TOPIC 2 - AERODYNAMIC FORCES & MOMENTS

Lift

Thrust

Drag

Weight

Aerodynamic Forces

Aerodynamic forces

the forces exerted by air as it flows over a surface.

Aerodynamic moments/ Pitching moment

refers to the torque produced by the aerodynamic force on an airfoil.

Lift

upward force perpendicular to weight

Thrust

forward force due to propulsion

Drag

backward force (wind resistance)

Weight

downward force due to gravity

Lift = Weight

Thrust = Drag

FOR LEVELLED UNACCELERATED FLIGHT

Pitching moment

a rotational aerodynamic effect that causes an aircraft to tilt nose-up or nose-down around its lateral axis (the axis running from wingtip to wingtip).

Pitching moment

It's not a force like lift or drag, but a moment, which means it causes rotation, not straight-line motion.

wind engineering

focuses on how natural or simulated wind interacts with aircraft surfaces.

Lift

determines if an aircraft can maintain or gain altitude under varying wind conditions.

Drag

impacts speed, fuel efficiency, and how the aircraft handles against headwinds or crosswinds.

Pitching moment

affects the aircraft's nose-up or nose-down attitude, crucial for stable flight and safe landings.

Lift

Drag

Pitching Moment

Aerodynamic forces in wind engineering

Lift

aerodynamic force that acts perpendicular to the direction of the airflow around an object or wing which supports the weight of an aircraft and allows it to rise and stay in the air

Lift

mainly produced by the way pressure is distributed across an airfoil or a wing in general. This occurs because there's a difference in pressure between the upper and lower surfaces.

Bernoulli's equation

states that as velocity increases, pressure decreases.

True

[True or False] As air flows over the curved upper surface of an airfoil, it must speed up to a reduction in flow area.

True

[True or False] High velocity implies low pressure, low velocity implies high pressure, and a pressure differential between the upper and lower surfaces leads to LIFT

Boundary Layer

A thin layer of air that clings to an airfoil's surface due to air's viscosity.

Within this region, airflow speed increases from zero at the surface to the freestream velocity just beyond it.

True

[True or False] excessive boundary layer separation can significantly reduce lift and increase drag, affecting aerodynamic performance

Laminar Boundary Layer

Occurs at low Reynolds numbers. Flow is smooth and layered, with gradual velocity change. It produces less surface friction but is more prone to separation due to low kinetic energy.

Transition Point

The position where flow becomes turbulent. At low angles of attack, it lies near the trailing edge and moves forward as the angle increases.

Turbulent Boundary Layer

At higher Reynolds numbers, the flow becomes turbulent past the transition point. It has greater kinetic energy and friction but resists separation better than laminar flow.

Symmetrical Airfoil

At zero angle of attack—with equal curvature on both

surfaces—produces no lift, since the negative pressures above and below are equal and opposite.

The forces balance at the aerodynamic center (AC), resulting in zero net force, no pitching moment, and only minor parasite drag.

True

[True or False] As the angle of attack increases, the upper surface generates stronger negative pressure while the lower surface sees less. This pressure difference creates upward lift, which grows with angle of attack up to the stall point.

Cambered Airfoil

has a more curved upper surface, causing faster airflow and stronger negative pressure above than below.

This pressure difference—combined with positive pressure at the lower front edge—produces a reactive force.

Negative Angles of Attack

Small Positive Angles of Attack

Large Positive Angles of Attack (Beyond

Stall)

three different groups of angles of attack

Negative Angles of Attack

Where pressure above and below equalizes. Though there's no lift, a nose-down pitching moment occurs due to the offset of pressure vectors, with the aerodynamic

center (AC) midway between. (a cambered aerofoil at zero angle)

Small Positive Angles of Attack

The negative pressure on the upper surface becomes greater than that on the lower surface. This pressure imbalance generates a total reactive force directed

upward, perpendicular to the chord line.

The large negative pressure, or "suction," on the

upper surface is the primary driver of lift.

Large Positive Angles of Attack (Beyond

Stall)

When the angle of attack exceeds the stalling

angle—typically around 15°—the smooth, laminar airflow over the upper surface begins to separate due to excessive curvature and flow instability.

Drag

resistance to forward motion and is assumed to act along a line parallel to the longitudinal axis.

Total drag

sum of all aerodynamic drag forces acting on an aircraft as it move through the air.

Parasite Drag

The element of the total drag not directly attributed to the procurement of lift. Sometimes called profile or zero lift drag

Surface-Friction Drag

Form Drag

Interference Drag

Induced Drag

Wave Drag

Types of Parasite Drag

Surface-Friction Drag

caused by the viscosity of air as it flows over the surface of an aircraft

Form Drag

caused by the shaped of the airplane and its airfoils

Interference Drag

occurs when airflows from different component of an aircraft interact with each other

Induced Drag

The drag caused by the generation of lift. Caused by the pressure difference above and below the wing. Forms wingtip vortices and downwash

Wave Drag

Caused by a change in static pressure and loss of total pressure, produced by shock waves that occurs as aircrafts approach the speed of sound.

The Effect of Altitude on Total Drag

The density of the atmosphere and air pressure decrease with increased altitude, resulting in a reduction in total drag.

The Effect of Mass on Total Drag

A heavier aircraft needs more speed to stay airborne

The Effect of Flap on Total Drag

Extending the flaps increases both lift and drag, with drag increasing more significantly. As a result, maximum drag occurs at a lower airspeed.

Pitching Moment

The turning effect that causes an aircraft's nose to tilt up or down.

Occurs due to a lift acting at a distance from a reference point on the wing.

It influences the aircraft's stability and control around the lateral (pitch) axis.

Lift

Angle of Attack

Center of Pressure

Moment Arm

What affects pitching moment?

True

[True or False] longer arm = stronger moment

Leading Edge

produces strong nose-down moments, generally stable. (pitching moment a this reference point)

Trailing Edge

can cause nose-up moments, often unstable. (pitching moment a this reference point)

Aerodynamic Center

is a special point where the pitching moment is nearly constant, regardless of angle of attack. (pitching moment a this reference point)

True

[True or False] Increasing AoA increases lift and pitching moment—up to the stall angle.

Trailing-edge flaps

Increase effective AoA. Cause CP to shift forward, increasing nose-down movement