Top

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
  • The basis for these principles are in the four forces acting on an aircraft:
  • 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
Bernoulli's Principle
Figure 3: Bernoulli's Principle
Cambered Airflow
Figure 4: Cambered Airflow
Newton's Third Law
Figure 5: Newton's Third Law
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 Airfoil which brings an aircraft to fly
  • Lift always acts in a direction perpendicular to the Relative Wind and to the lateral axis of the aircraft [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 [Figure 4]
        • 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
    • 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]
  • 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
    Pilot Handbook of Aeronautical Knowledge, Airfoil Designs
    Figure 9: Pilot Handbook of Aeronautical Knowledge
    Pressure distribution on an
    airfoil and CP changes with AOA
    Pilot Handbook of Aeronautical Knowledge, Tip vortex
    Figure 10: Pilot Handbook of Aeronautical Knowledge
    Tip vortex
  • Airfoil Design:

    • An airfoil is a structure designed to optimize Newton's and Bernoulli's principles
    • Air acts in various ways when submitted to different pressures and velocities; but this discussion is confined to the parts of an aircraft that a pilot is most concerned with in flight—namely, the airfoils designed to produce lift
    • 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
    • NOTE: The two extremities of the airfoil profile also differ in appearance. The rounded end, which faces forward in flight, is called the leading edge; the other end, the trailing edge, is quite narrow and tapered
    • A reference line often used in discussing the airfoil is the chord line, a straight line drawn through the profile connecting the extremities of the leading and trailing edges. 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
    • An airfoil is constructed in such a way that its shape takes advantage of the air’s response to certain physical laws. This develops two actions from the air mass: a positive pressure lifting action from the air mass below the wing, and a negative pressure lifting action from lowered pressure above the wing
    • As the air stream strikes the relatively flat lower surface of a wing or rotor blade when inclined at a small angle to its direction of motion, the air is forced to rebound downward, causing an upward reaction in positive lift. At the same time, the air stream striking the upper curved section of the leading edge is deflected upward. An airfoil is shaped to cause an action on the air, and forces air downward, which provides an equal reaction from the air, forcing the airfoil upward. If a wing is constructed in such form that it causes a lift force greater than the weight of the aircraft, the aircraft will fly
    • 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
    • Low Pressure Above:

      • In a wind tunnel or in flight, an airfoil is simply a streamlined object inserted into a moving stream of air. 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
    • 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. However, 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. Both Bernoulli’s Principle and Newton’s Laws are in operation whenever lift is being generated by an 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
    • Airfoil Behavior:

      • Although specific examples can be cited in which each of the principles predict and contribute to the formation of lift lift is a complex subject. 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 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
  • You can control lift in 2 ways:
    • Increasing AoA
    • Increasing Speed
  • 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
  • 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
    • 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:
The Airfoil
Figure 7: Instrument Flying Handbook, The Airfoil

Weight:

  • Weight is simply the force of gravity on the aircraft which acts vertically through the center of gravity
  • 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
  • It opposes lift and acts vertically downward through the aircraft’s center of gravity (CG)

Thrust:

  • Forward acting force that opposes drag and propels the airplane
  • Measured in pounds of thrust and/or horsepower
  • 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
  • Excess thrust makes an airplane climb
  • Provided by a propeller in a small aircraft
  • Thrust must overcome total drag in order to provide forward speed with which to produce lift
  • Increasing the power allows thrust to exceed drag, causing the airplane to accelerate
  • Reducing the power allows drag to exceed thrust, causing the airplane to slow
  • 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
Drag Curves
Figure 8: Drag Curves

Drag:

  • Drag is the rearward, retarding force caused by disruption of airflow, opposing thrust
  • 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
  • Classified as either parasite or induced
  • Total drag is the combination of the two
  • Induced Drag:

    • Induced drag is drag due to lift
    • Causes wingtip vortices
    • Decreases with airspeed
    • Induced drag = 1/V
  • Parasite Drag:

    • Parasite drag is drag due to the surface of the aircraft disturbing smooth airflow
    • Is not associated with lift in any way
    • Increases with airspeed
    • 3 Types of Parasite Drag:

      • Profile/Form Drag:
        • Created because of the shape of a component or the aircraft
        • Turbulent wake caused by separation of airflow (burbling) created by the shape of the aircraft
        • Newer aircraft are generally made with consideration to this by fairings along the fuselage so that turbulence and form drag is reduced
      • 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
        • That is, 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
        • Learn more about the effects of interference drag here
      • Skin Friction Drag:
        • The force of the boundary layer meeting the free stream air retards motion due to the viscosity of the air
        • Because skin friction drag is related to a large surface area its affect on smaller aircraft is small versus large transport aircraft where skin friction drag may be considerable
    • Parasite = V^2 (Same as Lift produced)
    • Drag can be intentionally caused by speed brakes, spoilers, or dive brakes

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
  • You can see the four forces of flight are inter-related
    • In order to achieve flight, we must overcome drag, and resist gravity
  • A balanced aircraft is a happy aircraft (fuel burn, efficiency, etc.)

References: