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Principles of Flight

Introduction:

  • The principles of flight are the aerodynamics which deal with the motion of air and the forces acting on a body moving relative to that air
  • thrust, drag, lift, and weight are forces that act upon all aircraft in flight
    • Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight
  • In un-accelerated, level flight, the four forces are in equilibrium
    • Equilibrium is defined as lift equaling weight, and thrust equaling drag, but by changing these forces we can affect climbs, descents, and other maneuvers
The Four Forces and Three Axes of Rotation
Figure 1: Instrument Flying Handbook, The Four Forces and Three Axes of Rotation
Lift vs. Relative Wind
Figure 2: Lift vs. Relative Wind
Instrument Flying Handbook, Angle of Attack and Relative Wind
Figure 6: Instrument Flying Handbook, Angle of Attack and Relative Wind

Lift:

  • Lift is the key aerodynamic force on an which brings an aircraft to fly
  • Lift is produced by the dynamic effect of the air acting on the Airfoil, and acts perpendicular to the flight path through the center of lift (CL) and perpendicular to the lateral axis
  • Lift always acts in a direction perpendicular to the Relative Wind and to the lateral axis of the aircraft and opposes the downward force of weight[Figure 2]
  • Produced by the wings, flaps, and slats
  • In order for lift to be effective, it must be a force greater than that of gravity, directed opposite the direction of gravity
    • It is important to note however, 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
  • Lift is concentrated from the Center of Pressure [Figure 2]
    • The term Center of Pressure is synonymous with the Center of Lift
  • 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 states 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 area is constant, you get: V1P1 = V2P2 [Figure 3]
      • The formula shows that as the velocity of 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, its the decreased pressure which causes lift
      Bernoulli's Principle
      Figure 3: Bernoulli's Principle
    • Newton's Third Law
      Figure 5: Newton's Third Law

      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 it is 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)
        • This law may be expressed by F=MA, for example, Speeding up, slowing down, entering climbs or descents, and turning

      • Newton's Third Law:

        "For every action, there is an equal and opposite reaction"
        • 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 5]
  • Lift Equation
    Figure 5: Lift Equation

    Lift as an equation:

    • When examining the equation, lift (L) we must review the relationship of the air density (ρ), the airfoil velocity (V), the surface area of the wing (S) and the coefficient of lift (CL) for a given airfoil

    • 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 the density factor is decreased and the total lift must equal the total weight to remain in flight, it follows that one of the other factors must be increased
      • The factor usually increased is the airspeed or the AOA because these are controlled directly by the pilot

    • 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, this is accomplished by the rotation of the rotor blades
      • For other types of aircraft, such as airplanes, weight shift control, or gliders, air must be moving across the lifting surface
      • This is accomplished by the forward speed of the aircraft
      • 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

    • Wing Area:

      • 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
  • 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, if increasing weight, lift must also be increased. 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 density of the air has one-half the density of air at sea level. In order to maintain its lift at a higher altitude, an aircraft must fly at a greater true airspeed for any given AOA. 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:

    • Pilot's can control lift principally with two factors:
      • Angle of Attack
      • Velocity (speed)

    • Angle of Attack:

      • Any time the control yoke or stick is moved fore or aft, the 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 critical AOA
        • Figure 5-5, shows how the CL increases until the critical AOA is reached, then decreases rapidly with any further increase in the AOA

    • Velocity:

      • 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
      • Taking the equation further, one can see an aircraft could not continue to travel in level flight at a constant altitude and maintain the same AOA if the velocity is increased. The lift would increase and the aircraft would climb as a result of the increased lift force or speed up. Therefore, to keep the aircraft straight and level (not accelerating upward) and in a state of equilibrium, as velocity is increased, lift must be kept constant. 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, of course, a limit to how far the AOA can be increased, if a stall is to be avoided

  • Lift/Drag Ratio:

    • 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 5-5]
    • 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 also dimensionless and is 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, which 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, as well as changes in the AOA, affects the production of lift. Notice in Figure 5-5 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 is operated 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 5-6 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
    Figure 7: Pilot Handbook of Aeronautical Knowledge
    Typical Airfoil Section
    Pilot Handbook of Aeronautical Knowledge, Airfoil Designs
    Figure 8: Pilot Handbook of Aeronautical Knowledge
    Airfoil Designs

  • Airfoil Design:

    • An airfoil is constructed in such a way that its shape 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 the upper surface of an airfoil 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. Many thousands of airfoils have been tested in wind tunnels and in actual flight, but no one airfoil has been found that satisfies 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 type of airfoil 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 for which it is designed. [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 obvious characteristics of design [Figure 7]
      • 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 quite 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 magnitude of the upper and lower 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 the manner in which air flows underneath the airfoil, a positive pressure results, particularly at higher angles of attack
      • There is another aspect to this airflow that must be considered
        • At a point close to the leading edge, the airflow is virtually stopped (stagnation point) and then gradually increases 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, where the airflow was slowed beneath the airfoil, a positive upward pressure was created (i.e., as 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:

      • If the airfoil profile were 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
        • But if the teardrop shaped airfoil were cut in half lengthwise, a form resembling the basic airfoil (wing) section would result
        • 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 the speed of the air across the top of an airfoil produces a drop in pressure. 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 angles of attack (AOA), 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 9]
      • The average of the pressure variation for any given AOA is referred to as the center of pressure (CP). 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 very important, since it affects the position of the air loads imposed on the wing structure in both low and high AOA conditions. An airplane’s aerodynamic balance and controllability are governed by changes in the CP
      Pilot Handbook of Aeronautical Knowledge, Airfoil Designs
      Figure 9: Pilot Handbook of Aeronautical Knowledge, Pressure distribution on an
      airfoil and CP changes with AOA

    • Airfoil Behavior:

      • The production of lift is much more complex than a simple differential pressure between upper and lower airfoil surfaces. In fact, many lifting airfoils do not have an upper surface longer than the bottom, as in the case of symmetrical airfoils. These are seen in high-speed aircraft having symmetrical wings, or on symmetrical rotor blades for many helicopters whose upper and lower surfaces are identical. In both examples, the only difference is the relationship of the airfoil with the oncoming airstream (angle). A paper airplane, which is simply a flat plate, has a bottom and top exactly the same 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 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 being placed outside the car window at a high speed. If the hand is inclined in one direction or another, the hand will move upward or downward. This is caused by deflection, which in turn causes the air to turn about the object within the air stream. As a result of this change, the velocity about the object changes in both magnitude and direction, in turn resulting in a measurable velocity force and direction

    • Angle of Attack (AoA):

    • 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 5]
    • 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 maximum lift occurs (stall)
  • The magnitude of the force of lift is directly proportional to the density of the air, the area of the wings, the airspeed, shape, and AoA
  • Total lift must overcome the total weight of the aircraft, which is comprised of the actual weight and the tail-down force used to control the aircraft's pitch attitude
  • Pilot Handbook of Aeronautical Knowledge, Tip vortex
    Figure 10: Pilot Handbook of Aeronautical Knowledge
    Tip vortex

    Wingtip Vortices:

    • 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
    • In order 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 10]
    • 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
  • To learn more see:

Weight:

  • 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 lift is greater than weight, the vertical speed will increase

Thrust:

  • 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, it is said to act parallel to the longitudinal axis

  • Measuring Thrust:

    • Propeller & rotor driven aircraft are generally rated in horsepower
    • Turbine driven aircraft are generally rated in 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:

    • Engine power is reduced, 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:

    • 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 in three categories:

      • 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 5-3]
        • If thrust decreases and airspeed decreases, lift will become less than weight and the aircraft will start to descend
        • To maintain level flight, the pilot can increase the AOA 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, and

        • 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 is increased, 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 is decreased too fast, the aircraft will descend, and if the AOA is decreased 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 is reduced and airspeed decreases, the AOA must increase in order 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 the airplane is equipped with an AOA indicator, it should be referenced to help monitor the proximity to the critical AOA
  • Some aircraft have the ability to pivot the engines or vector the exhaust, thereby changing the direction of the thrust rather than changing the AOA [Figure 5-4]
Drag Curves
Figure 8: Drag Curves

Drag:

  • 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
  • Their are two basic types of drag (induced and parasite) with total drag being a combination of the two

  • 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:

    • An airfoil (wing or rotor blade) produces the lift force by making use of the energy of the free airstream. Whenever an airfoil is producing lift, the pressure on the lower surface of it is greater than that 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 low pressure area on the upper surface. In the vicinity of the tips, there is a tendency for these pressures to equalize, 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 5-9] As the air (and vortices) roll off the back of your wing, they angle down, which is known as downwash. Figure 5-10 shows the difference in downwash at Figure 5-9. Wingtip vortex from a crop duster. 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 that is 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 5-11, you can see that when you have less downwash, your lift vector is more vertical, opposing gravity. 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 consequent downwash component on the net airflow over the airfoil, 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 therefore 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 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 separation of airflow (burbling) created by the shape of the aircraft
      • 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 5-7]

    • Interference Drag:

      • Generated by the collision of air-streams creating eddy currents, turbulence, or restrictions to smooth flow
      • For example, landing gear meeting the fuselage
      • The most interference drag is created when two surfaces meet at perpendicular angles
      • The drag of each item individually, added to that of the aircraft, are less than that of the two items when allowed to interfere with one another
        • 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 create and 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 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 through which the wing or airfoil is moving, 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 just as it would to the physical surface of an object
        • 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
      • In order 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
        • In addition, a smooth and glossy finish aids in 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 intentionally caused by 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, in steady state, as airspeed decreases to near the stalling speed, the total drag becomes greater, due mainly to the sharp 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

Ground Effect:

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

Stability:

Trim:

  • Trim refers to employing adjustable aerodynamic devices on the aircraft to adjust forces so the pilot does not have to manually hold pressure on the controls
  • This is done either by trim tabs (small movable surfaces on the control surface) or by moving the neutral position of the entire control surface all together
    • Trim tabs are likely to be on the aileron, elevator and rudder
  • Trimming is accomplished by deflecting the tab in the direction opposite to that in which the primary control surface must be held
  • The force of the airflow striking the tab causes the main control surface to be deflected to a position that corrects the unbalanced condition of the aircraft
  • Because the trim tabs use airflow to function, trim is a function of speed. Any change in speed results in the need to re-trim the aircraft
  • An aircraft properly trimmed in pitch seeks to return to the original speed before the change due to its stability
  • Trimming is a constant task as soon as you change any power setting, airspeed, altitude, or configuration
  • Proper trimming decreases pilot workload allowing for attention to be diverted elsewhere, especially important for instrument flying
  • In the pattern, if you have trimmed appropriately, you shouldn't have to use back stick at all, which should also prevent you from exceeding approach speed/on-speed

  • Rotor Blade: spinning "wings" which allow for lift on helicopters or "rotor-craft"
  • Stabilizer: a control surface other than the wings which 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): formed by the chord of the airfoil 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 and 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 which propels the airplane
  • Drag: Retarding force which limits speed

Conclusion:

  • The principles of flight are those basic characteristics which act upon an aircraft
  • Both Bernoulli’s Principle and Newton’s Laws are in operation whenever lift is being generated by an airfoil
  • You can see the four forces of flight are inter-related
    • In order to achieve flight, we must overcome drag, and resist gravity
    • In order 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 lift is generally controlled through AoA and velocity, 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.)

References: