Vertical stabilizer alignment. (Long post)
Danny,
Roberta has a different horizontal stabilizer on the RV-7 than we have on the RV-9 and -9A airplanes. The only guess work is not in the OFFSET, but in the vertical alignment.
http://www.n2prise.org/rv9a031.htm
You can also look at my page 30 for some hints, but the big thing is to be sure that you have the fuselage LEVEL laterally and longitudinally before you start. You will want to check the TWIST of the fuselage also. Hopefully you don't have any, but there is a way to recover from that issue if needed. You can see where I was checking for twist at the bottom of my page 30.
OK, it helps to have a LONG golden Stanley aluminum level with a V-groove on one side. I found mine at Wal-Mart for less than $10. The V-groove is important as you will learn shortly.
First, put the long level across the longerons at the seat back location above the F-705 bulkhead. Adjust your saw horses as needed to get this level. You will also need to check the leveling from firewall to tail along the canopy deck of the cabin area. Then check the level condition across the aft deck near the tail as I did on page 30. If there is a twist shown here, you can fit the HS with some very thin shims to level it up, since you have probably already riveted the aft deck plate and whatever twist exists there is there to stay. Don't worry about it, but take note of it if it exists.
Have you already set the incidence of the horizontal stabilizer? The instructions tell you about using a particular size of drill bit to get the stabilizer rear spar to the correct height while you have the two 1/8" spacers under the attach angles at the front main spar. Roberta was correct about the need for the centerline along the aft deck plate, but only at the rear between the two 6061-T6 F-711C bars that are sticking up waiting to be match-drilled to the rear main spar of the HS. Incidence correct, then drill those holes ONLY IF the horizontal stabilizer is LEVEL from END to END.
My fuselage had a slight twist between the seating area of the fuselage and the aft deck plate. As it turned out my two HS-908 attach brackets were slightly different in height to offset the twist, so no shims were required for mounting my horizontal stabilizer in a level position horizontally.
Put your level on top of the horizontal stabilizer across it at a point just aft of the front spar and confirm that it is level, then drill through two of the holes in the F-711C upright bars to the aft spar of the horizontal stabilizer. Two will be enough to start. That way if you screw up, you can always remove those bolts and drill the other two accurately, but hopefully you have done this right.
NOW, we get to alignment of the horizontal stabilizer along the centerline of the fuselage before drilling those two holes down though the attach angles on the front spar of the HS, through the spacer plates, and through the longerons and that other angle and spacer that goes across under the front spar of the HS. I used the two fresh air vents way up on the front of the forward side skins as my reference points. They were cut by machine at the factory and are at the exact same location on each side of the fuselage. They also will hold the end of a 25-foot tape measure as long as you keep it tight. Get help to hold the tape in place if you need it.
Pick a rivet hole at the tip of the horizontal stabilizer, any hole, but use the same hole for the measurement on the other end of the HS. Check the distance to the AFT edge of the fresh air vent on the fuselage on BOTH sides. When you are sure the distance is the same within 1/16", you can drill one hole down through the correct location on the HS-908 mounting angle. But READ the FOLLOWING FIRST.
When you built the horizontal stabilizer, it said the two holes in the HS-908 would be drilled in assembly. Now you are going to LEARN WHY that is done. Even if you have marked the "supposed" location where the hole should be as shown on drawing 3, take a good look at the situation before you drill that hole. WHY? You want to be sure that you have picked a drill location that will allow room for a flat washer and a nut to clear the longeron angle that is at the bottom of this stack of metal. I had to grind off one edge of one of my washers to get it to fit on one of the bolts that goes through the longeron. The nut fit fine, but I had to hold it with an open end wrench and tighten the bolt from the top side since a socket would not go over the nut since it was too close to the longeron.
OK, now, we start heading to your question about the setup of the vertical stabilizer. You finished drilling ALL the other holes and the HS is now square with the airplane. You put on the vertical stab as shown on drawing 27A if you are building an RV-9A. It is important again to drill in place the holes through the tie-down bar and the F-912D elevator stop AFTER you have confirmed the vertical stabilizer is truely squared with the horizontal stabilizer. Measure from the top of the stabilizer skin near some reference point of your choosing near a particular rivet hole or the front spar DOWN to the outer edge of the horizontal stabilizer at another suitable reference point on each end of the HS. When the dimensions are equal, you are ready to drill the holes on the vertical stabilizer to the tie-down bar and the F-912D. If you are worried, just drill one hole in each and double check that the vertical stabilizer to horizontal stabilizer dimensions are still equal. When you are satisfied, drill the other holes.
Drawing 27A, view A-A indicates one AN-960-10 washer is glued between the F-912D and the rear spar of the vertical stabilizer at two places to get the lower and center rudder attach brackets in vertical alignment. All these bolts should now be in place and tightened. This is where the long Stanley level with the V-groove comes in handy. The front spar of the vertical stabilizer should be floating around allowing the vertical stabilizer to move fore and aft, pivoting slightly above the attach point with the F-912D. The F-981D vertical attach plate is shown in the fourth photo on my page 31. It will be used to match drill to the front spar of the vertical stabilizer in a later step. For now, it may be clamped to the vertical stabilizer front spar centered in the spar. The prints show it on the back side of the vertical stabilizer spar just as you see it in my photo. Somewhere in all this fitting you have to trim the vertical stabilizer front spar to the proper length to clear the front spar of the horizontal stabilizer, etc.
Now the long Stanley level does its thing for you. Put it up against the three rudder attach brackets and verify that all three are aligned vertically. All three sets of rudder attach brackets must touch the V-groove of the Stanley level at the same time. NOW, clamp the F-981D vertical attach bracket to the vertical stabilizer front spar so that the bracket is also firmly against the F-714 aft deck.
Way back on drawing 3, view A-A, it said to leave four rivet holes open for fuselage attachment. Well, now you get to see what gets connected there. The vertical slots on the F-981D must align with those four open rivet holes on the front spar of the vertical stabilizer. The aft edge of the vertical stabilizer and the rudder attach brackets are truely vertical and at a right-angle to the longerons of the fuselage and you have clamped the F-981D to insure the front spar of the VS stays in the correct vertical position. If you have also centered the F-981D laterally on the VS front spar, then when you move it left or right to align with the open rivet holes in the horizontal stabilizer front spar, you have aligned the vertical stabilizer offset and everything is in alignment. Back drill and cleco two holes through the VS front spar using two of the pre-punched holes in the F-981D as your guide. Double check that everything is still properly aligned vertically at the rudder attach brackets. Enlarge one of the rivet holes through the horizontal stabilizer to accept a bolt through the slot of the F-981D. This keeps the VS from moving from side to side. The jog in the F-981D sets the vertical stabilizer offset angle correctly. You can also drill one of the holes down through the F-981D, but you may have to remove the Vertical stabilizer to get clearance for your drill. You will be doing this after you have separated the F-981D from the front spar of the vertical stabilizer, using the F-981D as the drill guide for the holes that go down through the aft deck plate, spacer, and angle. That hole we just drilled in the front spar of the horizontal stabilizer will assure the correct placement of the F-981D when it is put back in place to drill the two holes down throught the deck plate, etc. After that you can cleco the F-981D back to the vertical stabilizer front spar and finish drilling the remaining NINE rivet holes. Deburr all eleven holes in the spar and rivet the F-981D to the spar.
I hope this has not bored you, but what you need to know is in there and it shows the ingenious simple design and the logical sequence the engineers at Van's have given us.
Jerry K. Thorne
East Ridge, TN
RV-9A N2PZ
www.n2prise.org
