Almost all children are intrigued by planes and birds flying in the sky. At one time or another, they all want to fly, and so, in turn, are curious about how things fly. This fact makes this unit appealing to students because they have the chance to explore that question and to answer it themselves.
How Can a Heavy Plane Lift Off the Ground?
The Bernoulli Effect is the effect that is credited for lifting a plane off the ground. Daniel Bernoulli was a Swiss mathematician who first discovered it in 1783. Bernoulli noticed that as air moves, its pressure drops. This is true for air, water, or any other liquid. The faster the air moves, the more the pressure drops.
Planes fly because they are lifted off the ground by a force called "lift." Lift happens because the pressure is greater below the wing of the airplane than it is above the wing. Air pressure is lower when the air travels faster, and air pressure is higher when the air travels more slowly. This can happen because of the shape of the wing. The wing of an airplane is usually flat along the bottom and curved on the top, and tapers to a point at the back end of the wing. Since the top of the wing is curved, the air has further to travel than on the bottom of the wing. The same amount of air goes above the wing as below the wing, the air on top is spread out more and traveling faster, creating lower pressure above the wing. The air below the wing has a shorter distance to travel and travels more slowly, this air has a higher density, so this creates a higher pressure below the wing. When an airplane travels faster, the pressure difference becomes greater. In order for the plane to actually come off the ground and into flight, the lift force must be greater than the weight of the plane.
The only way for wings to lift a plane off the ground is if they are moving very quickly through the air. Gliders can achieve this same effect by flying downward and gaining speed. Gliders will also use upward air currents to gain some additional height. (See figure ii)
The 'angle of attack' is also important for lift to occur. The angle of attack is the angle at which the wing meets the airstream. At high speeds, the wing needs only about a 4 degree angle of attack to produce enough lift. As the planes travels more slowly, the angle must increase to produce enough lift to equal its weight. However, if the angle of the wing reaches 14 degrees, the wing stalls and the airplane descends. Stalling occurs when the airstream above the wing becomes turbulent and the wing loses lift. (See figure iii)
What Makes a Plane Move Forward?
Thrust is the force that moves the plane forward in the air. Thrust must overcome drag, the resistance of air, or the plane will not move. Thrust is typically measured in pounds or kilograms. Airplanes create thrust using propellers, jet engines, or rockets. For the purpose of the unit, rockets are not discussed with the students. Rocket engines are extremely powerful providing a tremendous amount of thrust, and large ones are often powered by liquid oxygen and ethyl alcohol.
When thrust is created by a propeller, it acts as a giant fan, pulling air past the plane. The blades of the propeller slope at an angle and curve. The propeller spins and pushes some air backwards. Since the front of the blade is curved, it acts much like a wing, forcing the air to move faster over the front of the blade than behind it. With a lower pressure in front of the blade, combined with a backward flow behind the propeller, this is the thrust that moves the plane forward.
Jet engines are used on larger aircraft to create thrust and to overcome drag. Surprisingly, the main thrust involved with jet engines is not the air that is heated by combustion and pushed out through the jet engine, it is from the cooler air that bypasses the duct. At the front of the engine is a fan that rotates to suck air into the engine. Some of this air enters the compressor of the engine where the air then increases in pressure. Next, this high-pressure air enters the combustion chambers of the engine where the air is combined with kerosene and burns. This burning produces hot, high-pressure gas which is then passed through the turbines. The turbines are the part of the engine that drives the compressor near the front of the engine, and drives the fan at the very front of the engine. From the turbines, the hot high-pressure gas rushes out the backend of the jet engine with great force. This thrust of hot gases mixes with cool air that has bypassed the engine and together creates enormous thrust.
How Does Air Create 'Drag'?
Drag is what needs to be overcome by thrust in order for a plane to move through the air. Drag is the force that resists the motion of the plane moving through the air, that is, friction with the air. Friction is caused when two things rub together, in this case, it is the air rubbing against the surface of the airplane. Planes, birds, or anything else that moves through the air or water can be specially shaped to minimize drag, this shape is a "streamlined" shape. The idea of a streamlined shape is to have the air be able to flow around the plane as smoothly as possible. This can be seen by watching downhill skiers, whenever they have the chance to, they crouch down into a ball, making themselves as small as possible and therefore reducing drag. Pilots do the same thing, when they finish with takeoff, they retract the landing gear on the plane, they want to make the plane as small as possible so that the drag on the plane is reduced. If the landing gear of the plane remained extended during flight, the drag on them at cruising speeds would be so great that the landing gear would be ripped right off the plane. As the speed of a plane increases, so does the drag. The best streamlined shape is similar to a slim teardrop with the rounded end pointing in the direction that the plane is traveling and the pointed end at the back. This shape parts the air smoothly in the front, and the gentle tapering allows the air to come together again with the least amount of disturbance.