Principles of Flight

The Four Forces and Three Axes of Rotation


  • The principles of flight are the aerodynamics dealing with the motion of air and forces acting on an aircraft
  • Lift is the most apparent force, as it's what we think of as giving an aircraft the ability to fly
  • Thrust provides a method with which to move the aircraft
  • Drag and weight are those forces that act upon all aircraft in flight
    • Understanding how these forces work together and knowing how to control them with the use of power and flight controls are essential to flight


  • Lift vs. Relative Wind
    Lift vs. Relative Wind
  • Instrument Flying Handbook, Angle of Attack and Relative Wind
    Instrument Flying Handbook, Angle of Attack and Relative Wind
  • Lift is the critical aerodynamic force that brings an aircraft to fly
  • The dynamic effect of the air moving across an airfoil produces lift
    • Common airfoils include not just the wings but the flaps/slats and stabilizers too
  • Most see a lift vector as acting "up," but instead, it acts perpendicular to the relative wind and the lateral axis of the aircraft [Figure 1]
    • This means "up" is relative to the aircraft, and being in a turn or even upside down changes the direction the lift vector points (a key principle in understanding turn performance and aerobatics)
  • Lift concentrates from the Center of Lift/Pressure [Figure 2]
    • Note that the center of lift is almost always behind the center of gravity, resulting in a nose-down attitude if not countered by the tail
  • Lift vs. Relative Wind
    Lift vs. Relative Wind
  • In straight and level flight, to be effective, the total lift must overcome the total weight of the aircraft, comprised of the actual weight and the tail-down force used to control the aircraft's pitch attitude
  • It is important to note that lift has no reference to Earth
    • This means that when performing a loop, for example, the lift vector is still perpendicular to the relative wind, which would have the lift vector pointing toward the ground as the aircraft becomes inverted
  • Instrument Flying Handbook, Angle of Attack and Relative Wind
    Instrument Flying Handbook, Angle of Attack and Relative Wind
  • Creation of lift can be understood by observing Bernoulli's principle as well as Newton's Laws of Motion:
    • Bernoulli's Principle:

      • Bernoulli's Principle
        Bernoulli's Principle
      • Bernoulli's principle demonstrates that as the velocity of a moving fluid (liquid or gas) increases, the pressure within the fluid decreases
      • A Venturi [Figure 3] demonstrates Bernoulli's principle: A1V1P1 = A2V2P2
        • A = Area, V = Velocity, and P = Pressure
      • Assuming the area is constant, you get: V1P1 = V2P2
      • The formula shows that as the velocity of the fluid (air) increases, its pressure must decrease
      • Relating this principle to an airfoil, we see a similar shape
        • The rounded upper surface increases the velocity of the air, which causes pressure to decrease
        • As pressure above the wing decreases, the relative pressure below it is higher, creating a pressure differential, which we know as lift
      • Note: with regards to rotary-wing aircraft, lift and thrust are both in the vertical direction
      • Note: We say lift is created by air moving faster over the top of the wing, but more specifically, it's the decreased pressure which causes lift
      • Bernoulli's Principle
        Bernoulli's Principle
    • Newton's Laws of Motion:

      • Newton's first law:

        "Every object persists in its state of rest or uniform motion in a straight line unless compelled to change that state by forces impressed on it"
        • A body at rest tends to remain at rest, and a body in motion tends to remain moving at the same speed and in the same direction
          • This means that nothing starts or stops moving until some outside force causes it to do so
        • An aircraft at rest on the ramp remains at rest unless a force strong enough to overcome its inertia is applied
        • Once it is moving, its inertia keeps it moving, subject to the various other forces acting on it
        • These forces may add to its motion, slow it down, or change its direction
      • Newton's second law:

        "Force is equal to the change in momentum per change in time. For a constant mass, force equals mass times acceleration"
        • When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force
          • This takes into account the factors involved in overcoming Newton's First Law
        • It covers both changes in direction and speed, including starting up from rest (positive acceleration) and coming to a stop (negative acceleration or deceleration)
        • The equation F(force)=M(mass)A(acceleration) may express this law where the force is equal to the mass times the acceleration
      • Newton's Third Law:

        "For every action, there is an equal and opposite reaction"
        • Newton's Third Law
          Newton's Third Law
        • In an airplane, the propeller moves and pushes back the air; consequently, the air pushes the propeller (and thus the airplane) in the opposite direction (forward)
        • This principle applies whenever two things act upon each other [Figure 4]
        • Newton's Third Law
          Newton's Third Law
  • Lift as an Equation:

    • Lift Equation
      Lift Equation
    • The magnitude of the force of lift (L) is directly proportional to the Coefficient of Lift (CL), the density of the air (ρ), the area of the wings (S), and the velocity (airspeed) (V) [Figure 5]
    • Coefficient of Lift:

      • The lift coefficient is a number that aerodynamicists use to model all of the complex dependencies of shape, inclination, and some flow conditions on lift
    • Air Density:

      • If air density decreases and the total lift must equal the total weight to remain in flight, it follows that another factor needs to increase
      • The factor usually increased is the airspeed or the Angle of Attack (AOA) because these are controlled directly by the pilot
    • Wing Area:

      • The lift varies directly with the wing area, provided there is no change in the wing's planform
      • If the wings have the same proportion and airfoil sections, a wing with a planform area of 200 square feet lifts twice as much at the same AOA as a wing with an area of 100 square feet
    • Velocity:

      • The shape of the wing or rotor cannot be effective unless it continually keeps "attacking" new air
      • If an aircraft is to keep flying, the lift-producing airfoil must keep moving
      • In a helicopter or gyroplane, the rotation of the rotor blades creates the necessary lift
      • For other types of aircraft, such as airplanes, weight shift control, or gliders, air must be moving across the lifting surface by way of forward speed
      • The forward speed of the aircraft accomplishes this
      • Lift is proportional to the square of the aircraft's velocity, meaning that an airplane traveling at 200 knots has four times the lift as the same airplane traveling at 100 knots if the AOA and other factors remain constant
  • All other factors being constant, for every AOA, there is a corresponding airspeed required to maintain altitude in steady, unaccelerated flight (true only if maintaining level flight). Since an airfoil always stalls at the same AOA, the lift must increase if weight increases. The only method of increasing lift is by increasing velocity if the AOA is held constant just short of the "critical," or stalling, AOA (assuming no flaps or other high lift devices). Lift and drag also vary directly with the density of the air. Density is affected by several factors: pressure, temperature, and humidity. At an altitude of 18,000 feet, the air density is one-half the air density at sea level. An aircraft must fly at a greater true airspeed for any given AoA to maintain its lift at a higher air density altitude. Warm air is less dense than cool air, and moist air is less dense than dry air. Thus, on a hot, humid day, an aircraft must be flown at a greater true airspeed for any given AOA than on a cool, dry day
  • Controlling Lift:

    • Pilots can control lift principally with two factors:
      • Angle of Attack
      • Velocity/airspeed
    • Angle of Attack:

      • Airplane Flying Handbook,Coefficients of lift and drag at various angles of attack
        Airplane Flying Handbook,
        Coefficients of lift and drag at various angles of attack
      • AOA is fundamental to understanding many aspects of airplane performance, stability, and control
      • AoA is the acute angle measured between the relative wind or flight path and the chord of the airfoil [Figure 2]
      • Any time the control yoke or stick is moved fore or aft, the Angle of Attack, or AOA, is changed
        • As the AOA increases, lift increases (all other factors being equal)
        • When the aircraft reaches the maximum AOA, lift begins to diminish rapidly
          • This is the stalling AOA, known as CL-MAX (maximum CL)critical AOA
        • The CL increases until reaching the critical AOA, then decreases rapidly with any further increase in the AOA [Figure 6]
      • Lift created (or reduced in the case of negative AoA) is measured with the coefficient of lift, which relates to the AoA
      • Every airplane has an angle of attack where the maximum lift occurs (stall)
    • Velocity/Airspeed:

      • For instance, in straight-and-level flight, cruising along at a constant altitude, altitude is maintained by adjusting lift to match the aircraft's velocity or cruise airspeed while maintaining a state of equilibrium in which lift equals weight
      • In an approach to landing, when the pilot wishes to land as slowly as practical, it is necessary to increase AOA near maximum to maintain lift equal to the weight of the aircraft
      • An aircraft could not continue to travel in level flight at a constant altitude and maintain the same AOA if the velocity increases. The lift would increase, and the aircraft would climb due to the increased lift force or speed up. Therefore, to keep the aircraft straight and level (not accelerating upward) and in equilibrium, lift must be kept constant as velocity increases. This is normally accomplished by reducing the AOA by lowering the nose. Conversely, as the aircraft is slowed, the decreasing velocity requires increasing the AOA to maintain lift sufficient to maintain flight. There is a limit to how far the AOA can be increased if a stall is to be avoided
  • Lift/Drag Ratio:

    • Drag vs. Speed
      Airplane Flying Handbook,
      Drag vs. Speed
    • The lift-to-drag ratio (L/D) is the amount of lift generated by a wing or airfoil compared to its drag
    • A L/D ratio is an indication of airfoil efficiency
    • Aircraft with higher L/D ratios are more efficient than those with lower L/D ratios
    • In unaccelerated flight with the lift and drag data steady, the proportions of the coefficient of lift (CL) and coefficient of drag (CD) can be calculated for specific AOA [Figure 7]
    • The coefficient of lift is dimensionless and relates the lift generated by a lifting body, the dynamic pressure of the fluid flow around the body, and a reference area associated with the body
    • The coefficient of drag is dimensionless, used to quantify the drag of an object in a fluid environment, such as air, and is always associated with a particular surface area
    • The L/D ratio is determined by dividing the CL by the CD
      • This is the same as dividing the lift equation by the drag equation, as all of the variables, aside from the coefficients, cancel out
    • The lift and drag equations are as follows (L = Lift in pounds; D = Drag; CL = coefficient of lift; ρ = density (expressed in slugs per cubic feet); V = velocity (in feet per second); q = dynamic pressure per square foot (q = 1⁄2 ρv2); S = the area of the lifting body (in square feet); and CD = Ratio of drag pressure to dynamic pressure)
    • Typically, at low AOA, the coefficient of drag is low, and small changes in AOA create only slight changes in the coefficient of drag
      • At high AOA, small changes in the AOA cause significant changes in drag
      • The shape of an airfoil and changes in the AOA affect the production of lift
      • Notice in [Figure 6] that the coefficient of lift curve (red) reaches its maximum for this particular wing section at 20° AOA and then rapidly decreases
      • 20° AOA is, therefore, the critical angle of attack
      • The coefficient of drag curve (orange) increases very rapidly from 14° AOA and completely overcomes the lift curve at 21° AOA
      • The lift/drag ratio (green) reaches its maximum at 6° AOA, meaning that at this angle, the most lift is obtained for the least amount of drag
      • Note that the maximum lift/drag ratio (L/DMAX) occurs at one specific CL and AOA
      • If the aircraft operates in steady flight at L/DMAX, the total drag is at a minimum
      • Any AOA lower or higher than that for L/DMAX reduces the L/D and consequently increases the total drag for a given aircraft's lift
      • [Figure 7] depicts the L/DMAX by the lowest portion of the blue line labeled "total drag
      • The configuration of an aircraft has a great effect on the L/D
  • Pilot Handbook of Aeronautical Knowledge, Typical Airfoil Section
    Pilot Handbook of Aeronautical Knowledge,
    Typical Airfoil Section
  • Pilot Handbook of Aeronautical Knowledge, Airfoil Designs
    Pilot Handbook of Aeronautical Knowledge,
    Airfoil Designs
  • Airfoil Design:

    • Airfoil construction takes advantage of the air's response to Newton's and Bernoulli's principles
    • Air acts in various ways when submitted to different pressures and velocities: a positive pressure lifting action from the air mass below the wing and a negative pressure lifting action from lowered pressure above the wing
    • If all the lift required were obtained merely from the deflection of air by the lower surface of the wing, an aircraft would only need a flat wing like a kite
      • However, the balance of the lift needed to support the aircraft comes from the flow of air above the wing
      • Herein lies the key to flight
      • It is neither accurate nor useful to assign specific values to the percentage of lift generated by an airfoil's upper surface versus that generated by the lower surface
      • These are not constant values. They vary, not only with flight conditions but also with different wing designs
    • Different airfoils have different flight characteristics
      • No one airfoil can satisfy every flight requirement
      • The weight, speed, and purpose of each aircraft dictate the shape of its airfoil
      • The most efficient airfoil for producing the greatest lift is one that has a concave or "scooped out" lower surface
      • As a fixed design, this airfoil type sacrifices too much speed while producing lift and is not suitable for high-speed flight
      • Advancements in engineering have made it possible for today's high-speed jets to take advantage of the concave airfoil's high lift characteristics
      • Leading-edge (Kreuger) flaps and trailing-edge (Fowler) flaps, when extended from the basic wing structure, literally change the airfoil shape into the classic concave form, thereby generating much greater lift during slow flight conditions
    • On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance sometimes does not have enough lifting power to take the airplane off the ground. Thus, modern airplanes have airfoils that strike a medium between extremes in design. The shape varies according to the needs of the airplane
      • [Figure 8] shows some of the more common airfoil designs
    • Airfoil Construction:

      • By looking at the cross-section of a wing, one can see several prominent characteristics of design [Figure 8/9]
      • Notice that there is a difference in the curvatures (called cambers) of the upper and lower surfaces of the airfoil
        • The camber of the upper surface is more pronounced than that of the lower surface, which is usually somewhat flat
        • The two extremities of the airfoil profile also differ in appearance as the rounded end, which faces forward in flight, is called the leading edge; the other end, the trailing edge, is relatively narrow and tapered
      • Chord Line:

        • A straight line connecting the extremities of the leading and trailing edges denotes the Chord Line
        • The Chord line is a reference line often used in discussing the airfoil
        • The distance from this chord line to the upper and lower surfaces of the wing denotes the upper and lower magnitude of camber at any point
        • Another reference line, drawn from the leading edge to the trailing edge, is the mean camber line
        • This mean line is equidistant at all points from the upper and lower surfaces
    • High Pressure Below:

      • A certain amount of lift is generated by pressure conditions underneath the airfoil
        • Because of air flows underneath the airfoil, a positive pressure results, particularly at higher angles of attack
      • There is another aspect of this airflow to consider:
        • At a point close to the leading edge, the airflow nearly stops (stagnation point) and then gradually increases in speed
        • At some point near the trailing edge, it again reaches a velocity equal to that on the upper surface
        • In conformance with Bernoulli's principle, positive upward pressure is created where the airflow slowed beneath the airflow (i.e., the fluid speed decreases, the pressure must increase)
        • Since the pressure differential between the upper and lower surface of the airfoil increases, total lift increases
    • Low Pressure Above:

      • With an airfoil in the shape of a teardrop, the speed and the pressure changes of the air passing over the top and bottom would be the same on both sides
        • If the airfoil were then inclined so the airflow strikes it at an angle, the air moving over the upper surface would be forced to move faster than the air moving along the bottom of the airfoil
        • This increased velocity reduces the pressure above the airfoil
      • Applying Bernoulli's Principle of Pressure, the increase in airspeed across the top of an airfoil produces a pressure drop. This lowered pressure is a component of total lift. The pressure difference between the upper and lower surface of a wing alone does not account for the total lift force produced
      • The downward backward flow from the top surface of an airfoil creates a downwash
        • This downwash meets the flow from the bottom of the airfoil at the trailing edge
        • Applying Newton's third law, the reaction of this downward backward flow results in an upward forward force on the airfoil
    • Pressure Distribution:

      • As air flows along the surface of a wing at different AOAs, there are regions along the surface where the pressure is negative or less than atmospheric and regions where the pressure is positive or greater than atmospheric
      • This negative pressure on the upper surface creates a relatively larger force on the wing than is caused by the positive pressure resulting from the air striking the lower wing surface [Figure 10]
      • The average pressure variation for any given AOA is called the center of pressure (CP). The aerodynamic force acts through this CP. At high angles of attack, the CP moves forward, while at low angles of attack, the CP moves aft. In the design of wing structures, this CP travel is essential since it affects the position of the air loads imposed on the wing structure in low and high AOA conditions. Changes in the CP govern an airplane's aerodynamic balance and controllability
      • Pilot Handbook of Aeronautical Knowledge, Airfoil Designs
        Pilot Handbook of Aeronautical Knowledge, Pressure distribution on an
        airfoil and CP changes with AOA
    • Airfoil Behavior:

      • The lift production is much more complex than a simple differential pressure between upper and lower airfoil surfaces. Many lifting airfoils do not have an upper surface longer than the bottom, as in symmetrical airfoils. These are seen in high-speed aircraft with symmetrical wings or symmetrical rotor blades for many helicopters whose upper and lower surfaces are identical. In both examples, the only difference is the airfoil relationship with the oncoming airstream (angle). A paper airplane, which is simply a flat plate, has a bottom and top shape and length. Yet, these airfoils do produce lift, and "flow turning" is partly (or fully) responsible for creating lift
      • As an airfoil moves through the air, the airfoil is inclined against the airflow, producing a different flow caused by the airfoil's relationship to the oncoming air. Think of a hand placed outside the car window at high speed. If the hand inclines in one direction or another, the hand will move upward or downward. Deflection causes the air to turn about the object within the air stream. The velocity about the object changes in both magnitude and direction, in turn resulting in a measurable velocity force and direction
  • Wingtip Vortices & Lift:

    • Pilot Handbook of Aeronautical Knowledge, Tip Vortex
      Pilot Handbook of Aeronautical Knowledge
      Tip Vortex
    • While the biggest consideration for producing lift involves the air flowing over and under the wing, there is a third dimension to consider
    • Consider the tip of the airfoil also has an aerodynamic effect
    • To equalize pressure, the high-pressure area on the bottom of an airfoil pushes around the tip to the low-pressure area on the top [Figure 11]
    • This action creates a rotating flow called a tip vortex or wingtip vortices
    • This downwash extends back to the trailing edge of the airfoil, reducing lift for the affected portion of the airfoil
    • Manufacturers have developed different methods to counteract this action
    • Winglets can be added to the tip of an airfoil to reduce this flow (essentially decrease induced drag)
    • The winglets act as a dam preventing the vortex from forming
    • Winglets can be on the top or bottom of the airfoil
    • Another method of countering the flow is to taper the airfoil tip, reducing the pressure differential and smoothing the airflow around the tip
    • Pilot Handbook of Aeronautical Knowledge, Tip Vortex
      Pilot Handbook of Aeronautical Knowledge
      Tip Vortex
  • To learn more see:


  • Weight is simply the force of gravity on the aircraft, which acts vertically through the center of gravity
  • It is the combined load of the aircraft itself, the crew, the fuel, and the cargo or baggage
  • Weight varies based on load, passengers, and fuel
    • A Load is essentially the back pressure on the control stick required, the G-loading, which an aircraft experiences
    • Passengers and fuel are more obvious
  • Opposing lift, as an aircraft is descending
  • Weight vs. Lift:
    • Weight has a definite relationship to lift
    • This relationship is simple but important in understanding the aerodynamics of flying
    • Lift is the upward force on the wing acting perpendicular to the relative wind and perpendicular to the aircraft's lateral axis
    • Lift is required to counteract the aircraft's weight
    • In stabilized level flight, when the lift force is equal to the weight force, the aircraft is in a state of equilibrium and neither accelerates upward, or downward
    • If lift becomes less than weight, the vertical speed will decrease
    • When the lift is greater than the weight, the vertical speed will increase


  • Thrust is the forward-acting force that opposes drag and propels the airplane forward
    • It is through excesses or deficits of thrust that accelerations and decelerations can occur
    • The aircraft will continue to speed up/slow down until thrust again equals drag, at which point the airspeed will stabilize
  • In powered aircraft, thrust is achieved through the powerplant, be it a propeller, rotor, or turbine
  • With a glider, thrust is created through the conversion of potential energy (altitude) to kinetic energy (airspeed) by pitching toward the ground
  • Newton's second law: When a body is acted upon by a constant force, its resulting acceleration is inversely proportional to the mass of the body and is directly proportional to the applied force
    • This law may be expressed by F = MA (Force equals Mass times Acceleration), for example, speeding up, slowing down, entering climbs or descents, and turning
  • Acts parallel to the center of thrust to overcome drag, F = MA
  • Note: with regards to rotary-wing aircraft, lift and thrust are both in the vertical direction
  • As a general rule, thrust acts parallel to the longitudinal axis
  • Measuring Thrust:

    • Propeller & rotor driven aircraft are generally rated in horsepower
    • Turbine-driven aircraft are generally rated in pounds
  • Thrust During Acceleration:

    • Increasing engine power increases thrust (now exceeding drag), thereby accelerating the aircraft
    • As long as the thrust continues to be greater than the drag, the aircraft continues to accelerate
    • When drag equals thrust, the aircraft flies at a constant airspeed
  • Thrust during Deceleration:

    • When reducing engine power, lessoning thrust, thereby decelerating the aircraft
    • As long as the thrust is less than the drag, the aircraft continues to decelerate
    • To a point, as the aircraft slows down, the drag force will also decrease
    • The aircraft will continue to slow down until thrust again equals drag, at which point the airspeed will stabilize
  • Straight-and-level flight:

    • Pilot Handbook of Aeronautical Knowledge, ngle of Attack at Various Speeds
      Pilot Handbook of Aeronautical Knowledge
      Angle of Attack at Various Speeds
    • The pilot coordinates AOA and thrust in all speed regimes if the aircraft is to be held in level flight
    • Remember, (for a given airfoil shape) lift varies with the AOA and airspeed
    • Therefore, a large AOA at low airspeeds produces an equal amount of lift at high airspeeds with a low AOA
    • The speed regimes of flight can be grouped into three categories:
    • Pilot Handbook of Aeronautical Knowledge, ngle of Attack at Various Speeds
      Pilot Handbook of Aeronautical Knowledge
      Angle of Attack at Various Speeds
      • Low-Speed Flight:

        • When the airspeed is low, the AOA must be relatively high if the balance between lift and weight is to be maintained [Figure 12]
        • If thrust decreases and airspeed decreases, the lift will become less than weight, and the aircraft will start to descend
        • To maintain level flight, the pilot can increase the AOA by an amount that generates a lift force again equal to the weight of the aircraft
        • While the aircraft will be flying more slowly, it will still maintain level flight
      • Cruising Flight

        • Straight-and-level flight in the slow-speed regime provides some interesting conditions relative to the equilibrium of forces
        • With the aircraft in a nose-high attitude, there is a vertical component of thrust that helps support it
        • For one thing, wing loading tends to be less than would be expected
        • In level flight, when thrust increases, the aircraft speeds up, and the lift increases
        • The aircraft will start to climb unless the AOA is decreased just enough to maintain the relationship between lift and weight
        • The timing of this decrease in AOA needs to be coordinated with the increase in thrust and airspeed. Otherwise, if the AOA decreases too fast, the aircraft will descend, and if the AOA decreases too slowly, the aircraft will climb
      • High-Speed Flight

        • As the airspeed varies due to thrust, the AOA must also vary to maintain level flight
        • At very high speeds and level flight, it is even possible to have a slightly negative AOA
        • As thrust reduces and airspeed decreases, the AOA must increase to maintain altitude
        • If speed decreases enough, the required AOA will increase to the critical AOA
        • Any further increase in the AOA will result in the wing stalling
        • Therefore, extra vigilance is required at reduced thrust settings and low speeds so as not to exceed the critical angle of attack
        • If equipped with an AOA indicator, it should be referenced to help monitor the proximity to the critical AOA
  • Aircraft like the V-22 and F-35, pivot the engines or vector the exhaust to change the direction of the thrust rather than changing the AOA


  • Drag Curves
    Drag Curves
  • Drag is the rearward, resisting force caused by disruption of airflow
  • Drag is the net aerodynamic force parallel to the relative wind
  • Drag is always a by-product of lift and thrust
  • Always a by-product of lift
  • There are two basic types of drag (induced and parasite), with total drag being a combination of the two [Figure 13]
  • Induced Drag:

    • In level flight, the aerodynamic properties of a wing or rotor produce a required lift, but this can be obtained only at the expense of a certain penalty
    • That penalty, induced drag, is inherent whenever an airfoil is producing lift
    • Causes wingtip vortices
    • Decreases with airspeed
    • Induced drag = 1/V
    • as AOA increases, induced drag increases proportionally
      • To state this another way, the lower the airspeed, the greater the AOA required to produce lift equal to the aircraft's weight and, therefore, the greater induced drag. The amount of induced drag varies inversely with the square of the airspeed
    • Wingtip Vortices & Drag:

      • Pilot Handbook of Aeronautical Knowledge, Wingtip Vortex From Crop Duster
        Pilot Handbook of Aeronautical Knowledge,
        Wingtip Vortex From Crop Duster
      • Pilot Handbook of Aeronautical Knowledge, The difference in wingtip vortex size at altitude versus near the ground
        Pilot Handbook of Aeronautical Knowledge,
        The difference in wingtip vortex size
        at altitude versus near the ground
      • Pilot Handbook of Aeronautical Knowledge, The difference in downwash at altitude versus near the ground
        Pilot Handbook of Aeronautical Knowledge,
        The difference in downwash at altitude
        versus near the ground
      • An airfoil (wing or rotor blade) produces the lift force by using the energy of the free airstream. Whenever an airfoil produces lift, the pressure on the lower surface is greater than on the upper surface (Bernoulli's Principle). As a result, the air tends to flow from the high-pressure area below the tip upward to the upper surface's low-pressure area. There is a tendency for these pressures to equalize in the vicinity of the tips, resulting in a lateral flow outward from the underside to the upper surface. This lateral flow imparts a rotational velocity to the air at the tips, creating vortices that trail behind the airfoil
      • When the aircraft is viewed from the tail, these vortices circulate counterclockwise about the right tip and clockwise about the left tip. [Figure 14] As the air (and vortices) roll off the back of your wing, they angle down, known as downwash. [Figure 15] shows the difference in downwash at altitude versus near the ground. Bearing in mind the direction of rotation of these vortices, it can be seen that they induce an upward flow of air beyond the tip and a downwash flow behind the wing's trailing edge. This induced downwash has nothing in common with the downwash necessary to produce lift. It is, in fact, the source of induced drag. Downwash points the relative wind downward, so the more downwash you have, the more your relative wind points downward. That's important for one very good reason: lift is always perpendicular to the relative wind. In [Figure 16], you can see that your lift vector is more vertical, opposing gravity when you have less downwash. And when you have more downwash, your lift vector points back more, causing induced drag. On top of that, it takes energy for your wings to create downwash and vortices, and that energy creates drag
      • The greater the size and strength of the vortices and the consequent downwash component on the airfoil's net airflow, the greater the induced drag effect becomes. This downwash over the top of the airfoil at the tip has the same effect as bending the lift vector rearward; therefore, the lift is slightly aft of perpendicular to the relative wind, creating a rearward lift component. This is induced drag
  • Parasite Drag:

    • Parasite drag is comprised of all the forces that work to slow an aircraft's movement
      • As the term parasite implies, it is the drag that is not associated with the production of lift
    • Parasite drag includes the displacement of the air by the aircraft, turbulence generated in the airstream, or a hindrance of air moving over the surface of the aircraft and airfoil
    • There are three types of parasite drag: form drag, interference drag, and skin friction
    • Profile/Form Drag:

      • Form Drag
        Form Drag
      • Interference Drag
        Interference Drag
      • Form drag is the portion of parasite drag generated by the aircraft and components (antennas, wheels, etc.) due to its shape and airflow around it
      • Turbulent wake caused by the separation of airflow (burbling) created by the shape of the aircraft [Figure 17]
      • When the air has to separate to move around a moving aircraft and its components, it eventually rejoins after passing the body
      • Newer aircraft are generally made with consideration to this by fairings along the fuselage so that turbulence and form drag is reduced [Figure 18]
    • Interference Drag:

      • Generated by the collision of air-streams creating eddy currents, turbulence, or restrictions to the smooth flow
      • For example, landing gear meeting the fuselage
      • The most interference drag is created when two surfaces meet at perpendicular angles
      • If a jet fighter carries two identical wing tanks, the overall drag is greater than the sum of the individual tanks because both of these generate interference drag
      • Fairings and distance between lifting surfaces and external components (such as radar antennas hung from wings) reduce interference drag. [Figure 5-8]
      • Learn more about the effects of interference drag here
    • Skin Friction Drag:

      • Skin friction drag is the aerodynamic resistance due to the contact of moving air with the surface of an aircraft
        • Every surface, no matter how apparently smooth, has a rough, ragged surface when viewed under a microscope
        • The air molecules, which come in direct contact with the surface of the wing, are virtually motionless
        • Each layer of molecules above the surface moves slightly faster until the molecules are moving at the velocity of the air mass moving around the aircraft
        • This speed is called the free-stream velocity
        • The area between the wing and the free-stream velocity level is about as wide as a playing card and is called the boundary layer
        • At the top of the boundary layer, the molecules increase velocity and move at the same speed as the molecules outside the boundary layer
        • The actual speed at which the molecules move depends upon the shape of the wing, the viscosity (stickiness) of the air, and its compressibility (how much it can be compacted)
      • The airflow outside of the boundary layer reacts to the shape of the edge of the boundary layer
        • The boundary layer gives any object an "effective" shape that is usually slightly different from the physical shape
        • The boundary layer may also separate from the body, thus creating an effective shape much different from the physical shape of the object
        • This change in the physical shape of the boundary layer causes a dramatic decrease in lift and an increase in drag
        • When this happens, the airfoil has stalled
      • To reduce the effect of skin friction drag, aircraft designers utilize flush mount rivets and remove any irregularities that may protrude above the wing surface
        • Also, a smooth and glossy finish aids in the transition of air across the surface of the wing
        • Since dirt on an aircraft disrupts the free flow of air and increases drag, keep the surfaces of an aircraft clean and waxed
    • Drag can be caused intentionally through the use of speed brakes, spoilers, or dive brakes
    • Additionally, normal procedures such as lowering flaps can increase drag
    • Parasite drag increases as the square of the airspeed (V^2)
      • Thus, as airspeed decreases to near the stalling speed in a steady state, the total drag becomes greater due mainly to the exponential rise in induced drag. Similarly, as the aircraft reaches its never-exceed speed (VNE), the total drag increases rapidly due to the sharp increase of parasite drag

Interaction Between the Four Forces of Flight:

  • The four forces of flight do not exist in isolation as each influences the other
  • Lift opposing weight, and drag opposing thrust is only true in level flight
  • For example, a pitch-up requires thrust to not only overcome drag but also weight
  • Ground Effect:

    • Reduction of induced drag during takeoffs and landings
    • Caused by a reduction of wingtip vortices
    • It occurs at about a wingspan above the ground
    • Up-wash and down-wash decrease
    • Down-wash can hit the ground and push the wing from below, forming what feels like a cushion
    • Causes floating if fast on approach
    • More noticeable in a low-wing aircraft
    • Ground Effect:
      • Increases lift while decreasing drag (induced)
      • The opposite is true when leaving ground effect

Critical Definitions:

  • Rotor Blade: spinning "wings" which allow for lift on helicopters or "rotor-craft"
  • Stabilizer: a control surface other than the wings that provide stabilizing qualities
  • Chord: Chord line longitudinal length (length as viewed from the side)
  • Chord Line: The chord line is the straight line intersecting the leading and trailing edges of the airfoil
  • Angle of Incidence (AoI): the angle formed by the airfoil chord and the longitudinal axis of the aircraft, which is designed into the aircraft and cannot be changed by the pilot
  • Attitude: relationship of the aircraft's nose with the horizon
  • Flight Path: The course or track along which the aircraft is flying or is intended to be flown
  • Lift: A component of the total aerodynamic force on an airfoil that acts perpendicular to the relative wind
  • Center of Gravity: The average weight across an aircraft through which gravity is considered to act
  • Weight: Opposes lift via gravity
  • Thrust: Forward force that propels the airplane
  • Drag: Retarding force which limits speed

Principles of Flight Airman Certification Standards:


  • The principles of flight are those basic characteristics that act upon an aircraft
    • Although simplified as thrust, lift, weight, and drag, we know that there are more upward forces than lift, and there are more downward forces than just weight
  • Although the pilot can only have limited control of some of these factors, principally, lift is affected by wing design, angle of attack, velocity, weight and loading, air temperature, and humidity
  • Both Bernoulli's Principle and Newton's Laws are in operation whenever an airfoil generates lift
  • You can see the four forces of flight are inter-related
    • To achieve flight, we must overcome drag and resist gravity
    • To maintain a constant airspeed, thrust, and drag must remain equal, just as lift and weight must be equal to maintain a constant altitude
  • Although AoA and velocity allow a pilot to manipulate lift, other factors are slightly under pilot control, such as air density (as a pilot could change altitude)
  • A balanced aircraft is a happy aircraft (fuel burn, efficiency, etc.)
  • Several books are available in digital and hard copy to help you learn more [Amazon]
  • In un-accelerated, level flight, the four forces are in equilibrium
    • Equilibrium is defined as lift equaling down-force (weight+tail downforce [which makes up ~5% of aircraft weight]) and thrust equaling drag, but by changing these forces, we can affect climbs, descents, and other maneuvers
  • To learn more, consider AOPA's Essential Aerodynamics online course
  • Check out the FAA's Angle of Attack fact sheet for more information about AoA
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