Basic Aerodynamics: Atmosphere, Airflow and Aerodynamic Force
The atmosphere as an aerodynamic variable
Every aerodynamic quantity you will compute depends on the air mass you are flying through. Density falls with altitude, temperature and humidity; pressure falls with altitude. Candidates must be able to state that lift and drag both scale with density, that true airspeed for a given indicated airspeed rises as density falls, and that engine power output falls with density while the propeller's ability to absorb power changes too. The standard atmosphere is a reference model, not a forecast: know the sea-level values (1013.25 hPa, 15 °C, density about 1.225 kg/m³) and the lapse rate of about 2 °C per 1000 ft in the lower layers.
Airflow and the aerodynamic force
Air flowing past a body forms a boundary layer that may be laminar or turbulent, attached or separated. Separation is the root cause of most exam questions on stall, buffet and control loss. The resultant aerodynamic force acts at the centre of pressure and is conventionally resolved into lift perpendicular to the relative airflow and drag parallel to it. Relative airflow is the airflow experienced by the aerofoil, not the aircraft's flight path through the air mass; this distinction trips candidates when a question mentions wind or climb angle.
Traps and a rule of thumb
Typical traps: confusing pressure altitude with density altitude; assuming indicated airspeed gives constant lift regardless of load factor; and treating the centre of pressure as fixed. Remember that the centre of pressure moves, whereas the aerodynamic centre of a conventional aerofoil stays near 25% chord.
Rule of thumb: a 1% change in density changes lift and drag by about 1% at constant true airspeed; a 2% change in true airspeed changes them by about 4%.
Worked example: at 10 000 ft density is roughly 74% of sea level, so at the same true airspeed lift is about 26% lower. To hold the same lift, true airspeed must rise by about 1/√0.74, approximately 16%.