Aircraft

Aircraft Physics

How does an airplane fly?

The top of the wing is curved and the bottom is flat. Therefore the air over the wing has to travel farther than the air under the wing. One of the laws of physics states that air will always try to stay in equilibrium, or in balance with itself. This means the air flowing over the wing has to travel faster than the air under the wing. The fast air over the wing causes a difference in pressure or pressure differential. It is like the air above the wing is stretched out. Because it is moving faster there is less of it in a given space and therefore less pressure above the wing than below it. This is called lift. The low pressure pulls the wing up like a vacuum and the high pressure beneath it gives it a push. In fact about 75 percent of lift comes from the pull and 25 percent from the push.

(Sorry about poor image quality.)

However, there is doubt as to whether this theory is true. Using Newton's law of motion that every action has an equal and opposite reaction, we can assume that the air hitting the bottom of the wing is being deflected downward.

For more information on lift theories, visit the following pages:

http://travel.howstuffworks.com/airplane5.htm

http://www.straightdope.com/mailbag/mairplanesfly.html

What is attitude?

Attitude is described in terms of it's pitch, yaw, and roll. Imagine a plane flying level then goes into a nose dive strait down. It just changed its pitch. Yaw is the flat wise direction of the plane's nose and tail. Roll is just that the roll of the plane whether it rolls to the left or rolls to the right. In other words: pitch is the up down movement of the nose, yaw is the left right movement of the nose, roll is the up down movement of the wing tips. Different control surfaces work together to control the plane's attitude. Pitch is controlled by the elevators, yaw by the rudder, and roll by the ailerons. For any of these control surfaces to work, however, the plane has to be moving forward quite fast, because deflecting air doesn't mean much unless the force of the air being deflected is quite strong itself.

What is lift, weight, thrust, and drag?

There are are always four forces working on a plane when it's in the air: lift, weight, thrust, and drag. Lift is the upward force generated by the wings. Weight is the downward force generated by gravity. The plane climbs whenever lift exceeds weight. Thrust creates forward momentum. Drag is the opposite of thrust, just as weight is the opposite lift. The drag created by a plane's movement through air is called parasite drag because it has no other purpose except to slow the plane down. There are three types of parasite drag: form drag, skin friction, and interference drag. Form drag comes from the fact that hitting air oncoming air slows and object down. The same thing happens when you run against the wind. On a plane form drag would be the plane itself-the fuselage, wings, engines, antennas, and all the other parts that come in contact with the wind. Skin friction is the loss of energy that occurs whenever two surfaces rub together, friction is resistance. On the plane the friction is between the air and the metal surfaces of the plane. Interference drag comes from the areas of the plane that are not aerodynamically efficient. Such as the engines which disrupt the air flow past the plane. Though it is impossible to eliminate drag completely, it is up to all the designers of planes, cars, boats, and trains to keep friction to a minimum. In boats air friction is not such a big factor but water friction is. New improved designs would be more streamlined designs to reduce form and interference drag, as well as smoother surfaces to reduce skin friction. Swept wings on planes help reduce drag. The reduction of drag is an important aspect of aircraft designs because it can dramatically affect fuel consumption. Drag also increases as air speed increases. So a plane flying at 200 knots experiences four times the amount of drag traveling 100 knots. Tripling airspeed increases drag by nine times. That is why Boeing engineers put the first 747 through 12,000 hours of wind tunnel tests before they found just the right shape.

What are the parts of a wing?

Airplane wings are actually very simple. In fact, there are only two requirements for a wing: it must be curved on top to generate lift, and it must have a pivoting section called an aileron to help the plane turn. Of course the 747 wing is a little more complex. Far from being a fixed shape, the wing of a 747 is made up almost entirely of moving panels. To begin with there are not one but two ailerons on each wing: A low-speed aileron on the outer edge of the wing (for more leverage) and a shorter, high-speed aileron closer to the fuselage. At slower speeds, only the combined force of the two ailerons is enough to turn the plane. But at the fast cruising speeds of the 747, the slight deflection caused by the inboard, high-speed aileron is more than enough to bank it. In addition to the two ailerons, each 747 wing also has three sets of leading-edge flaps, two sets of trailing-edge flaps, and six spoilers. The flaps work at the command of the pilot to adjust the shape of the wing. Extending the flaps on takeoff and landing increase the wings ability to generate lift. Retracting them at cruising altitude reduces the planes drag. In this way, the flap allows the pilot to make the wing bigger or smaller as necessary. The vertical flight spoilers have a dual function. During a turn, the five outboard spoilers help the ailerons bank the plane. On landings, all six spoilers rise up to break the airflow over the wing and (spoils) the lift.