Airfield Models - Plot an Airfoil

Plot and Draw an Airfoil

January 21, 2009



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Airfield Models (http://www.airfieldmodels.com/)Plotting and Drawing an Airfoil

This is the third and final installment of this series.

Part 1 of this series provides some background, explains coordinate standards and provides sources for obtaining airfoil ordinates.

Part 2 explains how to calculate the ordinates that are absolutely required before the airfoil can be plotted.

This part provides a details how to plot the ordinates on paper and then draw the airfoil.

 
 

Plotting the Airfoil

Drafting tools to draw airfoil. An airfoil can be drawn with a minimum of drafting instruments.

You will need a sharp pencil, accurate scale (ruler), and a good curve.  I use ship curves because they better match the shape of an airfoil.  French curves are more common, but tend to have curves that are too sharp.

If you do not want to buy ship curves then an adjustable curve might work.  I've tried few different types of adjustable curves and none of them were satisfactory to me.  Your results may vary.

If you must use French Curves, try to find one that is at least twice the length of the airfoil you are drawing.  You can also bend a stick of wood which is surprisingly accurate.  I use a piece of 1/8" x 1/4" spruce to draw long curves, such as fuselages, when I draw plans.

The calculator only needs to be able to multiply, so any calculator will work.

Draw the centerline, leading edge and trailing edge locations. Draw a centerline slightly longer than the airfoil chord.

Draw lines to represent the front of the leading edge and the rear of the trailing edge.

The chord of this airfoil is 9" so that is the distance the lines are spaced.

Make tick marks along the centerline to locate the ordinate stations. Make tick marks along the centerline to indicate the station locations.

The intersection of the leading edge and centerline is point (0, 0) for this ordinate standard.

Some ordinate standards have the trailing edge as point (0, 0).  If you aren't sure what standard you're using, just plot the points.  If you are using the same standard as I am here, the airfoil will point to the left.

If you plot the points backwards, the airfoil will point to the right.  Either way you end up with the same airfoil.

Draw the station locations. Draw vertical station lines through the ticks you made in the previous step.

If the stations are different for the upper and lower portions of the airfoil then you should probably make ticks for one side.  Then draw the lines.  Repeat for the underside of the airfoil.

Plot the airfoil ordinates. Tick off the ordinate locations at each station.

The trailing edge of this airfoil tapers to 0" thickness.  However, I will sheet this wing with 1/16" balsa.  That means I will have to fudge the airfoil somewhat to account for the sheeting.

The next image actually represents two steps combined into one.  I neglected to scan the drawing between steps.

Draw the Airfoil Outline. Draw the slope of radius through the leading edge.

Slope = Rise over Run = y divided by x.  In this case the Slope is 0.1.

To draw the slope line, start at point (0, 0).  Measure back 1" (x) along the airfoil centerline and from there measure up .1 inch (y).  Draw a line through point (0, 0) and the point you just marked.

The center of the circle representing the leading edge is found on the slope line by measuring from point (0, 0) to a distance equal to the radius of the leading edge.

For example, if the diameter of the leading edge is 1", then measure back 1/2" (radius) along the slope line.  That is the center of the circle that represents the leading edge.  Draw the circle.

Using curves that match the point best, draw the airfoil outline.  Normally I use several different curves by selecting the curve that best matches the airfoil in any given section.

The airfoil is tangent to the leading edge.

Because of the thickness of the sheeting, it is not possible to draw the exact outline through the plotted ordinates.  However, the finished product will be close enough that in our realm, nobody would notice the difference in the flight characteristics.

Be as accurate as you can but do what needs to be done to make the wing something that can actually be built and not just a theoretical ideal.

 
 

Establishing the Rib Pattern

The airfoil outline is complete but it can't be used as is.  The actual outline will not be a part of the pattern unless the wing has no sheeting.

What we need to do is subtract the thickness of the sheeting from the pattern and draw the location of structural details such as the Leading Edge, Sub-Leading Edge, Spars, Ailerons and Trailing edge.

The order in which you do these things in does not matter as long as you know what's what.

Draw the Leading Edge, Spar and Trailing Edge locations. In this example, the Leading and Trailing Edges are 1/4" wide.  The Sub-Leading edge is 1/8" wide and the Main Spars are 3/8" wide.  The Aileron is 1-1/4" wide.

Main spars are located where the wing is thickest for maximum strength.

Draw the Rib Pattern. Work your way around the perimeter of the airfoil and make tick marks inside the airfoil outline to indicate the thickness of the sheeting.

Arrange the scale so that it is perpendicular to the airfoil at the point from which your are measuring for best accuracy.  I normally eyeball this, but better would be to use an adjustable triangle that is adjusted to be perfectly tangent to the airfoil at each point.

Make enough tick marks to draw an accurate outline.

The completed Rib Pattern. Draw the rib pattern using the tick marks.  The rib pattern outline should be parallel to the airfoil.

Finish any other details necessary.  If some ribs are different than others, which is usual, then you should probably cut two patterns at the same time.

For example, you may want to add landing gear cut-outs to the second pattern.  I usually draw unique cut-outs directly on the ribs after cutting them out using a single master pattern.  Matching ribs from each wing panel are stacked and cut at the same time.

I tend to draw my pattern and glue it directly to whatever will be used for the template.  If you think you might want to save the original drawing then make copies.

Most copiers do not make exact size reproductions

Copiers either enlarge or reduce from the original to a small degree.  Usually it is by such a small difference that it is not a problem.  Be sure to check before you start cutting patterns and ribs.  If the pattern off by an unacceptable amount you'll have a really bad day if you find out that the wing you just built doesn't fit.

Also see

 
 

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How to Calculate Airfoil Ordinates
Using Shear Webs in Model Aircraft Wings

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Copyright 2004 Paul K. Johnson