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Science by Laurisa

The Science of Flight

by Laurisa White Reyes

n the morning of December 17, 1903 Orville Wright became the first man to fly in a motor-powered lighter-than-air machine. It was 10:35a.m. and, according to Orville’s own diary, the wind at Kitty Hawk was blowing 27 mph. The truck that carried his plane accelerated to 7 or 8 miles an hour when the plane lifted off the truck bed and took to the air where it remained for a brief but magnificent twelve seconds. The craft flew only 120 feet but it marked the beginning of a new era.

That historic morning at Kitty Hawk was the culmination of years of research and hard work by many men who had previously attempted and failed to discover the key to manned flight. Orville and his brother, Wilbur, built upon what had already been learned by their predecessors and continued to improve on the technology after 1903.

Aerodynamics

Now, imagine it is a clear, breezy day and you are standing in a wide-open field with a kite in your right hand. You hold it high above your head with a ball of twine tucked firmly in your left hand. You turn so that you are standing into the wind then you begin to run. As you pick up speed you release the kite. You are running now at full speed and you feel the kite tug at the twine, which you are now holding with both hands and letting out a little at a time. You stop running as you turn towards the kite and watch it climb higher and higher into the air. The experience is exhilarating and magnificent. The forces that take your kite into the air are the same that lifted the Wright brother’s plane off Kitty Hawk and that lift giant airplanes off runways today. These forces together are called Aerodynamics.

According to NASA, "Aerodynamic forces are generated by an object moving through a fluid (liquid or gas.)" In the case of airplanes, the airplane is the object and the air is the fluid. To generate lift, the object must move through the air or air must move past the object. While any object can generate lift, the amount of lift required to get a large object off the ground and keep it there depends on several factors, including the shape of the object and wind velocity. What keeps it in the air is the relationship between four forces which constantly pull the airborne object in different directions: Weight, lift, drag and thrust. The Wrights learned through trial and error how to combine these elements together so that flight could be achieved and sustained.

Wind Velocity

Wind Velocity is the relationship of Wind Speed to Ground Speed to Air Speed. To generate lift, there must be a positive velocity.

Wind Speed is the rate by which air moves past a fixed point of reference on the ground. Wind Speed has two components: Speed and direction (i.e. North-South, East-West, Up-Down).

Ground Speed is the rate by which an object moves past that same point of reference.

Air Speed is the difference between Wind Speed and Ground Speed and can be computed by a simple equation: Air Speed (A) = Ground Speed (G) – Wind Speed (W, or [A= G – W]

Imagine you are now sitting at the controls of an airplane, which waits on a runway. You are not moving. Your Ground Speed is zero. If there is no wind at all, the Air Speed is also zero. Since A = G – W, then your Airspeed would be zero [0 = 0 – 0] and there is no lift.

Let’s say now, that to get your airplane off the ground would require a positive velocity of 100 miles per hour. On that windless day your Air Speed equals your Ground Speed. Your plane would need to travel at 100 miles per hour to generate lift.

When there is wind you must compute your Air Speed. If you are facing North and the wind is blowing South, you have a headwind, the wind is blowing in your face. A headwind is a negative velocity. If the wind speed is 20 mph and your required airspeed is 100 mph, your airplane will need to reach a ground speed of 80 mph to generate lift. [100 = 80 – (-20)).

If, however, the wind is blowing North, you have a tailwind, the wind is blowing at your back. The tailwind is a positive velocity. This time, with the wind speed of 20 mph your airplane will need to reach a ground speed of 120 mph to generate lift. [100 = 120 – 20].

In other words, your plane must travel faster and farther in a tailwind in order to get off the ground. The ideal aim would be to take off into the wind, which requires a lower ground speed and a shorter runway.

Wing Geometry

The shape and size of a wing affects the amount of lift an object will have. If you were to look at the cross-section of a wing you would notice that the upper and under sides are curved. The leading edge (front end) of the wing is thick and tapers to the thinner trailing edge (back end). This basic shape is called an airfoil.

Airfoils serve to turn the flow of fluid, or air. Following Newton’s Third Law of Motion "For every action, there is an equal and opposite reaction," as the air is displaced by the airfoil, the wing generates lift. The amount of lift is determined by the wing area, the area between the leading edge, the trailing edge and the wing tip.

Four Forces

There are four forces that affect an airplane in flight: weight, lift, drag and thrust.

Weight is the gravitational pull toward Earth.

Lift is the force that overcomes weight by moving the airplane into the air (as explained earlier).

Drag is the air’s resistance to the plane’s movement.

Thrust is the force that overcomes drag by use of propulsion (i.e. Engine, propellers, rockets, etc.)

When these forces are in balance, the airplane will travel at a constant velocity. If these forces become unbalanced, the airplane accelerates in the direction of the largest force. In other words, if there is insufficient lift, the plane will succumb to its weight and go down. If there is not enough thrust, the airplane will slow down.

Conclusion

The science of flight is highly complex. The explanation offered in this article is meant only as a beginner’s introduction. For more detailed information, please refer to the following websites:

A detailed history of the Wright brother’s development of the airplane can be found at:

http://www.wam.umd.edu/~stwright/WrBr/taleplane.html

and at http://www.centennialofflight.gov/wbh/experience.htm

The information for this article was drawn primarily from NASA’s website on flight at http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html, which is a comprehensive collection of material on Aerodynamics and the science of flight.