Monday, February 5, 2007

How Model Airplanes Fly

After the successful flight of the man-carrying vehicle made by the Wright brothers, interest in aviation spread rapidly and many models were made. Model airplane enthusiasts are already existing in the early 1900s'. Most of the models are rubber powered, twining type with double stick fuselages that are common in Europe. But even in the early days of model flying, small petrol and compressed air engines are already being used. The materials used in model constructions are birch strips, veneer, spruce, piano wire or bamboo and oiled silk covering.

Then balsa structure and tissue covering appeared in the United States in the late 1920s'. So much for the history of model aircraft. So you see, even today, the airplane structure and how it fly is no different from the one we are flying today. The wings, fuselage, vertical and horizontal stabilizers, propellers, engines, landing gears are the same. The airplane, to fly and have control during flight uses them. The wings are obviously responsible why the airplane can stay in the air for a long time. With proper design of the airplane, dimensions, weight considerations and aerodynamic design characteristics it will fly successfully. The aerodynamic principles behind it is what really makes it fly. But even though it has a good design, weight and balance plays a major role. There was a saying that “a feather flies better than a brick” which is true because a very heavy airplane won’t fly if it cannot be sustained by its power plant (engine, propeller, and fuel tank). And with regards to balancing, a well-balanced airplane is controllable during flight. Usually the fulcrum or center of gravity is located ¼ of the wing chord. Balancing it is by placing your fingers on both sides of the wing, then relocating the receiver, batteries and servos until the airplane is balanced.

You may ask why the center of gravity is located ¼ of the wing chord? It has something to do with aerodynamic center, neutral point that can be explained in detail with some illustrations by following the link on the bottom.

When you look at the cross section of the wing, the shape is called an airfoil. Basically the airfoil consist of upper and lower camber, leading and trailing edge. When the airplane is flying, there are aerodynamic forces that interact with the wings, vertical and horizontal stabilizers because the airplane is going against the air or commonly called “relative wind”. Then it creates a variance of pressure on the upper versus the lower camber of the airfoil (or the wing itself) which generates lift. The air that passed the lower camber should have a higher pressure against the upper camber to sustain flight. This has something to do with law of continuity. The air molecules that separates from the leading edge, going to the upper and lower camber, should meet at the trailing edge at the same time. Since the upper camber has a greater curve than the lower camber, the distance on the upper camber is longer and therefore requires more velocity to meet the air on the lower camber. This creates a lower pressure on the upper camber based on the Bernoulli's theorem, "as the velocity of air increases, pressure decreases"