Jig built foam rib wings--how to.

[jbdesigns]

good afternoon my fellow modellers. due to some interest from Mike Chandley building his 50% extra, i thought i could post information on how to build a jig built foam rib wing. anyone reading this will take away something from this thread, even if some simple entertainment.

Understand that this technique is not my technique. I learned the basics by talking to Bob Godfrey in person at a couple Toledo shows. other tricks i used are my own. i will at this point say that Precision Aviation has not approved or disapproved of what i have done. I will simply describe what i have done, while as i have already done, give Bob credit for the approach. BTW, Wayne Ulrey also uses this technique in many of his designs and may have even shown Bob how do do it. Doesn't really matter to me. It is sound construction and will yield a straight and light weight wing.

so here goes.

[n8rloves2fly]

Wow I have this on the top of my list fot the winter I talked to Jack Strickland about his 40% laser and he gave me very helpful and spasific directions on hoe to build this type of wing and am very interested in his plans.. However I would like to build a set for my 33% laser as well as one I'm planning to build, I also want to build a set for one of my 330's from carden which are 40% I would also like to build a HUGE trainer and feel this is a strong yet light. The problem is I don't really know how to do it. I will be watching this tread and would like any tips or advice you can give.Good luck,and thank you!

[jbdesigns]

First off, some information about this build.

It is of a precision aviation (godfrey) 32.5% extra 300L. i built this plane in the winter of 1998-1999. i haven't built anything since that because i was busy building the most treasured things any man can "build": Three children! My oldest son was born after i finished it.

anyways, it is a bit of an outdated design but still flew well. the techniques are applicable to any size airplane i think. below is a picture of me nearly 9 years ago with the plane. I envy all of you a little bit for all of the building and flying you all have been doing while i took my sabatical. Doing this thread and reading FG forums is getting me motivated to get back into it.

[jbdesigns]

If you plan on doing work with foam, you need some tools. in this picture is my hot wire transformer and one of my bows. the transformer uses a 24/12V dc transformer from radio shack. the heat control is the dimmer switch. i have set it up with a switch to go from 24v or 12v depending on how long the bow is. more volts for the long bow. and there is an on/off switch.

the bows are simple. a stick of 1x2 poplar wood and some 1/4" music wire. the holes for the music wire are drilled at an angle. the ends of the rods are grooved with dremel to accept the wire. the rods are simply bent a little to provide the tension to keep the wire taught.

also in the picture are some of the various templates. they are all made from formica material i got from a counter top store as scrap. i cut them with the band saw and then sand them to final shape. final sanding with 400 grit to remove any scratches that would cause the wire to hang up on.

the most used templates are the triangles. there is a left and right tool. on one side is a 1/4" square stick glued to the edge to act as a stop, sort of like a speed square. these templates are used to cut foam to final shape before coring the block.

[jbdesigns]

one of the first things i did was to fabricate a bunch of rib blanks. i believe that i heard some say that you can get foam in certain thickness sheets. But when i did this, i wanted to use 5/8" thick ribs, just as Godfrey had on the plans. after much thought, i decided to use 5/8" instead of the readily available 1/2" sheet. that little bit extra seemed to be important enough for him, so it was for me too. so, i had to make them.

i first got some 4" thick blue foam from my foam supplier. look in the yellow book, you will likely find someone local to cut you custom foam. i asked them for a 2'x8' block, 4" thick.

from the end of the block, we will cut off rib blanks at whatever thickness you want. i made a small jig with two upright formica templates to guide the hot wire. this was probably screwed to the table. in front of this, i clampled a block for a stop. the foam is pushed up against the stop. in this position, the uprights are exactly 5/8" away from the stop.

Now, just cut down holding the wire agains the vertical templates as seen in the picture. i used a weight to keep the block from moving during the cut. it probably takes 30 seconds or so to make one rib. if you need ribs longer than 24", then get your raw block to that width.

[jbdesigns]

i thought it would be helpful if i gave a little information about my airfoil templates. some of my old time rc buddies taught me this style of template 15 years ago. i believe it is the absolute best way to make foam cutting templates for wings. some may already now about this anyways but there are many out there that will not.

a wing airfoil template is made using two pieces that are held together with a couple screws. basically, one of the pieces is for the top of the airfoil and the other piece is for the bottom of the airfoil. the reason it is is two pieces is so that the wire can always "rest" on the template when cutting. and the reason we don't make a template that looks resembles the whole airfoil is so we don't have to flip the foam block over to cut the other side of the wing. to often, foam blocks are no symetrical and if you flip it over and weight it down, then you might put some bow or twist into the wing when you go and cut it. what we want is to keep the block exactly in the same spot, remove one half of the template and then cut the bottom.

it does take a little time to make these templates and it is usually reserved for when you think you will make several wings. i did make two sets of wings back then and probably thought i would be making more (silly of me to think that).

looking at the pictures carefully will answer some of the questions you may have on how they are designed. i printed the template twice onto paper. i drew on the paper the center line of the wing and drew another line below the center at the distance i wanted the core centered in the block. for the root, that might be 2" for a wing cut from a 4" block. and think about this, for the tip root, you make the distance whatever you need to get the dihedral you want. 2" spacing for none and something larger than 2" for your dihedral. i put the dihedral into the templates and cut the wing tube parallel to the table. make sense?

the top of the airfoil template piece will have it's bottom cut less than this distance. look close at the pictures i just took of a couple templates. we don't want this template touching the table. and in fact, we need some space below the bottom of this template to put the tooth picks in to fasten the bottom airfoil template to the foam block.

there is a line that marks where the edge of the foam block goes. i also put a hole right on this line and look through the hole to see when i have the core perfect on the line when i tooth pick the template to the foam.

once you have the template done, it is very simple to place the template against the table on the end of the core block and pin it in place. you cut the top of the core and then remove one half and then cut the bottom airfoil.

this will all make a bit more sense later in the thread when you see the template against the foam.

[jbdesigns]

One thing that just struck me in the head like a brick is how much effort i spent doing this 9.5 years ago. i am looking forward to buying a dalton kit and working on that next winter.

i also went through a lot of work because i was building two planes together and at the time, my time.

need to scan a few more pictures and i will continue on. I wasn't digital 10 years ago.

[jbdesigns]

you must also creat a jig board on which to build your wings. you need a left and a right. use the plans to size up the board. the outer edge of the board is a bit larger than the wing planform. do not make it exactly the size of the wing but allow maybe 3/4" all around. i used partical board and for extra stiffness, i made it two layers thick with some strips below which were glued and stapled to the bottom of the board. in hind sight, i don't think this is necessary. i actually screwed the board to my table anyways, so if your building table is straight and flat, then this jig board will conform to the table.

on top of the board with a marker, you lay out the rib locations and the leading and trailing edges. you also need to mark the center line of the wing tube and the center of the aileron hinge line.

not visible in these pictures yet are strips of wood that i nailed on top of the board to hold the rib blanks in place. i ripped in the table saw strips that were 1/2" x 3/4" from a 1" pine board (measures 3/4" thick) these strips were located on either side of a rib and were just tight enough around a foam blank to hold the rib in place but still be removable. if you doing just one wing, you can actually just glue each rib blank down to the board. for that matter, you can glue them down to your building board and skip making a jig board if you don't mind sanding off the mess later on and also the marks you would have to make on your building board. just giving you options.

here are two pictures i took of my jig boards.

[jbdesigns]

second picture of the jig board. had to make it smaller to get it to upload. funny, second picture was just as big but the first photo wouldn't go.

[jbdesigns]

this picture shows the rib blanks in position on the jig board. the blanks are not cut to final length yet, we do that all in one step with the hot wire. photo two shows the ribs cut to final length after they are all in place on the jig. the triangle templates are used to cut the leading and trailing edges straight, cutting off the excess of the blank. now the jig has all of the ribs in place and they are all the perfect length. (make sure to cut the little sticks that hold the ribs in place shorter than the length of the rib they are holding in place, otherwise in this step, the wire will hit any protruding stick and make your day a bit foul)

note the thicker root rib and the second little half rib for the aileron root. visible in the second photo.

[jbdesigns]

In this picture, i have cut the top airfoil on one of the wings. We will only cut the top half now.

in the picture, you also see my bow for cutting tapered wings. one end of the bow has the 1/4" rod for wire tension. the other end of the bow has a leg coming down. on this leg is a long dowel. this dowel will allow the bow to be pivoted at one end in my simple looking aparatus which is clamped to the end of the table.

the idea is that the end of this bow is set to pivot exactly where the leading edge and trailing edge would meet if you extended the lines. i find this point with trial and error. placing the bow at the leading edge, the wire should touch the root and the tip template together. notice that i can also raise or lower the pivot point to also make sure that the wire touches the tip template just a little bit before the root when i lower the wire down onto the templates. this will ensure that the wire is always touching the tip template. you have to make this check on the leading and trailing edges.

once you have it set right, it is a one man operation to place the wire at the leading edge of the foam, turn on the power to the wire, follow the root rib with one hand and the wire will cut this tapered wing.

BTW, my wire is tapered in thickness from where it cuts the root template to the where the tip template is. i grind the wire with a dremel to gradually reduce the thickness of the wire as you go towards the end that cuts the root. the wire at the root has to cut faster than the tip, so if the wire is thinner there, it will get hotter and thus, cut faster. this will keep the wire from excessively burning the foam closer to the tip of the wing. there the wire is travelling slower during the cut and the wire has longer time at any given location. if the wire was just as hot as the wire at the root rib, the foam will be melted way too much and distort the airfoil. you can always tell if someone used a tapered wire or not. if the tip of the wing core is all melty looking and extra crusty, the wire was not tappered. if it is tappered, there will be little fuzzy strings at the tip, just like there are at the root area.

[mike chandley]

Very impressive!
I have learned several things already. The pivot point for the bow was a ingenious idea. I looked at the photo until I realized how simple it was
I have all the respect in the world for Bob Godfrey he was my idol back in 1994 when I scratch built my first 35%er. I met Bob on several ocassions and he was always a first class gentleman. Bobs airplanes were always light and flew very well he was way ahead of his time. I myself wonder why he chose 5/8 for the ribs, I did not know that but I was designing my ribs at 1/2 inch. My thoughts are you are spreading the load out on the rib and sheeting, also the way I will cut mine out with the cnc I can lighten them up alot. Thinning the wire out was also a new one on me. Keep going on this thread others like me are sure to learn more.
Thank you very much!
The pic below shows the lightened rib. The top rib was hand cut the bottom was cnc cut.