Generating Lift

Airfoils

As viewed as a cross-section, the upper surface of an airfoil has more camber (curve) than lower surface.  A straight line from leading edge to trailing edge is referred to as the chord.  With this structure, lift is generated by two process—what are referred to as pressure differential and ram air.

Lift by pressure differential is based on the theory of Daniel Bernoulli (Bernoulli’s Theorem)—the faster a fluid flows (including air), the lower will be the pressure surrounding it; given the difference of the camber of the upper and lower surfaces, the air passing over the foil has greater distance to travel than the air passing under the airfoil; the air passing over must therefore travel faster than the air passing under the foil; a low air-pressure region is created above the accelerated air flow; the foil is displaced toward the lower pressure (upward) above the wing.  This pressure differential accounts for about 50% of the lift, while the remaining lift is generated by ram air.  With ram air, air is rammed under the foil, creating downwash and resultant upward pressure—this is based on Isaac Newton’s Third Law (the application of force causes an equal opposite force).

Angle of Attack

Lift varies with the angle of attack—the angle between the relative wind (parallel to flight path) and the chord line (line between leading and trailing edge).  Generally, the greater the angle of attack, the greater the lift—lift increases because the distance the air must flow along the upper camber increases, and the ram air and downwash increase.  An excessive angle of attack, referred to as the critical angle of attack (usually about 20°) will produce a stalled condition—laminar airflow above the wing is displaced by turbulent airflow, and differential pressure collapses.

Parasitic Drag

Parasitic drag is drag created by those parts of an aeroplane that do not contribute to lift—e.g., the tires, windshield, rivets, etc.  There are, in turn, three forms of parasitic drag—form drag, skin-friction drag, and interference drag.  Form drag is caused by the frontal areas of the aeroplane, and is reduced by streamlining.  Skin-friction drag is caused by the air passing over the aeroplane surfaces, and is reduced by smoothing the surfaces (flush riveting, smooth paints, and waxing).  Interference drag is caused by the interference of airflow between parts of an aeroplane (wings and fuselage or fuselage and empennage) and is reduced by filleting interference areas.

Induced drag

Induced drag is created by those parts of the aeroplane that create lift—the wings and the horizontal tail surface; induced drag is said to be the by-product or cost of lift—that is, the greater the angle of, attack, the greater the induced drag.  Induced drag does not increase with speed; instead, as speed decreases induced drag increases.

Induced drag is associated with difference in pressure that exists above and below a wing surface—as airspeed decreases, an airfoil must produce an increased low pressure above the wing, and an increased high pressure below the wing.  At the wingtip these disparate pressures meet in the form of a vortex as the high pressure flow around the wingtip is sucked into the low pressure above the wing; the greater the pressure differences (such as in the case in slower flight), the greater the vortices are at each wing tip, and the greater the drag caused by these vortices.

Ground Effect

Ground effect is a term used to describe the reduced drag and increased lift experience when an aircraft is flying close to the ground—as is the case, for example, during landings and takeoffs; the reduce drag associated with ground effect is the result of the ground interfering with the formation of the wingtip vortices.  Ground effect exists when the aircraft is within one wingspan distance from the ground, but is most effective at distances equal or less than ½ wingspan (i.e., ½ the distance between the wingtips).

Boundary Layer

During flight, there are two types of airflow along the upper camber of an airfoil—turbulent and laminar (smooth).  Turbulent and laminar flow are separated by a point of transition or separation point; as the angle of attack is increased, the portion of the upper airflow that is turbulent also increases (it migrates forward from the trailing edge) and therefore produces increased drag.

Vortex generators are small fins (approximately 1 inch tall) that are placed along the leading edge of an air foil; the vortex generators are themselves small air foils that are placed perpendicular to the upper wing surface, and are positioned so as to meet the laminar flow coming over the wing with a slight angle attack; the vortex generators, as the name implies, generate vortices which regenerate the boundary layer and delay turbulent flow (boundary layer separation).

Stalls

Stalls occur at the critical angle of attack, where induced drag (airfoil drag) exceeds lift—the wing can no longer produce sufficient lift to counteract weight.  As the airfoil approaches the critical angle of attack, the point of transition, or separation point, moves forward enough to exceed the design factor of the wing.  In contrast, the centre of pressure moves forward as the angle of attack is increased until the critical angle of attack is achieved; when a stall occurs, the centre of pressure moves rearward, causing the instability associated with stall phenomena.

As said earlier, the stalling angle is usually 20°.  Since most aircraft lack angle-of-attack indicators, airfoil angle is measured by indicated airspeed (IAS)—our best estimate of the actual angle of attack.  As a rule, aircraft will usually stall near the stalling speed published in the Pilot Operating Handbook; however, IAS does not always accurately indicate angle of attack, as in the case of a high-speed stall.

Factors that affect the Stall

Contaminants.  Snow, frost, ice and dirt—all of these disrupt the laminar flow and therefore reduce airfoil lift capability.  It is illegal to fly with snow, frost, or ice adhering to critical surfaces of the aircraft—“wings, control surfaces, rotors, propellers, horizontal stabilizers, vertical stabilizers or any other stabilising surface of an aircraft (CAR 602.11).”  Contamination to the extent of medium to coarse sandpaper will reduce lift by 30% and increase drag by 40%.

Weight.  Increased weight requires increased lift and an increased angle of attack; therefore the critical angle of attack (stall) will occur at higher airspeeds.  Stated another way, if two aircraft are travelling at the same airspeed, but one is heavier than the other, the angle of attack of the heavier aircraft is greater than the lighter aircraft and therefore that much closer to the critical angle of attack.

Centre of Gravity.  Stalling speed increases as the aircraft C of G moves forward.  As the C of G moves forward, the negative lift generated by the horizontal tail surface will have to be increased.  Any increase in the negative lift produced by the tail will effectively increase the aerodynamic weight of the aircraft—producing the same effect as described above with respect to weight.  Conversely, stalling speeds decrease as the C of G moves aft as less negative lift is required from the tail and the aircraft is aerodynamically lighter.  While the benefits of a rearward C of G is a lower stall speed, the adverse result of a rearward C of G is less stability as there is less tail force that can be manipulated by the pilot through elevator or stabilator control.

Turbulence.  Upward vertical gusts abruptly increase the angle of attack beyond the stalling angle, irrespective of airspeed.

Turns.  During a turn in level flight, greater lift is required to offset increased load factor; the critical angle of attack is therefore reached at higher airspeeds.  The formula is as follows—normal stalling speed times the square root of the load factor equals banked stall speed; accordingly, an aircraft with a stall speed of 50 KTS in a 60°-banked turn (load factor of 2.0) will stall at 71 KTS.

During a climbing turn, the inner wing has a smaller angle of attack than the outer wing; the outer wing will therefore stall first.  The reverse is the case for descending turn, where the inner wing has a larger angle of attack and will therefore stall first.

Flaps.  An increase in airfoil lift is produced by the use of flaps, and the stall speed is decreased by their use.

Spins

Spinning is defined as autorotation that develops after an asymmetrical or aggravated stall (a wing dropping during a stall)—the downward moving wing has a higher angle of attack and more induced drag than the upward moving wing and therefore acquires a greater stalled condition.  Spinning involves simultaneous roll, yaw, and pitch as it develops a helical or corkscrew path nose down.  An incipient spin is the autorotation prior to a vertical descent path, while a fully developed spin begins once the vertical path is achieved.