generates a restoring rolling moment; wing dihedral and sweep both contribute to it.

Why are swept-wing aircraft prone to Dutch roll?

Strong coupling between directional and lateral stability produces a combined yaw-and-roll oscillation.

What is the purpose of a yaw damper?

To automatically suppress unwanted yaw oscillations, particularly Dutch roll.

Why are transport-aircraft wings swept?

Primarily to delay compressibility effects and increase the Mach number at which significant high-speed effects occur.

How does wing sweep delay compressibility effects?

It reduces the airflow component normal to the leading edge, reducing the effective Mach seen by the aerofoil section.

What is critical Mach number?

The lowest aircraft Mach number at which local airflow somewhere on the aircraft first reaches Mach 1.

Can local airflow be supersonic while the aircraft is still subsonic?

Yes. Local airflow accelerates over the wing and can exceed Mach 1 before aircraft Mach reaches 1.

What happens across a normal shock wave?

Flow decelerates abruptly from supersonic to subsonic while static pressure and temperature rise and total pressure decreases.

What is wave drag?

The additional drag associated with compressibility and shock-wave formation.

What is Mach tuck?

A nose-down pitching tendency at high Mach caused by aft changes in aerodynamic pressure distribution and shock-related effects.

What is high-speed buffet?

Buffeting caused by shock-induced flow separation as compressibility effects increase.

What is coffin corner?

A high-altitude condition where the margins between low-speed buffet/stall and high-speed buffet/MMO become small.

Why does manoeuvring reduce buffet margin at high altitude?

Increased load factor requires higher CL and AoA, moving the aircraft closer to the low-speed buffet boundary.

Why can a heavier aircraft have a smaller high-altitude buffet margin?

More lift is required at the same flight condition, requiring higher CL and AoA.

What is the basic aerodynamic condition for a steady climb?

Thrust exceeds drag; the excess thrust balances the component of weight along the climb path.

What determines climb gradient versus rate of climb?

Climb gradient is primarily related to excess thrust; rate of climb is primarily related to excess power.

In a steady climb, is lift exactly equal to weight?

Usually not; lift approximately balances the component of weight perpendicular to the flight path, so it is slightly less than weight for a conventional climb.

What determines the best still-air glide angle?

Maximum L/D.

How does higher weight affect best-glide performance?

The best-glide angle is essentially unchanged, but the corresponding speed and sink rate are higher.

What is the region of reversed command?

The low-speed region where flying slower requires more thrust/power because induced drag rises rapidly.

What is the key limitation of manoeuvring speed VA?

It is not a universal “safe speed”; its protection applies only under defined certification assumptions and does not protect against every control combination, gust or structural load.

How does VA change with aircraft weight?

It decreases as aircraft weight decreases.

What is the difference between pitch attitude and AoA?

Pitch attitude is measured relative to the horizon; AoA is measured relative to the local airflow.

Why can AoA change significantly without a large pitch-attitude change?

Because the flight-path angle and relative airflow can change rapidly.

What is the key aerodynamic principle in an upset with excessive AoA?

Reduce AoA first; attempting to recover the flight path by pulling can deepen or maintain the stall.