Side Rails
Bending Side Rails
for the
Hummel UltraCruiser
Created 12/20/2014
Note: Most images can be enlarged by clicking on the image

Bending the side rails is relatively easy ... once you have done it!

Note: there are some errors on the drawings:
- The radius for the arc on the plywood bending form is actually the diameter ... so divide by 2 to get R52.0
- The depth of the arc is shown as 3.625 inches.  This will result in an over-bend ... I used 3.25 inches.
Charles Snyder notes this on his outstanding blog: 

Below are the steps to make the bend ... in the order of the best first ... with earlier missteps shown further down.

The first step is to layout the outline of the fuselage from the aft portion of the side rails to the forward potion.
Note: The black gaffers tape denotes the straight portions of the side rails.  One rail has been bent and is being checked against the layout.  Below is a table for making the layout.  Make certain the numbers entered match your aircraft.
Setup for bending the left side rail.  The plywood bending form is screwed to the table with 1/8-inch spacers to allow the side angle to slide under the form as the bend is made.
The second angle is used to determine the depth of the arc.  Once 3.25 inches is achieved, the bending is stopped and the bent angle is removed from the setup and tested against the outline drawn on the workbench.  Since this was the second bend, I was able to complete the operation in one quick step.
Here I am measuring the depth of the arc.
Note: The angle used to facilitate the measurement was originally a spar cap ... unfortunately, it was the last spar cap to be drilled and it was a mirror image of what was intended!

Now it is a handy tool for jigs, hole patterns, & measuring.

      And for the final result ... more images below:
      - In the background: two completed wing panels, center spar in Cleco stage and two 8-foot, one-piece ailerons.
      - Rudder operation is very smooth.  Note: the oversize rudder
      - The fuselage stand is a converted wing stand.  Rollers are from a Northern Tool dolly for $14.99 cut in half.
Front view of fuselage with revised temporary stand for the nose until the rolling stand can be modified.

Used a scrap spar cap (yes, another one that I ruined) attached with one Cleco and gaffer's tape  

      The aft portion of the side rail doesn't quit fit flush.  Situation may improve during build.

This was the setup for the first bend. Note the heavy use of clamps.  It is much simpler to use construction screws to hold things in place.

The plywood curve was cut several more times until the desired bend was achieved ... BUT, unfortunately an over-bend.
The twist created by the bend is noticeable.  From this view one can see the spacing under the plywood form created by placing 1/8-inch pieces of aluminum under the form.

In addition to twist, an upward bend is also created ... Considerable force is required on the machinist's vise to remove the twist.  Spring-back of the twist is very strong.
Measuring the depth of the arc.  Some of the clamps had to be removed to make the measurement ... the advantage of using screws as above.

Some of the scrap plywood from trimming the form.  The form was trimmed five times.  Four would have sufficed.  As a result, I ended up with an over-bend.
An additional short angle was clamped to the "straight" portion of the side rail to ensure that no bending occurred in this area.

Note: the use of the Cleco style clamp in the upper left corner.  This had to be replaced with a small clamp ... the Cleco spring-force was too weak.

The large clamp was temporarily holding the side rail to measure the arc depth ... then, place a mark on the table for a quick check on depth after spring-back without having to actually make another arc depth measurement.
The final bend ... over-bend actually.  It is clear that the radius of the arc is much smaller.  This is the 4th of 5th re-cut of the form.

Note: the small clamp replaces the Celco style clamp.  Notch in form allows one to make a complete, flush bend.

Railroad tie and a multitude of clamps are replaced by a few construction screws in the second bend operation. The doubled-angle to protect the straight section is easily seen.

Layout for unbending.

The removal of the over-bend was easily accomplished.  The trick was to do the unbending in such a way that the aft portion of the side rail would fit flush with the side of the fuselage.  If one fails to achieved this goal ... one can just cut the last 6+ inches off the rail which is allowed per the plans.  The third or lower board was  tested in various positions to accomplish this goal. 

Mere finger pressure was required to test various positions of this board until the best position was determined.  Then force was applied until the spring back resulted in the side rail matching the lines of the fuselage layout on the work table.

If you have made it this far ... fuselage with "C" to "B" skin and gas tank:


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