Gavin Magills Online KR2S Build Log

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Vertical Stabiliser

My Vertical Stabilser and rudder will utilise the NACA63009 profiles sourced from Mark Langfords web site. These profiles differ from the standard KR2S profiles by being 2" higher at 45" and use a different airfoil cross section. The increased height should improve the effectiveness of the rudder and using the NACA profile should also make the vertical stabiliser more effective while reducing drag.

Tuesday 13th October 2009. Given I had some time available I made a start on measuring up the positioning of the rudder post on the rear of the upper longerons. The rudder post is part of the Vertical stabiliser so I have started a new page to detail the build of the Vertical Stabiliser.

Wednesday 14th October 2009. I initially needed to work out how much longer the top longerons will need be than the lower ones given the angle of the tail post is 7 degrees. To do this I use the following trig calculation.
Using a triangle with an adjacent side of 11 inches (the height of the fuselage at the tail) and a theta angle of 7 degress (the angle of the tail post from vertical) I need to find the length of the opposite side

Trigonometry says  tan(theta) = Opposite / Adjacent
        therefore  tan(7)     = Opp / 11
              ==>  0.123      = Opp / 11
              ==>  Opp        = 0.123 * 11
              ==>  Opp        = 1.35" or 1 7/20"

This tells me that given the total fuselage length of 145.5 inches as shown on the plans, I need to add 1.35" to the top longeron to allow for the angle of the tail post. I had already worked out the angles manually and measured a figure of 1 11/32 which you can see in the photo. This is equal to 1.34" which was pretty close to the 1.35" calculated above.

After marking in the total length of 145.5" + 1.35" on the top longeron, I measured back 5/8" for the width of the tail post itself and then 6mm for the tail post doubler. This gave me the cutting line for the top of the upper longerons. I then drew in a 7 degree line from this line. I checked and triple checked my figures before starting these cuts as there is no going back once I start. I used a hand coping saw as shown in the photo to make the cuts.

And this is the result. I still need to clean up the ends of the longerons to remove the cut marks but will wait to do this once I have the cut out the doubler to use as a guide.

After cutting the top longerons I then made up the 6mm doubler for the rudder post. I checked the plans and then confirmed my calculations for the height and width of the doubler with those on Darren Cromptons site. The doubler is 18" in height and is 1.625" (1 5/8") wide for the bottom 12", narrowing to 1.52" inches at the point where the rudder template meets the doubler (5" above the top longeron). The forward face of the doubler is offset from the rear of the fuselage by 22mm. (5/8" (approx 14mm) + 6mm = 22mm) so this needed to be allowed for when drawing in the lines where the longerons meet the doubler. Also the sides of the doubler are cut at an angle of 2.54 degrees to match the sides of the rudder post itself. The picture shows the doubler with the lines where it engages the longerons drawn in.

Thursday October 15 2009. Tonight I drew up a diagram to show all the various angles and so I could work out the height for a temporary spacer to sit between the upper and lower longerons. This will give me the the correct distance between the longerons.

This is the spacer fitted between the longerons.

Sunday October 18 2009. With the temporary spacer fitted, I placed the doubler against the existing top longeron cut to check the angle and to make sure the doubler engages the bottom longeron at the line I have calculated it should intercept to give a 7 degree angle. It was pretty close but still required a little sanding to get the angle correct. This gave me an opportunity to clean up the cutting lines on the top longerons.

With the top angle sorted I then clamped the lower longeron firmly and used a laser to check the alignment of the top and bottom set of longerons. You can see the laser line on the forward face of the spacer block between the two longerons. This task required the assistance of my wife as you need at least two sets of hands to secure everything in place while the clamps are tightened.

With everything checked, I then cut the lower longerons using my hand coping saw.

Once I had cleaned up the ends of the lower longerons with a file and coarse sand paper I then set up the doubler for glueing. I used the laser to again ensure the doubler was aligned vertically and centered. Once I was satisfied I had everything aligned, I drilled four 1.6mm holes through the doubler and into each longeron to use as guides for panel pins when glueing.

With the glue applied I lightly hammered the four small panel pins into the guide holes to ensure everything was aligned. You can just see the head of the pins in the photo at right.

Once I had everything set up I checked the angle of between the top longeron and the doubler and found it to be slightly less than 7 degrees for some reason. It took me a bit to figure out that the weight of the clamps was bending the longerons down sufficiently to reduce the angle. I quickly cut a support to lift the top longerons back up to straight and level and the angle between the longerons and the doubler came back to 7 degrees. You can see the support in the photo at right and the builders bevel set at 7 degrees on top of the longeron up against the doubler.