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Old Aug 04, 2013, 03:25 PM
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Extreme Sports's Avatar
South Africa, GP, Johannesburg
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Beyond gutter pipes and yardsticks: Share your new S.P.A.D. build tips and techniques

Browsing through the foamies/parkflier/electric forums gives one the impression that correx is an outdated material that is not really suitable for serious plane building any more. Responses to any mention of correx invariably seem to include words and expressions such as "Ugly", 'Heavy", "Not suitable for electric power - use for combat IC power only", "Not for scale building" etc.

The words that someone once used to describe the bagpipes often spring to mind when I read these comments: "Nice from far, but far from nice"!

This general perception of correx seems at odds with many of the 'very far from ugly' SPADs appearing on the "Let's see the S.P.A.Ds" thread (especially Spaderman's creations), the PSS corroscale models built by Flying Beagle and the PE Slope Rebels MIG7 thread.

Clearly SPAD has come a long way from the days of heavy 4mm wings, yardstick spars, gutter-pipe fuselages and ultra simple, 'build in one evening' designs. While the newest offerings may not be quite as simple to build as the original SPADS, clearly the correx building techniques have advanced to the point where the material is a viable option for semi-scale builds and electric park flyers. Also, it seems that there are clever new techniques being developed all the time.

Unfortunately, many of these 'beyond simple SPAD' techniques are spread around many different build/ idea threads and buried in amongst tons of other information.

How about a thread where anyone can post details of any new ideas or techniques for building with correx and/or utilising the unique features of correx in combination with other materials?


I would suggest that we do not post details of techniques that are already well known (see Flying Beagle's corroscale PSS thread for a good list of links to the more conventional correx build techniques - http://www.rcgroups.com/forums/showthread.php?t=1445391), but keep this thread focused on techniques and ideas that push the boundaries of what is possible with correx.
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Old Aug 04, 2013, 04:18 PM
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South Africa, GP, Johannesburg
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A technique for streamlined ailerons and trailing edges

OK, let me start:

As much as I like correx as a build material, the conventional method of adding ailerons has bugged me since my first MIG7 build. I was always taught that sharp, smooth trailing edges are one of the most important elements of good aerodynamics. Although this is apparently not so critical at model scales, the 'steps' created in the conventional correx wing method (at the joint between the wing and aileron) simply do not look like a real plane...even when smoothed over with vinyl. The 3 or 4mm thick TE of the aileron does not look particularly great either.

Here is a method to create sharp, streamlined trailing edges and ailerons that look as good as anything in the foam or balsa world, while being reasonably simple to construct. The technique should work with any wing shape that has a straight TE (i.e. its not suitable for an elliptical spitfire wing, but should work for just about all other wings)

Put simply, the method involves moving the joint between the top and bottom wing skins from the trailing edge/ aileron transition line to become the aileron hinge itself. In other words the joint ends up being out of sight on the underside of the wing and part of the aileron hinge.

Basic steps (refer to the photos - my explanation is likely to sound more complex than the reality)

1) Mark out the bottom wing skin so that its TE is at the aileron hinge line. Mark out the top wing so that it includes additional material for the top and bottom of the aileron. The bottom skin of the aileron should be about 5mm shorter than the top skin (this being the approx. width of the gap needed to allow the aileron to hinge freely)

2) In addition to scoring and folding the leading edge in the normal way, carefully score and fold along the line that will become the TE of the aileron . The quality of this step will determine the neatness of the TE - use a straight edge and score firmly enough to ensure the folded edge does not become 'wavy'.

3) Cut out a hinge groove on the inside of the top wing skin. The easiest way to do this is to just cut through the inner layer of the correx and fold the correx back on this cut line - the flutes will tear as the correx hinges back on the intact layer. Holding the correx back to back, very carefully trim off a 45 degree slice from each side of the cut. Allow the correx to unfold and you have a nice hinge line cut in the underside (but across the flutes rather than along the line of flutes as is conventionally done). The trick is to not cut right through the correx when doing the slicing.

4) The wing will be folded and joined in the normal way...except now the joint will be at the hinge line rather than at the TE and you will need a wedge of foam or similar material at the hinge joint to keep the top and bottom the right distance apart (about 6mm for a typical size correx wing) - I used an offcut of Depron, but balsa or polystyrene will work as well. Make up a 20mm x 6 mm strip that runs the full span of the wing and glue onto the bottom wing skin (I used UHU Por for the joint between the Depron and corrrex - it is more than strong enough). The foam should overhang the edge of the bottom wing skin by ~10mm. I sanded a slight taper onto this strip to match the airfoil shape, but this is not critical (this is still SPAD after all).

5) Now fold the wing to create an airfoil shape in the conventional way...the only difference being that the joint is at the hinge line rather than the TE. (Use UHU Por or similar CA if you are using Depron, ordinary CA if you used balsa in step 4.) [note that in the pictures I am using a CF arrow and foam strips for the spars, but use whatever spar material suits your build]

6) Turn the wing over and fold and glue the front edge of the bottom aileron skin to the foam. You now will have a nice neat airfoil with a sharp trailing edge, but with a strip of foam visible at the hinge line.

7) Finally, cut out a wedge of foam to allow the aileron to have as much throw as you need. Just be careful to not cut though the correx hinge that you created in step 3.

This probably sounds more complicated than it is...try on some scrap correx first - its very simple once you get the knack of it. The technique also works for wings without ailerons - simply omit the steps where the hinge line is cut and run some tape over the resulting joint on the underside of the wing.

PS: Ignore all the other stuff in the wing - it was for a lightweight twin and so needed a few extra items to make provision for the motor cables and servo wires. I also used CF and foam for the spars (more on this in a later post).
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Old Aug 04, 2013, 08:25 PM
...missing the PE Slope Rebels
Flying Beagle's Avatar
United States, MI, Manchester
Joined May 2004
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ES. I think you are on to something here!
Very innovative, yet simple and still maintaining the basic correx wing folding methods.
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Old Aug 05, 2013, 01:36 AM
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South Africa, GP, Johannesburg
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Constructing profiled tail surfaces

The technique above can also be used to resolve one of the other criticisms of correx planes. While flat 3mm or 4mm sheets make for very quick and easy tail surfaces, the rough correx edges don't make nice LEs and TEs. The usual technique is to neaten up the edges with tape, but here is another option. I'll let the pictures do the talking this time - the basic technique is exactly the same as for the wing construction, but you just bend the LE fold far more, so as to get a much thinner airfoil.

I suspect that applications for this are a bit more limited. Apart from the requirement for a straight LE and TE (i.e will work for a Mustang, but not a MIG or Spittie), the resultant surface is probably slightly heavier than a flat 4mm surface, so I would only try this on a large correx plane. The pictures are for the horizontal stabiliser of a 1.6m span B25 that is currently on the backburner (I have not yet developed all the new techniques I need to build it!)

Have fun
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Old Aug 05, 2013, 01:44 AM
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South Africa, GP, Johannesburg
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Quote:
Originally Posted by Flying Beagle View Post
ES. I think you are on to something here!
Very innovative, yet simple and still maintaining the basic correx wing folding methods.
Thanks. It is a little more involved, but I think just the aesthetic benefits are worth the extra effort. The key is to get really good folds - my first few experiments had very wavy/wobbly trailing edges...but this should be well within the capabilities of most correx builders.

I also think there might be a material drag reduction /speed improvement - you can literally hear the poor airflow on most correx planes. My next project will probably be to make a full house (i.e. flaps and retracts) electric version of your Corrostang using this technique for the wing - will be interesting to see how it compares to the MIG and Spitfire in flight.
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Old Aug 05, 2013, 12:58 PM
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Old Aug 05, 2013, 02:13 PM
Call me Frosty
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Lawton, Oklahoma - USA
Joined Sep 2004
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Quote:
Originally Posted by Extreme Sports View Post
very carefully trim off a 45 degree slice from each side of the cut.
What kind of tool are you using to very carefully trim?
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Old Aug 05, 2013, 07:30 PM
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This is hardly pushing boundaries, but the slot in the delta plane here has some thin rubber tubing in to make it a bit cleaner. I didn't make this one, but there was a bunch of guys racing them a while ago. It looks like two slots were needed for enough flexability. It is a bit stiff now, I guess the rubber is hardening. The leading edge of the little .15 combat is carbon fiber, saving a bit of weight without using a spar and two thicknesses of coro. These aren't pretty, but just a couple of thoughts.
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Old Aug 06, 2013, 01:52 AM
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South Africa, GP, Johannesburg
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Originally Posted by TheRealFrosty View Post
What kind of tool are you using to very carefully trim?
Just a normal hobby knife. Fold the corrrex back on itself and run the blade at 45 degrees to the surface of the correx to trim off a small 'triangle' of correx. This is the one occasion that having a super sharp blade is not a good idea as if you are not careful you can accidently cut the intact skin of the correx.

The technique is the same as I use for cutting hinge lines in foamboard wings.
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Old Aug 06, 2013, 02:07 AM
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Aspeed, not sure if I understand your technique correctly, but it reminded me of another way to make neater ailerons that I have seen - this one is more applicable to delta wings or wings where the top flutes run parallel to the hinge line. Basically the top skin is used for the hinge in the same way as one would do for an elevator hinge, but the correx is then folded over to create a double thickness aileron/elevon and a flush hinge joint. The picture should explain this better.

If I have misunderstood, maybe you could post a sketch of the rubber technique?
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Old Aug 06, 2013, 04:14 AM
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South Africa, GP, Johannesburg
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Different hinge options

For what its worth, here are the different types of aileron/elevon hinge methods that I know of.
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Old Aug 06, 2013, 10:13 AM
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It is basically the first and third drawing with a piece of tubing in the slot. It was likely too stiff to get free action, so two flutes were cut with two pieces of tubing. Thin weak tubing would be good. Neoprene was used on the one that I have. (I didn't actually build that one) On a different thought, I had aften thought of experimenting with a piece of downspout, heating up the rear, and stretching it. That may give a tapered rear part to the fuselage. It could maybe not work, or be brittle. Never tried it, too busy/lazy to do it in the near future. I would like to hear if someone else does it successfully though. Even the coro may do some different things if heated, compound curves? I am trying to get away from balsa lately. Tired of painting.
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Old Aug 06, 2013, 10:23 AM
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OK, another separate thought, for the trailing edge. Heat it up with a soldering iron and/or using some type of forming tool to go to a pointed edge. Don't know about vacuum forming coro, but used separately from coro, a nice looking plane can be made. I have seen some local .25 rc combats that look passable using that method (heat shrink) with cola bottles on WW1 and WW2 planes as well. Some of them it seems shameful to wreck them, they look so good.
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Old Aug 06, 2013, 02:32 PM
Building with TLAR precision
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near Madison, WI
Joined Jan 2009
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Gnat design update

Greetings,

The original SPAD Gnat combat design (pdf below) calls for creating the airfoil by wrapping the leading-edge coroplast around the fuselage. While this is simple, replacing a damaged wing is not easy, as it requires removal of one or more servos and the engine mount.

My modification is to create a KFm4 airfoil by wrapping a piece of coro around the flat base wing -- see the photos below -- with 4-6" overlap on each side. The wrapped piece is scored to make the LE curve, and then Gooped to the base wing. Insert a metal rod (or the stiffener of your choice) into the wing, add a 2" coro spacer for under the fuselage, and screw it from below with a couple of sheet metal screws with plastic reinforcements.

Changing a wing now means only to disconnect the control rods, removing the mounting screws, and screwing on a new wing.

This change has gained quick acceptance of a number of RC Combat flyers for the new provisional Gnat combat class, and comments have all been positive with regard to the flight capabilities (not that the original was all that great! ).

-= Dave
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Old Aug 06, 2013, 03:14 PM
.....till the wind drops.
Sean Oelofse's Avatar
Port Elizabeth, South Africa
Joined Oct 2006
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Quote:
Originally Posted by Extreme Sports View Post
.............keep this thread focused on techniques and ideas that push the boundaries of what is possible with correx.
Brilliant!!
My next Mig will have your hinge/aileron modification!!.........for the "light" version!......as I use 2mm for the wing skins......will have to see if it can handle "heavy abuse"!
FB.........can you still "adjust" the plan for us?
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