How Do Airplanes Fly . . .
. . . without flapping their wings?
Forces on an Airplane in Flight The four aerodynamic forces that act upon an airplane in flight are lift (the upward acting force), weight (or gravity, the downward acting force), thrust (the forward acting force), and drag (the air resistance or backward acting force). These four forces are continuously battling each other while an airplane is in flight.
Gravity opposes lift, thrust opposes drag. In order to take off, the aircraft's thrust and lift must be sufficient to overcome its weight and drag. In level flight at constant speed, thrust exactly equals drag and lift exactly equals the pull of gravity. To land, an aircraft's thrust must be reduced safely below its drag, as its lift is reduced to levels less than its weight.
What makes the plane go up? How an Airplane Generates Lift Lift is the aerodynamic force that counteracts gravity and holds an airplane in the air. Most of the lift required by an airplane is created by its wings. But, a certain portion is also generated by other parts of the aircraft, such as the body, or fuselage of the plane. But how do the wings actually create the lift? In very simple
terms, lift is generated by the wings in two different ways. The
first is by the angle of attack. The second is by the airfoil. 1. Angle of Attack. Some lift is created by the angle that the wing makes with the oncoming airflow. This angle is called the angle of attack. You can feel this effect when you hold your flat hand outside of a moving car window. Past a certain point, however, the increased angle of attack will cause the wing to suddenly lose all of its lifting ability. This is called a stall. When a stall happens, the plane will begin to fall from the sky and could very well crash.
But the effect of the angle of attack alone cannot generate enough lift to fly a plane. Most of the lift needed to fly a plane is created by a second method called the airfoil. 2. The airfoil. Back in the 1700's, a Swiss scientist named Daniel Bernoulli determined that the faster a fluid moves across a surface the less pressure it exerts on the surface. This effect is called the Bernoulli Principle. Let's see how that can cause lift with an airplane wing. First, understand that air is a fluid, just like water, and that all fluids adhere (stick) to the same physical and mathematical principles. Next, realize that lift can only be generated when a fluid is in motion. For example, a wing must be either passing through the air, or the air must be moving around a stationary wing, in order for lift to occur. The way it usually happens of course is that the wing is doing most of the moving, although the air may be moving too (wind) at the same time. Most airplane wings have a special, basic shape when viewed from the end. Their upper surfaces are curved and their lower surfaces are flatter. This shape, called an airfoil, is what works with the fluid motion of the air to create lift. As air moves around a wing, some goes over the top and some goes underneath. Since the top of the wing is curved, the air must travel over the top of the wing farther, and therefore faster, in order to reach the back of the wing at the same time as the air beneath the wing.
According to the Bernoulli Principle, the faster moving air on top of the wing exerts less pressure on the top surface than the slower air does on the bottom of the wing. Since there is more pressure on the bottom of the wing, the wing is pushed up, causing lift.
How do you guide the plane through the air? Airplanes are guided through the air by changing the angles of control surfaces on the tail and wings of the plane. Control surfaces are hinged flaps that can be moved either up or down or back and forth from inside the airplane. Let's look at how, by changing the angles of these surfaces, we can guide the airplane in the directions we want it to go. An airplane in flight moves around three axes of rotation: longitudinal axis, lateral axis, and vertical axis.
Look at the picture above. These axes are imaginary lines that run perpendicularly (90 degrees) to each other through the center of the airplane. Rotation around the longitudinal axis (the line from the nose of the plane to the tail) is called roll. Rotation around the lateral axis (the line from wingtip to wingtip) is called pitch. Rotation around the vertical axis (the line from beneath to above the plane) is called yaw. The pilot guides and controls the aircraft by controlling its pitch, roll, and yaw by using what's called the control surfaces. The ailerons, elevators, and rudder are three important types of control surfaces. 
Basic Control Surfaces on an Airplane
Ailerons The ailerons on an airplane's wings control roll around the longitudinal axis. When the control wheel in the cockpit is turned left, the aileron on the left wing goes up and the one on the right wing goes down. The opposite occurs when the wheel is turned right. But how does this make the airplane roll? 
The ailerons alter the lifting ability of the wings slightly. When an aileron is lowered, the lift on the outer portion of that wing increases, causing that wing to rise a little. When an aileron is raised, the lift on the outer portion of that wing is decreased slightly, causing that wing to drop a little. Since the ailerons on an airplane work together, their action causes the airplane to roll.
Aileron Position When an airplane rolls to the right, it causes the airplane to turn right. When the plane rolls the left, it turns left. Therefore the ailerons are used to control the direction the airplane is flying.
The elevators are located on the horizontal portion of the tail of an airplane. They control the pitch of the plane, or its motion around the lateral axis. They are moved by the control wheel in the cockpit. When the wheel is pulled back, the elevators move upward, causing the tail of the plane to move downward and the nose to pitch upward. When the wheel is pushed forward, the elevators move downward, causing the tail of the plane to rise and the nose to pitch downward. 
The elevators work like the ailerons on the wings, in that they cause changes in the lift generated by the tail of the plane. Also, the elevators work together, simultaneously, like the ailerons, but they do not work in opposition to one another. Both go up when the control wheel is pulled back and both go down when the control wheel is pushed forward.
Elevator Position
Rudder The rudder on the rear edge of the vertical fin on the airplane's tail controls yaw around the vertical axis. It is connected to the pedals at the pilot's feet. Pushing the right pedal causes the rudder to deflect to the right. This makes the tail of the airplane move toward the left, causing the nose to move to the right. Pushing the left pedal makes the rudder deflect to the left, the tail moves to the right, and the nose points to the left. 
Rudder Position Although the rudder pedals and control wheel in the cockpit are not linked together, they must be used simultaneously (at the same time) to control the plane. The pilot guides the airplane by careful and precise movements of the control wheel and rudder pedals, as well as adjusting the thrust of the aircraft with the throttle (engine speed control).
What makes the airplane go forward? How Does an Airplane Produce Thrust? Thrust is the force created by a power source (engine) that overcomes the airplane's aerodynamic drag (its resistance to passing through the air) and gives it forward motion.
Reciprocating Engines with Propellers A reciprocating engine is an internal-combustion engine in which pistons moving back and forth act upon a crankshaft to create rotational movement. (This is the same type of engine that powers most family cars.) A mixture of fuel and air is compressed by the pistons, an electric spark causes the mixture to explode, driving the pistons downward. This motion is transferred to the crankshaft by connecting rods. The rotating crankshaft turns the propeller.
A propeller is a type of airfoil (similar to a wing) that turns and accelerates air. As the blades of the propeller rotate they create lifting forces just as a wing does, only working in the horizontal plane instead of the vertical as with wings.
Thus, the propeller creates a propulsive force (thrust) that moves the aircraft forward as a reaction. Jet Engines A jet engine operates on the application of Sir Isaac Newton's third law of physics: for every action there is an equal and opposite reaction. In a jet engine, this is called thrust. This law can be demonstrated in simple terms by releasing an inflated balloon and watching the escaping air propel the balloon in the opposite direction.
A jet engine is any engine that ejects a jet or stream of gas or fluid, thereby obtaining thrust. A jet aircraft engine gets oxygen from the atmosphere for the combustion of its fuel, creating thrust in reaction to the rapid exhaust of the combustion products. There are several types of jet engines. Click here to learn about the various types of jet engines.
All jet engines, which are also called gas turbines, work on the same principle. The engine sucks air in at the front with a fan. A compressor raises the pressure of the air. The compressor is made up of fans with many blades and attached to a shaft. The blades compress the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine. As the jets of gas shoot backward, the engine and the aircraft are thrust forward. Jet engines move the airplane forward with a great force and thereby fly very fast.
|
