Tuesday, August 14, 2012

Tight Fit – Almost Tight As OJ’s Gloves

At DOG Aviation we have a knack for making the easy complicated. Case in point, striving to place as many solid rivets as possible into the stabilator assembly. The idea was kicked around for a few days and what I feel is an effective approach was devised.

To be honest, had it not been for the fact that Van’s CNC parts fit like a glove, I would not have attempted changing the assembly order of the stabilator box spar to accommodate a hand full of solid rivets. The procedure I used was fairly simple and easy to accomplish once finalized and checked by a dry run to make sure things would work out correctly.

Having a plan at hand, the spar components were dimpled to accommodate the flush rivets I’m using on the skins… the spar caps were already dimpled in a previous session so the dimpling began on the forward and aft spars but only at those locations where the left and right skins have rivets setting into the spars … basically everything between the two innermost inspar ribs is left undimpled along with the holes that Van’s called out for countersinking. The reason I chose not to countersink is because in order for a 1/8” flush rivet to sit flush in .040 material the countersinking process leaves a knife edge at the base of the hole. (Not good for shear forces on the rivets). The spars are .040 and there is just no metal left at the base of the hole … if the spars were any thicker, I would have countersunk them.

Because I wanted to distort the metal on the forward and aft spars as little as possible I tried to devise a method whereby I could ensure the pneumatic squeezer was as square as possible with the spars before setting the dimples. I came up with what I thought was a clever solution … I bent a piece of metal to a perfect 90 degrees and clamped it onto the pneumatic squeezer to use as a guide. Then the metal piece was parallel with the spar I set the dimple … it worked out quite well.
Metal piece bent at 90 degrees to use as a guide to ensure the squeezer is held perpendicular to the spar. As many dimples as possible were made with the squeezer clamped to the bench … however the guide worked quite well for the rest of the dimples made holding the squeezer by hand.

After marking all the spar box components for proper orientation, the assembly order used was the following:

First the two innermost HS-1209 spar cap spacers were riveted onto the HS-1211 upper and lower spar caps with solid rivets.
                    Upper and lower HS-1211 spar caps with innermost HS-1209 spar cap spacers riveted in place.

Note: To prevent the thin flanges from lifting during the riveting with solid rivets, remember to slide a small piece of vinyl tubing or a rubber “O” ring over the shaft of the rivet tall enough to compress the parts together before the rivet begins to set. This makes consistently tight joints.

Next the two innermost HS-1212 inspar ribs were riveted onto the HS-1202 forward spar using solid rivets. Note: The innermost HS-1212 inspar ribs can be riveted to either the front or aft spar (but not both) with solid rivets so it is builders choice. I chose the front spar because the rear spar will have almost all solid rivets connecting the ribs to the spar.
                                   The two innermost HS-1212 inspar ribs riveted to the HS-1202 forward spar.

At this point the two innermost HS-1204 ribs were riveted to the HS-1202 forward spar with solid rivets in the holes outboard of the each of the HS-1212 inspar ribs previously riveted.
                                        HS-1202 forward spar with the two HS-1204 ribs attached along with
                                        the two innermost HS-1212 inspar ribs.

The HS-1211 upper and lower spar caps were then mated to the HS-1202 forward spar/HS-1212 inspar rib/HS-1204 rib assembly and Clecos used to hold alignment. Once good alignment was achieved, the HS-1211 upper and lower spar caps were riveted to the HS-1212 inspar ribs with solid rivets. I was able to reach the center hole with my squeezer so did not use a pop rivet there and that hole was previously countersunk to 100 degrees for the solid rivet I used as opposed to 120 degrees for a countersunk rivet.
                                            HS-1211 upper and lower spar caps Clecoed to the HS-1212 inspar
                                            ribs and HS-1202 forward spar assembly.

Before mating the HS-1203 aft spar to complete the spar box, the two HS-1214 inboard rib clips were riveted onto the HS-1203 aft spar with solid rivets. The rest of the clips will be installed with solid rivets after the remaining two HS-1212 inspar ribs are attached later.
                                Two inboard HS-1214 rib clips riveted onto the HS-1203 aft spar with solid rivets.

Because I’ve dimpled the spars as opposed to countersinking, I know it will be a tight fit getting the HS-1203 aft spar to seat in the area of the two HS-1212 inspar ribs.  In an effort to reduce the pressures necessary to get the HS-1203 aft spar properly seated, the two outermost HS-1212 inspar ribs have been purposely left out. By doing this, the only contact point with pressure will be just the one hole in each of the HS-1212 inspar ribs the HS-1203 spar mates to. (Actually there are two holes, but I seated one row of holes first with the spar on a small angle so it seated into the dimples then slowly worked the other side of the aft spar onto the box assembly). Once the HS-1203 aft spar is mated creating the sopar box, Cleco the aft spar to the two HS-1212 inspar ribs.

With the dimpling I knew it would be a tight fit … Yep, it was. In fact, the fit was almost as tight as the gloves in the OJ Simpson trial.  And I almost produced the blood to prove it when my fingers slipped … fortunately the good job of deburring parts resulted in just a little skin abrasion.

I love it when a plan comes together nicely.
                                      HS-1203 aft spar successfully mated to complete the spar box assembly.