The Clark Y Airfoil is Ideal for RC Model Aircraft

Complete instructions on how to plot out a Clark Y airfoil Outline of a Clark Y airfoil

Clark Y is the name of an airfoil that is widely used in aircraft design. With its predictable and gentle stall characteristics and flat bottom outline, the Clark Y airfoil is an ideal choice for sport RC model aircraft. I have used the Clark Y airfoil for all my model airplane designs, from the Electro Aviator to the Finch.

See later on in this article for complete instructions on how to plot out a Clark Y airfoil from tabulated ordinates. Drawing up your own airfoil shape is very easy to do. You can use the tabulated ordinate method to draw a Clark Y airfoil by hand, or use TurboCAD for the drafting task. You can then use TurboCAD to draw additional versions of the finished Clark Y wing rib shape for any length wing rib your model aircraft design requires.

 

Table 1: Ordinates to draw a Clark Y airfoil. All figures are a percentage of rib chord (top line), with percentage of chord upper and lower airfoil points on next two lines

 Selecting an airfoil is one of the most important decisions that a model airplane designer has to make. Airfoils can vary from a complete symmetrical shape, usually used for aerobatic RC models, to a no airfoil at all. The employment of no airfoil, in other words a flat wing, is used only on lightweight electric powered flyers. The flat wing usually lends itself best to smaller aircraft used for indoor 3D flight. With this type of model, the thrust actually exceeds the weight of the aircraft. As a result, the shape of the wing’s airfoil is really not all that critical. A flat wing will not apply for larger or heavier model aircraft, where an airfoil shape must be employed.

 

Table 2: Percentages from Table 1 are used to determine the upper and lower marks for this example Clark Y airfoil which is ten inches in length

Antique airplanes, incorporating designers’ early knowledge of airplane building techniques and aerodynamics, often had an under cambered airfoil shape. An under cambered airfoil can be captured for lighter weight scale RC model airplanes of antique aircraft. I used the top outline of a Clark Y airfoil for my 1912 Blackburn monoplane wing’s airfoil shape.

 

Initial plot of upper and lower points for the sample airfoil

As a general rule an under cambered wing cross section does not have enough depth for the strong spars needed for a larger aircraft. To maintain wing strength, the two options are to use a deeper wing section, as available with a Clark Y airfoil, or to employ functional wing to fuselage rigging wires in your model airplane design.

Initial bezier curve outline of the airfoil

The Clark Y airfoil was designed in the early 1920s by Virginius E. Clark. As you can see from the Clark Y ordinates, the airfoil bottom is flat from essentially 20 percent of the chord to the airfoil’s rear end. This flat bottom shape makes for simplified RC model airplane construction, as the wing ribs can be pinned directly onto the building board to guarantee a warp free wing.

 

Construction lines are removed from airfoil sketch

The Clark Y airfoil is a great choice for the RC model airplane designer. The Clark Y has high camber, which allows for an efficient lift to drag ratio on a typical lightweight RC model. For the benefit of all users of this airfoil, the Clark Y offers predictable and gentle stall characteristics.

Finished Clark Y outline and two examples of a complete wing rig outline

The Clark Y airfoil has a deep mid-section that allows a model airplane designer to add whatever structure and wing spar arrangement that is needed for a particular wing layout. In addition, the generous volume within the Clark Y wing shape allows the placement of other functional equipment such as aileron or flap servos and retractable landing gear.

The tabulated ordinates are easy to use to draw your own airfoil. The top set of figures is the percentage of the chord back from the leading edge of the airfoil. The second line (Upper Mark) is simply the top of the airfoil from a datum line as a percentage of the airfoil chord. The third line (Lower Mark) is the bottom of the airfoil shape, measured from the datum line, as a percentage of the airfoil chord.

Draw with CAD or by hand

For our example of drawing a Clark Y airfoil, I will use an airfoil chord of 10 inches to make the math easy. I used TurboCAD for the drawing, although you can easily draw the airfoil by hand on paper with a pencil and a French curve.
Division lines are drawn back from the airfoil leading edge at 2.5 percent, 5 percent, 10 percent, etc. as shown in the ordinates chart. Next add the calculated upper and lower marks on each division line. I then used TurboCAD’s Bezier curve to join the intersections for the final Clark Y airfoil shape. If you are drawing the Clark Y airfoil by hand, use a French curve to connect the upper and lower marks on the division lines.