Flight Theory and Aerodynamics. Joseph R. Badick
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Название: Flight Theory and Aerodynamics
Автор: Joseph R. Badick
Издательство: John Wiley & Sons Limited
Жанр: Техническая литература
isbn: 9781119772415
isbn:
MOMENTS
If a mechanic tightens a nut by applying a force to a wrench, a twisting action, called a moment, is created about the center of the bolt. This particular type of moment is called torque (pronounced “tork”). Moments, M, are measured by multiplying the amount of the applied force, F, by the moment arm, L:
The moment arm is the perpendicular distance from the line of action of the applied force to the center of rotation. Moments are measured as foot–pounds (ft‐lb) or as inch–pounds (in.‐lb). If a mechanic uses a 10 in.‐long wrench and applies 25 lb of force, the torque on the nut is 250 in.‐lb.
The aircraft moments that are of particular interest to pilots include pitching moments, yawing moments, and rolling moments. If you have ever completed a weight and balance computation for an aircraft, you have calculated a moment, where weight was the force and the arm was the inches from datum. Pitching moments, for example, occur when an aircraft’s elevator is moved. Air loads on the elevator, multiplied by the distance to the aircraft’s center of gravity (CG), create pitching moments, which cause the nose to pitch up or down. As you can see from Eq. 1.2, if a force remains the same but the arm is increased, the moment increases.
Several forces may act on an aircraft at the same time, and each will produce its own moment about the aircraft’s CG. Some of these moments may oppose others in direction. It is therefore necessary to classify each moment, not only by its magnitude, but also by its direction of rotation. One such classification could be by clockwise or counterclockwise rotation. In the case of pitching moments, a nose‐up or nose‐down classification seems appropriate.
Mathematically, it is desirable that moments be classified as positive (+) or negative (−). For example, if a clockwise moment is considered to be a + moment, then a counterclockwise moment must be considered to be a − moment. By definition, aircraft nose‐up pitching moments are considered to be + moments.
Figure 1.8 Balance Lever.
EQUILIBRIUM CONDITIONS
Webster defines equilibrium as “a state in which opposing forces or actions are balanced so that one is not stronger or greater than the other.” A body must meet two requirements to be in a state of equilibrium:
1 There must be no unbalanced forces acting on the body. This is written as the mathematical formula ∑F = 0, where ∑ (cap sigma) is the Greek symbol for “sum of.” Figure 1.2 illustrates the situation where this condition is satisfied (lift = weight, thrust = drag, etc.)
2 There must be no unbalanced moments acting on the body. Mathematically, ∑M = 0.Moments at the fulcrum in Fig. 1.8 are 5000 ft‐lb clockwise and 5000 ft‐lb counterclockwise. The weight (force) of A is 100 lb and is located 50 inches (″) to the left of datum (fulcrum), thus 100 lb × −50″ = −5000 lb‐in. The weight of B is 200 lb and is located 25 inches to the right of datum, thus 200 lb × 25″ = 5000 lb‐in. So, ∑M = 0.
NEWTON’S LAWS OF MOTION
Sir Isaac Newton summarized three generalizations about force and motion. These are known as the laws of motion.
Newton’s First Law
In simple language, the first law states that a body at rest will remain at rest and a body in motion will remain in motion, in a straight line, unless acted upon by an unbalanced force. The first law implies that bodies have a property called inertia. Inertia may be defined as the property of a body that results in its maintaining its velocity unchanged unless it interacts with an unbalanced force. For example, an aircraft parked on the ramp would not even need chocks unless an unbalanced force (such as wind, or gravity if parked on a slope) acted on it. The measure of inertia is what is technically known as mass.
Newton’s Second Law
The second law states that if a body is acted on by an unbalanced force, the body will accelerate in the direction of the force and the acceleration will be directly proportional to the force and inversely proportional to the mass of the body. Acceleration is the change in the velocity of a body in a unit of time. Consider an aircraft accelerating down the runway, or decelerating after touchdown. For our discussion, the primary forces acting on an aircraft accelerating or decelerating down a runway are thrust, drag, and friction. Future chapters will discuss the net force on an aircraft during the takeoff and landing regimes.
The amount of the acceleration a is directly proportional to the unbalanced force, F, and is inversely proportional to the mass, m, of the body. For a constant mass, force equals mass times acceleration.
Newton’s second law can be expressed by the simple equation:
Then, solving for a,
EXAMPLE
An airplane that weighs 14 400 lb accelerates down a runway with a net force of 4 000 lb, what is the acceleration (a) assuming constant acceleration?
Newton’s Third Law
The third law states that for every action force there is an equal and opposite reaction force. Note that for this law to have any meaning, there must be an interaction between the force and a body. For example, the gases produced by burning fuel in a rocket engine are accelerated through the rocket nozzle. The equal and opposite force acts on the interior walls of the combustion chamber, and the rocket is accelerated in the opposite direction. As a propeller aircraft pushes air backward from the propeller, the aircraft is pushed forward.
LINEAR MOTION
Newton’s laws of motion СКАЧАТЬ