Principles of Flight


  • 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 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]
  • 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: occur based on wing design when the pressure differences attempt to equalize over the wingtips
    • Winglets are designed to protect against this
  • To learn more see:
The Airfoil
Figure 7: Instrument Flying Handbook, The Airfoil


  • 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)


  • 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 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
  • Drag Curves
    Figure 8: Drag Curves
  • 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



  • 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


  • 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.)