The Fundamentals of Flight – How does an aircraft fly?

Sep 14

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?



Lift

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. 




 Drag

 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. 

 

 





Weight

 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. 

 


 


Thrust

 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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22 comments

  1. Roy Dale /

    I acquired my basic knowledge of aerodynamics by designing and building model aircraft. Pilots don’t need to have a basic knowledge of aerodynamics, if they did there would be no pilots. All a pilot needs is a bias knowledge like the one he gets from his pilot handbook or this web page. Wouldn’t it be nice to have a basic knowledge of aerodynamics not because you fly an aircraft but because you are a solid object that has personally been generating aerodynamic force from the day you were born as a result of at the very least the process of respiration?

    There are only two aerodynamic forces not four so I have already made aerodynamics twice as simple than you thought it was. The two aerodynamic forces are lift and drag. These two forces are very, very similar. By accurate definition the only difference between the two is there direction in relation to the relative airflow that caused them. Because the direction of relative airflow is infinite so are lift and drag. This means each force can cause motion or resist motion in any direction. Some aircraft have flown around the world in spite of drag while others have done so as a result of drag.

    If lift were only created by the flow of air over an “airfoil” airplanes would not be able to fly on days the wind is not blowing (flowing). When an airplane is flying in still air the air that is generating aerodynamic force is still, static, not moving, motionaly inert, no airflow over or under the wing. It’s a good thing that the air that causes lift does not require it to be in motion or that lift does not require the motion of an object through the air or the motion of an object while in the air (rotation) although any one or combination of these can cause lift by causing relative airflow. Relative airflow only requires the motion of air in relation to an object, it is the difference in velocity between the object and the air.

    The main use of lift in aeronautics is to resist gravity but drag is exclusively referred to as a resistance force. Aircraft have circumnavigated the earth using drag exclusively as a means of horizontal acceleration but drag is exclusively referred to as a resistance force. To move through the air you must overcome drag but to remain still in moving air you also must overcome drag and if you cannot you will move as a result on drag. Even when drag opposes the motion of an object through the air it can cause motion. The drag from a canoe paddle is in the forward direction, the canoe is effectively using drag as a means of thrust. Drag is in the direction of the relative airflow that caused it so any time the relative airflow that is influencing an aircraft has an upward component there is a component of drag helping to support its weight. Upward relative airflow can be caused by downward flight or thermal or updraft. When a bird flaps its wing downward it generates an upward relative airflow which propels it forward (lift) and upward (drag). The trick for the bird is to pull its wing down at max drag and up at min drag.

    Thrust is not the force that pushes an aircraft through the air. Gravity can push an aircraft through the air and gravity is not thrust. Thrust is the force created by an onboard power source. Thrust is the force that generates a relative airflow for the wings. It does this by moving the wings through the air and by holding the wing in moving air. Sometimes thrust cannot hold the wing in the moving air and the wing cannot over come drag so the wing starts to move backwards as a result of drag. Not bad for a force that is synonymous with air resistance.

  2. Hi there, I found your blog via Google while searching for first aid for a heart attack and your post
    looks very interesting for me.

  3. The ground school portion of the Private Pilots course does have the basics of aerodynamics. The test for a private pilots license does have test questions relating to aerodynamics.

  4. Thanks for the input Roy. This definitely makes for an interesting read.

  5. Thanks! Glad you like it.

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