Saturday, March 04, 2006

Setting Washout - 1 hour (186 Total)

With the help of a couple of friends, I set the washout of my wings. Rans design calls for 1/4" of washout measured between the root of the wing and where the lift struts meet the wing. This raising of the trailing edge spar gives the wing tip a slightly reduced amount of angle of attack. This makes the plane more stable by causing the wing tip to stall at a lower speed than the root of the wing so when a stall occurs the plane will be less likely to "drop a wing."

Since my wing fabric is already installed, I needed to find another method to set the washout other than the method Rans describes in their manual. My approach was to attach a large piece of paper to a straight edge and hold it against the bottom of the wing near the root. I drew a horizontal line on the paper 43.5" from where the straight edge meets the wing. This is the same distance as the center to center spacing of the Leading Edge Spar and Trailing Edge Spar. I then used a plump-bob to mark off where the string intercepted the long horizontal line. This is my base line mark.

I then moved the jig to the location the lift struts meet the wing and made a second mark. The goal is have the second mark be 1/4" forward of the base line mark. I adjusted my adjustable lift strut until I achieved this.

Because of the fuel tank and the fuselage got in the way of my straight edge, I wasn't able to make my baseline mark directly at the root. I took my base line mark ~33" from the root. Since the lift struts meet the wing at ~108", my washout measurement should be 70% of 1/4" or 0.175".

I ended up setting the washout to pretty close to 1/4" because one half turn (minimal adjustment) of the eyebolt in the adjustable lift strut mechanism caused a 1/8" change in the washout! I decided it was better to have a little more washout than too little. Since it's adjustable I can always make changes later!