Thread Tools
Aug 19, 2013, 04:06 PM
Random slope flyer
Mike_'s Avatar
Originally Posted by villapilote
I mount aileron servos on the bottom of the 4 MM Coroplast, inside the wing. The Coroplast flutes are in the chord direction. I make a hinged trap door on the bottom of the wing, .... The trap door is held closed with a piece of bamboo placed in a flute from the TE.
Thanks - just used this yesterday, works great. 82MG servos and small horns just fit
Sign up now
to remove ads between posts
Aug 20, 2013, 09:07 AM
Registered User
Hi Mike_
Glad to see my idea used by others. Here is another: When I use the clevis you show in your photo, I add some safety by tying a piece of string around the clevis to keep it closed, and further tie the string to an empty hole in the servo arm to keep the string in place. Others use a piece of fuel tubing to do the same thing. I thing that is just a hope, since the fuel hose is terrible stretched, and is known to split in normal service.
Aug 26, 2013, 06:23 AM
Registered User
Extreme Sports's Avatar
Thread OP

Uses for the humble electrical connector...

Be interested to hear if anyone else has found further uses for the metal insides of ordinary electrical connector blocks. They are available at hardware stores and are cheap, so fit 100% into the SPAD concept. We get 3A, 6A and 15A sizes here; Once the black plastic has been cut away a brass (or sometimes steel) inner remains, which can be cut or used as is for several different uses:

1) Cut in half for ultra cheap wheel collars (kind of obvious)
2) The larger ones make good adjustable connectors between the piano wire and wood dowel of a pushrod
3) Similar to the above, I have used them to make wing struts adjustable
3) If cut in half, the smaller ones can be used to make a wire pushrod adjustable
4) A few cuts and a bit of filing makes a low cost blind nut...our 3A fits a 2.5mm bolt, while the 6A takes a 3mm bolt.

Anyone using these in other applications?
Aug 26, 2013, 06:33 AM
Registered User
Extreme Sports's Avatar
Thread OP

Tail wheel bearing tube idea

Was scratching around in my workshop trying to find something to make a bearing tube for the tail wheel of my new C180 when a pack of pop rivets caught my eye. Removed the inner lugs and the combination of two outers made a perfect bearing tube; the rivet shape removes the need for any additional washers. AFAIK, they are available in many different sizes so should work for different wire gauges.
Aug 26, 2013, 06:44 AM
Registered User
Extreme Sports's Avatar
Thread OP

Simple, cheap nosewheel

On the subject of wheels, here is something that worked very well for a steerable nose wheel. I cut a rectangle from a plastic kitchen chopping block (sometimes used in the SPAD-world to make engine mounts), drilled a 2.5mm hold through it long-ways to take the nose wheel wire, and bolted the assembly to the front former. In my application I used a free castoring wheel (twin with differential thrust steering, so no need for a servo), but it should be easy enough to attach a servo to the wire arm (see the second picture) to make it fully steerable.
Aug 26, 2013, 12:55 PM
Registered User
Extreme Sports's Avatar
Thread OP

Another trailing edge/ aileron option

Originally Posted by TheRealFrosty
I used the following technique to hide the flute end of 90 degree 4mm coro joints on my SPAD Twist-VSF (
Originally Posted by robw_london
How about routing the correx for the hinge and TE folds. 1/8 dia cutter set at (say) 25 thou / 1/4mm to just leave the top skin. I know its been done (spadtothebone?) but I haven't tried it yet.
Gents, the combination of your posts gave me another idea - cut and sand out the inner skins and flutes as per TheRealFrosty's 90 degree joint method, except make the cut much wider...then glue onto a foam wedge to get the desired TE profile. Have not tried it out on a plane yet, but did a quick test on some scrap correx and it looks neat. Needs some care as it is very easy to cut through the top skin by mistake, but on the plus side, I can see this working for an elliptical wing and, with some imagination, it could be used to make passable split flaps (should be pretty obvious by now what my next project could be...).

Have updated the diagram of alternative TE techniques.
Aug 27, 2013, 06:52 AM
Rusty Nail's Avatar

Corro Bat

A similar method is used for the leading edge on the Corro Bat (RC Soaring, March 08)
Aug 27, 2013, 11:18 AM
Jer. 29:11
jeffsch's Avatar
Rusty, thanks for the pointer. It's an interesting article. Here's a direct link to the PDF.
Sep 29, 2013, 05:36 AM
Registered User
Extreme Sports's Avatar
Thread OP

Lightweight Corro-Scale diet

The most common reaction when one suggests building out of correx is that it is heavy and not a suitable material for building smaller and scale planes. Some time back someone posted on this forum looking for a park-flyer electric SPAD. He was advised to use foam.

Given the origins of SPAD in .40 - .60 glow powered combat, where strength and ease of construction are paramount, I guess it’s not surprising that there has been less development of light-weight, more scale looking designs using correx (not that none exist; it’s just that there are far more heavy, easy-build SPAD designs).

Even the popular MIG7 design is relatively heavy and with an AUW of 900g upwards for an electric version (with corresponding wing cubic loading in excess of 10), it needs a bit of speed and space to enjoy safely. There is very little on offer around the 400g-700g mark outside of flying wings such as the Mugi (generally between 400g and 550g).

The reality is that correx planes can be built substantially lighter without a significant loss of crash survivability (particularly electric powered ones, given the gentler characteristics of electric motors). The lower strength of a light plane is offset by the lower energy carried into the crash (lower weight and speed). A light plane also means a cheap plane since you can use a smaller and cheaper power system (which will more than offset the extra cost of using some 'exotic' materials ). Finally, the properties of correx can also be exploited to make semi-scale models that are easier to build than if using foam or balsa (e.g. easy curves, nice smooth surfaces without hours of sanding and painting, simple hinges, clever uses of the flutes etc etc). Here are some examples of my recent corroscale parkfly projects:

1) 1100mm Piper Cub – 450g
2) 1150mm Cessna 180 – 550g
3) 1150mm MIG7 – 620g to 750g (still experimenting with power combinations)
4) 900mm SE5A biplane – 700g
5) 1250mm Twin – 875g

While correx will never match foam scratch-builds gram for gram, these weights are comparable to many RTF foam models of the same approximate size and feature list.

In the posts that follow, I will discuss some of the techniques that can be used to save weight on correx builds. Several are blindingly obvious; some are unashamedly borrowed from foamies, while others may have SPAD aficionados convinced that the unwritten rules of SPAD are being violated!

As with all diets, this one may not suit everyone’s tastes. But if you aspire to build planes that are relatively cheap and easy to build, fly well, look good and can be flown at your local park, then read on.

Here are some pictures and videos.

SE5A Video std (1 min 50 sec)

Cub video (2 min 4 sec)
Last edited by Extreme Sports; Sep 30, 2013 at 02:00 PM.
Sep 29, 2013, 06:00 AM
Registered User
Extreme Sports's Avatar
Thread OP

General dieting tips

To paraphrase the immortal words of Monty Python “Every gram is sacred...” None of these tips on their own will result in a significant weight reduction; the power lies in using as many of them as are applicable to your particular design and removing any and every source of excess weight.
  1. Set up a spreadsheet with the approximate unit weights of the materials you plan to use and calculate the area/size of each major part. I find that doing this has helped me target the heaviest items, select the right power combination and get the CoG correct before building. Adding weight to correct for a poorly balance model is like eating a large packet of crisps and a chocolate cake when you are supposed to be watching your calorie intake.
  2. Use contact adhesive (CA) rather than hot glue wherever possible.
  3. Use mini-servos only. 5g servos will work for most elevator/ rudder controls in this size range; If in doubt use no heavier than the 9g ones (e.g. if using a single servo for the ailerons or flaps).
  4. Mount your servos using a foam or foamboard ‘shelf’ – and glue them in with a small tack of hot glue (if using foamboard, the screws that come with the servos can be used instead of glue). No need for heavy ply or similar wood servo rails.
  5. Spray paint is lighter than vinyl if you intend to colour the whole plane, while permanent marker pens add negligible weight for smaller trim details. My experiments indicate that a full coat of spray paint adds about 25g for a ~1m span parkflyer, whereas a full covering of vinyl will add close to 100g. Rustoleum has a range specifically designed to adhere to plastic and it seems to be far more scratch and flaking resistant than some of the other brands I have used. Of course the lightest option is to select the right colour correx from the start (unfortunately we only get black and white locally) and then only add trim colours using permanent marker pen.
  6. A lightweight build enables a lighter power pack – To my surprise I have found that I can use the very cheap 39g E-max 2822 and 2812 series motors with a 20A ESC and an 80g – 110g 3S Lipo if I keep the correx airframe light. A 2S lipo can even be used for the slower flyers. Believe it or not, these motors provide more than enough power for a ~1m span correx model if built lightly – I suspect that further weight could be saved by using the 24g Blue Wonder type motors and a smaller ESC/ battery combination if you don’t require excess power on the smaller models.
  7. Try to get away with no more than a 1300mAh Lipo (~110g for 3S, 80g for 2S)...above this the wing loading starts to get too high.
  8. The landing gear adds quite a bit of weight (e.g. over 10% of the weight of my SE5A). If you really have to have them, use 2.5mm piano wire and lightweight wheels.
  9. Minimize weight adding scale details (exhaust manifolds, guns, pilots etc) and, if you must have them, use foam and other very lightweight materials (e.g. drinking straws for the exhaust pipes on the SE5A)
  10. Foam is also useful for making light nose cowls, nose cones and other areas where compound curves are required. A coat of WBPU-acrylic will give some ding-resistance. Vacuum formed parts are an option, but that is probably pushing the boundaries of SPAD a bit (well at least they are made from plastic )
  11. Don't ignore the weight of props and spinners. I found to my horror that my 65mm spinner weighed over 50g and the prop I had initially selected for the MIG7 weighed another 30g. Needless to say, I made my own vacuum formed spinner (<10g) and used a lighter prop.

Time invested in keeping the airframe light is time well invested. Remember, building the airframe itself represents only a portion of the total build time – the rest being connecting the control surfaces, installing the electronics and adding the trim and other details. I estimate that the airframe build time is generally around 50% of the total build time, so increasing this by say 20% to build light and neat only increases the total build time by ~10%. That is an investment I am more than happy to make if the plane looks and flies better.
Last edited by Extreme Sports; Sep 30, 2013 at 02:06 PM.
Sep 29, 2013, 06:14 AM
Registered User
Extreme Sports's Avatar
Thread OP

Meal plan 1: Getting that lean, ribbed torso you've always dreamed of

The conventional 3mm or 4mm correx fuselage with ply formers and 3mm/4mm correx empennage is super-strong, but also unnecessary heavy if your primary ambition is to fly around the park at medium speeds without hitting any other planes. Careful choice of materials and build techniques can result in a fuselage that is only ~20g heavier than one built from 6mm Depron, but still substantially tougher.
  1. Build the fuselage from 2mm rather than 3mm correx or gutter pipe/aluminium – 2mm correx is more than strong enough for the section aft of the wings (after all, how often does this area get damaged, even in full size crashes?) and can be doubled up in areas where more strength is required. I use 2mm doublers to strengthen any areas in front of the wings that need it, particularly areas where the flutes have been cut to form curves, as well as for any joints. 2mm correx is also easier to fold and create curves for semi-scale fuselages.
  2. Use scrap Depron/ foam/polystyrene for most of the fuselage formers – foam safe CA such as UHU-Por works well for gluing foam to correx. Any plywood formers (e.g. firewall, landing gear mounting) should be from thin plywood (e.g. <2mm) or foamboard. If you feel that foam is not strong enough for a particular former (e.g. the former needs a large cut-out area), then a ‘ply’ of 2 layers of 2mm correx will generally be lighter than plywood.
  3. To attach the motor to a plywood firewall, use the small mounting screws that come with servos...anything larger is unnecessary weight. Remember, the result of the ‘diet’ is that you should be using a less powerful propulsion system.
  4. I usually add a slice of foam behind the firewall to protect the Lipo from being punctured by the screws and to give some shock absorption in event of a crash.
  5. Try to minimise the number of fuselage joints. A folded correx edge is not only neater than an edge joint, but is lighter and far stronger (monocoque effect) . For example, where applicable, I replace the usual fuselage top and bottom pieces aft of the wings by folding the fuselage sides over to meet along the centreline (visible in pictures 2 and 3 below). This replaces 2 corner joints (hard to do neatly) with a single seam that is easy to make neat and strong by adding a thin doubler on the inside (visible in picture 2) and maybe even a strip of tape/vinyl on the outside (picture 3). Origami meets SPAD!
  6. Use 2mm correx for the tail surfaces – its more than strong and rigid enough for park flying speeds and seldom needs any additional bracing. 3mm foamboard is a slightly heavier but more rigid alternative that is still marginally lighter than 3mm correx and which can be sanded to give a rounded leading edge. Interestingly, the 5mm foamboard we get locally is pretty much the same weight as 3mm correx.
Last edited by Extreme Sports; Oct 04, 2013 at 10:32 AM.
Sep 29, 2013, 06:22 AM
Registered User
Extreme Sports's Avatar
Thread OP

Meal plan 2: The 'Hot Wings' diet

The wings are the most difficult component to match the lighter weight of foam. One simply cannot escape the fact that 2mm correx weighs about 340g/m^2 as opposed to ~130g/m^2 for 3mm Depron and ~260g/m^2 for 6mm Depron. For any model, the wing area is a given, so two skins of 2mm correx are always going to be heavier than the equivalent 2 skin Depron wing and heavier than a flat plate Depron or foamboard wing. The only real options are to lighten the spar, use less correx, or replace the correx with a lighter material. I’d love to hear from others on ways to make wings lighter...the following are the ideas that have given me some benefits.
  1. Try alternatives to the normal two x 2mm skin correx wing. For my smaller semi-scale park flyers (Cub, SE5A), I used a single 2mm curved plate correx wing with the flutes running spanwise. Unfortunately, this technique is only suited to slower, more docile RET designs that don’t need to do axial rolls (since ailerons are not very effective on curved plate wings); however, flight can be quite realistic since full size Cubs and WWI biplanes generally fly slowly and have a limited aerobatic repertoire. I still want to experiment further with a correx top surface plus a taped bottom surface to create flat bottomed airfoils. An alternative that I used on the Cessna 180 was to use a built up foam wing mated to a correx fuselage (I guess one could also use a foam KFM wing to simplify construction)...this at least gives one the option of adding functional ailerons.
  2. I often enlarge the wing slightly relative to scale to give me my desired overall wing loading (I generally try to target a Cubic Wing Loading below 6.5 for park flyers.)
  3. Take some lessons from foamy wings – ditch the yardstick/ pine/ ply spar and use CF or GRP tube. Those yardsticks or ply equivalents are surprisingly heavy. See the next post on some ideas for building lighter weight spars for double skin correx wings.
  4. Curved plate wings can be made with minimalist spars – the bent shape (held in place by gluing the correx to the balsa/foam ribs) provides some rigidity, while a 2mm or 2.5mm CF or GRP rod inserted into the flute for the full span at the LE and the thickest point, adds further to the rigidity. If your design has wing struts, remember that the struts can add substantial strength, permitting a correspondingly lighter spar. Struts also add torsinal strength. I expect that the addition of a wing strut could eliminate the spar entirely if you plan gentle flying only (I have not tested this yet). My Piper Cub uses only a 4mm CF strip for a spar, and it’s plenty strong.
  5. Biplane wings only need a spar on the lower wing – the inter-plane struts and bracing rods transfer the rigidity of the spared wing to the upper one. I suspect it may even be possible to eliminate the spars altogether in a curved plate, fully braced biplane wing (that’s what they do in some foamy designs).
  6. In most curved plate wings, a full length spar is not needed, since the curve provides rigidity for the outer wing panel - my SE5A only has a 6mm scrap GRP tube spar running as far as the inter-plane struts. Since this spar weighs about 15g, so I saw no benefit in pushing the boundaries by trying a spar-less wing!
Last edited by Extreme Sports; Sep 29, 2013 at 10:48 AM.
Sep 29, 2013, 06:28 AM
Registered User
Extreme Sports's Avatar
Thread OP

Meal plan 3: Light bone structure: Ideas for simple spars from foam and GRP/CF rods

Yardstick type spars are quick and simple to build, but they are heavy. They can also be difficult to make for tapered wings unless you have access to power tools. While you can simply use a CF tube as a spar, you still need something to create the airfoil shape of a typical double skin correx wing, and CF is expensive.

Fortunately, there are lessons to be learned from the old world of built up spars used in some balsa designs. The key thing about a spar is that it is simply an I-Beam and a strong I-Beam has as much material as possible on the outsides flanges and as little as possible in the centre web. As a kid, I built a balsa glider that has a spar that is constructed from two spruce strips separated by a thin balsa web, and it is very light and strong. In contrast, a solid ply spar has an equal amount of material at the centre as at the top and bottom 'flanges', so there is a lot of weight in the centre that adds no bending strength. Yes, one can drill out the centre of the spar, but this takes time and you are unlikely to lose more than 25% of the weight.

Some theory on spars: In the ideal wing design, the spar and the wing skin should work together to act as a single I-Beam (the so called ‘semi-monocoque’ construction). The failure mode of an I-Beam supporting a uniform bending load (as in a wing) is most often compression failure at the outer point of the compressed flange. While the correx wing skin itself acts as a flange, it has very little compressive strength if it is used with the flutes running chord-wise. So the flutes in the top of the wing collapse when only lightly loaded (if you run the flutes span-wise as in some SPAD designs, you get quite a strong wing, but you lose the nice airfoil shape of the chord-wise method.)

Interestingly, the popular Mugi Evo flying wing can be built without any spar at all. However for some reason the designer has the flutes running chord wise on top and span wise on the a result most Mugi’s have characteristic ‘creases’ in the outer top wing skin. Fixing this is as simple as building the plane upside the top skin has the flutes running span-wise and you will no longer get any creases (unless you do lots of negative G manoeuvres such as outside loops).

So, for a light spar, you need 2 things:
1) A light material with reasonable compressive strength to act as the web of the I-Beam, in effect keeping the wing skins a set distance apart, and providing the shape of the airfoil, but not really adding any bending strength ...foam works well for this;
2) A strong material that can add compressive and tensile strength to the wing skins...CF or GRP rods work well for this. Wood dowels are OK, but not as strong.

Two light spar ideas: The attached sketch shows two options to construct a strong, but light spar for a correx wing. In both methods, cut a foam spar in exactly the same shape as you would the normal plywood spar and glue it to the bottom wing in the normal way (UHU Por works well and about 10mm thick foam gives a good surface for the bond). When you fold over the wing, make sure that you also apply glue to the top, so that the foam ends up being glued to both skins.

If you have a large GRP tube (>6mm) and want a quick and simple spar, use the first method – it’s what most foam builders do. Just glue the CF/GRP tube solidly to the bottom wing skin and the dihedral joiner and you are done - the tube provides the strength and the foam provides the airfoil shape. You can even add a second foam spar if you want a specific airfoil shape.

The second option gives a slightly lighter and more technically elegant solution (yes, I once was an engineer ). Use 2 thinner rods (~3mm) glued to each of the top and bottom skins as close as you can to the foam spar. Glue them in place before you fold the wing. The important thing here is that when you make the dihedral joint, you MUST make sure that all 4 of the rod ends at the joint are well bonded to the dihedral joiner (thin plywood is probably best for the joiner). By using this method, you have in effect made the wing into a single I-Beam - the foam is the web and the correx skins and GRP rods together become the flanges. If you are nervous about the strength or plan to pull massive G's, you can re-enforce the foam spar with a second spar cut from 2mm correx with the flutes running vertically. Its highly unlikely that you will ever pull enough G's for the GRP rods to fail in tension or compression - more likely the 'beam' will buckle as the foam fails in compression and adding correx will stop this from happening.

Apart from the weight saving (about 50%), I find it much easier and faster to cut a tapered spar from a scrap of foam than shaping one from 6mm ply.

Also, see posts #2 and #3 in for some thoughts on correx spars in mugi-style flying wings.
Last edited by Extreme Sports; Sep 29, 2013 at 11:21 AM.
Sep 29, 2013, 08:25 AM
...missing the PE Slope Rebels
Flying Beagle's Avatar
Great techniques ES. Thanks for the info. !
Sep 30, 2013, 10:39 AM
Call me Frosty
TheRealFrosty's Avatar
You can re-define SPAD as Sophisticated Plastic Airplane Design instead of Simple Plastic Airplane Design and the acronym still works.

I'm sure my SPAD Twist-VSF build was in that boat because it was not simple like the original SPAD designs.

I like your weight saving ideas.

Originally Posted by Extreme Sports
In the posts that follow, I will discuss some of the techniques that can be used to save weight on correx builds. Several are blindingly obvious; some are unashamedly borrowed from foamies, while others may have SPAD aficionados convinced that the unwritten rules of SPAD are being violated!

Quick Reply

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Build Log Peanut Miles Sparrowhawk build testing Vac Forming Depron Cutting Techniques agricola1 Scratchbuilt Indoor and Micro Models 27 Jul 14, 2013 11:12 AM
Discussion Best building techniques for a newbie? ennobee Scratchbuilt Indoor and Micro Models 5 Apr 08, 2013 03:19 PM
Discussion Building Technique Questions. highnpressurized Balsa Builders 7 Mar 12, 2013 06:24 PM
Advanced Covering Techniques with Solite jreed Electric Plane Talk 22 Aug 07, 2003 07:55 AM