the wing for a model airplane is not the most exciting part of airplane
construction. And production of the wing ribs tends to bore me more than
I’m willing to accept. However as long as our airplanes are expected
to fly we need a wing. So . . .
There are several ways to make ribs. Some people cut out each single rib from a sketch given on a plan. Some make solid rib patterns for each rib, cut from cardboard, plywood, or metal. I don’t like this method because: firstly I don’t ever build one design twice; secondly it’s a lot of work for one airplane only; and thirdly I don’t trust my building skills to produce exact ribs with this method. I still prefer the “sandwich” method; that means I shape the ribs altogether while they are stacked in a block. This is not much more work than shaping them individually. But this method guarantees that all ribs have truly the same shape ( in a constant chord wing ) or have a perfect taper for a tapered wing planform. The following story deals with the tapered wing design only, since the rectangular configuration is all too easy and doesn't need explanation. For the tapered planform some hints might be helpful.
First we need two templates, one for the root rib and one for the tip rib. This usually means to draw the airfoil on paper, cut it out, and glue it to whatever you prefer to make some rigid templates. Long ago I’ve given up drawing the complete airfoil . Maybe you already know that a tiny difference in airfoil outline can make a huge difference in flight characteristics. We should make sure that the airfoil is truly symmetrical. I only draw one half of the airfoil with a centreline. Then I scratch the centreline, fold the paper, and cut out the airfoil shape ( picture 1 ). That way I have a 100% symmetrical airfoil. The cut out patterns are glued ( in my case ) to 2 mm plywood, carefully sawn, and precisely sanded to final shape. The cutouts for control wires are not cut at this stage.
Before cutting any ribs we should select suitable wood. I do not recommend the lightest balsa sheets. For this purpose we need some rigid stuff with long running grain, or even quarter grain if it's light enough. Actually I choose stiff but light sheets for those ribs close to the wing centre and for the inboard wing, and heavier wood for the outboard ribs since we usually can benefit from some outboard weight. Also I do not use lightening cutouts in the outboard ribs.
Using the rib templates I cut out the required number of ribs. I do not cut rectangular pieces since this is a waste of material, and I don't like that irksome work to shape the ribs out of a rectangular block . I roughly cut out all ribs individually, not spending much care to do this precisely. Depending on rib size and balsa sheet width we can save a lot of material since we can get more ribs from one sheet ( pic 1a ). The ribs near the wing tip are shorter and we can arrange some ribs by just cutting them shorter.
all ribs are put on one pile with both templates placed on each end, and
clamped in a vice. I start shaping the pile with a razor plane, beginning
with the front end ( pic 2 ). Preferably the ribs should be stacked with
the templates located exactly at 90 degrees to the rib block. If we dont
do this the front end of the ribs are strongly bevelled and thus have a
smaller gluing area to fit to the leading edge. This is especially true
if there's only a small number of ribs. That's why I put all ribs of one
wing into one block. When the front end is roughly done the cutout for the
leading esge is shaped. Even if the cutouts in the tamplates are correct
I prefer to use a scrap piece of the leading edge to precisely shape the
cutout ( pic 3 ).
we get to the rear end. Again - in order to get the block end at right angles
to the templates - we have to arrange the rib pile accordingly. I do this
by holding the pile very loosely in my hand, hold it on a piece of sandpaper,
shake slightly so all ribs will slide down and touch the sandpaper, and
shake loosely until the rear end of the rib pile is “squared”
to the templates ( pic 4 ). The rib rear ends can be shaped now. Of course
the rib templates will now have all the correct cutouts for the spars and
the control wires( pic 5).
|When cutting the spar cutouts again arrange the rib stack so that the cutouts in the templates are rectangular to each other, to avoid bevelled cutouts, thus having bad gluing joints. Again scrap pieces of the main spar help to produce exact dimensions ( pic 6 ). Next on the agenda are the cutouts for the control wires. Depending on preference we can decide whether we do cutouts on all ribs ( to save weight ) or on the inboard ribs only ( pic 7 ). I leave the outboard ribs as they are since we need outboard wing tip weight anyway. So you’ll have to decide.|
any case the ribs are numbered now, beginning with number 1 at the root
rib ( pic 8 ). In case our rib stack falls down we’ll have an exciting
time to get the ribs back in correct order ( don’t ask how I know
!). If only the inboard ribs are hollowed we have to divide the whole rib
stack into two piles. All odd numbers go to the inboard pile, the even numbers
make up the outboard pile ( pic 9 ). The easiest way to do this work is
with an electric drill. I first drill holes with about 8 mm diameter, the
rest is done with rough files. Final work is done with balsa knife and fine
files to reduce as much material as desired.
most inner ribs ( in my case number 1 and 2 ) are usually placed exactly
in place where the wing is glued to the fuselage ( sides !), in order to
provide a very rigid connection between wing and fuselage. I prefer to use
thicker ribs in this place. I do this by gluing these ribs on some thick
( about 4 mm ) balsa sheet and cut out again ( pic 10 ).
By now, clever minds will have noticed that my outboard wing will have a slightly smaller chord than my inboard wing half : all even numbered ribs are a little shorter than the odd ribs. That’s right ! But I don’t care. Since I believe to the theory that the outboard wing panel should be a little smaller anyway, the result is exactly what I want ( I do not explain this problem here since it belongs to another topic ). I have to admit that I do have a problem : the leading edge spars don’t meet exactly in the middle of the wing ( depending on wing design the outboard wing half may have a smaller chord by about 2 to 3 mm ). So I have to “fudge” somehow to find a satisfying solution. But that shouldn’t be a big problem. The two leading edges are simply joined a little left off centre line. They will be connected by suitable joiners anyway.
Pic 11 shows the parts which I use for a wing jig. Using the rib templates I cut “negative” rib shapes, two each for the wing root and wing tip. Precisely shaped little balsa pieces ( fixed to the building board ) support the leading edge and trailing edge during construction phase.
|A picture is worth a thousand words. I’ve included a few drawings which help to understand what is difficult to explain in words. Sketch 12 shows how the rear end of the block is sawn off (A) with the “bevelled” rib ends as a result (B). These are squared after relocation of the ribs in the stack. The next sketch (13) shows another method to overcome this problem: the rib stack is held together with two bolts. By shifting the rib stack it’s easy to align the ribs to the correct position as required for exact shaping the rear end.|
the next sketch (14) you can see how I mount the line guide. It’s
a 1 mm ply doubler which is glued to the most inner tip rib before installing
the rib. Finally there’s a drawing (15) for the centre rib. For a
profile model this is absolutely essential to provide a solid connection
for the wing to the fuselage. In this case it will be a full rib with (
almost ) the same thickness as the fuselage.
full body airplanes I still insist on a centre rib for securely connecting
the two wing halves. Also this “rib former” is combined with
the leading edge joiners, the bell crank bearer plate, the top and bottom
main spars, and the wing sheeting to make for a very rigid central unit
which can stand any pull test. It’s installed before planking the
wing. For ease of installation it’s composed from three parts: rear,
front bottom, front top. The sketch shows my interpretation of building
this section. For other applications it has to be changed according to the
preference of the builder.
|As I’ve already pointed out there are different ways to produce ribs. What I’ve explained here is my way of doing things, which I don't claim to be a superior method. The topic of building the wing wasn't covered here because rib production is a complete story in itself. For the moment I'll leave it to you to make a wing out of those ribs.|