Hat section stiffener fitting: The hat section stiffeners have a joggle stamped into each end as they fit under the spar flange and then are flush with the skin. The joggle isn’t deep enough. Even when they look flush (I checked mine with a straight edge and it was perfect from the spar flange to the hat section), there will be a small dip when you rivet the skin. Make a small stamping jig from wood and deepen the joggle so the hat section extends about 1/16″ above the spar flange (outboard either up or down depending on wing side.)
Hat section stiffener fitting problems: In particular most of us know that the forward and aft joggle are off substantially and this caused uneven skins and high spots at the spar to hat section joint. In addition, the curvature of the hat section does not match the curvature of the wing as compared to the main ribs. In order to work these by hand I realized it would take too much time to achieve a satisfactory job.
So I fabricated male and female dies with enough spring back angle in the flanges to achieve virtually no hand work necessary to straighten the flanges. The dies also have the ability to straighten the curvature of the hat section to conform to the shape of the wing. Regarding the joggles, using 20 tons of pressure I was still unable to correct the joggles with my first set of dies and found it necessary to make a second set just for the joggles.
I think it is beyond some of our abilities to end up with a decent hat section which in turn greatly affect the appearance of the finished product. If there is any misalignment in the hat section when you rivet the skins it would be magnified in the finished product.
–Danny – Airlink Technologies
Hat sections stiffeners: Hat sections are getting a lot of attention. Quite frankly they suck. SH is working on a solution, but don’t expect one anytime soon.
There is a lot of work in getting them to fit properly. Not only are the flanges at the wrong angle etc. but, they are not properly curved to fit the contour of the wing. We did not have the benefit of the dies mentioned by Danny (Airlink Tech). I had to use a less elegant process. It was kind of nerve wracking, but it worked. I took the suckers by the ends in both hands and placed them on a high tech, precision engineered sand bag and pressed down until I could just feel them start to give. They will deform slightly just before they reach the point where they will buckle- that is where the tenseness comes in. This will usually take care of the contour, and it will help some with the flanges and the offset.
You must continue to adjust the flanges until you get them absolutely flat. Check the stiffeners for fit along side one of the outboard ribs where the skin is the thinnest and you can be sure it is following the right contour. Continue to fit and try until they lie absolutely flush with no gaps anywhere before you install them. Support the underside of the stiffeners to insure that they are lying close to the skin while you drill them. Install them starting at the outboard end and work inboard. That way if you have one that is not quite right and you are too chicken to try another trip to the sand bag you can install it under the thicker inboard skin. The hat section will conform to the contour of the skin in this area but in the outboard two sections the skin will be pulled in to fit the contour of the stiffeners. If you buckle a stiffener order a new one. It will never have its same strength once it has reached its limit of elasticity.
Alternate hat stiffener riveting method: This method assumes that you are ready to attach/rivet the lower wing skins. The leading edge skins are already installed and the rivets are squeezed with the exception of the lower inboard LE skin that overlaps the LE of lower inboard main skin. The basic idea is to rivet the hat stiffeners before installing the skins using a riveting frame and rivet gun. Riveting with the gun attached to frame is a one person job, much faster and produces superior results compared to using a helper to buck the rivets while the skins are installed. Also, for you back riveting fans, this technique is a form of back riveting. The aft end of main ribs are left unriveted and aft end of the strut support bracket is left unbolted to allow the forward spar to be lifted up 1/4″ so that the spar flanges will clear the ends of the upper hat stiffeners. If you’re confused, read on. Here’s the method:
- DO NOT RIVET THE AFT ENDS OF THE MAIN RIBS TO THE AFT SPAR or bolt the aft end of strut support bracket until later. You’ll see why.
- Take a lower main skin and cleco the appropriate hat stiffeners from the inside. Every third hole is sufficient. With the hat stiffeners installed, the skin has rigidity, but be careful handling it so it doesn’t buckle. Cleco from the inside so that the clecos do not interfere with the 2X4 supports (described next). Make sure that you have spent adequate time straightening the hat stiffener flanges. This riveting technique produces superb results as long as the stiffeners were properly prepared.
- Take your dimpling/riveting frame and put it down on the floor with a large area cleared around it, enough to work with the largest wing skin. Take four scrap pieces of 2X4s around 2 1/2′ to 3′ long. Build them up with the same length of 1X4 to match the height of your riveting frame lower tool. You may want to pad them for added protection to the outside of the wing skins. The 1/8″ packing foam that came with the kit should work fine. Spread the 2X4s around on either side the riveting frame to support the wing skin.
- Set up your riveting frame as follows: Install the riveting frame upper and lower tools for either flush or universal rivets. The shop head tool goes on the upper side. I am flush riveting just my hat stiffeners, so I have installed a 1″ wide flush riveting tool in the lower tool hole. It’s the one that I usually use in the rivet gun. (My riveting frame is from Avery. I removed the normal lower tool holder and drilled up the center hole in the channel to accept a standard riveting tool shank.) For the upper tool, use the shop head tool that came with the frame that mates to your rivet gun. It comes standard with the Avery riveting frame. This is mandatory. Don’t try to set the rivets using a hammer. I already tried it. It produces lousy results. The rivet gun approach produces a perfect shop head every time (almost). If you purchased the return spring and collar use it. The return spring is strong enough to support the upper tool and the rivet gun. It will make things much easier for single person riveting. If you don’t have one, recommend you purchase it from Avery. It’s not expensive, around $5.00. Attach your rivet gun to the upper tool. It works best to set the air pressure a little lower than normal to prevent over driving.
- With the wing skin standing up, fill every open hole with a rivet of the appropriate length. Tape over the rivets with riveter’s tape. If you have never used riveter’s tape, it is just like magic transparent tape but has removable adhesive similar to Post-It notes. The tape keeps the rivets from falling out when the skin is upside down in the riveting frame and it also protects the area around the rivet from marring.
- Now it’s time to start riveting. Place the skin upside down in the riveting frame. Adjust the 2X4s to provide adequate support. Line up a rivet head with the lower tool and drive the rivet with the riveting gun. Start easy and then add some pressure as a straight shop head starts to form. Be careful not to over drive them. It’s easy to do. With your free hand, put pressure on the hat stiffener flange on either side of the rivet until the shop head is upset enough to hold the sheets together. Drive the rest of the rivets. Then remove the clecos and fill/rivet the remaining holes. The riveting frame produces excellent results. Note, be careful if you have installed a universal head tool in the lower hole, it has more potential to mar the outside skin surface as you slide the skin around to the next rivet.
- Now it’s time to install the skin. Carefully pick it up and position the skin with the lower hat stiffener joggles inside the lower spar. A little pressure just above the joggle to slightly flex the skin makes it easier for the hat stiffener joggles to slide inside the lower spar flange. Slide the skin down as far as it will go with out buckling the lowest set of driven hat stiffener rivets. Also be careful not to scratch your spar primer. At this point the skin will probably support itself, but a helper would be beneficial. With the skin slid as low as it will go, the upper ends of the hat stiffeners are about 1/4″ too long to fit under the upper spar flange. So, gently lift up the upper spar just enough for the hat stiffeners ends to clear the upper spar flange (this is why the aft ends of the main ribs were left unriveted for now). A little pressure may be required to flex the skin enough to make the joggles clear the inside of the spar flange as the spar is lowered. With the spar lowered, the skin is secure. Now slide the skin back up into alignment and cleco the upper spar flange rivet line and shared hat stiffener holes. This will prevent the upper end from popping out when the spar is lifted 1/4″ for the next skin. Don’t cleco the lower spar holes until all the skins are in place. To help prevent scraping the spar primer, you may want to either slightly flair the last 1/16″ of the hat stiffener joggles or apply a short strip of riveter’s tape to the last 1/4″.
- That’s all there is to it. Continue with the rest of the lower skins. Be careful not to lift the spar too high on subsequent skins. This will prevent the lower hat stiffener joggles from the previous skin from popping out. I recommend a helper. Once all the skins are in place, cleco the lower spar line of rivets and main ribs and continue riveting according to the instructions. Leave the aft flanges of the main ribs unriveted until we can determine if the same technique can be used on the upper skins. See note below.
Note: I’m not sure if this method will work on the upper skins yet. I don’t have the instructions that show the top skin riveting order yet. My guess is that it may work if the aft ends of the main ribs are left unriveted and the ends of the hat stiffeners are trimmed down to the minimum edge distance length. There should be just enough flex in the lower wing skin to pop the upper skin in place. Maybe someone further along knows the answer. This technique has the potential to dramatically reduce the requirements for a helper and it produces nicer results. I hope it works for the rest of you. Drop me a line with your questions/comments.
Aligning & drilling hat stiffeners: We came up with a much faster way of aligning and drilling the Hat stiffeners for the wings. The manual recommends drawing centerlines on the stiffeners on both of the flanges. This is not easy and can be time consuming as it cannot be done with a compass and come out right. (you will see why when you see how the lines are to be drawn).
Instead of drawing any centerlines, the stiffeners can be lined up from the predrilled holes in the skin. The Wing skins are mounted and drilled. When this step is done the bottom holes for the hat stiffeners are already drilled into the spar. (You can drill these when drilling the holes through the spars). The hat stiffeners are clecoed to the forward spar as per the manual. But, instead of aligning the stiffeners with the centerlines you use the two bottom holes drilled through the rear spar for the hat stiffeners to center the stiffeners. The distance from the aft holes in the stiffeners to the nearest hole on the rear spar can be used to center the stiffener. Be careful here though as it does not always work out to be in the center of the two holes.
We used this method and they all came out perfectly straight and I am sure saved a lot of time.
Hat stiffener centerlines: A while back Rhett posted a note saying to not bother trying to mark the centerlines on the hat stiffeners. He was right on the money. Just to sum up, here’s what to do: The aft spar already has two holes in it for each stiffener (you drilled them when aligning the skins.) The stiffener has a pre-drilled hole in it for mounting it to the forward spar. Drop a vertical line on the inner surface of the aft spar flange from the centers of each of those two holes. Cleco the stiffener to the forward spar using its pre-drilled hole. The flange of the stiffener now covers up the two holes in the aft spar, but you know where they are because you can see the vertical lines you made. It is now very easy to center the stiffener on those two lines. Any misalignment is very obvious. Once you have the stiffener centered on those two holes/lines it’s a simple matter to drill the stiffener flange (from the skin side) and cleco it to the aft spar. Now drill the second hole through the forward end of the stiffener flange, again using the hole you drilled when aligning the skins. The stiffener is now securely clecoed to both the forward and aft spars. I tested the first three of these I did by putting a pen mark on the stiffener flange through all of the predrilled holes between the spars, then disassembling and checking. All of the dots made a nice line right in the center of the stiffener flange. Just in case anyone is getting discouraged thinking about doing those stiffeners, I have to say that I’m either incompetent or very lucky because all of the stiffeners ended up laying nicely flat against the skin. I didn’t have to do any of the manipulation that the manual suggests to get gaps out of the joggle areas. I spent about an hour getting the stiffener flanges to lay flat on a flat surface, and that was it. They went in easy as pie. Does that mean my wing is going to fall apart? Can it possibly be that easy? Has anyone else had an easy time with these things? Thanks for the use of the bandwidth. I know there’s no new information here but I just wanted to reinforce Rhett’s good idea.
Hat section installation: Re- steps 35, 36, & 37 Hat shaped stiffeners A. Skip step 35. B. Mark with ultra fine Sharpie a line straight down (aft) inside the aft spar from each previously drilled hole in the aft spar flange where the hat shapes will be riveted. Install hat shapes with one cleco as per step 36. Have partner eyeball the hat shapes into alignment with the Sharpie line peeking out below them on the spar flange. Partner’s backup board should be notched on the end to accommodate the “crown” of the hat. This worked great for me; the only trouble was I did’t think of it till the second side of wing no. 2.
Installing hat sections single-handedly: Looking in the SH manual, it advises that you will need an extra pair of eager hands to hold the hat section in place while you drill the mounting holes. SH says you will have some eager friends who will be only too glad to give you a hand. I checked around and the only hands I found were clapping. All those folks who thought they would be given a ride in 600 Hours are long gone after 2400.
I finally talked it over with Necessity who is the mother of Invention. The following was suggested. Make three holding clips as follows. You will need three pieces of aluminum about 1/8″ thick, 3/4″ wide and 1 3/4″ long. Using you band saw, slot it down the center for about 1 1/4″ and a “loose” 1/8 inch in width.
Put your hat section Unit in place with the locating hole clecoed to the proper hole in the forward spar flange. Now locate the two skin holes that are opposite from each other and next to the rear spar. Move the unit sideways until the edge lines up with one of the holes. With your marking pen, go to the opposite side of the unit and mark the skin next to the edge. Now move the unit the opposite direction to the other hole and do the same thing on the other side. Now center the unit between the two lines and mark both edge lines onto the skin. Using one of the clips you made, clip it into the closest spar rivet hole and overlap the toe of the hat section. With the hat section held in place, go to the other side and mark the two holes next to the forward spar and also the two next to the rear spar. Remove the one cleco and just relieve tension on the clip cleco enough to slip the unit out for inspection of the hole markings. Note any adjustment of hole placement that is needed and clip it back into position between the marked lines. Tap it sideways if further alignment is needed. Usually only a marking line width is needed, if any. Using the rest of the clips, Clamp the rear spar end and the one hole area on the forward spar. Note: cleco the clips near the edge so that the drill will miss the clip. I next drill a hole through the hat section midway between the spars. My reasoning is that this area is braced from both ends and is less liable “crawl” from drilling vibration. Once that hole is clecoed. Check the other side and if everything looks centered drill out the rest of the holes.
–Orville Eliason, Ontario, California Kit 5298
Hat Section Straightening: I found a pretty good way to increase curvature:
Obtain a standard issue 2×4 scrap and cut it into two pieces about 6″ long. Lay these down flat on the bench with their facing sides spaced apart the same distance as the wing spars.
Lay the hat section onto these 2x4s with the curvature “downward.” (Sort of looks like a catenary bridge at this point) Now, all you have to do is push down at the center or in two places, say, placed 6″ apart, with your hands. You will be able to first feel the springiness of the hat section as you push down. (Engineers call this the elastic portion of the stress/strain curve.) Depending upon how straight the section was to begin with, as you push further you will feel a slight yield, where you have actually (inelastically) deformed the hat section giving it more curvature. Some trial and error by comparing with an adjacent wing rib and you’ll have it.
The good thing is that since the center of the hat section is only about 3/4″ above the workbench, I never over-bent any of my 33 sections I worked for the left wing. The concept has a built-in safety stop (the workbench) to prevent buckling the part. But, although I did it twice, it is difficult to undo the increased curvature, so proceed with each cautiously.
I found that the curvature was easy to get right. Most of the time spent was working the joggle at the ends to match the skin contour so the skin would still be tight against the hat immediately adjacent to the spars, near the first rivet. I feel I did a moderate job at this, but its one of those things where you can spend many hours for only marginal gain.
–Mark Neubauer kit #5733