Aerodynamics & Bernoulli's Principle

• Aerodynamics is the study of air in motion and how it interacts with solid objects, such as aircraft.
• It explains how an aircraft is able to fly, and how its shape and design affect its performance.
• A commercial pilot needs a strong understanding of aerodynamics to safely and efficiently operate an aircraft.

Four Forces of Flight

• Lift, weight, thrust, and drag are the four fundamental forces acting on an aircraft in flight.
• Lift opposes weight and is produced by the dynamic effect of the air acting on the wing.
• Weight is the force of gravity acting on the aircraft.
• Thrust opposes drag and is the force produced by the engine.
• Drag opposes motion and is the force that resists the movement of the aircraft through the air.
• In steady, level, unaccelerated flight, lift equals weight and thrust equals drag.

Bernoulli's Principle

• States that as the speed of a fluid (air) increases, the pressure decreases.
• Air flowing over the curved upper surface of an aircraft wing travels faster than air flowing under the flatter lower surface.
• This difference in speed creates a pressure difference, with lower pressure above the wing and higher pressure below.
• The pressure difference generates an upward force called lift.

Angle of Attack

• Angle of attack is the angle between the wing chord line and the relative wind.
• Wing chord line is an imaginary straight line from the leading edge to the trailing edge of the wing.
• Relative wind is the direction of the airflow with respect to the wing.
• As the angle of attack increases, lift increases up to a certain point.

Stall

• Stall occurs when the angle of attack exceeds the critical angle of attack.
• Critical angle of attack is the angle at which the airflow separates from the wing's upper surface, causing a sudden loss of lift.
• Stalling angle of attack is constant regardless of airspeed, weight, or load factor.
• Stall speed varies with weight, load factor, and altitude.
• During a stall, the wing no longer produces enough lift to support the weight of the aircraft.
• Stall can occur at any airspeed or attitude.

Wing Design

• Aspect ratio is the ratio of the wing span to the wing chord.
• A high aspect ratio (long, narrow wings) results in lower induced drag.
• Wing area affects the amount of lift produced.
• Wing planform (shape of the wing) affects stall characteristics and handling qualities.
• Winglets are small, vertical extensions at the wingtips that reduce induced drag by disrupting wingtip vortices.

Lift Augmentation Devices

• High-lift devices increase the lift coefficient of the wing, allowing the aircraft to fly at lower speeds.
• Flaps increase both the wing area and the camber, increasing lift and drag.
• Slats are leading-edge devices that increase the angle of attack at which the wing stalls.
• Slots are fixed openings in the wing that allow high-energy air to flow from below the wing to the upper surface, delaying stall.

Drag

• Parasite drag is the resistance of the air to the movement of the aircraft.
• Form drag is caused by the shape of the aircraft.
• Interference drag is caused by the intersection of airflow streams.
• Skin friction drag is caused by the friction of the air against the surface of the aircraft.
• Induced drag is a byproduct of lift and is created by wingtip vortices.
• Total drag is the sum of parasite drag and induced drag.
• As airspeed increases, parasite drag increases and induced drag decreases.

Ground Effect

• Ground effect is the phenomenon that occurs when an aircraft flies close to the ground.
• Within about one wingspan of the surface, the ground interferes with the airflow around the wing, reducing induced drag.
• Ground effect also increases lift.
• Aircraft feels like it is floating.

Stability

• Stability is the tendency of an aircraft to return to its original attitude after being disturbed.
• Static stability is the initial tendency to return to equilibrium.
• Dynamic stability is the way the aircraft returns to equilibrium over time.
• Longitudinal stability is stability around the lateral axis (pitch).
• Lateral stability is stability around the longitudinal axis (roll).
• Directional stability is stability around the vertical axis (yaw).

Factors affecting Air Density

• Temperature, altitude, and humidity affect air density.
• As temperature increases, air density decreases.
• As altitude increases, air density decreases.
• As humidity increases, air density decreases.
• Density altitude is the altitude the aircraft "feels" based on the density of the air.
• High density altitude results in reduced aircraft performance.

Load Factor

• The ratio of the total load supported by the aircraft's wing to the actual weight of the aircraft.
• It is measured in "G's".
• Load factor increases during turns, pull-ups, and other maneuvers.
• Increased load factor increases stall speed.
• Aircraft are designed to withstand certain load factors.
• Exceeding these load factors can result in structural damage or failure.
• Load factor is greatest during abrupt control movements or turbulent air.
• The amount of excess load that an airplane can carry is affected by: air density, airspeed, and angle of attack.

Turns

• When an aircraft is in a turn, a component of lift is directed horizontally, providing the centripetal force needed to turn the aircraft.
• The steeper the bank angle, the greater the horizontal component of lift, and the faster the turn rate.
• As bank angle increases, the vertical component of lift decreases, so the pilot must increase the angle of attack to maintain altitude.
• Increasing angle of attack increases induced drag, which requires more thrust to maintain airspeed.

Wake Turbulence

• Wake turbulence is caused by the wingtip vortices created by an aircraft.
• Wingtip vortices are swirling masses of air that trail behind the wingtips.
• The strength of the wake turbulence depends on the weight, speed, and wingspan of the aircraft.
• Wake turbulence can be hazardous to following aircraft, especially smaller aircraft.
• Pilots should avoid wake turbulence by maintaining adequate separation from other aircraft and by landing and taking off upwind of larger aircraft.
• The greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow.
• Vortices sink at a rate of several hundred feet per minute and level off about 900 feet below the generating aircraft.

Wind Shear

• Wind shear is a sudden change in wind speed or direction.
• Wind shear can occur at any altitude, but it is most dangerous near the ground.
• Wind shear can cause sudden changes in airspeed and altitude, making it difficult to control the aircraft.
• Pilots should be aware of the potential for wind shear and take appropriate precautions.
• Low-level wind shear alert system (LLWAS) is used at airports to detect wind shear.
• Doppler radar can also detect wind shear.
• During approaches and landings, a sudden increase in headwind (or decrease in tailwind) results in increased performance and vice versa.