LAST EDITED ON Oct-19-06 AT 04:08 PM (CST) Randy wrote:
What stopped me from building them was the
>massive amount of ribs to cut that had to be cut very
>accurately.
True enough Randy. But the ribs are where I believe Dave was ahead of his time. Had CAD and Laser cutting been around back then we might all have been building "Beamers" differently today.
Here are a few thoughts on the subject.
Dave used a set of lofted ribs that slid onto the I-beam. This makes for a much more accurate airfoil for the entire legnth of the wing, compared to strip ribs. Strip ribs make better use of a smaller amount of balsa gold, but the airfoil suffers towards the tips.
Others, apparently, were also aware of this design flaw with the "Beamers" because several have tried methods to use lofted ribs combined with an I-beam.
I've seen a lot of methods where wings were built, semi-I-beam style. Some were better than others, but almost all of them required very accurate and tedious cutting to make them work.
Let's compare the different styles of Beams, and explore what makes them work, and why it's sometimes possible to over engineer them with resultant increases in weight.
As an example, I recently saw a great looking box style beam, to be used in an I-beam design. Very strong, well built, but weighed in at over 4 ounces by itself. For a `550 square inch wing, that's a lot of weight, and most likely more than needed.
First, as to engineering for stregnth, I was always taught that the design should be strong enough to support the plane in flight, handle the minor excesses that it maight be subjected to, but when it came to a serious crash, if the plane didn't all turn to dust, I had built it over engineered, and thus, too heavy.
When the fellows in Detroit began building the I-beam ships, it was to utilize the weight savings, accuracy, speed, and less balsa needed for their ships. They were very successful and managed to keep the method secret for several years. The designs depended partly upon the stregnth of the covering to help support the wing in flight.
For the sake of this letter, I'm assuming that we are discussing all wood structures. It's possible to do some amazing things with modern composite construction techniques.
I-beams were mainly one to three laminations of balsa, with cap strips on top and bottom, hence the name, "I-beamers", the beam looked like a capital I. This is a good design for flex stregnth, but lacks a bit for torsion, thus the need for additional support from the covering.
From what I remember from my design class, an I beam puts the maximum area or section modules at the widest part of the structure. This makes for the greatest stregnth in expansion or compression when the beam is flexed. The wider the top and bottom caps are, the greater the section modules, and the resistance to extension and compression.
Choosing the I-Beam for the Detroit Stunters was simply good engineering for the time.
I've seen beams with no caps. These will not be as strong, in flex resistance, as the I-beam until their widths are as wide or wider than the caps on the I-beam, (section modules)and by the time it gets there, it'll be much heavier than the I-Beam.
Dick Williams used a form of I-Beam in his Electra design, that was notched. This idea was good for locating the rib stations, but not so good for stregnth. He solved the problem by also sheeting the leading edge. It was not, and is not, considered an I-beam design.
Many others have used some form of a beam to build their wings on. I-beams, Box-beams, (which have a bit more torsional rigidity)and other combinations, but to me, the simple I-beam is still the winner for light, yet strong enough, structures. Adding shear webbing to a C tube wing, resulting in a D tube, is the same as using an I beam with the cap strips of the beam at the surface of the wing.
Back to Gierke's design. He used a spar, made up of 3 laminations. A center of 1/8" balsa, with the grain oriented to the legnth of the wing, and 2 outside layers of 3/32" balsa oriented at angles to the center. As in most I-beam designs from the classic era. the center was plywood and mounted the gear.
When we first looked at the design, knowing we would be laser cutting the ribs, we decided to install a 1/8" X 1/2" spar below the surface of the wing, as if it were the cap strip to a normal I-Beam. Gierke didn't use this in his design, instead he depended on the stregnth of the laminations to carry the load.
We figured that the spars, oriented as they would be, would give us the maximum section modules, and therefore the highest stregnth we could get using wood structures. It was decided that we would hedge our bets, and use a spruce spar, 1/8" X 1/2", and install shear webbing between the ribs, therfore making up the I-beam.
This worked pretty good, until we started installing the shear webbing. It took 2 nights to cut and fit all theses little pieces of vertical grained wood. It did wind up very strong, and relativly light weight.
About the same time, the second plane was ready to have the wing built. Being lazy, I didn't want to spend all that time installing the shear webbing, and knowing myself, and my building habits, I wasn't sure I would put enough effort into the project as was put into the first one.
They say necessity is the mother of invention, and it proved itself once again. I could simply cut a 1/8" slot between the 1/8" X 1/2" caps, and slide a 1/8" full legnth shear web into place. It only took a few minutes to make these changes, and we were ready to start assembling the wing.
The typical lines were laid out. One center line for the fuse, a crossing line at right angles to the center line, for the trailing edge of the wing, and locating lines parrallel to the center line, where the last ribs would be located. These last ribs, and one in the middle of each wing panel were set up the same as the tip rib for a nomal I-beam.
After positioning the fuse, the trailing edge, (Angled braces were glued to the flat table top)and sliding in the 1/8" X 1/2" balsa spars, (yes, we used balsa spars on the second plane) We began sliding on, and positioning the ribs and half ribs. We made several tools to help. One was used to support the ribs as we slid them onto the spars. Others were used to set up the spacing.
It soon became very apparent that this was going to be a strong wing spar. The fits were tight, due to the accuracy of the laser cutting, and even before any glue was applied, the spar was very resistant to flexing.
Once every part was in place, glue, in the form, of thin CA was applied. The struture is quite rigid in flex, and very light weight. The fuselage weighed in at about 7 oz's before the wing was added. Now the entire fuse with the wing assembled to it weighs in at 11.8 oz's. When the controls are added, as well as wing tips, flaps, horns, etc. the weight wil go up, but not much more than 4 oz's, about 16 oz's total. Not bad for 630 sq inches.
Pictures are coming, but since I'm leaving in a few minutes for Clovis CA. they'll have to wait until I get home.
I'm wondering why everything is spinning around?
(Actually, cutting out conventional ribs is not much enjoyed: hooray for lazers for that.)