Lift
  • Artificial forces manipulated by pilot.

  • Generated through the wings.

  • Acts perpendicular to the relative wind and wingspan.

  • Lift is exerted through the centre of pressure.

  • Opposes weight: during level cruise, lift equals weight; during climb, lift is greater than weight; and during descent, weight is greater that lift.

 

Weight
  • Natural (uncontrollable) force generated by gravity (g force) that acts perpendicular to earth’s surface.

  • Weight is exerted through the centre of gravity.

  • Opposes lift (see above).

 

Thrust
  • Artificial force manipulated by pilot and generated through engine(s) that acts horizontally, parallel to flight path.

  • Opposes drag: when airspeed constant, thrust equals drag; when airspeed accelerating, thrust is greater than drag; and when decelerating, drag is greater than thrust.

 

Drag
  • Natural resistance of aeroplane while moving through air, partially controlled by pilot.

  • A horizontal force, parallel to flight path.

  • Opposes thrust (see above).

 
 
 
 
Generating Lift

 

 
 
Airfoils           
  • Viewed as a cross-section: upper surface has more camber (curve) than lower surface.

  • Chord line: straight line from leading edge to trailing edge.

  • The airfoil generates lift by two means: pressure differential and ram air.

 
Bernoulli’s Theorem
 

 

 

  • Lift by pressure differential is based on the theory of Daniel Bernoulli: 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.

 

Ram Air
  • The pressure differential accounts for about 50% of the lift, while the remaining lift is generated by ram air.

  • Air is rammed under the foil, creating downwash, and upward pressure (Isaac Newton’s Third Law: the application of force causes an equal opposite force).

 

Angle of Attack and Speed
 
 

 

 

  • Lift varies with the angle of attack and speed.

  • The pilot controls speed during straight and level flight through the use of throttle (power), while the pilot controls angle of attack throught the use of pitch inputs (nose-up and nose-down—control column rearward and forward, respectively)

  • The angle of attack is 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.

  • However, an excessive angle of attack—referred to as the critical angle of attack—will produce a stall (usually 20°) and this will be discussed below.

  • With respect to speed, generally speaking, the faster a wing surface (airfoil) travels through the air, the greater the lift force that will be generated. 

  • Generally, the relationship between lift and speed—velocity—is exponential such that lift depends on the square of velocity—a doubling of speed will quadruple the lift generated by the wing surface.

  • Similarly, the greater the angle of attack of a wing, the greater the lift force that will be generated.

  • Accordingly, as an aircraft slows during an approach for landing, for example, pilots have to increase the angle of attack to maintain the same amount of lift necessary to offset the aircraft’s weight.

  • Similarly, as an aircraft accelerates in level flight (where the aircraft’s weight is equal to the lift being generated by the wings), pilots have to decease the angle of attack to prevent the aircraft from climbing to a higher altitude.

  • Here is an important simplication of the lift formula that converts the factors to only those variable factors controlled by the pilot during flight—speed (throttle) and angle of attack (pitch):

     

 

 
Drag

Importantly, there are two types of drag: parasitic and induced.  Parasitic drag is straight forward and intuitive, while induced drag is more aerodynamically significant in understanding lift, but a little more complicated

 

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.

  • The greater the speed of the aircraft, the greater the parasitic drag.

  • There are 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 creates lift—the wings and the horizontal tail surface.

  • Induced drag is said to be the by-product of lift.

  • 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, and airfoil must produce an increased low pressure above the wing, and an increased high pressure below the wing; at the wing tip these disparate pressures meet in the form of a vortex as the high pressure flows around the wing tip 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 is a term used to described 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 reduced drag associated with ground effect is the result of the ground interfering with the formation of the wing tip 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).

  • Wing tip vortices can produce extremely hazardous turbulence and there are crucial operational considerations (to be reviewed later in Flight Operations).

Boundary Layer
  • Along the upper camber of an airfoil, there are two types of airflow: turbulent and laminar (smooth).

  • The turbulent and laminar flows 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).

 
Aileron Drag
  • A type of adverse or undesirable yaw is aileron drag; aileron drag is created when a pilot manipulates the ailerons when rolling into a turn.

  • When the pilot induces a roll, one aileron is deflected upward so as to decrease the angle of attack associated with that portion of the wing, the same portion of the opposite wing is subject to an increase in the angle of attack as the aileron is deflected downward.

  • As the downward deflected aileron enters the high-pressure airflow below the wing it creates drag that causes the wing to move rearward; the upward moving aileron enters low-pressure airflow and is subject to less drag and allowing this wing to move forward.

  • Aileron drag can be reduced by flap design, including Differential Ailerons (in which the movement of the downward aileron is reduced relative to the aileron that is deflected upward), and Frise Ailerons (in which the upward deflected aileron projects the forward portion of the aileron structure into the airflow below the wing).

  • Aileron drag can be controlled by the use of sufficiently adequate opposite rudder during the rolling movement.

 
Stalls
  • Stalls occur at the critical angle of attack, at which point the airflow over the wing becomes chaotic and the wings can no longer produce sufficient lift to counteract weight.

  • As the airfoil approaches the stalling speed, the point of transition, or separation point, moves forward enough to exceed the design factor of the wing.

  • The stalling angle is usually 20°.  Since most aircraft lack angle-of-attack indicators, airfoil angle is measured by indicated airspeed (IAS).

  • As a rule, aircraft will usually stall near the stalling speed published in the Pilot Operating Handbook (bottom of green line on an Airspeed Indicator); however, IAS does not always indicate angle of attack, as in the case of a high-speed stall.

 

I'm

a paragraph. Click here to add your own text and edit me. It's easy.

I'm

a paragraph. Click here to add your own text and edit me. It's easy.

I'm

a paragraph. Click here to add your own text and edit me. It's easy.

I'm

a paragraph. Click here to add your own text and edit me. It's easy.

 
Factors that affect the Stall

 

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 stabilizing surface of an aircraft.”  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; 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.

 

I'm a paragraph. Click here to add your own text and edit me. It's easy

 

.

 

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 effective aerodynamic weight of the aircraft in the turn.  Because the angle of attack of an aircraft must be increased to offset the increased aerodynamic weight (resultant force of a turn), the wing is closer to the critical angle of attack and therefore closer to a stall, despite a constant airspeed.  Accordingly, there is an increase in the relative stall speed of an aircraft in a level turn.  The formula to determine increased stall speed is as follows: normal stalling speed times the square root of the load factor equals banked stall speed.  Thus, an aircraft with a stall speed of 50 KTS and in a 60°-bank turn (load factor of 2.0) will stall at 71 KTS.

 

 

Flaps

An increase in airfoil lift is produced by the use of flaps, and the stall speed is decreased by their use (bottom of white line on an Airspeed Indicator).  The extension of flaps has the effect of increasing the relative angle of attack of the airfoil.  Induced drag is also increased.  Some flap designs incorporated slots, which permit the compressed air under the wing to migrate through the slot to the upper surface of the flaps.  These are referred to as slotted flaps.  Some flap designs allow the flap surface to move rearward as it extends—the feature is referred to as a Fowler Flap.

 
 
Spins

Spinning is defined as autorotation that develops after an 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 helical or corkscrew path is achieved.

 

I'm a paragraph. Click here to add your own text and edit me. It's easy.

I'm a paragraph. Click here to add your own text and edit me. It's easy.

I'm a paragraph. Click here to add your own text and edit me. It's easy.

I'm a paragraph. Click here to add your own text and edit me. It's easy.

 
Lift-drag ratio
  • The amount of lift/induced drag can be graphically plotted (lift/drag coefficients vs. angle of attack).

  • As the angle of attack increases, lift increases to the critical angle of attack, and then falls off suddenly.

  • As the angle of attack increases, induced drag increases slowly at first, and then in ever increasing proportions.

  • The lift-drag ratio defines the proportion of lift to drag at given angles of attack.

  • The study of an aircraft’s lift-drag ratio helps engineers map out the maximum range and maximum glide-distance configurations.

 
Wing Design

There are numerous wing-design features that affect performance: laminar versus conventional airfoils, angle of incidence, washout, stall strips, and airfoil variation.

 
 
 
Laminar and conventional airfoils
  • There are two types of airfoils commonly used in wing design: laminar and conventional.

  • As a rule, the laminar foil is faster, but the cost is more adverse stalling characteristics.

  • They differ with respect to location of the maximum camber: while the maximum camber on a conventional airfoil is located 25% behind the leading edge, the laminar maximum camber is located at 50% chord.

  • On the laminar foil, a greater portion of the upper camber is dedicated to laminar airflow, and there is therefore less drag; the cost of this, however, is that the transition or separation point “jumps” rapidly forward at the approach of a stall; additionally, the laminar foil is more susceptible to surface contamination.

 
Angle of incidence
  • Defined as the angle between the chord line and the longitudinal axis of the aeroplane; designers select an angle that provides optimum lift/drag ratio.

 
 
Washout
  • A design trait that pacifies or softens the stall characteristics of an aeroplane whereby the wings are “twisted” such that the wing tips have a lower angle of incidence than the wing root.

  • The effect is that the entire wing will not stall simultaneously; instead, the stall will progressively move from the roots to the tips.

  • Because the wing tips are the last to stall, the ailerons will remain effective longer during the stall.

 
Stall Strips
  • Stall strips are triangular strips placed on a portion of the leading edge of wing.

  • Stall strips create a gentle stall because the portions of the wing behind the strips stall first.

 
 
Airfoil Variation
  • This involves spanwise airfoil variation whereby a thin high-speed airfoil is designed near the roots, and a low-speed airfoil near the tips.

  • The high-speed roots stall before the low-speed tips.

 
Stability
  • Aeroplane movement is based on 3 axes: the vertical (normal) axis, the lateral axis, and the longitudinal axis.

  • Movement around the vertical (normal) axis is yaw; movement around the lateral axis is pitch; and movement around the longitudinal axis is roll.

  • All three axes pass through the aircraft C of G.

  • Stability is defined as the tendency of an aircraft to return to, stay at, or move farther from its original position relative to the earth's horizon—referred to as the aircraft's attitude—after it has been displaced.

  • There is positive, neutral, and negative stability; additionally, there are two classifications of stability—static and dynamic.

 

Static Stability

  • Static stability refers to the initial tendency of an aircraft in flight to move back to its previous state of balance following a disturbance—a disturbance causing of yaw, pitch, or roll movement—for example, following a disturbance caused by turbulence.

  • An aircraft with positive static stability to return to the attitude associated with its original state; an aircraft with neutral static stability will remain in its new attitude following the disturbance; and an aircraft with negative static stability will continue moving away from the original state without achieving a stable attitude.

 

Dynamic Stability

  • Dynamic stability concerns the ability of an aircraft to return to its equilibrium state over time following a disturbance.

  • An aircraft with positive dynamic stability will return to it original equilibrium state through a series of oscillations of decreasing magnitude; an aircraft with neutral dynamic stability will continue in a series of oscillations of equal amplitude; and an aircraft with negative dynamic stability will simply continue to diverge from its original equilibrium state.

 
 
Longitudinal
  • Movement around the longitudinal axis is roll, which produces bank, and is produced by the ailerons.

  • Longitudinal stability (of the axis) is provided by a nose-heavy design and a “negative-lift” tail.

 

I'm a paragraph. Click here to add your own text and edit

me. It's easy.

 
Lateral
  • Movement around the lateral axis is pitch, and is produced by the elevator or stabilator.

  • Lateral stability (of the axis) is provided by dihedral, which lowers C of G relative to the lifting surfaces (wing tips are positioned higher than the wing roots).  Additionally, the lower wing produces more effective lift than the raised wing, causing the lower wing to naturally rise, and higher wing to descend.

 
Directional
  • Movement around the vertical (normal) axis is yaw, and is “controlled” by the rudder.

  • Stability of the vertical axis is referred to as directional stability, and is produced by the vertical stabilizer and sweptback (sweepback) wing design.  When a sweptback wing moves forward, it acquires increased lift, as the airflow is more perpendicular to the wing’s leading edge.  This increased lift simultaneously produces increased induced drag, which encourages the wing to migrate back to is original position.  The effect is the reverse for the opposite wing.  

  • Adverse yaw is undesired yaw that occurs naturally during certain movements of the aircraft normal flight—for example, during roll movement (because of aileron drag), during power changes (in single-engine aircraft because of slip stream), and during climbs (in single-engine aircraft, caused by asymmetric thrust)—for all of these instances of adverse yaw, the pilot attempts to minimize yaw movement (keep the nose of the aircraft straight) through usually gentle and subtle rudder inputs.  While these instances of yaw can occur during any phase of flight, they are especially evident to the pilot during takeoff.

 
Forces during Takeoff

There are a number of forces exerted on the aircraft during the take-off roll and climb, and they all contribute (in single-engine aircraft) to left yaw tendencies, which must be controlled by the pilot with the use of rudder pedals. 

 

Torque

The force of Torque is based on the principle of physics that a movement in one direction will cause a movement in the opposite direction—Newton’s Third Law of Physics—for every action, there is an equal and opposite reaction.  The force referred to here is the force of the spinning propeller which, from the pilot perspective in the cockpit, rotates clockwise.  This movement of the propeller produces an opposite rotation of the aircraft—from the pilot perspective, counter-clockwise.  This translates during the takeoff roll as a left-yaw tendency, and requires the pilot to exert right-rudder pressure.

 

 

Precession

Precession is a gyroscopic force, which again originates from the movement of the propeller.  According to the law of precession, a pressure exerted on a spinning mass will cause a reaction 90° along the direction of rotation.  The force of precession during takeoff is primarily associated with conventional-gear (“tail-dragger”) aircraft, with the pilot exerting nose-down pressure on the control column to raise the tail wheel off the ground—when this happens, the spinning propeller converts this force into a left-yaw movement, which again must be countered by the pilot with right-rudder input.

 

Slipstream

Slipstream refers to the spiral-like movement of the air flowing from the propeller over the fuselage of the aircraft.  The spiral flow migrates around the fuselage as it travels rearward and strikes the left side of the vertical fin.  This again translates to the pilot as left-yaw tendency, which must also be countered with right rudder.

 

Asymmetric thrust

The force referred to as asymmetric thrust (referred to as P-factor in the US) must be managed by the pilot whenever a single-engine aircraft is in a climb or nose-up attitude.  When the longitudinal axis of the aircraft is inclined upward, the propeller produces different levels of thrust whereby the down-going blade (the right side of the spinning propeller disk, as viewed by the pilot) produces more thrust than the up-going blade (the left side of the propeller disk).  The relative wind (or flight path) of the aircraft with a nose-up attitude means that the down-going blade has a greater angle of attack (and therefore greater thrust) than the up-going blade—which has a less angle of attack.  The greater the angle of attack of the blade, the greater the thrust.  A pilot learns that whenever the aircraft is in a nose-up attitude—whenever the aircraft is climbing—compensating right rudder must be used to counter the resultant left-yaw tendency.  The greater the power (thrust) of the propeller, the more adverse the yaw; for this reason, power changes made by the pilot are typically slow and smooth.

 

Best Angle Climb (Vx)

The best angle climb speed provides the greatest gain in altitude over a given distance.  This speed would be used, for example, when the pilot must clear obstacles that exist off the end of a runway.  An aircraft manufacturer usually specifies that partial flap settings must be used when flying best angle speed—with the Piper Cherokee, for example, 25° flaps are required. 

Climbs

There are three climb configurations commonly used in flying.  The climb configurations are flown on the basis of air speeds (the pilot’s indication of angle of attack), and these speeds, as well as the associated flap configurations, are specified by the aircraft manufacturer in the Pilot Operating Handbook.   They are summarized as follows:

 

 
 

Best Rate Climb (Vy)

The best rate climb speed provides the greatest gain in altitude over a given period of time—it will get you to the highest altitude the quickest.  This speed is typically flown immediately after rotation during a takeoff where pilots, not faced with obstacle clearance concerns, typically seek to get safely clear of the ground as quickly as possible during a departure.

 

Normal or Cruise Climb

The normal climb speed is usually specified by the manufacturer to combine efficiency in climb performance with effective cooling of the engine (the steeper the angle of attack, the less effective the airflow over the engine).  This speed is usually flown after the airport departure is completed—say 1000’ AAE —and obstacles are no longer a factor.  The total time en route is decreased when a normal or cruise climb is flown.

 

I'm a paragraph. Click here to add your own text and edit me. It's easy.

© 2021 Paceline Pilot Training, Inc.

 

Choose a job you love, and you will never have to work a day in your life—Confucius

 

Home Page Photo Credit: Algkalv (talk) - Own work, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=9925021