The Clark Y is an airfoil widely used in aircraft wing design. The Clark Y airfoil provides a smooth stall entry, well suited for sport RMC models. The airfoil has a flat bottom shape that makes for easy building directly on your construction board. I have employed the Clark Y for all my model plane designs.
Drawing an airfoil is not hard. You can use the ordinate method described below to draw a Clark Y airfoil. An alternative approach is to use TurboCAD. A significant advantage of the TurboCAD method is that you can quickly draw versions of the Clark Y rib shape for any size design you might prepare in the future.
Selecting an airfoil is a crucial design decision. Airfoils vary from a symmetrical shape, typically used with aerobatic aircraft to the use of a flat wing. Flat wings are used with lightweight 3D indoor flyers. Their flight performance is excellent due to their small size and high thrust to weight ratios. The motor power actually exceeds the weight of the aircraft.
The Clark Y is an ideal choice for a wide range of RC models. The Clark Y has reasonably high camber. This allows for an efficient lift to drag ratio on a typical sport RC flyer.
Airplanes flown in the first decade of flight often had a highly under cambered airfoil shape. With an under cambered airfoil you can clearly see the inward curvature on the underside of the wing.
An under cambered airfoil can be usefully employed for lighter weight RC models of antique aircraft. As an example, I used the top outline of a Clark Y airfoil for under cambered airfoil of my 1912 Blackburn Type D monoplane design.
An under cambered wing cross section usually does not have sufficient depth for the structural wing spars. To maintain wing strength the designer has two choices. You can either use a deeper wing section to embed the spars or employ functional wing to fuselage rigging wires.
The Clark Y shape was developed in the early 1920s by Mr. Virginius Clark. The airfoil bottom is flat from around 20 percent of the chord to the airfoil’s end. The flat bottom permits easy wing construction as the wing ribs are pinned directly onto the building board. This building approach certifies a warp free wing.
The Clark Y airfoil has plenty of mid-section volume. This allows a model plane designer to include spar structure as required. The volume within the Clark Y airfoil allows the placement of other equipment. These functional items typically include aileron and flap servos plus retractable landing gear.
Use tabulated ordinates to sketch the airfoil. The top numbers are the percentage of chord back from the airfoil’s leading edge. The second line (Upper Mark) is the top of the airfoil from a datum line as a percentage of the airfoil’s chord. The third line (Lower Mark) is the airfoil bottom. This is measured from the datum line as a percentage of the airfoil’s width (or chord).
For this demonstration let’s use an airfoil chord of 10 inches. This will make the calculations easy. I’ll use TurboCAD. You can certainly draw the airfoil by hand with a pencil and a French curve.
Division lines are drawn aft of the airfoil’s leading edge. Prepare these lines at 2.5%, 5%, 10%, etc. as shown in the chart. Add the upper and lower marks on each division line. Click on TurboCAD’s Bezier curve to join the intersections to draw the final airfoil shape.
You will find it an easy task to use a French curve to connect the upper and lower marks on the division lines if drawing the airfoil by hand.
Author: Gordon McKay