Chapter 4: Lift, Drag, and the Angle of Attack

4.1 Influence of Dynamic Pressure

•  If the static pressure on one side of a body is reduced more than the other side, a pressure
• differential will exist.
•  Increased dynamic pressure (IAS) increases this pressure differential between the upper and
• lower surfaces of the wing, which increases the upward acting force (lift).

4.2 Influence of Angle of Attack (AOA)

•  At a constant dynamic pressure (IAS), increasing the AOA (up to the critical angle, approx.
• 16°) increases the pressure differential and thus increases lift.
•  The greatest positive (high) pressure occurs at the stagnation point, where the relative flow
• velocity is zero. As AOA increases, this point moves from the upper surface down around the
• leading edge to the lower surface. 80808080
•  The greatest contribution to overall lift comes from the negative (low) pressure on the upper
• surface.

4.3 Centre of Pressure (CP) Movement

•  As the AOA increases from 0° towards the stall, the point of lowest pressure on the upper
• surface moves forward.
•  Consequently, the point where the total lift is effectively concentrated, the CP, also moves
• forward.
•  The CP is at its most forward location just before the stall.

4.4 The Lift Formula and its Variables

• The total lift generated by a wing depends on several combined variables. The formula for lift
• is:
• L = Cʟ × ½ρv² × S
• Where:
•  L = Lift force
•  Cʟ = Coefficient of Lift (determined by AOA and aerofoil shape)
•  ½ρv² = Dynamic Pressure (where ρ is air density and v is True Air Speed)
•  S = Wing surface area
• To maintain constant lift (e.g., in level flight where Lift = Weight):

•  As altitude increases, air density (ρ) decreases. To keep lift constant, True Air Speed (v) must
• be increased. For example, at 40,000 ft, air density is about one-quarter of its sea level value,
• so TAS must be doubled to maintain the same lift.
•  As speed (v) is changed, the Angle of Attack (Cʟ) must be adjusted. If IAS is doubled,
• dynamic pressure increases by a factor of four. The AOA must be decreased so that Cʟ
• reduces to one-quarter of its initial value to keep lift constant.

4.5 The Lift Curve

•  A lift curve is a graph that plots the Coefficient of Lift (Cʟ) against the Angle of Attack (α).
•  Cʟ increases linearly with AOA up to a maximum value, known as
• Cʟₘₐₓ.
•  The AOA at which Cʟₘₐₓ occurs is the
• critical or stalling angle of attack (around 16° for a typical aerofoil).
•  Beyond the critical angle, the airflow separates from the wing, Cʟ decreases rapidly, and the
• wing stalls.

4.6 The Lift/Drag Ratio (L/D Ratio)

•  The L/D ratio is a measure of the aerodynamic efficiency of the wing. A higher ratio means
• more efficient lift production.
•  The L/D ratio increases with AOA up to a maximum at the
• ptimum angle of attack (about 4 degrees).
•  Beyond this optimum AOA, the L/D ratio decreases.
•  Flying at the AOA for L/D max provides the greatest gliding distance and maximum
• endurance. This AOA does not change with aircraft weight, but the IAS required to achieve it
• will. 97979797