Canvas Wings II

[Oceaneer99]

I'm posting this for Lars Opland:

Greetings All,

This is an approximate re-post of a technique that disappeared into the cybervoid awhile back.

The basswood wings in these photos were carved as masters for a vac-formed 1:72 Waco YKS-6 kit, so I had no need to sharpen trailing edges or carve the bottom surfaces from the same blanks. For a solid wood model, the whole airfoil will of course be shaped from one piece. To preserve a thin wooden trailing edge once formed, it can be permeated with CA glue, epoxy or similar. Wing bottoms, having less depth, exhibit little sag between ribs & can be detailed with only rib tapes, fuel tank supports & whatever else is needed on your subject.

Some have claimed from time to time that properly covered fabric wings exhibit no sag or "catenary" between ribs but that just isn't so. How much sag exists will depend on the depth of the airfoil curve, distance between ribs, type of fabric & "dope" used, & skill of those doing the job, but some catenary will always be present between ribs on the tops of fabric wings. Consider for a moment that shrinkage of the covering will induce tension in every direction, & that the fore-&-aft component of this tension inevitably pulls the fabric down between the ribs no matter how much the spanwise tension is trying to straighten it. So, to capture those contours:

First, copy your scaled airfoil section, flipped for both ends. Select or prepare a balk of clear grained basswood (others may work but this is best) of the right size & length, then glue your "handed" airfoil diagrams to either end of it, making sure both are precisely oriented & not twisted in relation to one another, & allow to dry.

I go for the "fast track" at this point & take the prepared balk, now double-sticky-taped onto a block for easier handling, to a friend's table belt sander to rough out the basic airfoil, to just outside the lines. The final basic shape should be worked out with medium & fine grit sand paper on a flat surface.

[Oceaneer99]

I'm posting this for Lars Opland:

Tapered & elliptical wings will present some sculpting problems in these early stages, but most of the early birds only need such special attention at the tips. Once you have your basic airfoils, lay out tip shapes, ribs, tanks & control surfaces from the plan view with a soft pencil; sketch or rule all lines lightly so you don't engrave what should be raised. On the other hand, I emboss the aileron hinge lines with the pencil while I have the ruler out.

Now for the most important part: With a fine brush, cover just the pencil lines with fast-drying clear lacquer. While you're at it, use a broader brush to lacquer all areas that won't be fabric, such as wing tanks & plywood-faired leading edges. The lacquer will preserve these guidelines through the rest of the process & prevent chunks of wood grain from being pulled out of the raised ribs by masking tape as you go.

[Oceaneer99]

I'm posting this for Lars Opland:


Now you can cut out the tip shapes with a fret saw or jeweler's saw. If the upper surfaces of the wing tips curve downward in the head-on view, it's time to establish that finished edge around your tips with a pencil & eyeball, & shape the upper & lower surfaces to it. (The Waco project spared me this step because the upper airfoil crown runs straight out to the tip.) Re-establish & lacquer any rib lines removed while shaping tips, & you are ready to begin sculpting fabric details.

Mask leading edge fairings, tanks & control surface outlines, then mask around each rib bay in turn, leaving it's outlines just visible & barely out of reach against the edge of the tape. Using your fingertip & a patch of 320 grit sand paper, sand into the rib bay just a bit; not too deep but as far as you can get into the corners. You're after a subtle but visible effect. Don't cut into trailing edges or control surface openings. Mask & sand one bay at a time, from one end of the wing to the other. Once you have a feel for what's needed, it doesn't take very long to dish out one rib bay.

[Oceaneer99]

I'm posting this for Lars Opland:


Once you've finished the first pass, remove all tape & lacquer the whole wing with a broad brush. Stroke across the grain from leading edge to trailing & try to cover each rib bay with a single stroke. When the lacquer has dried, re-mask leading edges & all as before, & go another pass with the tape & sandpaper to begin removing burrs & grain texture.

[Oceaneer99]

I'm posting this for Lars Opland:

Repeat as needed, using finer paper & always lacquering across the grain until wood grain no longer appears in the reflections when you hold your wings at an angle to a light. If you are getting broad, flat tops on your ribs, mask a little farther away from them while sanding. With the heavy masking tape I was using, the ideal distance was only about half a millimeter.

[Oceaneer99]

I'm posting this for Lars Opland:

These Fairchild wings, seen here on a built-up vac kit, were originally mastered in basswood the same way. This was my first attempt at this method, so I can attest that it's nearly foolproof:

[Balsabasher]

Beautifully done and described and the end results speak for themselves,as a fabric worker myself in real life aviation many years ago I can attest to the sag between the ribs of the wing,this takes on many forms especially if the wing has a plywood leading edge or even a metal wing with box spar and metal leading edge panel whereby the fabric is actually stuck to,the other important facts are the pinked edge rib tapes which go over the semi tautened fabric ( or serrated torn edges on WW1 aircraft as they did not use pinking shears back then ) just after one coat of red tautening dope,these traverse from the leading edge across the ribs and terminate or join at the bottom,once doped down then the real tedious business of stringing takes place,for this a long needle is used that literally goes either side of the rib or capping rib,it goes underneath and then across via a simple loop then pulled upwards to he top of the rib where we started,then a process of knots are done to keep the beeswaxed thread taught throughout the process,once fully completed ( yes the process is very labour intensive ) doped tapes are placed over the stringing which produce a sort of lumpy effect as can be imagined,succesive coats of dope build up to semi blend in the fabric and build up that distinctive starved horse effect ! although the trailing edges on post WW1 aircraft are usually wood as opposed to the distinctive piano wire which was the reason for the pulled in trailing edges,they do in fact still pull in slightly if you study a full sized machine,all part of the overall effect we are looking for in our models.
Other factors are the rib spacing on different types of aircraft and the chord of the wing which creates different effects on the droop of the fabric,best to look at pictures of the real aircraft to determine the amount of sag needed,just like anything if overdone it can look wrong,some plastic kit manufacturers have overdone things in the past producing thin wings with just too much sag between them making the wing look wrong.
Traverse doping effects such as when fabric is applied around acute tail surfaces or even wingtips need checking first to determine which way the sag appears,a typical example is the distinctive De Havilland tail shape for example on the Tigher Moth,take a look at various examples and you will soon see that they difffer a lot due to techniques or even repairs altering the shape of the fin or rudder.
I hope these observations will give anyone a better understanding of the principles involved,fabric work is a skill and an art form that needs to be kept alive,with the number of vintage aircraft still flying this is assured for the future.
Barry.

[lastvautour]

Thank you Lars and Garet for a very informative article.

Lou

[Lars_Opland]

Thank you, Garet, for somehow finding & re-posting the original version of this article, which is longer & more detailed than what I was trying to "re-post" yesterday!

-Lars

[cliff strachan]

Thank you Lars. I must admit that I have to go through your very informative description once, at least, again. But at this point you've answered a great number of my questions. A very well done exposition of what is expected. And nice model displayed also.

Cliff.