Basic Glider Physics Washout

Originally by Kurt Krempetz for AMA Glider

When discussing glider designs, whether indoor or outdoor hand launch gliders (HLG) or catapult launch gliders (CLG), the subject of washout is typically brought up. Washout is usually thought of as twisting, shaping/sanding or adding an up tab to the trailing edge of the tips of a wing, so wing tips are at a negative angle of attack, compared to the root of the wing. It is believed that washout is desirable because the tips stall first and putting the tips negative, compared to the rest of the wing, prevents this from happening. One can view the “Lift Coefficient vs. Angle of Attack” graph to understand why this theory is believed to be true. Also, it is important that both wingtips are washed out equally; otherwise, a roll or aileron effect is created. Yaw makes the washout issues even more complicated, so to keep things simple, no yaw is made. 

Currently, there are two methods typically used in glider designs to add washout to a wing.  One method is the typical way of shaping/sanding the wingtips. The other is to cut the dihedral joint skew to the centerline of the wing.    

Typical Washout Method

For years, washout appeared in many glider designs. Bending or sanding the wingtips at the trailing edge up about 1/16” added washout. This was thought to improve the glide of the model, along with the transition from launch velocities to glide velocities. The disadvantages of washout are added drag, and the possible added roll to the model. Bending tabs on any flying surface is thought to be very velocity dependent. This means that the characteristics of the model will change dramatically with velocity. Since gliders go through a large range of velocities, this is thought to be a concern. Also, at high velocities, the concern of the up tab bending or flexing is an issue. When parts of a glider flex/bend or the model is velocity dependent, the trimming of the model becomes very difficult.

To quantify the washout using this typical washout method, it was decided that the change in angle of attack is the parameter of most interest. To calculate this change in angle of attack, some trigonometry was applied.  

The tangent function is defined as:  

          Tan q = Opposite/Adjacent

To calculate the washout in terms of angle of attack, set a reference line that passes through the front point of the leading edge and trailing edge of the normal airfoil. Next, set a line that passes through the front point of the leading edge and trailing edge where the trailing edge has washed up. Calculate the angle between these two lines.          

          Tan = height washed up (H)/wing chord at this location (Wc)

Or      

          q = Arctan (H/Wc)

The following dimensions are taken from Super Sweep plans. To get an average angle of attack, the dimensions were taken at the midpoint between the dihedral joint and the end of the wing.  

          Up tab = 1/16

          Wing Chord at wingtip locations = 2 “

Therefore, the change in angle of attack is:

          q = Arctan (.0625/2)

Or doing the math

          q =1.79 0

Now, this is an approximation, since wings are typically tapered or some other interesting elliptical shape. The angle of attack typically decreases as you move to the tip of the wing; this is not a constant number using the typical washout method. Still, an approximation is better than nothing and some numbers are needed, so intelligent design choices can be made.

The Dihedral Washout Method

Recently, in the last 30 years, many glider designs cut the dihedral joint (poly-dihedral designs, outer dihedral joints only) skew to the centerline of the wing.  Ron Whitman’s Super Sweep model had this feature, but it’s unknown whether he originated this idea.** After spending many hours talking to some great modelers about this subject, it was concluded that cutting the dihedral skew to the centerline of the wing can add washout or wash-in.

** I recently received correspondence from Lee Hines. He informed me that he used the dihedral joint method of glider washout technique in the early 1950’s. He believes to be the originator of the technique. (Update as of May 2007)

Some paper models best illustrate the concept.

  1. Take two pieces of paper that are stiff enough to hold some shape. With a pen, draw lines that are parallel to the centerline of the wing/fuse on the paper. This is basically the way the air flows across the wing when the wing is at flying level and there is no yaw (This assumes a 2D model with no circulation around the wingtips).
  2. Bend one piece of paper into a dihedral angle with the dihedral joint parallel to the centerline of the wing. On the second piece of paper, put a skew angle outward (45 degrees).
  3. Next, lay these pieces of paper on a flat board and measure the height from the paper to the board at both the leading and trailing edges on each of the pen lines. Since the paper is lying flat on the board, these measurements are 0 for both the leading and trailing edges, until you get to the dihedral joint.
  4. After you are finished measuring, note what happens. For the one with the dihedral joint that is parallel to the centerline of the wing/fuse, the height changes as you measure to the tip, but the height of the trailing edge and leading edge are equal at each pen line. The angle of attack of the wingtip has not changed, compared to the rest of the wing.
  5. Now, measure the one with the dihedral joint pointing outward, like the Super Sweep design. After the dihedral joint, the leading edge height is smaller than the trailing edge height at a specific pen line, essentially putting the tip negative, compared to the rest of the wing. From this, it can be concluded the wing has been washed out.

The advantage of adding washout by using this method is that it eliminates many of the disadvantages mentioned with adding washout the typical way. The disadvantage of using this method to add washout is that the whole tip is at the same angle of attack, where with the typical washout method, the angle of attack decreases as you move out further on the tip.

Again, to quantify the washout using this dihedral washout method, it has been decided that the change in angle of attack is the parameter of most interest. To calculate this change in angle of attack, you need to apply some trigonometry.  

Most plans do not give the angle, just the distance off of the dihedral joint, which is parallel to the centerline of the wing (X).

          Tan q = Distance off of the dihedral joint (X)

                                    Wing Chord (Wc)

Now, Tan f = Y/X, where f is equal to the dihedral angle.

Substituting in for X where X= Tan q* Wc and solving for Y, the following equation is derived:

          Y = Tan f * (Tan q * Wc)


Now, calculate the angle of attack.

          Tan Y = Y/Wc

 

Substitute in for Y; Y = Tan f * (Tan q * Wc) and solving for Y:

          Y = Arctan (Tan f * Tan q)

 

Again, the following dimensions are taken from Super Sweep plans:

          X = 1/16

          Wing Chord (Wc) = 2.945 “

          Dihedral angle = 17 degrees (1.25” high tip)

 

The skew angle for the dihedral cut is:

          Tan q = .0625/2.945 = .02122

 

Therefore, the change in angle of attack is:

           Y = Arctan (Tan 17 * .02122) 

Or doing the math

           Y = .37 0

The issue whether a good glider design should have washout in a wing is still not understood or settled. What is clear is that there are at least two ways to create washout in a wing. The two methods that were described both offer some advantages and disadvantages.

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