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Pilot training

Principles of Flight

 

Description: Principles of flight are in essence the fundamental principles that govern flight.

 

Objective: To determine that the applicant exhibits instructional knowledge of the elements of principles of flight by describing:

 

References: FAA-H_8083-3, FAA-H-8083-25

 

Equipment: PHAL, handouts, assessments

 

Instructor actions: 

 

Discuss the following topics: 

  1. Airfoil design characteristics 

  2. Airplane stability and controllability 

  3. Turning tendencies

  4. Load factors in airplane design l

  5. Wingtip vortices and precautions to be taken

 

  1. Intro-Forces of Flight

    1. Lift: The upward force created by the effect of airflow as it passes over and under the wing. 

    2. Weight: Opposes lift, and is caused by the downward pull of gravity. 

    3. Thrust: The forward force which propels the airplane through the air.

    4. Drag: Opposes thrust, and is the backward force.

The four forces of flight from faa phak

 

 

  1. Lift: A force that 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 and perpendicular to the lateral axis. 

  1. Principles of lift

        1. Newton’s first law: Nothing starts or stops moving until some outside force causes it to do so. A plane at rest remains at rest until a force strong enough can overcome its inertia. (Plane on an ice ring, when we push it it will keep going assuming there is no friction until an outside force stops it)

        2. Newton’s second law: Acceleration is produced when a force acts on a mass. The greater the mass, the greater the acceleration needed. F=MA.

        3. Newton’s third law: For every action, there is an equal and opposite reaction. (airflow bounces of the airfoil and pushes the bottom part of the wing) 

      1. Bernoulli’s principles: An increase in speed of the air across the top of an airfoil produces a drop in pressure.  

    1. Airfoils: A structure designed to obtain reaction upon its surface from the air through which it moves or that moves past such a structure.

      1. There is a difference in the curvature (camber) of the upper and lower surface of the airfoil. The camber of the upper surface is more pronounced compared to the lower surface. 

      2. An airfoil is designed in such a way that its shape takes advantage of the air’s response to certain physical laws.

        1. The greater curvature of the upper portion causes air to accelerate as it passes over the wing.

        2. The increase in the speed of the air on the top of an airfoil produces a drop in pressure and this lowered pressure is a component of lift. 

      3. A positive pressure lifting action from the air mass below the wing, and a negative pressure lifting action from lowered pressure above the wing. 

      4. As air strikes the flat lower surface of a wing, the air is forced to rebound downward causing an upward reaction in positive lift. 

      5. The air stream striking the upper curved section of the leading edge is deflected upward. 

      6. At a point close to the leading edge, the airflow is stopped and then gradually increases towards the trailing edge. Where the airflow is slowed beneath the airfoil, a positive upward [pressure was created. 

    2. Thrust: The forward force produced by the power plant/prop which is opposed by drag. 

      1. The force is provided when the engine turns the prop. Under Newton’s third law, when the propeller moves and pushes back the air; consequently, the air pushes the propeller and the plane in the opposite direction. Just like the wing, creates lift forward. 

      2. Acts parallel to the longitudinal axis.

      3. In order to maintain constant airspeed, thrust and drag must remain equal. 

      4. If engine power is reduced, thrust is reduced, then the plane will decelerate.

      5. When the plane is in steady flight, the forces are equal. The drag and thrust can be equal because you don’t need more thrust just to keep moving, the engine running is enough for you to maintain level flight. 

      6. To go faster, you push the throttle so have more thrust but eventually drag will catch up too. 

 

 

 

    1. Drag: The force that resists movement of an aircraft through the air. 

      1. Parasite Drag: All the forces that work to slow an aircraft’s movement. The speed and the shape of the plane affects the amount of drag.There are three types of parasite drag. 

        1. Form drag: Drag generated by the aircraft due to its shape and airflow around it. When the air to separate to move around a moving aircraft and its components, it eventually rejoins after passing the Examples include the engine cowling, antennas. 

        2. Interference Drag: The intersection of airstreams that creates eddy currents, turbulence, or restricts smooth airflow. The most interference drag occurs when two surfaces meet at a perpendicular angle. For example, the interference of the wing and the fuselage. 

        3. Skin friction drag: The aerodynamic resistance due to the contact of moving air with the surface of an aircraft. 

Interference Drag fom FAA PHAK

 

 

 

 

      1. Induced Drag

        1. As the angle of attack is increased, in an effort to create more lift, an additional amount of drag is created and known as induced drag. We can see in the picture below that the total aerodynamic force for a wing at a low angle of attack is directed almost completely vertical. However, as the wing is tilted upward, and the relative airflow begins to strike the lower airfoil surface, an increase in drag will cause the aerodynamic force to change direction and begin pointing further horizontal than vertical.

        2. Whenever an airfoil is making lift, the pressure on the lower surface is greater than the pressure above so that high pressure tends to flow from the high pressure area below to the low pressure on the upper surface. In the area of the tips, there is a tendency for these pressure to equalize resulting in a lateral flow outward from the underside to the upper surface. This lateral flow imports a rotational velocity to the air at the tips, creating vortices that trail behind the airfoil. 

            1. As the air and vortices roll off the back of your wing, they angle down and are called downwash. Downwash points the relative downward which causes a problem because your lift is always perpendicular to your down wash so your will lift vectors points back more which causes induced drag. 

            2. It takes energy for your wings to create downwash and vortices, and that creates drag. 

            3. 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 which is induced drag. 

 

 

3.Turning Tendencies

Induced Drag From FAA PHAK

  1.  Torque effect: As applied to the aircraft, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the aircraft in the opposite direction. 

    1. Torque reaction involves Newton’s Third Law of Physics

    2. With the propeller rotating clockwise, a force is produced to toll the entire plane about its longitudinal axis in a counterclockwise (left) direction. 

    3. Pilots have to press the right rudder to adjust for this tendency. 

  2. Spiraling Slipstream: The high speed rotation of an aircraft propeller gives a spiraling rotation to the slipstream.

    1.  At high propeller speeds and low forward speeds, the spiraling rotation is very compact and exerts a strong sideward force on the aircraft’s vertical tail surface. 

    2. When this spiraling slipstream strikes the vertical fin, it causes a yawing moment about the aircraft’s vertical fin. 

    3. It forces the airplane’s tail to the right and the nose to the left causing the airplane to rotate around the vertical axis. 

    4. Pilots must use the right rudder to correct for the turn. 

  3. Gyroscopic precession: A rotating propeller makes a very good gyroscope. All practical applications of the gyroscope are based upon two fundamental properties of gyroscopic action which are precession and rigidity in space. The one of interest is precession for this topic. 

    1. Precession is the resultant force or deflection, of a spinning rotor when a deflecting force is applied to its rim and this effect takes effect 90 degrees ahead of in the direction of rotation. 

    2. The rotating propeller makes a very good gyroscope and has similar properties. 

    3. The resultant force causes a pitching moment, a yawning moment dependent upon the point at which the force was applied. 

    4. If the plane has a tailwheel, the force will cause a yaw to the left around the vertical axis. A normal plane will have the yaw to the right. 

    5. A result of gyroscopic action, any yawing around the vertical axis results in a pitching moment so the correct rudder and pitch action. 

  4. Asymmetric Loading. : When the plane is flown at a higher AOA, the downward moving blade which is on the right side of the propeller arc has a higher angle of attack, greater action and reaction , and therefore higher thrust than the upward moving blade on the left

    1. The down swinging blade has more lift and tends to pull the plane’s nose to the left.

    2. The tendency is for the plane to yaw to the left.

    3. When descending at a low AOA, the tendency is a yaw to the right.

Torque effect From FAA PHAK
Slipstrem from FAA PHAK

 

 

Gyroscopic precession from FAA PHAK
Tailwheel airplane from FAA PHAK

 

 

4. Load Factors: The ratio of the total load supported by the airplane’s wings to the actual weight of the plane and its contents. 

  1. Load factor is measured in Gs(acceleration of gravity), a unit of force equal to the force exerted by gravity on a body a rest and indicates the force to which a body is subjected when it is accelerated. 

  2. For example, a load factor of 3 means the total load on an aircraft’s structure is three times its weight.

  3. Any force (centrifugal force) applied to an aircraft to deflect its flight from a straight line produces a stress on its structure. 

  4. Load factors are important because the pilot may impose an overload on the aircraft structure. Increased load factors increase stalling speed.

    1. Load Factors in Steep Turns

      1. At a constant altitude, during a coordinated turn in any aircraft, the load factor is the result of two forces: centrifugal force and weight. 

      2. For any given bank angle, the rate of turn varies with speed- the higher the speed, the slower the rate of turn. This compensates for added centrifugal force, allowing the load factor to remain the same. 

      3. The load factor increases significantly at a terrific rate after bank reaches 45 to 50 degrees. 

      4. Load factor for a 60 degree turn is around 2gs while and 80 degree bank is 5.76 Gs since the wing must produce lift equal to this load factor is attitude is to be maintained. 

    2. Load Factors and Stalling Speed

      1. An aircraft’s stalling speed is proportional to the square root of the load factor, 

        1. So an aircraft with a normal unaccelerated stalling speed of 50 knots can be stalled at 100 knots by inducing a load factor of 4 Gs. 

        2. Pilots should be aware that stalling a plane above its maneuvering speed produces a tremendous load factor. 

        3. The design maneuvering speed is the speed at which you can move a single flight control, one time to its full deflection, for one axis of airplane rotation in smooth air without risk of damage to the airplane.

        4. The more you bank the more back pressure you need to apply so you can be flying 110 KIAS and still be close to stall speed. 

    3. Load Factors and Flight Maneuvers

      1. Turns: Load factors become significant for in a constant altitude, coordinated turn is load factor beyond 45 degrees. 

        1. The more you bank the more back pressure you need to apply so you can be flying 110 KIAS and still be close to stall speed. 

      2. Stalls: The normal stall entered from straight and level flight, or an unaccelerated straight climb, should not produce added load factors beyond the 1G of straight-and-level flight. As the stall occurs, the load factor may be zero. 

        1. An abrupt nose down recovery may result in a negative load. 

        2. An abrupt pull up recovery, significant load factors are encountered. These may be increased by excessive steep diving, high airspeed, and abrupt pull ups to level flight. 

          1. May produce secondary stalls.

      3. High speed stalls: The load factor necessary for these maneuvers produces a stress on the wing and tail structure. 

        1. A stall above the normal stalling speed may be accomplished by a severe pull on the elevator control.

      4. Chandelles and Lazy eights: In a chandelle, the plane is in a steep climbing turn and almost stalls to gain attitude while changing direction. 

        1. The load factors incurred depend directly on the speed of the dives and the abruptness of the pull-ups during these maneuvers. 

        2. The smoothest pull up possible with a moderate load factor delivers the greatest gain in altitude and a better overall performance. 

      5. Rough Air: Gust load factors and rough air around storms produces gusts that can exceed the load limits. 

        1. Speeds up to the maneuvering speed allows an aircraft to stall prior to experiencing an increase in load factor that would exceed the limit load factor of the plane.

Load Factor from FAA PHAK