Airfoil Terminology:

• Pitch Attitude (q ) – the angle between an airplane’s longitudinal axis and the horizon
• Flight Path – the path described by an airplane’s center of gravity as it moves through an air mass
• Relative Wind – the airflow the airplane experiences as it moves through the air and is equal in magnitude and opposite in the direction to the flight path
• Angle of Attack (a ) – the angle between the relative wind and the chordline of an airfoil
• Flight path, relative wind, and angle of attack should never be inferred from the pitch attitude
• Mean Camber Line – Line drawn halfway between the upper and lower surfaces
• Airfoil Positive / Negative / Symmetrical Camber – determines whether the mean camber lines is above or below or equal to the chordline
• Airfoil Thickness – is the height of the airfoil profile. The point of maximum thickness corresponds to the aerodynamic center
• Spanwise Flow – Airflow that travels along the span of the wing, parallel to the leading edge and is normally from root to tip. This airflow is not accelerated over the wing and therefore produces no lift
• Chordwise Flow – is air flowing at right angles to the leading edge of an airfoil and is the only flow accelerated over the wing producing lift

Aerodynamic Force – a force that is the result of pressure and friction distribution over an airfoil and can be resolve into lift and drag

• Lift – the component of aerodynamic force acting perpendicular to the wind

• Drag – the component of the aerodynamic force acting parallel to and in the same direction as the relative wind
• Leading Edge Stagnation Point – the area of high static pressure where the air strikes the leading edge of the airfoil and its velocity is slow to near 0

• Trailing Edge Stagnation Point – the point where the upper and lower airflows

meet. The velocity slows to near zero, forming an area of high static pressure

• Aerodynamic Force Equation – a product of dynamic pressure (q), the surface area of the airfoil (S), and a variable (C_f) which is the coefficient of aerodynamic force AF = ½ r V² S C_f

• Lift – controlled by eight factors that affect aerodynamic force
• Density, Velocity, and Surface Area
• Angle of Attack and Shape Effects
• Aspect Ratio (deals with the shape of the wing), Viscosity (affects the aerodynamic force by decreasing the velocity of the airflow immediately adjacent to the wing’s surface), and Compressibility (air compresses when it hits the wing)
• An increase in density or velocity will produce greater lift
• An increase in wing surface area produces greater lift
• Flaps are devices used to change the camber of an airfoil and are used during take off and landings
• Velocity and Angle of Attack are inversely related in level flight
• C_lmax is the most effective AOA and remains constant regardless of weight, dynamic pressure, bank angle, etc

The Boundary Layer – the layer of airflow over a surface that demonstrates local airflow retardation due to viscosity, it is usually no more than 1mm thick at the leading edge of the airfoil and grows in thickness as it moves aft over the surface

• Laminar Flow – the air molecules move smoothly along in streamlines and this layer produces little friction but is easily separated from the surface
• Turbulent Flow – streamlines break up and the flow is disorganized and irregular, this produces high friction drag, but adheres to the upper surface of the airfoil
• Favorable Pressure Gradient – assist the boundary layer in adhering to the surface by maintaining its high kinetic energy
• Adverse Pressure Gradient – impedes the flow of the boundary layer
• If the boundary doesn’t have significant kinetic energy to overcome the adverse pressure gradient, the lower levels of the boundary layer will stagnate. The boundary will separate from the surface and cause the airfoil to lose the suction pressure that creates lift

Stalls – a condition of flight where an increase in AOA has resulted in a decrease in C_l

• Regardless to flight speed and air conditions, the wing will always stall beyond the same AOA
• The only cause of a stall is excessive AOA
• The only action necessary for stall recovery is to decrease the AOA below C_lmaxAOA
• The T-34C AOA indicator is calibrated so that the airplane stalls between 29.0 and 29.5 units AOA regardless of airspeed, nose attitude, weight, or altitude. It is self adjusting to account for differences in full flap or no flap stalls. The T-34 also contains AOA indexer and rudder shakers that receive their input from an AOA probe on the left wing. The rudder pedal shakers are activated and airframe buffeting will occur at 26.5 units of AOA. Stalls at idle in a clean configuration are characterized by a nose down pitch with a slight rolling tendency at near full aft stick. The effect of the landing gear on stalls is negligible, however extending the flaps will aggravate the stall characteristics by increasing the rolling tendency. Increased power will degrade the stall characteristics by increasing nose up stall attitude, increased buffeting and increased roll tendency.

Stall Speeds – the minimum true airspeed required to maintain level flight at C_lmaxAOA. It is greatly controlled by weight (as weight decreases, stall speed decreases), altitude (an increase in altitude, increases stall speed), power and maneuvering

• Power On Stall Speed – is less than power off stall speed because at high pitch attitude, part of the weight of the airplane is actually being supported by the vertical component of the thrust vector
• The T-34 Power On Stall Speed is 9 knots less than its Power Off Stall Speed

High Lift Devices – affect stall speeds since they increase C_l as we approach C_lmaxAOA. The primary purpose of high lift devices is to reduce takeoff and landing speeds by reducing stall speed. There are two types: those that delay boundary layer separation and those that increase camber.

Boundary Layer Control (BLC) Devices – operate by allowing the high static pressure air beneath the wing to be accelerated through a nozzle and injected into the boundary layer on the upper surface of the airfoil. As the air flows through the nozzle, the potential energy is converted into kinetic energy. There are many types of BLC devices but we will concentrate on slots

• Fixed Slots – are gaps located at the leading edge of a wing that allow air to flow from below the wing to the upper surface. High pressure air from the leading edge stagnation point is directed through the slot, which acts as a nozzle converting the static pressure into dynamic pressure. The high kinetic energy air leaving the nozzle increases the energy of the boundary layer and delays separation
• Slats – moveable leading edge sections and used to make automatic slots. When the slat deploys, it opens a slot

Camber Change – increasing the camber of an airfoil increases C_lmax . Extending flaps increases the airfoil’s positive camber thus shifting its zero lift point to the left.

• Plain Flap – a simple hinge portion of the trailing edge that is forced down into the airstream to increase the camber of the airfoil
• Split Flap – a plate deflected from the lower surface of the airfoil and creates a lot of drag because the turbulent air between the wing and the deflected surface
• Slotted Flap – similar to plain flap, but moves away from the wing to open a narrow slot between the flap and the wing for BLC.
• Fowler Flap – used extensively on larger airplanes. When extended, it moves down increasing the camber and aft causing a significant increase in wing area as well as a slot for BLC
• Leading Edge Flaps – devices that change the wing camber at the leading edge of the airfoil.

Stall Pattern / Wing Design – the most desirable pattern on a wing is one that begins at the root. The primary reason for a root first stall pattern is to maintain aileron effectiveness until the wing is fully stalled.

• Rectangular Wing – lift distortion is due to low lift coefficients at the tip and high lift coefficients at the root. It has a strong root stall tendency
• Highly Tapered Wing – is desirable from the standpoint of structural weight, stiffness, and wingtip vortices. They produce most of the lift towards that tip
• Swept Wings – are used on high speed aircraft because they reduce drag and allow the airplane to fly at higher mach numbers. They have similar lift distribution as that of Tapered and have strong tip stall tendency and rapidly progresses over the rest of the wing
• Elliptical Wings – has an even distribution of lift from the root to the tip and produces minimum induced drag. All section stall at the same AOA.
• Moderate Taper Wings – have a lift distribution and stall pattern that is similar to elliptical. The T-34 uses tapered wings because they reduce weight, improve stiffness, and reduce wingtip vortices. The stall is undesirable and as the stall progresses, the pilot will lose lateral control
• Geometric Twist – is a decrease in angle of incidence from wing root to tip. T-34 is geometrically twisted at 3.1 degrees
• Aerodynamic Twist – is a decrease in camber from wing root to tip. The T-34 is aerodynamically twisted to create a reduced camber at the tip
• Stall Fences – redirect airflow along the chord, thereby delaying tip stall and enabling the wing to achieve higher AOAs without stalling
• Stall Strip – a sharply angled piece of metal mounted on the leading edge of the root to induce a stall at the wing root. T-34s have stall strips located near the root at the leading edge