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  #1  
Old 11-23-2009, 02:00 AM
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Ironflight Ironflight is offline
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Default RV-3 ? ?The Big Bend? (Longerons that is?)

I have written before about the ?bogeymen? of RV construction, and how easy it is to get worked up about trimming or cutting the canopy, setting wing incidence, fitting the cowling ? or today?s big event (at our shop) bending the longerons. While I try hard to dispel the ?myths? of the bogeymen, since so many mortals have managed to accomplish these tasks before, it is nonetheless hard not to get just a little palm sweat going before doing something that you?ve never done before, and probably won?t do again (at least not more than a couple of times). In fact, that is probably what sets these scary tasks apart ? most things we do in the RV build we do over, and over, and over (and over?) again, giving us the chance to practice and get good at them. But these special events are one time affairs, and most times, your first time through is with the parts you plan to fly with. Fortunately, these things are designed to be built by the average guy (or gal) with average tools, and history shows that it can be done!

That?s a long way to introduce the topic of the day, bending the longerons for our RV-3. I?m pleased to announce that it took us about an hour and a half from start to finish, and all turned out perfectly. I never picked up a hammer! Using the vice and steady pressure, it was easy to put the required two bends in each longeron, and it sure seemed simpler than what I have imagined ? I think that is probably because no curves are involved!

We started by doing a search here on VAF for ?Longeron+Bending?, and reviewed the threads in the archives. Most of them relate to the RV-9, but I was intrigued by references to the ?Orndorff Method?. As I understand it, this basically involved putting the two longerons back to back in the vice and then pulling them apart to get the bend you wanted. Sure sounded easier to me than beating with a hammer, and I figured it couldn?t hurt to try!

I first put the longerons on the jig bed, notched into the aft bulkheads to determine where the F-305 bulkhead needed to go. I marked this point, which is where the horizontal bend starts. We then taped the two angles back-to-back (with the two vertical webs next to each other) so that the bend would be created by pulling the two pieces apart. I chucked them up in the vice using a couple of nice hard pieces of wood with a relieved corner that fits into the radiused inside corner of the aluminum angle ? this way, the angle was fully in contact with the wood. I lined up the bend location with the end of the oak and the edge of the vice jaws, and cranked down hard. The plans specify the amount of bend as a distance off ?straight? at 12 inches, so I marked a line across both angles at the one foot point, then I simply grabbed the nearest angle and pulled it with a smooth force and a couple of pulsed tugs. It bent nicely, I got a feel for how much it moved, and I pulled until I got what looked to be the right amount, then measured from the remaining straight piece. When I had the full bend, I noticed that the longeron had twisted down just a little, so I pulled up to straighten it, and that side was done. The other one went just as quickly, and I simply measured twice the specified distance from the one that was already bent.





The second bend (in the vertical plain) was just as simple. I measured the location of the F-303, marked it, and then taped the two pieces back to back with the horizontal webs against each other. Louise and I both confirmed that the bends would go in the right direction, then repeated the process for the first bend ? and we were done! The least thing to do was cut off the excess (Van?s gave us a whole inch and a half extra material!!), and it was time to start assembling longerons and bulkheads in the jig. I was amazed how easily the bending process went, especially with all I have read about beating the heck out of them with a sledge. It is probably because the bends in the -3 are pretty small and no curves are involved, but this Bogeyman proved to be pretty much a non-event. I am REALLY glad that I took the time to research this in the archives before we got started, or we might be pounding away on them now?.

Additional Pictures are here: http://picasaweb.google.com/Ironflig...ctionPictures#

Paul
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  #2  
Old 11-23-2009, 01:04 PM
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REHughes REHughes is offline
 
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Location: Polson MT (8S1)
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Default Big Bends

Quote:
Originally Posted by Ironflight View Post

I lined up the bend location with the end of the oak and the edge of the vice jaws, and cranked down hard. The plans specify the amount of bend as a distance off ?straight? at 12 inches, so I marked a line across both angles at the one foot point, then I simply grabbed the nearest angle and pulled it with a smooth force and a couple of pulsed tugs. It bent nicely, I got a feel for how much it moved, and I pulled until I got what looked to be the right amount, then measured from the remaining straight piece. Paul
The perfect bend is one made over the smallest distance possible, and the desired result is two perfectly flat segments of the longerons on either side of the bend. When bending the angles in the manner you describe, there is a tendency for the region of the bend to 'spread out' in a gradual curve toward the free end that is being pulled by your hand. In the top picture in your post, there appears to be very slight curve as you move away from the bend in the longeron.

I have seen two methods used to combat this effect. One is to slide a tight fitting thick walled tube over the limb that you are bending off line (but it may be hard to find a tube with the right I.D. to be snug enough on the angle stock). Another way, and the one I used on my -3 project, is to bend the longerons in the vise one at a time, and as bending pressure is applied with your hand, repeatedly strike the area immediately adjacent to the forming bend with a heavy shot-filled poly hammer.

The hammer technique will confine the bend to a shorter segment, but may actually result in more twisting and out-of-plane bending that must then be corrected.
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