Thursday, January 12, 2012

The Fundamentals of Flight – How does an aircraft fly?

The Fundamentals of Flight – How does an aircraft fly?

Perhaps the most important things to understand before even attempting to build a model airplane, is the “fundamentals of flight”.  These are the same principles that are applied in full-sized aircraft as well. Once you understand these basic principles, I can assure you that RC flying would be a lot easier and a lot more fun.
“Flight” of an aircraft is based on pretty much the same principles as those that cause things in the natural world to “fly”. A bird flies not only by flapping its wings, but by gliding its wings outstretched for long distances. The flight is made possible by a careful balance of four physical forces: lift, drag, weight, and thrust. An aircraft’s “lift” must balance its “weight”, and its “thrust” must exceed its “drag”. The aircraft uses its wings for lift and its engines for thrust. Drag is reduced by a plane’s smooth shape and its weight is controlled by the materials it is constructed of.

 The Fundamentals of Flight   How does an aircraft fly?

 The Fundamentals of Flight   How does an aircraft fly?

For an aircraft to rise into the air, a force must be created that equals or exceeds the force of gravity. This force that is created is called LIFT. The lift is created by the flow of air over an “airfoil”. The shape of an airfoil causes air to flow faster on top than on bottom. The fast flowing air decreases the surrounding air pressure. Because the air pressure below the airfoil is greater than above the airfoil, a resulting lift force is created.

 The Fundamentals of Flight   How does an aircraft fly?The resistance that any physical body that is propelled through the air experiences is called DRAG. It is a result of a number of physical phenomena. Pressure drag is created when the wind in front of the airplane is greater than the thrust pushing it forward. Skin friction or viscous drag is created by rough surfaces. To reduce frictional drag, careful attention needs to be paid to smoothen the surface of an airplane as much as possible.
Usually special materials are used in building model airplanes that reduce the frictional drag.

 The Fundamentals of Flight   How does an aircraft fly?

The weight of an aircraft is a limiting factor in aircraft design. A heavy plane, or a plane meant to carry heavy payloads, requires more lift than a light plane. It may also require more thrust to accelerate on the ground. A small plane, and especially a model airplane, must be appropriately balanced for flight. Too much weight in the back will cause the aircraft to go nose up and too much weight in the front will cause the aircraft to go nose down. In both cases, the plane become unstable and difficult to control. 
Careful attention must be paid to get the Center of Gravity of the airplane at the correct position. The position of the Center of Gravity should be such that the when the plane is held from the ends of the two wings, the plane should be completely balanced. 

 The Fundamentals of Flight   How does an aircraft fly?
Thurst is the force that pushes an aircraft through the air. It must be greater than the drag to achieve the forward acceleration needed for takeoff and to increase an aircraft’s speed in level flight. An airplane’s engine is responsible for producing thrust, and in case of an electric-powered model airplane, the thrust is created by a brushless  motor. The engine or motor provides power to the propeller, which produces the thrust. Therefore, the weight of the aircraft usually determines the size of the engine or motor. 

Once all these four aerodynamic forces act on the aircraft in the intended manner, the result is FLIGHT. It all might sound a bit technical in the beginning, but understanding these basic principles will help you a great deal when you start building and flying your model aircraft. I learnt this the hard way by crashing my first RC plane at least a dozen times. In all the dozen crashes, the cause of the crash was an imbalance in one of the above four forces . Therefore, take a few moments and understand these concepts.

My next article will be an introduction to the basics of a radio controlled airplane. Things are getting exciting so please stay tuned!

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