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The Unofficial Bearhawk FAQ

Wing Construction Page

Forming Blocks by Del Rawlins

All of the 2024T3 ribs for the wings are formed around a "forming block" made of thick hardwood, usually by hand. The 1995 Bear Tracks newsletters included with the plans give a good explanation of how the blocks are used, but I've assembled here a collection of hints and tips from the Bearhawk list and my own personal experience forming my own set of wing ribs.

It is recommended that the block be *at least* 1" thick, from wood hard enough to resist the mallet blows from the rib forming process. Maple and oak seem to be popular choices. Depending on your location, you may have the same problem that I did, that of locating a piece of wood of the necessary size and thickness. If you find yourself in this situation, consider my solution: I bought 2 of the thickest oak planks the local building supply had, and ripped them into strips about 2" wide on a table saw, then squared the edges as best I could by running them across a jointer. I glued the strips together with several bar clamps, concentrating on getting a flat, square glue-up. After the glue dried, I took the resulting plank back to the lumberyard where they ran it through their thickness planer. This produced a plank just under 1.75" thick, and due to the laminated construction it should be less prone to warping. There is enough extra thickness there for several more passes through the planer, if I should somehow manage to damage the radiused edge.

The next challenge was transferring the airfoil outline from the mylar roll (drawing #7) to the block; I asked the email list for ideas and here are some of the responses I received:

On Sun, 25 Jan 1998, Bob Reeves wrote:

I transferred the outline of the rib using carbon paper, like u use to make a copy of something on a typewritter. Just lay the carbon paper over the wood and carefully trace the outline, I use a piece of stiff wire ground down on the end to a smooth round end. This way you don't mark up the pattern.

On Sun, 25 Jan 1998, Rick Girouard wrote:

I used tracing paper taped together and then taped down to drawing 7. Using french curves I traced out the pattern. Then I carefully cut the pattern out of the tracing paper and then taped it to the oak board. I took some black spray paint and sprayed around the egdes of the tracing paper pattern. When the paint dried I removed the pattern and had a nice outline on the board. I cut the board with a jig saw making sure to stay a little outside of the edge of the painted pattern. When done cutting there is a little rim of black paint all the way around the forming block. File and sand the remaining painted portion of the block away and you now have your forming block.

On Sun, 25 Jan 1998, Kirk Urbanczyk wrote:

I dont know if this will help but I traced the rib template on plastic coated paper with a special drafting pen that was the same width as ink on the drawing (I also traced all the markings like the spar CL). Then I just glued the paper on the wood and cut it out.

I ended up following the first suggestion since it seemed like the simplest, and it worked like a charm. After tracing the outline I took a center punch and marked all of the lightening hole centers, which I drilled in my drill press. I cut the block out with a handheld jigsaw, staying around 1/8" from the line, which I sanded down to with a benchmounted 4" belt sander. The newsletter says to put a 1/16" radius on the edge, but several members of the Bearhawk list have pointed out that it is really an insufficient bend radius for .025 and .032 2024T3, and that 1/8" should instead be used. I have a specially made hand plane that puts a 1/8" radius on corners, but others have used routers, sanders, files, or even hand sanding (ugh) to achieve the proper radius.

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Should I subtract the thickness of the rib material from my form block? by Del Rawlins

This question is raised from time to time, and the answer is a resounding NO. Drawing #7 *is* the form block, so you shouldn't subtract anything from it. Bob did us another big favor by doing it this way instead of giving us a drawing of the rib or the wing cross section, like some other airplane plans do (hence the confusion).

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Cutting Out Rib Blanks by Del Rawlins

Basically there have been two methods of cutting out the blanks for the wing ribs discussed on the email list. The first, and most primitive is described well in the Bear-Tracks newsletter, and consists of drawing the blank on the aluminum with a felt pen (ultra-fine sharpie markers work best), and cutting it out with the tin snips. While this process may seem at first to be simple (and it is), here are a few helpful pointers:

On October 18, 1999 Russ Erb wrote:

A few notes on the use of aviation snips (or "tin snips"). I'm not sure why this doesn't appear in the typical homebuilder's books:

First, get a right hand and left hand version of OFFSET (or "Duckbill") snips. Usually these have red and green handles. The left and right hand refers to which direction of curve that the snips will cut, not which hand you hold them in. The left and right hand snips are mirror images of each other. Why each one will cut curves in only one direction is tough to explain, but fairly obvious after using them.

Don't waste your money on the non-offset snips. The offset allows you to cut for any distance across a sheet without excessive bending of the sheet. Try that with non-offset snips and the sheet will run right into the pivot bolt. I have not found anything yet that could be done by an non-offset snip that couldn't be done by an offset snip.

To use the snips, do not cut right to the line on the first cut. Doing so may be very difficult and lead to distorted parts. Instead, rough cut to within about 1/8" of the desired line. Then go back and finish cut to the desired line. Doing this will let the thin piece of scrap bend out of the way while leaving the part undistorted. Additionally, the scrap doesn't get in the way of cutting curves.

Be prepared to swap back and forth between the left and right handed snips as required. In general, use the one that works for the current cut. If it's not working, try the other one.

At least one of the jaws of the snips typically will have small serrations on it. Presumably these serrations keep the snips from slipping while cutting. Unfortunately, these serrations will leave marks on the parts. You'll want to scotch-brite these marks out to remove the stress concentrations, just like you would for any other scratch. I have not tried grinding the serrations off of the snips to see what effect it would have.

The other method involves the use of a router with a laminate trim bit, as follows:

On March 25, 1996 Paul Beam wrote:

I tried [tin snips] once, then I gave up. I find it much easier to use a router with a trim off bit. You have to make a wooden form the size of the rib + the flange, but I think it is well worth it. The blank is very accurate and your hands don't get tired. Of course, it throughs aluminum shavings everywhere!

Finally, for a note on my own experience. I have an old Craftsman router so I tried that method first. I found that it does, indeed throw tiny bits of aluminum EVERYWHERE. I don't work in a particularly large area, crammed full of boxes and worktables and junk, which made cleaning up these bits a major hassle. In addition, the router is extremely loud. I also found that the router base plate left little drag marks over the aluminum, and while they weren't deep enough to be a problem, I didn't like the way the ribs came out looking. So I cut out all of my ribs with my offset tin snips and found the slightly slower process to be quite enjoyable, especially since I much prefer listening to cd's and not the racket my router makes.

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Fiberglass wing tips by Fred Muder and Bill Shellenberger
                        
    
    
         Even after completing those beautiful aluminum wings, and you think you are all done, 
    and they look so good, and you swore you were not going to have anything to do with fiberglassing 
    on this airplane, now comes the realization that you've got to finish the tips in some fashion 
    or other.  Maybe in sweepback, maybe in drooping tips, maybe winglets, or wedges, or maybe 
    just a flat plate   whatever   you've got to do something! 
    
         Well, fiberglass offers a fairly easy solution.  Granted, you have to get set up for
    glassing, and it is a chore; but once you have decided to do it, it is halfway done.  Here is a 
    fairly simple technique   though we didn't say easy or quick!
    
         1    Cut some .025 aluminum into 3 inch strips and make a rim which will attach to the
         inside surface of the wing skin.  Follow around the entire periphery leaving about 2" 
    extended outside the skins.
    
         2     Drill and secure the rim to the skin with clecoes about every five inches, or so. 
    
         3     Now perforate the rims outer aspect with some large holes which will increase the adhesion of
              the fiberglass to the aluminum.  We used a ¨ inch punch.
         
         4    Mark a finish line with a felt tip pen all the way around the outside of the rim as it abuts the skin
              so that you will know where to end the lay-up. 
         
    5    Make a wood rib (1/2 in. fiberboard) to fit snugly inside the rim so it is held 
    firmly in place.  Secure with small screws through the rim into the wood so that the wood rib 
    and rim can be removed as a unit from the skins. Lay a straight edge on the wing skins to check 
    the contours so that there is no flare or low spot in the rim.
    
    6     Press a large piece of foam (2" thick) firmly onto the rim so as to make and 
    indented pattern.  This will be your first piece of foam buildup.   Cut this with a
    sharp knife and it will fit nicely inside the rim and flat against the wood rib.
    Again, this is a good time to check the contours to make sure you haven't created any 
    bulges at low spots.  Glue the foam to the wood with a very small amount of glue
    since you will have to separate to remove it later.  By the way, a little glue goes a 
    long way - you don't need much on any of the foam.
    
             1.    Now carefully remove the rim, wood rib, and foam as a unit and lay it on the 
                 bench.   Cut and glue more foam until you have enough of a block to begin to sculpture.
              We use a hot glue gun, but keep the hot glue away from the edges   it gums up the 
              sculpting tools.   By the way, if you don't have foam blocks handy, you can use  
              construction insulation board.  It is not very good for sophisticated carving but will do
              for general rounded wing tips.
              
              8     Carve to the form you want.  Use a sharp knife, Stanley Surforms, and a sanding block 
              to make like Michelangelo!  Remember, there is probably a  magnificent sculpture
              in there just waiting for you to uncover it!  Leave a flat spot on the outer aspect
              where you will attach you wing tip lights.
              
              
              9    We use a 2" roll of glass cloth to lay-up a band of fiberglass around the outer surface
              to connect the rim to the foam.  This will cover your perforated holes but does not quite
              approach your finish line marking ( remember, your small screws are still in place). 
              Carry this completely around the workpiece.
              
              10   Now you can apply separator if you wish.  We use a plastic cling wrap held in 
              place with masking tape. 
              
              11    Apply 3 layers of fiberglass lay-up to the bulk of your sculpture.  We prefer to let
              each layer set so that we can sand it lightly and make sure there are no rough surfaces
              to mar the finish.
              
              12    Remove the wood screws and use the 2" strip to extend the lay-up to the finish
              line - 2 or 3 layers.  After it has set you can fit it back on the wing and make any 
              small corrections.
              
              14    Remove the wood rib.  It will probably come away easily, but if it doesn't, drill
              a couple of   " holes and work a jigsaw from end to end.  This will give you enough
              joggle to get the wood out.
              
              15   Now,  go for the finish you want.  You can do a light sanding and finish or go through
              a lot of work with sanding paint, etc. to get a gel finish.  Be aware of weight.
              
              16   Now the holes can be turned into floating nut plates and/or sheet metal screws for
              permanent attachment to the wing skins.
              
              17   If the tip must extend aft from the wing skin to approximate the aileron, simply
              put the aileron in a neutral position and extend the sculpture to match it.  You won't 
              need a wood rib here.
              
              18   You can now remove the foam by just chipping it out with a sharp putty knife.
              You will be surprised how easy it will come away from the lay-up.  You won't have
              to get all the way down into the trailing edge because the weight of that small amount
              of foam is insignificant.   It is probably wise to add a lay-up of  a single layer of  2"
              strip on the inside of the aluminum rim so as to further secure the wing tip through 
              the punched holes. 
              
              19   Mark and drill the holes for you wing tip lights.
              
              OK Hotshot - now you are an expert.  Now you get to repeat the process for the other wing.  
              The real challenge is in trying to duplicate your first tip as a reverse image. Then you can teach the
              others in your chapter how to do it.
              
                           
    

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Can I use Pulled or Pop Rivets on the Wings?

The short answer is, NO. Here's the long answer...

On August 14, 2006, Budd Davisson wrote:

FYI-
Speaking as an aeronautical engineer (structures) who spent two years on loan to Grumman/Lockheed/Canadair as a blind fastener specialist (the rivets were blind, not me), let me say a couple of things about blind rivets. Most of which Iíve said before, but some bear repeating.

A pulled-mandrel rivet of any kind is usually limited in its expansion because that expansion is driven entirely by the mandrel, where an AN rivetís degree of upset is actually determined by the hole. This is important because a drilled hole isnít round, itís slightly triangular with three flat spots. On a pull-mandrel rivet (except some very specific ones), the expansion is often not enough to flow into the ďcornersĒ of the hole. So the bearing/shear loads are carried by the smaller ďflatsĒ of the hole, rather than the entire circumference as with a driven rivet, which actually forms an interference fit with the properly prepared hole.

The result of the limited fit of the pulled rivet, especially in a less than perfect hole, is the localized loads cause local bearing failures in the holes which result in loose rivets due to vibration. A riveted join depends on total rigidity within each hole. If a rivet can move at all, itís no longer carrying its share of the load so the others have to take up the slackerís load.

Also, only some pulled mandrel rivets are of a ďlocked spindleĒ design, meaning, once pulled, a locking ring in the top of the rivet is forced into the mandrel. and it canít fall out. This is important because the strength of pulled rivets is usually determined with the mandrel in place. If it falls out, you donít have a rivet, you have an eyelet.

Cherry Bulblocs and Huck MLS rivets and a few others are locked spindle rivets specifically designed to give maximum expansion in thin sheets and as such, greatly outperform the original, and relatively inexpensive, Cherry MS rivet. A draw back to locked spindle rivets is that they require a double action puller: while the mandrel is being pulled and broken, an inner sleeve pushes down on the locking ring and locks the spindle in place. Another draw back is that they are expensive.

There are a number of pure pull rivets approved for aircraft use, notably the Avex and Avdels. They depend on punched, perfect holes. Otherwise they are only one notch up from a USM pop-rivet.

The shear and bearing strengths of all of these rivets are different than driven AN rivets, so in a lot of applications, itís not a hole for hole replacement. You need more of the pulled rivets than of the driven ones (although, not always). In the case of the BH wings, itís just easier to prevail up on your local RV guy for an afternoon of help.

bd
 

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Stainless Steel Pop Rivets by Del Rawlins

On November 12, 2001, "bearhawk39" wrote:

> Why the stainless [pop] rivets on the aileron and flap nose skins?
> Is this strictly a corrosion problem or is strength the issue? Does
> any one have a good source for these (a little pricey).

We've seen this question enough times now that I guess I need to get off my duff and put it in the FAQ. There was an engineering notice in the July '99 Bear-Tracks in which Bob said to use 1/8" aluminum pop rivets instead. If you have already used the 3/32" SS rivets to go ahead and leave them alone, since they work fine.

This applies to plans #395 and below.

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Power Scotchbriting by Del Rawlins

If you are going to alodine and/or prime your aluminum parts for corrosion protection, it is recommended that you first clean and scuff the parts with the maroon colored scotchbrite abrasive pads. This is an extremely tedious process, and for most of the parts like the ribs, there is just no getting around doing it by hand. But for other parts, there are enough large, flat areas to be worth investigating a faster way.

When I went to the local industrial hardware store to buy more of the scotchbrite pads I noticed that they had 5" round maroon pads for random orbital sanders. Unfortunately, they only fit the hook and loop style sanders, and my Porter Cable 334 sander takes pressure sensitive adhesive (PSA) disks. I could buy another sander or a hook and loop pad for it, but I figured there had to be a cheaper way. I thought about it on the way home, and decided to try making my own PSA scotchbrite disks.

I cut 5" circles out of the rectangular pads, and using some contact spray adhesive I glued them to some PSA sandpaper disks that were left over from another project. I used them to scuff the capstrips for my main spars and I am happy to report they worked a lot better than I expected them to and saved a lot of tedious hand scuffing. It probably isn't worth buying a random orbital sander just for this purpose, but if you have one you might try it.

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