Year: 2010

OK, we’ve put off bending the control surface for the vertical stabalizer (AKA the “rudder”) for some time. All the control serfaces, (rudder, elevators, ailerons, flaps) are very similar. The elevators and rudder (shape Z03-01) are the same cross section with different length wise dimensions and the ailerons and flaps (Z02-01 and Z02-02 respectively) are each made of unique cross sections but in general are not too dissimilar from the rudder shap.

The one thing we did prior to this was to take a scrap piece of 0.025 and try to mark it out and bend it to match the drawing dimensions. I had also made a full size plot from AutoCad of the cross section of the rudder to help identify the “sight lines” for each bend. Tim ultimately used his mechanical engineering expertise and provided a bend diagram that I used to mark the flat part.

So, Tim came over last week and we cut out a 26″ long section of 0.025 for the rudder and I’ve been looking at it all week trying to get psyched up to take a shot at bending the thing. This morning I looked at the things I could do and thought it was time.

First, I cut the final rough dimension from the 48″ wide piece of 0.025. According to the T13-01 dimensions this should be 42-63/64″ to the longest projected point. At the Sonex workshop they mention the highly precise dimensions on the plans and allude to the difficulty of cutting parts the this level of precision.

Just to save anyone reading this the trouble of digging out a calculator, 1/64″ is close to 16 mils (1 mil = 0.001″ so of course 16 mils is 0.016″). This is about the width of the line that the “ultra fine” Sharpie pens will make. Cutting sheet metal to this level of precision consistently is nearly impossible with hand tools without spending an inordinate amount of time doing it. Besides, the rudder will be cut again for final length with an angle after it’s bent to the basic shape so in my book, 42″ (+/- 1/32″) for an overall length is close enough.

As an editorial comment, I’ve tried like heck to be as accurate as I can with every part I make and have found that the angle aluminum parts can be filed to within 5 mils pretty easily but in most cases it does not pay to get the part any more precise than maybe 10 mils and that might be excessive for most parts. I say this with regard to small dimensions (less than 6″) becasue I only have a 6″ micrometer) You can try to mark and cut large pieces to a higher level of precision but without a large micrometer you are relying on your ability to see and mark off a ruler with a marker lays down a paint stripe that’s 16 mils wide.

Anyway, my 50+ year old eyes cannot mark much better than 1/64″ using a ruler or a micrometer if anyone wanted to check my work I’d not be surprised that I’m not within 1/32″ with most things I bend or cut.

Enough of that, what about the part……

After cutting the sheet to the overall dimensions needed for the rudder control surface, which is 42″ x 26″ I cleaned up the edges with a file and then scotch brite .

A close up of the deburred edge.

So one of the things that becomes apparent to anyone that has bent parts and something they teach in mechanical engineering school (and probably drafting technical schools as well) is a little of the science of sheet metal bending. Where as machined parts can be cut to a particular length and then two parts can be butted together and fastened with bolts with the resulting part adding up with the associated tolerances of the parts. The problem with bending sheet metal is there is a radius that is typically prescribed for a bend. Each bend has a couple of components that are important to identify when marking and bending parts.

Below is a sketch of a cross section of a “nose” on a brake and the noteworthy dimensions that are useful when bending a sheetmetal part:

bending calculations sht 1

This diagram shows a theoetical bend and defines the resulting dimensions for a 90 degree bend.  It is important to realize that these numbers are valid when clamping as shown in the diagram.   When clamping from the other end of the sheet metal, in the drawing it would be the short end, a symetrical relationship exists and what is important is that the sight line, which is what you need when placing the sheet metal in a brake, moves by 0.43 times the bend radius (assuming your final bend radius is what you planned on).

bending calculations sht 2

The thing to remember in the case of the more complex parts like the control surfaces is that the bends are often (or in the case of the rudder, always) not 90 degrees.

Tim generated a diagram of the complete sheet along with distances from the edge to each sight line.  This diagram turned out to be a very good guide for bending the rudder and elevators.

Final Rudder part before cutting

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