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About to build wings, AL shrinking vs. fluting

Posted By:
Jay Smith
Homebuilder or Craftsman
#1 Posted: 6/7/2011 08:05:07

What to do, what to do??

I'm about to start wing construction for my BD-4 using John Steere's wing building methods as found on the BD-4 website here: http://www.old.bd-4.org/wings_steere00.html

After forming and shrinking some practice ribs on some 032 5052 scrap (much easier to work than the heat treated alloys, I must say!), 2 issues have emerged about which the knowledge here at OK365 will be  highly valued.  Thanks in advance for ringing in.

 The shrinker literally grabs the metal edge and compresses it together, leaving several marks per squeeze.  Does this cause an acceptable amount of stress compared to fluting?  Seems like fluting would cause less but I already have a shrinker and stretcher and am familiar with their use.  I'm thinking the stretcher probably causes more stress by pulling the metal in the opposite direction but that is not what is needed for this operation fortunately.

For the real rib, I originally intended to use .025 2024 T3 but am now considering going to 6061 T6 for it's easier formability and greater resistance (I hope) to cracking from the shrinking operation.  I've planned to use an .020 rib with .032 skin in the wet wing area which is the first 4 feet of the wing from the root as John Steere did; then changing to .025 ribs and skin both outward from there.  I could save a bit of weight by using .025 skin on the whole wing but in the tank area, the skin is also the tank wall, thoughts on this?

Finally, given that the modulus of elasticity is about the same for different alloys, which alloy would offer the greatest resistance to hail denting?  We've just had a BUNCH of cars at local car lots get peppered here in north TX and that's been on my mind too and I'm not sure whether denting would be more related to the tensile strength or modulus.

BTW, a .032 "scrap" rib weighs 9.2 oz if anyone cares!  I haven't built any out of .025 or .020 yet.

Thought on these  issues before I jump in??


Jay Smith

Craig Cantwell
#2 Posted: 6/7/2011 12:59:02

Jay: If you are using a Lancaster style shrinker/stretcher, then go get some of the modified die sets from Kent White. They work just as good the originals, but with a lot less surface change.


As to jumping from 2024 to 6061, you have a loss of tensile strength of about 1/3 and a significant loss of fatigue resistance. If you are going to make the material change, then pony up the bucks for a aero structures guy to do the analysis for the alloy and thickness changes. You would be making some significant engineering changes to the wing and in that area, it's critical that you get it right. You don't want to find out when you are at gross and in some turbulence that you made a wrong choice on the changes.

I'm not trying to be a naysayer, but I break and maintain flight test aircraft for my job, so I get to see what happens when bad choices are made. Take the time and effort to do it right and make your BD-4 outlast your grandkids...


BTW...If you are still looking for a DAR and are on the Fort Worth side of DFW, I think Frank Strickler still a DAR.

Mark Usik
Homebuilder or CraftsmanAirVenture Volunteer
#3 Posted: 6/10/2011 06:57:38

If you only plan to use these shrinker jaws on aluminum, dull the teeth on the shrinker jaws with a hand held abrasive stone or up against a steel wire wheel  (or find a set of well used, dull jaws). This minimise the stress risers that would normally be pressed into the soft aluminum. BTW, the shrinker jaws won't work very well on steel anymore.