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Old Mar 16, 2014, 08:35 AM
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Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
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More thoughts on Correx airfoil shapes

I've been doodling again with some scraps of correx in the hope of finding the elusive SIMPLE technique to make correx airfoils that closely resemble actual airfoils. In particular, I was hoping to find a technique that builds on the simplicity of the standard folding technique with the flutes running chordwise, but which can produce something more like a Clark Y or RG15 airfoil. The next post will detail a combination of techniques that looks to be fairly effective, while being only marginally more effort than the standard method used by most of the folks building MIG and other Corroscale slopers.

But first I built several sample aifoils to better understand how correx behaves. Each airfoil had a 200mm chord and a 20mm spar set 60mm back from the LE. These are the approximate proportions for a Clark Y airfoil (Maximum thickness is 11.8% of chord and occurs at 28% of the chord). Flutes run chordwise in all samples.

See the attached picture of the results - each has been traced out from an actual airfoil.

Airfoil 1 is what you get if you just bend and fold the LE, but taper the TE (as in the initial posts of this thread). Note that the max thickness does not occur at the spar location, but at the mid-point, since the top skin must bend evenly.
Airfoil 2 is the normal correx airfoil with a simple sharp fold at the LE and an aileron pinched in between the two skins. Note how the max thickness is still behind the spar and how the stresses in the correx create the characteristic reflex and under-camber near the trailing edge.
Airfoil 3 is the same as airfoil 2, but the LE has been rounded using a variation of the LE crusher technique (more on this in the next post). The shape is slightly better, but the max thickness is still further back than a Clark Y.

The last 2 airfoils are one made using a combination of techniques and a Clark Y respectively. If you compare these carefully with the first three with the Clark Y, you will also notice that the LE skin forward of the spar has less curvature than the Clark Y. This is because the correx must bend evenly along its length, whereas most airfoils have a greater degree of curvature forward of the spar.

Up to now, I have focused only on getting a rounded LE and a tapered TE. The above suggests that, to get a recognisable airfoil, one also needs to consider the amount of curve in especially the top skin (on symmetrical airfoils, one needs to consider the curve in the bottom skin too, but my analysis and flight tests so far suggest that the conventional symmetrical correx airfoil performs pretty well relative to more recognised airfoils).

The above implies that, in addition to rounding the LE and tapering the TE, one needs to find a way to get the correx to bend differently along its length....

Fortunately there are simple solutions to each of these challenges....
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Old Mar 16, 2014, 09:22 AM
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South Africa, GP, Johannesburg
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Getting a Clark Y profile on a Correx wing

IIRC, so far, this thread has demonstrated three broad methods to build a Clark Y airfoil (or whatever profile is desired) on a correx wing:
1) Run the flutes spanwise and use ribs to achieve the desired profile.
2) Fold a simple LE and add a foam capping forward of the spar (which is carved/ sanded to the desired LE shape), plus add a wedge of foam to the TE.
3) Cut a foam core and cover this in correx.

Each of the above have advantages and disadvantages. Moreover, there were still a few suggestions made by various folk that I did not have a chance to test, especially the LE crusher and trying to 'heat form' curves using a heat gun.

At the same time, I noticed that wings built with the flutes running chordwise had too little curvature forward of the wing (see previous post).

So here is a combination of methods with which I was able to produce a pretty accurate rendition of the Clark Y using only marginal more effort than building a conventional correx wing:

1) Run the flutes chordwise.
2) Build a rounded LE using a simple variation of the LE crusher. Instead of just scoring the LE fold line with a blunt edge (such as a screwdriver or pizza cutter), I flattened the LE flutes using the handle of a cheap wire screwdriver (approx. 6mm diameter, but it would be easy to make up an equivalent tool). I heated the handle to the point where it had a 'slippery' feel when pressed onto the correx...but was not quite hot enough to melt the correx. I then bent the flattened area around a 6mm dowel....and got a pretty decent rounded LE for not much more effort than scoring with a screwdriver. One can leave the dowel in place, or remove it...the correx seems to retain the rounded shape quite well once the dowel is removed.
3) Before folding the wing, add in some curvature. To do this I gently heated the OUTSIDE of the top skin forward of the spar with a heat gun on low heat. If the temperature is right, the correx will bend on its own, and it will bend in the right direction too. If needed, you can help the process by rolling it over a thick dowel. With a bit of practice, one can bend in any amount of curvature. Do the same to the bottom skin if your desired profile has some curvature there (as for the Clark Y).
4) Build a wedge shaped TE using scrap foam, as detailed in the early posts on this thread. This is by far the most complicated and time consuming part of the build. I suspect that a pretty decent profile that is somewhere between an RG15 and a reflexed foil would emerge if one just used the conventional sandwiched aileron method.

If you are a total perfectionist, you can consider adding a capping strip of PVC to the LE, but my tests suggest that this is not needed.

Testing on some scraps is advised...especially for the steps involving heat .

Some pictures of the final airfoil and the construction steps below. Seems to me to me relatively simple but effective. It still needs to be tested on a full wing build.

EDIT:
1) I was worried that the airfoil might distort if left out in the sun, but it seems to be a non-issue - left it out in the full sunshine for a day and there was no change to the shape.
2) A Clark Y is probably not the ideal airfoil for models....if you look at the foils designed specifically for low Re, most of them have a sharper LE and are thinner than the Clark Y. But its a good start....my feeling is that if you can make a Clark Y from correx, it should be even easier to make a low RE foil such as an RG15 or any of the AG34-38 series.
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Old Mar 16, 2014, 10:02 AM
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South Africa, EC, Port Elizabeth
Joined Oct 2012
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Interresting developments ES! Working with heat on correx can be tricky. I often find the correx warping after applying the hot glue on a join. Maybe my glue gun is too hot!
Just another comment on the foam core wing with flutes spanwise - I accidentally flew into an Impala at Maitlands this week with my Foxx glider and the resultant mid-air severed the fin and rear stabilizers clean off the Impala! My wing with a foam core was split at the point of impact but the foam behind the correx provided significant support minimising the damage. I was able to continue flying and just need a little glue and vinyl to repair the damage. I'll post a pic later to show what I mean.
I'm about to make a set of wings for the new Hawk with a foam core and will post more pics when done.
Keep up the experimentation you never know when someone will come up with a winning idea! TS
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Old Mar 16, 2014, 10:58 AM
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South Africa, GP, Johannesburg
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Indeed, using heat adds risk....one bad mistake and you may have to start from scratch . That said, so far the temps required seem to be low enough to not cause any distortion. Crushing the LE requires some care to not melt through the inner skin, but curving the wing skins is a complete non-issue...just get the heat gun temp right on a piece of scrap first.

Glad to hear your foam core wing survived. I like the technique, but it is too heavy for my smaller park flying designs, which is why the experiments continue.....and why I use different techniques for different models.
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Old Mar 16, 2014, 10:57 PM
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Australia, WA, Beldon
Joined Jun 2013
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That is a damn nice airfoil ES! My feeling is that it will be hard to pick the difference in performance between this and a 'perfect' clark Y. You've hit the mark, I might have to try this technique out myself :-)
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