Aerodynamics

Aerodynamics

Four Aerodynamic Forces:

• LIFT • WEIGHT • THRUST • DRAG

In level flight… • Aerodynamic forces are balanced

– lift balances weight – thrust balances drag *an airplane climbs due to excess thrust*

• The airplane is in a state of dynamic equilibrium. • There is no acceleration in any direction.

LIFT • Newton’s Laws of Motion – Newton’s Second Law- A force results whenever a mass is accelerated – Newton’s Third Law- For every action, there is an equal and opposite reaction.

• Bernoulli’s Principle – When the velocity of a fluid increases, its pressure decreases. Likewise, when the velocity of a fluid decreases, it’s pressure increases.

Newton’s Laws Applied… • The wing causes the air to move past it to curve downward, creating a strong downwash behind the wing. • As the mass of air is accelerated downward, the reaction force pushes the wing up. • Lift is the reaction that results from the action of forcing air downward.

Bernoulli’s Principle Applied… • The air pressure above the wing is decreased because the shape of the airfoil and angle of attack cause the air to flow faster over the top of the wing. • The air moving underneath the wing slows down, and it’s pressure increases. • Lift results from a relatively high pressure below the wing and a relatively low pressure above the wing.

Relative Wind • Relative wind is directly opposite the flight path of the airplane. • Lift is the force acting perpendicular to the relative wind. • Drag acts parallel to the flight path, in the same direction as the relative wind.

Angle of Attack and Angle of Incidence

• Angle of attack is the angle between the chord line of the wing and the relative wind. – Lift generally acts perpendicular to the relative wind, regardless of the angle of attack.

• The angle of incidence is the small angle formed by the chord line and the longitudinal axis of the airplane – In most airplanes, the angle of incidence is fixed and cannot be changed by the pilot. – The angle of incidence provides for proper angle of attack at cruising speed so that the longitudinal axis can be aligned with the relative wind, helping to minimize drag.

Lift Equation L = C (V^2) (ρ /2) S • L = Lift • C = Coefficient of Lift (This changes with airfoil design and angle of attack) • V = Velocity (feet per second) • ρ = Air Density (slugs per cubic feet) • S = Surface Area of the Wing (Square feet)

• For lift to increase, one of the factors on the right side of the equation must increase. • Lift is proportional to the square of the velocity, so doubling your airspeed quadruples the amount of lift if everything else remains constant. • Also, if the coefficient of lift increases, lift will also increase if all other factors remain constant. • To generate the same lift at a certain altitude with a reduced airspeed, the equation shows that you must increase the coefficient of lift or angle of attack. • Likewise if you reduce your airspeed, you have to reduce angle of attack.

• For any particular amount of lift, there is a specific combination of angle of attack and airspeed. • Since air density decreases with altitude, the airplane must fly with either a higher angle of attack or a higher speed to generate the same lift at a higher altitude.

4 Ways to Control Lift • • • •

Increase airspeed Changing the Angle of attack Changing the shape of the airfoil Varying the total area of the wing

The most common way to control lift is to change the angle of attack. • Increasing the angle of attack causes the wing to generate more lift, but only to a certain point. • As the wing reaches a particular angle of attack, the airflow begins to separate, and the amount of lift begins to decrease. • When the airframe begins to buffet just before a stall, the angle of attack is Clmax

• An increase in lift also causes an increase in drag, which will slow the airplane unless you also increase the thrust. • The angle of attack directly controls the distribution of pressures acting on a wing. By changing the angle of attack you can control the airplane’s lift, airspeed, and drag. • The angle of attack at which a wing stalls remains constant regardless of weight, dynamic pressure, bank angle, or pitch attitude.

Center of Pressure • The pressure distribution pattern over a wing changes as the angle of attack changes. • Some areas have a decreased pressure and some have an increased pressure. • The vector of the net lift represents the center of lift, or center of pressure. • In a symmetrical airfoil, which has the same curvature on both sides of the chord line, the center of pressure does not move when angle of attack changes.

High Lift Devices • devices that allow you to change the camber of the wing and area of the wing through the use of trailing edge flaps and leading edge high lift devices • some of these devices also delay the separation of airflow and lower the stall speed

Trailing edge flaps • using flaps helps to increase rate of descent without increasing airspeed • used for approach to landing • flaps extend the coefficient of lift and decrease the stall speed

4 Types of Flaps • • • •

plain flap split flap slotted flap fowler flap

• plain flaps increase the wing camber • split flaps also increase the wing camber • slotted flaps increase camber and help prevent airflow separation • fowler flaps increase the camber, increase wing area, and help prevent airflow separation

Because flaps increase lift and reduce the stall speed, many airplanes use them for takeoff as well as landing. Used properly, they can reduce ground roll and increase climb performance. In general, the first few degrees of flaps provide the greatest lift benefit.

Leading Edge Devices • The leading edge of a wing can also be changed to increase airfoil camber, add more wing area or delay airflow separation at high angles of attack. • The simplest leading edge device is a fixed slot.

Fixed Slot Leading Edge Device • Allow the airflow to remain attached over the outer portion of the wings after the roots have stalled. • When the wing is at a low angle of attack, only little amounts of air flow from the bottom of the leading edge through the opening, so slots do not cause significant change to the effectiveness of the airfoil.

Slat Leading Edge Devices • Slats are portions of the leading edge which are moved forward and down to create a path for air similar to a slot. • The advantage is that a wing is clean when they are retracted. • Most slats also increase the effective wing area, providing additional lift as well as delaying airflow separation. • The main disadvantages are weight and complexity.

Leading Edge Flaps • Usually, leading edge flaps increase both wing camber and area. • Generally found on jets due to expense, weight, and complexity. • Also, airfoils of jets, which are optimized at high speeds, need them to create adequate lift at relatively low takeoff and landing speeds.