# Review of Basic Aerodynamics (Part Two)

• Interference Drag

Interference drag is generated by the collision of airstreams creating eddy currents, turbulence, or restrictions to smooth flow. For instance, the airflow around a fuselage and around the wing meet at some point, usually near the wing’s root. These airflows interfere with each other causing a greater drag than the individual values. This is often the case when external items are placed on an aircraft. 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.

• Form Drag

Form drag is the drag created because of the shape of a component or the aircraft. If one were to place a circular disk in an air stream, the pressure on both the top and bottom would be equal. However, the airflow starts to break down as the air flows around the back of the disk. This creates turbulence and hence a lower pressure results. Because the total pressure is affected by this reduced pressure, it creates a drag. Newer aircraft are generally made with consideration to this by fairing parts along the fuselage (teardrop) so that turbulence and form drag is reduced.

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. Thrust must overcome total drag in order to provide forward speed with which to produce lift. Understanding how the aircraft’s relationship between these elements and the environment provide proper interpretation of the aircraft’s instruments.

Newton’s First Law, the Law of Inertia

Newton’s First Law of Motion is the Law of Inertia. It states that a body at rest will remain at rest, and a body in motion will remain in motion, at the same speed and in the same direction until affected by an outside force. The force with which a body offers resistance to change is called the force of inertia. Two outside forces are always present on an aircraft in flight: gravity and drag. The pilot uses pitch and thrust controls to counter or change these forces to maintain the desired flightpath. If a pilot reduces power while in straightand- level flight, the aircraft will slow due to drag. However, as the aircraft slows there is a reduction of lift, which causes the aircraft to begin a descent due to gravity. [Figure 4-4]

Figure 4-4. Newton’s First Law of Motion: the Law of Inertia. [click image to enlarge]

Newton’s Second Law, the Law of Momentum

Newton’s Second Law of Motion is the Law of Momentum, which states that a body will accelerate in the same direction as the force acting upon that body, and the acceleration will be directly proportional to the net force and inversely proportional to the mass of the body. Acceleration refers either to an increase or decrease in velocity, although deceleration is commonly used to indicate a decrease. This law governs the aircraft’s ability to change flightpath and speed, which are controlled by attitude (both pitch and bank) and thrust inputs. Speeding up, slowing down, entering climbs or descents, and turning are examples of accelerations that the pilot controls in everyday flight. [Figure 4-5]

Figure 4-5. Newton’s Second Law of Motion: the Law of Momentum.

Newton’s Third Law, the Law of Reaction

Newton’s Third Law of Motion is the Law of Reaction, which states that for every action there is an equal and opposite reaction. As shown in Figure 4-6, the action of the jet engine’s thrust or the pull of the propeller lead to the reaction of the aircraft’s forward motion. This law is also responsible for a portion of the lift that is produced by a wing, from the downward deflection of the airflow around it. This downward force of the relative wind results in an equal but opposite (upward) lifting force created by the airflow over the wing. [Figure 4-6]

Figure 4-6. Newton’s Third Law of Motion: the Law of Reaction.

## Flight Literacy Recommends

William Kershner's Instrument Flight Manual - Everything you need to know to obtain an FAA instrument rating, or a great refresher for existing instrument pilots. Covered subjects include airplane performance and basic instrument flying, navigation and communications, clearances, planning IFR flight, and carrying out the instrument flight itself from preflight, takeoff and departure, en route, through to the approach and landing phases.

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