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Practical Model Design
Radio controlled model aircraft can be designed using some basic rules of thumb or more appropriately, design paramaters. These basic design parameters can be applied to a trainer or sport model. There are no complex or magic formulas to solve. These paramaters have been proven to work by a multitude of sport models that have been developed using these rules. A modeler who has built a few models and has gained some knowledge of common structures can design a plane that suits his individual needs.
The design begins with selecting the size of engine that will be used. This will become the determining factor for the entire design. The wing area is first selected from the table.
After selecting the engine size and wing area, the next step is to determine the wingspan and wing chord that will give this wing area and an aspect ratio between 5:1 and 6:1. If .40 size engine is selected, the wing area will be 500 - 700 sq. in. To make things simple, and area of 600 sq. in. and a span of 60" is chosen. This will give a chord of 10" and an aspect ratio of 6:1. The rest of the design will be based on the chord length.
The next step in determining the configuration of the wing is selecting the airfoil according to the purpose of the model.
Programs can be downloaded that will draw one of a multitude of airfoils. Airfoils can also be plotted manually using the coordinate dimensions to draw points on the airfoil and drawing the curve of the airfoil using a French curve or flexible rule. The airfoil that is selected should have a thickness of 15% - 18% of the chord at 30% - 40% from the leading edge and should have a blunt leading edge for gentle stall characteristics. The wing incidence is normally set to 0°. The dihedral will be 0° - 3° with ailerons and 3° - 5° without ailerons. Finally, the type of ailerons that will be used is selected and the size determined according to the chord.
The fuselage length is now calculated using the 10" chord. The nose will be 10" - 15" and the tail will be 20" - 24". Taking the median dimension of these, the fuselage length will be 44 1/2" (12.5" nose + 10" chord + 22" tail). The engine thrust is usually set for 0° - 3° down and 0° - 3° to the right. The landing gear is selected as a matter of preference. A conventional landing gear is set even with the leading edge of the wing. The main gear of a tricycle landing gear is placed 1 1/2" behind the center of gravity. The width of either main gear is 1/4 of the wingspan.
The stabilizer area will be 20% - 22% of the wing area. The area for the 600 sq. in. wing would be 126 sq. in. nominal. The aspect ratio for the stabilizer is 3:1. Using a stabilizer chord of 6 1/2", the length of the stabiler would be 19 1/2" and the area would be 127 sq. in. The elevator is 20%; of the stabilizer area or 25 sq. in.
The fin is 1/3 of the stabilizer area and the rudder is 1/3 - 1/2 of the total fin area. For the current example, the total area of the fin would be 42 sq. in. and the rudder would be 21 sq. in.
The type of structure that is designed will depend on the use for which the model is intended and the personal preference of the builder. The slab sided fuselages are easier to build than the truss work structures but are also heavier and stronger in most cases. Foam wings are easier to build than built up wings but are heavier and more accurate. A little knowledge of structure goes a long way in the design of a model. In many cases, a modeler will design using the structural configuration of another model and simply change the appearance or the size of the model.
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