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RC University: Airplanes



 


Basic Aerodynamics
Written by: Dr. Bob Motazedi
 

To put it plainly, good radio control fliers understand aerodynamics. Student pilots of real planes, of course must demonstrate a mastery of aerodynamic principles or they never earn their “wings.” Radio control pilots who neglect to learn basic aerodynamics soon discover them the hard way. Getting that plane off the ground and keeping it in the air requires more than just moving a transmitter gimbal (lever) to the left or right. You must know the forces involved in flying.

Four forces act on an airplane in flight: thrust, weight, drag, and lift. A variation of any of these forces will cause a change in the flight path of an airplane. Let’s discuss each if these factors in detail.

Thrust
Thrust is easy to understand. As the engine turns the propeller, air is forced backwards, creating thrust. This thrust then causes the airplane to be propelled forward.

Weight
Weight refers to the effect of gravity on a flying airplane. Gravity is constantly pulling the plane down towards earth.

Drag
Drag, ad the name implies, is the resistance the airplane must overcome to be propelled forward. Two types of drag affect airplanes. The first is parasitic drag. Any projection on the airplane that alters its shape, including microscopic pores on the covering, can slow down the plane. Other sources include the landing gear, radio switch, and engine. Parasitic drag increases as the airplane’s speed increases. The second form of drag is called induced drag, a by-product of lift. As a wing develops more total lift, some of that lift pulls the airplane in a rearward direction, causing it to slow down. As lift increases, the induced drag also increases.

Lift
Lift is the fourth force; it’s the most difficult to understand and remained mostly a mystery until the 18th century. In 1738, a mathematician named Daniel Bernoulli discovered an interesting phenomenon: The pressure of a moving fluid varies with its speed. For example, if fluid is forced to move quickly through a tube, the pressure it exerts on the tube would decrease. What does this mean in terms of aerodynamics? Because of the shape of the wing, the air that flows over the top of the wing moves much faster than the air that flows beneath it (fig. 1-4). This results in less pressure on the top of the wing and comparably more pressure below. The higher pressure below pushes the wing upward.

Another way that a wing develops lift has to do with the downward deflection of air. This is the same reason that causes anyone’s hand to rise and drop in relation to how the wind hits it when held outside of a moving car. As the wind deflects off of a hand in a downward direction, the equal and opposite force crated makes the hand rise.

The same thing happens with a wing. When it is at a great or positive angle, air is also deflected downwards causing it to rise. Both of these forces (deflection and differential air flow) are involved at different times and in different amounts in generating lift on a moving wing.

To order the full text of Dr. Bob's book, "Safe and Easy Flying" or any of the many titles by Dr. Bob, call 1.800.390.HAWK.

 
 



 



 
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