402.5 hours on the hobbs - fabricated 3 riblets for trim tab and elevator

After a ton of research on Steve Riffe's and several others builder sites and logs, and a break in our super chilly weather here in Denver, I was able to fabricate the two end riblets for the trim tab, as well as the end riblet for the left elevator cutout which sits adjacent to the outboard edge of the trim tab. I also have to say that I did get a little distracted listening to our Denver Broncos pull out another miracle finish against the Chicago Bears this afternoon, to take sole possession of first place in the western division. Go Broncos!

This weekend was slow in terms of airplane building. Had to do the annual Xmas shopping yesterday, and got a new Smart phone as an early present, which I of course I just had to mess around with this morning. So the weekend airplane work did not actually commence untlil about noon today.

As a preface to the coming photos, let me just say this: Did I save any time on my build by choosing to make riblets instead of bending the dreaded tabs - NO! Did I end up with more piece of mind using the riblet solution without ruining any parts, or having to build a second or even a third trim tab in the process -  YES! So, if you are builder who seeks info about bending the tabs, do not waste any time hunting my blog for that. However, if you choose to cut the tabs off and make the riblets, then read on.

During the process of making the riblets, I encountered many twists in the road that most other builders don't seem to comment about, and I have tried to mention them in this post for the benefit of future builders. If you read my blog, you know by now that I like the details, and I also like to dispell as much of the doggone mystery about some of this stuff that I can.

Here is what I learned and how I made my riblets:

I had already trimmed the edges of the trim tab skin very close to their final state, but I needed to trim the edges of the elevator cutout where the trim tab resides just a bit more to allow enough clearance between the trim tab and the elevator. In this pic I have the skin laid out on my table with the 2x4s slid underneath. I then clamped everything down, and performed a bit more "dremel surgery" with the cutoff wheel to carefully trim away about another 1/8 inch of material from the top and bottom outboard trim tab cutout edges of the elevator skin. My measurements for the long edge of the trim tab cutout were about 17 and 22/32 inches, so this is the minimum amount needed to allow the trim tab to just fit inside the cutout in the elevator so that the edges of the trim tab will line up with the edges of the elevator.

This setup provided enough stiffness and separation of the edges of the elevator to allow me to keep two hands on the dremel tool to keep from making a costly mistake. One slip up here and you will order a new skin, so take your time, secure the work, and move slowly with the dremel tool, and it works out fine. I cut just outside the reference line I drew to leave a bit more material than desired, and finish trimming by using a file to trim away the remaining material up to the reference line.

Next is something I am a bit concerned about, and I will need to talk to Vans tomorrow, before removing any more material on the edge. Vans has you draw a line on the elevator that is perpendicular to the long trailing edge to determine the bend/cut line for the tabs. I expected that the corresponding matching edge of the trim tab would also be perpendicular to the LE of the trim  tab so that both edges would line up evenly. Unfortunately, I found this not to be the case. Upon trial fitting the trim tab in the cutout of the elevator, I found the outboard edges were not aligned as expected, and the trim tab edges are anything but perpendicular.

You are supposed to end up with a minimum gap between the elevator edge and the outboard trim tab edge of at least 3/32 inches. To correct the problem,  I think I will be able to trim the extreme trailing edges of either the elevator or the trim tab to even out this line, and still have the appropriate gap. SO much for cutting straight lines on everything. The next pic shows the additional amount I originally marked that needed to be trimmed from the elevator. Advice to future builders is to avoid trimming anything additional from this area until you actually start fitting the trim tab and hinge assembly in place.

And here it is after cutting and filing the excess down to the new line:

Next, the riblet fabrications begins. What do you need?

2 Vans practice Kit ribs (I don't know the part number but there is one for them - call Van's to find out what it is. thanks again to Mike Rettig for providing me some brand new untarnished ones so I would not have to tear apart my completed practice kit like Steve did!

The riblets fabricated from the ends from these two ribs will go in the OUTBOARD edge of the trim tab and in the edge of the elevator that sits next to it.

1 E703 elevator tip rib. Order one from Vans, or use a discarded one if you messed one up during elevator construction.

The riblet fabricated from this rib will go in the INBOARD edge of the trim tab, or the edge closet to the rudder when mounted on the fuselage.

Here is a pic of the remnant from all three ribs to give you some idea of where I made my initial cuts.

Now to describe the process I went through. This was a bit painful, and involved a lot of measurements of the gaps between the skins, and length measurements from leading to trailing edge, followed by transfers of all those measurements onto the rib stock, followed by more measurements, and more trimming, etc. I must have measured everything about 20 times before deciding where to make the first cut to separate the ends of each rib from the original part.

I essentially used a "more is better" approach, meaning that I took my original measurements, and added a bit to them just to be sure I did not cut anything too short. The elevator riblet pretty much gets cut symetrically off the end of the practice kit rib, while the other one for the outboard edge of the trim tab tends to have a bit of an angle on the leading edge due to the angled nature of the trim tab forward spar. The web on that spar, which has a Z bend on both ends for the flanges that attach to the top and botttom of the trim tab skin,  is not vertical, but sits at an angle.

Next I will go ahead and provide some "finished" photos of each riblet. First is the OUTBOARD or smaller end riblet for the trim tab. On the bottom of the forward part of the riblet you see a small cutout for clearance of the trim tab spar flange. You have to make this cutout to clear the edge of the trim tab spar flange. I suggest measuring to obtain about a 1/8 inch clearance, which is a standard clearance for most other non-attached rib-to-spar clearances. The hole that you see is a tooling hole from the original rib - I consider it a lightening hole (haha). Just debur the hole, and that is all you need to do with that.

Also notice the clearance from the end of the riblet to the trailing edge of the trim tab, and a slight rise in the forward flange of the trim tab skin where it joins the top flange of the trim tab spar. I was not very happy about this. It seems that the angle of the prefabricated practice ribs does not quite perfectly match the angle of the trim tab skin after attaching it to the forward spar. I will need to decide how to manage this, but for now some scuffing for primer and trying to work the edge of the flange of the riblet a bit more to reduce the width are all I can think of. The trailing edge of each riblet will sit just a bit forward of the trailing edge of the trim tab and the elevator to achieve the correct angle to fill the gap between the skins. Of all three riblets, this one does the worst job of mating correctly to the trim tab skin. Options are to make a wood form, take some of your spare metal, and form your own, as many other builders do. I'll have to ponder that this week to decide what I will do - leave it as is or form my own.

You should also know that I did not trim anything off of the trailing edge of this riblet - I used the original trailing edge and only trimmed the forward portion to fit it. Lastly, and this is a personal choice - you can face the web of the riblets inward or outward. For the trim tab I chose to face the web of the OUTBOARD riblet toward the outside to create a difinitive edge that will have the same effect as bending the tabs.

Next is the other side of the trim tab, or the INBOARD side. This riblet is made from the E703 Elevator tip rib, and is much longer and a bit wider than the one on the outboard edge.It will be placed as shown with the web facing inward, to match the edge of the E705 root rib, and allow solid rivets to be used on the flanges instead of pop rivets.

Oh, I guess I should have mentioned something about the rivets. Some holes will be drilled in the flanges of the riblets, and in some cases existing holes will be used. Then they will be dimpled for flush rivets. On this one I can set the rivets with a squeezer, but on the other two, with webs facing outward and flanges facing inward, I will need to use flush pop rivets for those, since the web will close off the access to the flanges. This is all by design.

Again, notice the slight cutout on the bottom forward portion of the riblet, and the clearance from both the web of the forward trim tab spar as well as the bottom flange. Also notice where the riblet trailing edge ends, forward of the actual TE of the trim tab. For this rib, I actually had to trim off about 1/4 inch or so of the end to preserve the correct angle of the trim tab skin without bulging it. This one actually sat much better inside the skin without bulging the leading or the trailing edge too much.

One other very important thing about making this riblet is that you end up utilizing one of the existing holes in the original part without needing to drill a different one. this is the hole on the bottom where one of the two halves of the trim tab control horn is mounted to the bottom of the trim tab. In other words, You will be setting a rivet through the control horn, then the skin, and then the riblet flange which is directly underneath. You can use this existing hole as a reference mark to determine how much to trim off of the front an rear portions of the riblet. Just keep lining up the existing hole in the bottom flange of the riblet with the existing hole in the skin, and put a cleco in there once you get it lined up and trimmed up enough. Then you can fine tune the LE and TE of the riblet accordingly.

One other note - - The rivet holes for the OTHER half of the trim tab control horn assembly end up being very close to the web of this riblet. I will need to decide if I am going to cut a recess on the webof the riblet in order to clear that area, or if I will simply put some RTV over that rivet to prevent any potential cracking from vibration, etc. It appears as though it will sit just on the edge of the dimple in the skin, so there is just enough clearance for the shop head of the rivet, but it is VERY close to the web of the rib.

You may need to trim the flanges a bit to line them up with the edges of the trim tab skin. All rivets that attach the control horn, skin, and this riblet can be set with a squeezer using solid rivets. Just be sure to increase the rivet length where necessary to account fo the additional depth of the flange of the riblet.

Lastly is the riblet that goes into the end of the elevator. This one was fairly easy to fabricate from the other practice kit rib. You do NOT have to cut a fancy recess in the bottom of this riblet like you do with the other ones, but you do need to ensure that there is sufficient clearance from the edges of the spar flanges on the front of the riblet. Again, notice the gap in the trailing edge of the riblet. This one also used the original trailing edge from the original part. I did not have to trim any of the trailing edge off of this rib.

So here are a few more pics to point out specific areas of interest in this process. Here is a blurry pic (sorry) of the cleco inserted into the INBOARD root riblet hole of the trim tab riblet I mentioned earlier.

Next is a close up of the forward portion of the same riblet to show the clearance from the spar web and the bottom flange of the spar of the trim tab. This gap is only 1/8 inch or less, so don't let the close up fool you into thinking that it is bigger than that.

Next are the two trim tab riblets by themselves. The pic on the top is the outboard riblet, and the one on the bottom is the inboard riblet.You can see the original hole in the bottom of the flange of the inboard riblet. You can also see more detail of the recesses I cut in each forward portion to clear the bottom trim tab flange, and the two original holes in the outboard riblet. Not sure if I will be using these or not, but will let you know what I decide. The upper flange holes in the inboard riblet (bottom) are a different story. I may use the most forward hole, and then make new holes and space them evenly along the flange accordingly. It is OK to leave existing holes and make new ones as long as you maintain proper clearance of each hole.

Next are the same two parts flipped on the other side, again for better clarity.

I was able to rough cut the recesses in each flange using the dremel cutoff wheel, and cleaned up each cut and trimmed it to final size with the Scotch Brite wheel. I ensured that each part was securely clamped, and that I could keep both hands on the dremel tool at all times. Had to keep a close eye on each cut, so safety glasses were a must. Also wore ear muffs.
Next is the elevator riblet. This one ends up with three rivet holes that I will most likely use, but if I need to space them out differenty I will do that.

And here is a shot of the trailing edges of each riblet so you see the difference between the ones created from the practice kit ribs and the one from the E703 elevator tip rib.

And finally a pic of the inboard riblet that shows both the top and bottom original holes in each flange.

I ended up doing a lot of cutting and trimming and cutting and trimming. I wish I could spew out exact measurements for everything I did, but you really end up doing most of the fitting on a trial and error basis to get the exact fit that you desire. I ran out of steam after getting them to fit where I wanted them, so I will finish fitting everything tomorrow. The hard part is done. The only other intense activity is the measuring of the other half of the hinge that attaches the trim tab to the elevator, and positioning the trim tab servo mounting brackets. Then I can drill and dimple holes for all the riblets, prime everything, and finally get around to riveting this bad boy together.