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Old Nov 27, 2014, 10:10 AM
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Correx Parkjets 2: Replacing joints with folds and various joint techniques

In the standard Shumate park jet the fuselage is built up from four sides of 6mm foam glued together with a strip of foam inside to butt up the joint. The nose is built in a similar fashion, followed by some carving and sanding. In many cases using correx opens up opportunities to simply this:
  • Where the line of the joint is not rounded and straight or close to straight, it can be replaced by a simple fold...so the two parts become a single part with a ‘score and fold’ edge. On the F22 I was able to build the entire underside of the fuselage from a single piece of correx with folds instead of joints (except at the tail, where the curve was created using a butt joint.) Likewise the nose was built from 3 rather than about 9 pieces. In this case I sacrificed the slight curve in the profile of the nose for simplicity of build.
  • Joints that must follow a curve (when viewed side on or in plan view such as on the Polaris fuselage) can be easily be replicated in correx either simply using hot glue to glue the two parts together, or for a neater finish, by using a 20-30mm strip of correx with the flutes crushed to butt the inside of the joint. The photos should make this clearer. Because the flutes are crushed (by rolling the strip back on itself), the strip can be made to follow slight curves.
  • In some cases the edges of the foam strips need to have a chamfer cut to accommodate a joint that is less than 90 degrees or which needs to be sharp...for example where the top fuselage skins on the F22 and F16 fair to the wing base at the strakes. I found I could easily create chamfers by using the same heating tool I use for crushing leading edges, without much risk of over-melting the correx. Simply lay the edge to be chamfered up against the edge of your workbench and run the heated tool along the edge at the desired angle. Obviously its best to practice on some scraps, but I found it to be much easier than trying to cut a chamfer. Incidentally I also melted chamfers into the joint between the wing top skin and the aileron to remove the usual 2mm 'step', and on the jet intakes to give a sharp edge.
  • To get a really nice finish, tape joints using 10mm vinyl tape....the weight is negligible.
  • Where the correx has to form compound curves (i.e. bend in 2 planes at once), the normal technique of cutting slits works. However these types of joints are rare in the designs since foam does not like to form compound curves either...only the F16 had extensive need for this. What I did do differently was to back up the slit joints with strapping tape on the inside of the fuselage rather than using vinyl on the outside.

The trick is to look at photos of the real airplanes and decide which joint method will best capture the lines of the plane while still being strong and reasonably simple to construct.
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Old Dec 02, 2014, 12:10 PM
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Correx Parkjets 3: Formers

Most of the parkjet designs use formers in the nose section and a formerless ‘box’ construction for the jet inlets and rear fuselage. Correx construction needs a few changes:
  • The nose can be constructed in the same way, but you need to modify the formers to account for the use of 2mm correx rather than 6mm foam (i.e. enlarge by 4mm all round), and to match the final shape... remember that most foamies use flat sided formers and then carve and sand the curves into the 6mm foam sides. So just redraw the formers slightly larger and with the curves built in.
  • Although one could use ply or correx formers I primarily use foam since its light, easy to cut and gives a nice wide surface for the sides to bond to. In the F22 I braced F2 with correx as that one is at risk of being damaged each time you insert the battery.
  • The formerless construction of the jet intakes and rear fuselage can be directly replicated in correx in most builds (see note on folding above). This is important as this allows the free flow of air through the fuselage which aids in cooling the ESC and motor. However, the F16 presented a problem since the jet intake and fuselage is round, not square, and correx needs formers to hold any curves that you build into it. The solution I came up with was to bend strips of thin (0.4mm) aluminium to the shape of the formers and glue them to the inside of the fuselage. I did the something similar to get the curves of the engine shape on the rear underside of the F22 - this time I inserted a curved wire in the flute of a strip of correx, which was then glued inside the fuselage.
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Old Dec 13, 2014, 07:59 AM
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Correx Parkjets 4: Stabilators and control layout

Many of the more realistic depron jets use fully moving horizontal stabilisers (stabilators). These look great and are easy enough to replicate in correx (look at any of Steve’s build guides and just run the CF rod inside the correx flute), but they are very prone to damage on landing. As you flare for the final touchdown you pull up elevator and the front tips of the stabilator catch the grass. The extra toughness of correx might reduce the damage, but I long ago gave up on stabilators and replaced them with a more conventional elevator. A couple of pointers on these:
  1. Replacing the stabilator with an elevator/taileron means that, in most cases, you can cut the elevator as an integral part of the wing-fuselage rather than as a separate part...see the earlier pictures of the integrated wing and fuselage. The exception would be if the elevators do not operate in the same horizontal plane as the wing (e.g. the Super Bandit jet).
  2. The elevator area needs to be quite large – you are replacing an all moving surface with a partly moving one, so cut the elevator hinge suitably far forward. On mine the elevator area is in the region of half the combined stabiliser plus elevator area.
  3. On both my F16 and F22 I have used a single push rod for the aileron and elevator on each side. This means that I set my Tx up for elevons and the elevator acts as a taileron (i.e. controls both pitch and roll). The photos should make this clearer.
  4. Don't underestimate the amount of movement needed, especially in the tailerons, even with the seemingly oversized control surfaces. I'm not sure how much of the issue is related to the small size of the tailerons vs their long moment arm to the CG (and the weight/inertia of the motor being so far back) and how much is due to the pusher prop not providing airflow over the control surfaces, but I have found that its best to use lots of travel (i.e. ~45 degrees each way) and then use expo and dual rates to resolve the sensitivity issues if these bother you. As an example my Vulcan's throws are more Mugi like and the plane can barely complete a roll at full speed, while you will miss the rolls of my F16 if you are not watching intently. The F16's elevator response was similarly anaemic until I significantly increased the throws.
  5. Elevons/ Ailerons can sometimes be driven through torque rods hidden in the fuselage, which leaves you with a nice clean wing....easier to do with correx than in the original foamies. My Vulcan and Polaris work this way, and I could easily have done the same for the F16 and F22.
  6. Regardless of what system you use, you MUST minimise slop in the system. Because these planes have more mass in their tails than conventional layouts, any slop leaves you with very uncertain trim and control.

Quite a few jets also have twin tails (F22, F18, F35, F15, F14 etc). The foamy plans will usually suggest a method for doing the dual rudder control. For the F22 I devised a simple rudder system that is totally hidden within the fuselage. It works on the same principle as a torque rod – a rod extends from each rudder into the fuselage, where the two rods are connected so that they move together (using a length of 2mm GRP rod connected with some shrink tube). One of the ‘torque rods’ is then connected to the rudder pushrod – once again I hope a few pictures will say what a 1000 words cannot, even if they are not the best pictures.

Moving canards can easily be made in the same way as stabilators. I would suggest running a narrow strip of correx along the underside of the root of each canard to prevent the flutes from bending.

Balancing park jets is usually a problem given the motor location, so try to mount servos as far forward as possible, taking into account all the other factors as listed above (e.g. longer push rods being more susceptible to slop). Since correx is heavier than depron, and most of the airframe weight is behind the CoG, getting the plane to balance needs more deliberate planning on a correx build
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Old Dec 13, 2014, 12:01 PM
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I certainly like your F22.
I've printed the plans and I'm going to make a start over the Christmas break.
Do you have much success with gorilla glue? I was thinking it would really stiffen up the flutes as it expanded into them.
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Old Dec 14, 2014, 02:27 AM
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I've never used Gorilla glue and not sure if we even get it here. However I googled it and it seems to be some sort of polyurethane foam adhesive. I did try to use polyurethane expandable foam to seal the flutes to ensure waterproofness in my Polaris, but the results were disappointing - the foam did a very bad job of actually penetrating the flutes, added weight and made an awful mess (even though I masked off all but the areas to be glued). It did stick quite well, so maybe it has a place in SPAD building...I just can't figure out what that place is

Simple contact adhesive works so well on correx and adds no measurable weight, that I doubt anything will beat it. A polyurethane based glue might be a candidate for the depron to correx joints and for some of the joints that need to be 'filled' and where I currently use hot glue, though it will presumably be less tough (being a foam) and presumably one would have to be more skilled at using it than I am, given the messy results of my first experiment! The F22 does not have too many of those types of joints though.

One tip with the F22: The 'Aft Fuselage Top Piece' as shown in Steve's plans needs to be enlarged to be slightly larger than the corresponding area of the part labelled 'Wing' in the plans. The foam template is smaller, presumably because Steve bevelled and sanded it to fair nicely to the wing...but with correx you want it to line up exactly with the front section of the 'wing' outlines. You will see what I mean if you lay the one template over the other.

Otherwise, remember to question every gram that you are adding - if its not essential, leave it out!

A big part of the fun of correx parkjets is figuring out how each model must be built differently to match the properties of correx.

Just shout if you need any more build pictures or help - though suggest doing this on the original build thread rather than here. I'll be around for the rest of this week, but probably out of contact from then until mid Jan.

Out of interest, what else have you built with correx?
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Old Dec 14, 2014, 01:37 PM
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Yes gorilla glue is pu adhesive. It sets pretty hard even as a foam and is sandable.

I had a break from the hobby and took up running again, got injured and am just returning (I love your comrades marathon and would love to do it one day).
I've just built a couple of foamie kits and I'm always disappointed by the hanger rash. Corex is so cheap and the 2mm is available in lots of places I deal with for work so very easy to get. I wanted a project for Christmas the F 22 looks so good I'm probably going to start it. However I may change to an English Electric lightning and instead of a pusher go with a tractor prop.

Thanks for the tips. I'll keep you posted.
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Old Dec 15, 2014, 02:18 AM
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Quote:
Originally Posted by Andrew u.k. View Post
However I may change to an English Electric lightning and instead of a pusher go with a tractor prop.
Interesting idea. I never thought of the Lightning before, but if you leave out its ventral bulge, it looks like quite an easy shape to model with correx. However, I'm not sure I would do it as a first correx project - that highly swept wing looks like it may add a few structural and aerodynamic challenges.

If you have not seen it, this thread http://www.rcgroups.com/forums/showthread.php?t=1445391 it has links and videos on just about everything you need to know about building with correx.

Enjoy.
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Old Dec 15, 2014, 02:38 AM
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Correx Parkjets 5: Noses and canopies and other miscellaneous items

Noses: The noses of most full fuselage parkjets are formed by laminating lots of strips of 6mm depron and then carving and sanding the nose block to the right shape. These can look great when first painted, but tend to quickly get scuffed, creased or destroyed on even small impacts.

All my correx parkjets so far use pool noodle noses as described a few posts back. With a bit of practice they can be made as good looking as all but the neatest foam jobs. They are far less prone to landing damage and hanger rash, and if they need to be replaced, it’s a much quicker operating than rebuilding a depron nose. The trick is to use an overall colour scheme that matches the colour of your pool noodle - they don't paint well. That said, using permanent marker pens seems to give a more durable finish than paint.

Canopies: Canopies for the depron designs are similarly built from laminating strips of 6mm depron. Many builders seem to struggle to get a decent profile. Many folks are now using vacuum formed canopies, and this is the approach that I have taken, since the final product is lighter and better looking than a lump of foam.

However, most jet canopies are the wrong shape for the simple soda bottle canopies used on the MIG7 and similar warbirds, and it’s also difficult to vacuum form a soda bottle to such a long thin shape if you want to go that route. I have taken the plunge and built a simple vacuum forming box and this allows me to form very light canopies from 0.2mm clear PVC...around 10g for the canopy plus pilot and cockpit. I cut and form the blank from 50mm foam (Isoboard). This is much easier than trying to form it from a hard material such as wood, but the blank is fragile and does not have a shiny surface, which leaves a nasty finish on the first pull. I get around this by leaving the first pull on the plug (i.e. the ‘new plug’ is now a foam shape covered by clear PVC). I then form the final canopy over this new plug...it’s not 100% picture perfect but the whole operation takes only a few minutes longer than carving a foam canopy, so seems to be to be a reasonable compromise.

Attaching the canopy to the plane can be a bit more challenging with a correx build than a foam one – usually the canopy must be detachable to give access to the battery compartment. In the case of the F22, I was able to attach the canopy to a depron base and fit this into the correx fuselage with a locator pin and a magnet. This was not possible on the F16 given the curves of that plane....I am still working on something better than the current tape and magnet system.

Spars: – For parkjets you need to take the plunge and use CF or GRP since the wing profile is too thin for a ply spar to work. However, the short spans and high aspect ratios (or delta planforms) of many of the jets mean that the spar does not need to run full span - I can't offer a hard and fast rule as I have simply used GRP offcuts for my spars and the length of the spar was simply the length of the longest offcut I had. If I have to join the spars at the centre, I wire the spars to a brace, since experience has shown me that epoxy alone is not strong enough. I also have added an 'anti-crease' strip to the top wing skin of all my jets to add strength - see the earlier post on this technique.

Note also that it can be an idea to run a GRP 'spar' down part of the fuselage in those designs that have a very long nose - e,g, F16, F18, Denel Cheetah etc. Not only does this add impact strength, but can also keep the nose straight while you are building. I think there is an earlier photo of this for the F16.

Motor wire extensions: – Most pusher parkjets need the wires of the ESC extended. Correx is no different. Just remember to extend the wires between the ESC and motor side, not the ones between the ESC and the battery. Also, try to locate the ESC in the airflow. I find that ordinary insulated copper wire from the hardware store works just as well as the more expensive and less easily found silicon wire.
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Old Dec 15, 2014, 11:27 AM
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Correx Parkjets 6: Lightweight trim

Adding vinyl or paint is an easy way to make a half decent correx build look a lot better. Unfortunately vinyl is very heavy (around 100g/sm) and while paint is only about half that, it all adds up. The Polaris needed vinyl to waterproof the fuselage joints, but this came at the price of around 50g of additional weight. The Vulcan camouflage is all vinyl and that added about 100g...and the plane is not nearly as nippy as it was before I added it.

Folks in the UK seem to be able to get coloured packing tape that is pretty light and used extensively on foamies...unfortunately I have only found clear and brown tape in SA, which is not at all helpful.

So my best advice is to select a trim scheme that requires as little colour as possible, and where it is needed, try to use permanent markers rather than vinyl or paint. This does mean that you want to build as neatly as possible, since you can’t hide badly built joints and mistakes with vinyl. It also means thinking carefully about where your seams end up...try to get them in the same places as the real plane has joints, where they are not visible, or at worst where they need limited vinyl trim to hide them.

My F22 uses black correx specifically so that no additional colouring is needed. While most F22s use a 2-tone grey paint scheme, apparently some black ones do exist, so the scheme works The thin vinyl strips on the leading edges are for orientation, and add less than 20g.

The F16 uses the Thunderbirds scheme as the added colour is minimal and the red and blue matched the permanent markers I had. The vinyl portion of the trim (on the nose, underbelly and vertical stabilizer) weighed in at 14g, while all of the rest did not register on the scale.

As another example, I'm contemplating building an EDF Mirage...and the obvious scheme is the SA flag scheme used by the Air Force Museum's Cheetah. Vinyl and paint will look great, but will add too much weight for an EDF...fortunately doing the entire scheme with permanent markers looks viable and should add less than 5g.

That's probably all that's relevant to building these jets - just ask if you come across something that I have not covered. I am sure that each new model that is built will bring new challenges require new techniques, so please share them here.

Have fun.
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Old Dec 18, 2014, 01:06 AM
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Thank you for the link to that thread. I'm working my way through it. I've made the hinge tool, that's pretty cool.

I've seen some great Planes covered with aluminium foil tape. Gives a great aluminium finish.
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