Aircraft Theory of Flight

Before a technician can consider performing maintenance on an aircraft, it is necessary to understand the pieces that make up the aircraft. Names like fuselage, empennage, wing, and so many others, come into play when describing what an airplane is and how it operates. For helicopters, names like main rotor, anti-torque rotor, and autorotation come to mind as a small portion of what needs to be understood about rotorcraft. The study of physics, which includes basic aerodynamics, is a necessary part of understanding why aircraft operate the way they do.

Four Forces of Flight

During flight, there are four forces acting on an airplane. These forces are lift, weight, thrust, and drag. [Figure 3-53] Lift is the upward force created by the wing, weight is the pull of gravity on the airplaneís mass, thrust is the force created by the airplaneís propeller or turbine engine, and drag is the friction caused by the air flowing around the airplane.

All four of these forces are measured in pounds. Any time the forces are not in balance, something about the airplaneís condition is changing. The possibilities are as follows:

1. When an airplane is accelerating, it has more thrust than drag.

2. When an airplane is decelerating, it has less thrust than drag.

3. When an airplane is at a constant velocity, thrust and drag are equal.

4. When an airplane is climbing, it has more lift than weight.

5. When an airplane is descending, it has more weight than lift.

6. When an airplane is at a constant altitude, lift and weight are equal.

Bernoulliís Principle and Subsonic Flow

The basic concept of subsonic airflow and the resulting pressure differentials was discovered by Daniel Bernoulli, a Swiss physicist. Bernoulliís principle, as we refer to it today, states that ďas the velocity of a fluid increases, the static pressure of that fluid will decrease, provided there is no energy added or energy taken away." A direct application of Bernoulliís principle is the study of air as it flows through either a converging or a diverging passage, and to relate the findings to some aviation concepts.

A converging shape is one whose cross-sectional area gets progressively smaller from entry to exit. A diverging shape is just the opposite, with the cross-sectional area getting larger from entry to exit. Figure 3-54 shows a converging shaped duct, with the air entering on the left at subsonic velocity and exiting on the right.

Looking at the pressure and velocity gauges, and the indicated velocity and pressure, notice that the air exits at an increased velocity and a decreased static pressure. Because a unit of air must exit the duct when another unit enters, the unit leaving must increase its velocity as it flows into a smaller space.

In a diverging duct, just the opposite would happen. From the entry point to the exit point, the duct is spreading out and the area is getting larger. [Figure 3-55] With the increase in cross-sectional area, the velocity of the air decreases and the static pressure increases. The total energy in the air has not changed. What has been lost in velocity (kinetic energy) is gained in static pressure (potential energy).

In the discussion of Bernoulliís principle earlier in this chapter, a venturi was shown in Figure 3-46. In Figure 3-56, a venturi is shown again, only this time a wing is shown tucked up into the recess where the venturiís converging shape is. There are two arrows showing airflow. The large arrow shows airflow within the venturi, and the small arrow shows airflow on the outside heading toward the leading edge of the wing.

In the converging part of the venturi, velocity would increase and static pressure would decrease. The same thing would happen to the air flowing around the wing, with the velocity over the top increasing and static pressure decreasing.

In Figure 3-56, the air reaching the leading edge of the wing separates into two separate flows. Some of the air goes over the top of the wing and some travels along the bottom. The air going over the top, because of the curvature, has farther to travel. With a greater distance to travel, the air going over the top must move at a greater velocity. The higher velocity on the top causes the static pressure on the top to be less than it is on the bottom, and this difference in static pressures is what creates lift.

For the wing shown in Figure 3-56, imagine it is 5 ft wide and 15 ft long, for a surface area of 75 ft2 (10,800 in2). If the difference in static pressure between the top and bottom is 0.1 psi, there will be 1/10 lb of lift for each square inch of surface area. Since there are 10,800 in2 of surface area, there would be 1,080 lb of lift (0.1 ◊ 10,800).

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