Part-66 Module 8: ISA, lift and drag, turns and stability

Module 8 is a short paper with three choices per question and a 75% pass mark. It rewards a clear picture of a few relationships rather than memorising large tables.

The standard atmosphere

ISA at mean sea level is 15 °C, 1013.25 hPa and 1.225 kg/m³. Temperature falls about 6.5 °C per 1,000 m (about 2 °C per 1,000 ft) to the tropopause at 11 km, where it is −56.5 °C and then stays constant to 20 km. Density falls with height, and humid air is less dense than dry air. Hot, high and humid conditions give the poorest performance.

Lift and drag

Bernoulli: where airflow speeds up, static pressure falls. Lift = CL × ½ρV² × S, so doubling speed gives four times the lift at the same angle of attack. The angle of attack is between the chord line and the relative airflow; the angle of incidence is fixed between the chord and the fuselage axis.

• Induced drag comes from lift and the wingtip vortices; it falls as speed rises. High aspect ratio wings and winglets reduce it.
• Parasite drag (form, skin friction, interference) rises with the square of speed.
• Total drag is lowest where the two are equal, which is also the speed of best lift/drag ratio and best glide.
• Beyond the critical angle of attack the flow separates and the wing stalls. Frost, ice and snow raise the stall speed.
• Vortex generators energise the boundary layer; a turbulent boundary layer resists separation better than a laminar one.

Theory of flight

In straight and level flight lift equals weight and thrust equals drag. In a level turn the load factor is 1 / cos(bank): 2 g at 60°, and the stall speed rises by √n (about 41% at 60°). Glide ratio equals L/D. Flaps raise CLmax and lower the stall speed; Fowler flaps add area as well as camber; slats delay the stall to a higher angle of attack. Transport aeroplanes are usually designed to a limit load factor of +2.5 g, with ultimate load 1.5 times limit load.

Stability

• Longitudinal stability (pitch, about the lateral axis) comes from the tailplane with the CG ahead of the neutral point. An aft CG reduces stability and lightens control forces.
• Lateral stability (roll, about the longitudinal axis) comes from dihedral, sweepback and a high wing.
• Directional stability (yaw, about the normal axis) comes from the fin.
• Dutch roll is a yaw-roll oscillation damped by a yaw damper; spiral instability is a slowly tightening turn; the phugoid is a slow exchange of height and speed.