Principles of Aerodynamics

4 forces of flight

4 forces

1. Thrust

2. Drag

3. Lift

4. Weight

Drag

The force that resists the movement of an aircraft through the air.

Parasite Drag

Drag that is not associated with the production of lift.

• Form drag: generated by the aircraft due to its shape and the airflow around it.

• Interference drag: intersection of airstreams that create eddy currents, turbulence, or restricts smooth airflow (e.g. intersection of the wing and fuselage at the wing root). The most significant interference drag occurs when two surfaces meet at perpendicular angles.

• Skin friction drag: resistance due to the contact of moving air with the surface of an aircraft.

Induced Drag

Inherent whenever an airfoil is producing lift. Always present if lift is produced.

• Higher pressure on the bottom of the wing, lower on top. At the edges, there is a tendency for pressures to equalize, leading to rotational vortices trailing behind the edges.

• The greater the strength of the vortices, the greater the drag. The greater the pressure differential, the stronger the vortices.

• The lower the airspeed, the greater the angle of attack (AOA) required to produce lift equal to the plane’s weight, and therefore, the greater induced drag.

• The heavier and slower the aircraft, the greater AOA and the stronger the wingtip vortices. Strongest wingtip vortices are during takeoff, climb, and landing.

Avoiding Wake Turbulence

• Avoid flying through another aircraft’s flight path.

• Rotate prior to the point at which the preceding aircraft rotated when taking off behind another aircraft.

• Avoid following another aircraft on a similar path at an altitude within 1,000 ft.

• Approach the runway above the preceding aircraft’s path when landing behind another aircraft and touch down after the point at which the other aircraft’s wheels contacted the runway.

Wingtip vortices drift with the wind. They descend at a rate of 300-500 fpm for about 30 seconds and levels off between 500-900 ft below the flight path.

Approximately 3 minutes provides a margin of safety that allows wake turbulence dissipation.

Flight controls & maneuvering

Ailerons

Ailerons control motion around the longitudinal axis (roll). Ailerons are controlled differentially; raising them on one side of the aircraft will force them down on the other. Raising the aileron on one side will decrease the angle of attack, giving less lift, while at the same time increasing the angle of attack on the opposing side, increasing lift, and putting the aircraft into a bank.

Elevators

Elevators control motion about the lateral axis (pitch). Pulling back on the controls deflects the elevators upward, forcing the tail down and placing the aircraft in a nose-up attitude. The converse is also true.

Rudder

The rudder controls motion about the vertical axis (yaw). A left deflection pushes the tail to the right, point the nose towards the left.

Forces in turns

• In a slipping turn, the horizontal force exceeds the centrifugal force. The aircraft is banked too much for the rate of turn.

• In a skidding turn, the centrifugal force exceeds the horizontal force. The rate of turn is too great for the angle of bank.

Torque and P-Factor

Torque, and the left-turning tendency of the aircraft, are the result of:

1. Torque reaction from the engine and propeller. Turning the propeller causes an opposite roll tendency on the aircraft.

2. Corkscrewing effect of the propeller slipstream. At high propller speeds and low forward speeds, this spiraling motion is very compact and produces a strong sideward force on the vertical tail surface.

3. Gyroscopic action of the propeller. Any time a force is applied to deflect the propeller out of its plane of rotation, the resulting force is 90 degrees ahead of and in the direction of rotation. For example, when a tailwheel plane tilts the nose down to lift the tail, the force is applied 90 degrees ahead of the top of the propller and also in the same direction (forward), yawing the plane to the left.

4. Asymmetric loading of the propeller (P-factor). When the plane is flying at a high AOA, the “bite” of the downward moving blade is greater than that of the upward moving blade (relative wind). This moves the center of thrust to the right of the prop disc area, causing a left yaw.

(Most engines in the US rotate the propller clockwise, as viewed from the pilot’s seat.)

Ground effect, stability & load factor

Ground Effect

Ground effect is the reduced aerodynamic drag that an aircraft’s wings generate when they are close to a fixed surface. Reduced drag when in ground effect during takeoff can cause the aircraft to “float” while below the recommended climb speed. The pilot can then fly just above the runway while the aircraft accelerates in ground effect until a safe climb speed is reached (source).

Less thrust and a lower angle of attack are required in ground effect to achieve a given coefficient of lift relative to normal flight.

Stability

Stability is an aircraft’s ability to maintain/return to its original flight path.

Two kinds of stability:

1. Static stability: initial tendency of aircraft once disturbed

   1. Positive: Returns to original position

   2. Neutral: Stays at new position

   3. Negative: Continues away from original position

2. Dynamic stability: tendency over time (positive static is required for dynamic stability)

   1. Positive: Dampens towards original position

   2. Neutral: Oscillates a constant amount

   3. Negative: Unstable oscillation (response gets worse with time)

Three axes of stability:

1. Longitudinal: nose to tail (roll)

2. Lateral: wingtip to wingtip (pitch)

3. Vertical: rotation about CG (yaw)

Load Factor

(PHAK 5-33)

Stall awareness, spins & spin recovery

Stalls

An aircraft stalls from a rapid decrease in lift caused by the separation of airflow from the wing’s surface brought on by exceeding the critical angle of attack.

A stall can occur at any pitch or airspeed.

In most straight-wing aircraft, the wing is designed to stall at the wing root first. The stall then progresses out towards the wingtip. This is to maintain aileron effectiveness and therefore aircraft controllability.

The stalling speed of an aircraft is not a fixed value for all flight situations, but a given aircraft always stalls at the sae angle of attack, regardless of airspeed, weight, load factor, or density altitude.

There are 3 flight situations in which critical AOA is most frequently exceeded: low speed, high speed, and turning.

AOA indicators can help with stall margin awareness.

Spins

Spin Recovery

Letter	Stands for	Description
P	Power to idle	When you’re at a high power setting, airflow from your propeller strikes your horizontal stabilizer, causing a tail-down force and pitching your nose up.
A	Ailerons neutral	When you bring your ailerons to neutral, you help your wings reach the same angle-of-attack, which helps you reduce the rolling and yawing moments in the spin.
R	Rudder opposite spin	When you add opposite rudder, you stop the rolling and yawing moment of the spin.
E	Elevator forward	Breaking the stall. Once you have your plane configured to fly out of the spin (steps 1-3), it’s time to reduce your angle-of-attack and keep on flying.

(source)