CRAFTSMAN'S CORNER

Ben Owen

HOW TO DRAW AN AIRFOIL By HARRY RIBLETT

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416RiblettLn. Wilmington, DE 19808

First of all you need some paper to plot the airfoil on. I suggest that you buy a pad of 11x17 reproducible (vellum) graph paper, 10x10 squares to the inch, from your local drafting supply store. Now tape 3 or 4 sheets together lengthwise, depending on the length of the airfoil, keeping the lines nice and straight. Now, obtain the airfoil coordinates from a catalog of airfoil tables. These are standardized for an airfoil length of 100 inches, so we must scale them down to the desired airfoil length. For example, I have chosen the NACA 4415 airfoil at 60" length, so we multiply both the "station" (X values) and the "upper and lower ordinates" (Y values) by 0.6 to obtain the ordinates for plotting. Draw a straight horizontal line (chord line) and mark it off in inch increments. Now, simply plot the points, and connect the dots with a smooth curve, as shown on Figure 1. A set of long French curves, known as "ship curves," makes the job easy. For the leading edge, all NACA airfoils specify a "slope and radius" method for layout. The 4415 airfoil lists "L.E. radius= 2.48, slope of radius through the L.E.= .20". Lay out a construction line beginning at the origin (O-O position) with a positive slope (upwards and to the right) of .20, which is a 1" rise for 5" run. Notice that we do not scale down the slope value. Now mark off the radius, in this case 2.48(.6)=1.49 inches, from the origin, along the sloped line. This point is the center of the leading edge circle, which you draw with a compass. Notice that the circle passes through the O-O point on a slight angle. Fair the upper and lower surface curves into this circle. Now that I have told you how to plot the NACA "slope and radius" LE., my next advice is ... don't use it. Notice from Figure 1 that this method actually raises the true leading edge above the chord line, and this aggravates leading edge separation at high angles of attack. Unfortunately, this was an honest mistake made by the NACA engineers in the 1 920s and '30s when they designed the NACA airfoils, all of which contain this mistake. You 78 DECEMBER 1992

NACA 4415

60" Chord Length

Nose Section should use the corrected airfoils, designed using the direct combination of ordinates. Contact NASA Langley Research Center, Hampton, VA 23665-5225. Figure 2 shows the difference in shape. Better yet, use equivalent airfoils that have been designed using constant velocity mean lines (not shown) such as GA airfoils, available from the author. Distortion - Blueprints can now be made from your airfoil drawing, and they are sometimes slightly shorter or longer than the true length, depending on the accuracy of the blueprint machine, paper stretch or shrinkage, etc. Not to worry - simply

find the quarter-chord point of the actual outline, and make all your true measurements centered on that point. (Editor's Note - Always match spar centerlines to the drawing so that your wing matches the fuselage.) Fat Ailerons - Always make the ailerons slightly fatter than the true airfoil shape. Better to be slightly oversize than undersize. Blunt Trailing Edges - Cut the airfoil off sharp and square at 99% of chord. The air flow is separated beyond that point, so you might as well cut off the airfoil there. The airfoil won't know the difference, and it will still act as though it is 100% long.

NACA 4415

60" Chord Length Modified by Harry Riblett

Nose Section

Modified by Direct Combination of Coordinates

Figure 2