Aerodynamics

Bernoulli’s Principle of Differential Pressure

• As the speed of air increases, its pressure will decrease
• The upper surface of the wing is curved and the lower surface is relatively flat, thus decreasing the pressure on top of the wing and being the major source of the lift that makes an airplane fly
• At high altitude, the air gest thinner, and the airplane has to move faster to create the same amount of lift as it does in dneser air

Four forces

Airplane components

Airfoil

Angle of attack (AOA)

• The acute angle between the chord line of the airfoil and the direction of the wind

• At low speed, the airflow over the top of the wing slows down and lift becomes less than weight. To maintain level flight, AOA can be increased to generate extra lift

• Critical AOA: above a wing’s critical AOA, the flow of air separates from the upper surface and backfills which reduces lift and increases drag, effectively stalling the aircraft and can lead to loss of control and an abrupt loss of altitude if the AOA is not reduced.

Stall speed

• The minimum steady flight speed at which the airplane is controllable/able to generate lift

Spins

• A spin is a yaw aggravated stall which results in rotation about the spin axis. The aircraft follows a steep, “corkscrew” like, downward path.
• A stall must occur before a spin can take place.

Center of Pressure

• The average location of all of the pressure acting on the plane
• CP location changes with AOA. As AOA increases, CP moves forward and vice versa
• Since center of gravity is fixed, the location of CP affectsaerodynamic balance and controllability

Flap

• Mounted on the trailing edge of the wing.
• When extended, they increase in both lift and drag and a reduction of the stall speed. These factors result in an improvement in takeoff and landing performance.

  Airplane controls

• Ailerons control roll - turning the control yoke right, the right aileron is up and the left aileron is down, and the airplane banks right
• Elevator controls pitch - pulling the control yoke pitches the nose up and vice versa
• Rudder controls yaw - pushing the right rudder pedal moves the rudder to the right and the airplane yaws to the right

Adverse Yaw

• The tendency of an airplane to yaw in the opposite direction of the turn.
• When rolling airplane to the right, right aileron goes up, and left aileron goes down, changing the chord line of the wing. AOA increases for the left wing and decreases for the right wing

• Thus, lift increases on the left wing, as well as induced drag. With more drag on the left wing then the left wing, the airplane will yaw to the left
• To counter adverse yaw, pushing the rudder paddle to the right

Drag

Parsitic drag

• Produced by aircraft as it moves through air
• Increase exponentially with airspeed

Induced drag

• Produced by lift
• Highest at high AOA / low airspeed

Vortex

• The lower pressure occurs over the upper wing surface and the higher pressure under the wing. This pressure differential triggers the rollup of the airflow aft of the wing resulting in swirling air masses trailing downstream of the wingtips.
• Vortex Avoidance
  • Landing behind a larger aircraft - stay at or above the larger aircraft’s approach flight path and land beyond its touchdown point.
  • Takeoff/depart behind large aircraft - fly above the upwind from the heavy aircraft

Ground effect

• Within one wingspan of the group, the distortion of airflow below the wing due to proximity to the ground reduces induced drag
• During take off, the plane may be airbone at a lower speed than the recommended take off speed. But the plane leaving ground effect will experience an increase in induced drag and thrust required and thus not able to sustain flight.
• During landing, the plane may float much longer thus a reduction in power is necessary to offset the extra lift

Stability

Static Stability

• Positive static stability: the tendency to return to the original state of equilibrium after being disturbed.
• Neutral static stability: the tendency to remain in a new condition after its equilibrium has been disturbed.
• Negative static stability: the tendency to continue away from the original state of equilibrium after being disturbed.

Dynamic Stability

• Positive dynamic stability: oscillation decreases in amplitude with time
• Neutral dynamic stability: oscillations are constant in amplitude with time
• Negative dynamic stability: oscillations increase in amplitude with time

Longitudinal stability

• Stability about lateral (pitch) axis
• Depends on location of center of gravity relative to center of lift

Lateral stability

• Stability about longitudinal (roll) axis
• Influenced by
  • Dihedral wings: wings angled upward when viewed from the front
  • Sweepback: wings angled backward when viewed from the top
  • High vs. Low wing: high wing provide more stability

Directional stability

• Stability about the vertical axis
• Influenced by location of vertical stablizers

Torque

• Left turning tendency of the airplane
• Four sources of torque:
  • Torque reaction
    • Newton’s Third Principle, action - reaction
    • Most US aircraft engines rotate the propeller clockwise from the pilot’s seat
    • Torque reaction tries to rotate the aircraft to the left
  • Corkscrew Effect
    • Air pulled in by the propeller is sent backward, moving in a clockwise pattern
    • The air hit vertical stablizer causing left yaw
  • Gyroscopic Action
    • Precession: when a force is applied to a rotating disc, the effect of that force (the resultant force) is felt 90 degrees in the direction of rotation of the disc.
  • Asymmetric Loading (P-Factor)
    • When the airplane has a high AOA, the descending blade creates more thrust than ascending blade.
    • Center of thrust moving to the right and causing left yaw

Load Factor

• Equals to the lift/weight ratio
• Lift can be separate into two components, the vertical component and the horizontal compoenent. To keep the airplane flying level, the vertical component needs to remain the same
• When the aircraft is banking, lift needs to be increased to keep the vertical component constant

• The load factor increases at a terrific rate after a bank has reached 45° or 50°. The wing must produce lift equal to these load factors if altitude is to be maintained.

• Increasing load factor increases stall speed