|Sep 23, 2012, 11:51 AM|
Something different: Correx Avro Vulcan
About a year ago I built a 150% correx Mugi, but immediately had the sense that, with a few modifications, the design could become a nice semi scale Avro Vulcan bomber...at that size the Mugi lost some of its zippiness and instead became very stable...exactly what one would expect of a scale like bomber.
Fast forward a year: A few experiments with correx and 'pool noodle' foam tubes and a look at some of the Vulcan threads on RC Groups, and I decided that the combination of correx and foam would make a very simple to build but tough Vulcan look-alike.
The result has a 110mm span, weighs just over 1kg and is powered by a SuperTigre .10 1200KV motor with a 2200mAh Lipo. Flight is very stable, but with a nice turn of speed when throttled up. I suspect the design will make a good sloper.
Build time for someone with correx/SPAD experience is about a day. Total cost of the airframe is less than USD15. A little more effort could probably increase the realism quite a bit (e.g. panel lines, painted canopy, jet engines etc)
See what you think - Plans and build photos to follow.
30/09/12: Plans added in Sketchup - still can't figure out how to print to PDF without the dimension lines overlapping. Zoom in using Sketchup to see the detail
|Sep 27, 2012, 09:09 AM|
The correx Vulcan is built in a very similar way to the correx funjet (see http://www.rcgroups.com/forums/showthread.php?t=1727654), but is larger and some of the scale details make the build a little harder (but it is still an extremely easy and quick build for a semi scale plane)
The wing is built from 2mm correx and uses two correx spars in place of the more conventional wood spar or carbon tube. The main (front spar) is lamininated from 2mm and 4mm correx, while the rear spar is just a single piece of 2mm correx with the flutes running vertically. This creates a lightweight and cheap wing and is possible without losing strength because the correx wing skins provide the necessary strength in the same way as the flanges of an I-beam provide the bulk of the I-beam's strength. However, for this to work, two things are critical:
1) There is some mechamism to keep the two wing surfaces apart and stop them buckling (this is where the correx spar comes in), and
2) The flutes run in the correct directions, so that the top skin is orientated in the same direction as the correx's best compressive strength.
Anyone who has built a Mugi will be familiar with the creases on the top skin where the wing compresses when too many 'g's are pulled - the Mugi has the top wing skin flutes running chord-wise and correx does not have a very high compressive strength in this direction. The Funjet has the flutes for the top wing skin running spanwise so that it is better able to resist the compression that occurs when the wing tries to bend. The spar is simply used to keep the two skins the right distance apart in the same way as the web of an I-beam.
The final wing turned out to be very strong, partly as a result of the thick aerofoil of the Vulcan.
The elevons are made from 3mm or 4mm correx, with the flute being slit to create the hinge. 2mm correx doubled over should also work, but a single 2mm elevon will be too flimsy. I have used torque rods for control, but control horns and pushrods would also work (and would make it easier to adjust the throws)
Straight flute correx has been used for the prototype, but I think S-flute might be a better choice for the wing skins as it would prevent the span wise creases that I got on the top wing skin.
The fuselage is made of a combination of a pool noodle (see http://www.amazon.com/s/ref=nb_sb_no...s=Pool+noodles) and 2mm correx. This gives a simple to construct fuselage with a round cross section that also acts as a shock absorber to protect the electronics in crashes.
The canopy is made from a 500ml coke bottle heat formed over a plug (made from automotive body filler). There are various threads on RCgroups that discuss this technique
The camo scheme is created using sheets of vinyl (think it is used in the signwriting industry - its available in SA from the same place we get our correx and is reasonably cheap.
The canopy and the covering end up being by far the most complex and time consuming aspects of the construction.
I will attach plans as soon as I figure out how to create a PDF from Google Sketchup such that the dimension lines don't overlap!
|Sep 27, 2012, 09:28 AM|
Preparing the wing
Start by cutting out the two wing halves. The easiest way to do this is to make a cardboard template of the top wing. Trace the outline (remember to orienate the flutes correctly), then flip it over on the wing leading edge crease line and trace out the bottom skin.
Once both sides of the wing are ready, cut out the bottom doubler. Then glue the two wings together onto the bottom doubler. Make sure that the joint aligns with the centre line of the bottom doubler. I use contact adhesive for all joints - just remember to sand all surfaces to be joined first.
The Funjet's Main spar is made from laminating a piece of 2mm correx with a 'spar doubler' made from 3mm or 4mm correx. The main spar has the flutes running vertically and has a 10mm edge top and bottom that is folded over to provide a surface to glue onto the top and bottom wing skins. The spar doubler has the flutes running horizontally and does not have a border.
Start by cutting out the main spar and the spar doubler. Next, cut through one side of the correx on the main spar to enable the edges to be folded over (be careful not to cut right through). Now fold the edges 90 degrees as shown in the picture.
Sand and glue the spar doubler to the main spar. (ignore the elevons in the photos - I changed the design during construction and decided instead to enlarge them and use thicker correx)
The main spar can now be glued onto the bottom wing skin on the dotted lines shown in the plans. Mark out the spar position, sand and apply contact adhesive to both the wing bottom and the bottom spar flange. Note the bend in the centre of the spar - cut the spar flange and score the spar itself if needed to enable it to bend. Also note that the flat side of the spar is the bottom.
Repeat the above steps for the rear spar, except that there is no need for a doubler. I found that the rear spar was quite floppy so I added some correx braces that held it in place until the top skin had been folded over (see photo)
|Sep 27, 2012, 10:14 AM|
Like your work. Nice size as well...
Tried Correx before.... but maybe a bit impatient. Now just use foam-board. (paper on)
|Sep 29, 2012, 02:59 PM|
Closing the wing
There are a few more things to do before the wing can be folded closed.
1) Cut the two elevons from 3mm or 4mm correx. Make a hinge by slitting the underside flute at the hinge point. Glue in place as shown in the photo.
2) I used torque rods in order to create a clean wing. Horns and pushrods can work and will provide easier adjustment of throws, but can spoil the overall appearance. If using torque rods, install them now - I ran each rod through a sleeve of 2mm correx to ensure that it could not move, and glued this correx sleeve to the bottom wing surface (note that the torque rod must be threaded through the correx before making the final bend)
I find that adding a wedge of foam as shown in the photos creates a firm area onto which the motor mount can be bolted - without this the correx deforms after mount bolts have been tightened, and the mount comes loose. Cut the wedge of foam from polystyrene or equivalent foam and glue to the bottom wing using hot glue
Large wings are hard to launch without something to hold onto...I screw a small piece of aluminium U-section to the underside. To enable this, glue a piece of 6mm plywood or similar to the bottom wing just behind the main spar. The launch handle screws will later be screwed into this plywood
Closing the wing
1) Score the leading edge fold lines using a screwdriver and a metal rule and fold the top wing skins over, being careful that the fold lines remain straight. Check that the trailing edges will match up once folded and practice getting a neat trailing edge. Cut out a small slot for the torque rods if applicable.
It is generally best to complete one side of the wing before gluing the other side:
2) Sand and spread contact adhesive on all joint areas - including the tops of the two spars.
3) Fold the top wing skin over, being careful to keep the bottom skin flat on the table surface and make sure that the trailing edges align. Check that the spars remain perpendicular to the bottom wing. This should create a nice aerofoil shape (see photos). Gently press down on all glued joints to ensure good adhesion.
4) Repeat for the other side of the wing
5) Lastly, hot glue the tips closed. Place the wing on a flat surface and hold the tips as flat as possible onto the surface while gluing in order to get some washout into the wing. I placed a heavy battery on the tips to keep them as flat as possible while I added the glue and pressed the tips closed.
|Oct 04, 2012, 11:44 AM|
Step 3: Installing the jet inlet ducting
Adding the jet inlet ducting is by far the hardest part of the build.
But first glue on the top wing doubler. This is just a 50mm wide strip of correx that covers the joint in the top of the wing. It runs from the front of the cut-out for the torque rods to the nose of the wing - trim off any excess correx at the nose.
Now for the inlet ducting:
1) Cut out the inlet ducting, mark and cut out the inlet holes as well as the sliver of correx that is cut out from the centre line.
2) Mark and carefully score the fold lines for the leading edge of the ducting. Refer to the plans and the photos for this.
3) Carefully fold the correx on all the fold lines - the aim is to get a rounded edge on the inboard side and a sharper edge towards the outboard side
4) Also pre-fold on the longitudinal fold lines than run from the LE to the TE. These are not full folds - just slight kinks in the correx to allow it to attach to the wing top surface on the outboard side, but sit above the wing surface on the wing centreline. Look at the head on picture in the Vulcan 3-views to see this transition.
5) Cut and glue the inlet ducting ribs in place. These are not shown in the plans - just make them from some scrap 3mm or 4mm correx. The exact shape is not critical - simply refer to the photos for their approx location and shape. Tack them in place with hot glue.
6) Now experiment with fitting the inlet ducting. This is the hard part because you will ideally need more sets of hands than you were born with! Carefully fold into place and check that the ribs are in the right place so they don't protrude through the inlets. Also check that the bottom doubler is not obstructing the inlet ducting (trim the doubler if needed) and that the inlet ducting mates properly to the wing LE and will sit flat at the back of the wing once the ducting flaps have been folded over. Look carefully at all the pictures to see what you are trying to achieve and don't get too frustrated because you don't have enough hands Practice installing it, such that you can get in into position before the glue sticks. Once again some extra hands would be nice
7) When you feel ready, lightly sand all surfaces to be glued and apply contact adhesive. Refer to the photos for help (it should now be clear why the inlet ducting needed the longitudinal folds). Do not put glue on the flaps that fold round to join with the bottom wing doubler yet.
8) Carefully glue the inlet ducting in place, doing your best to get in the right position before the contact adhesive prevents any adjustment.
9) Ok, that’s the hard stuff done - now just check that the flaps align nicely, trim if necessary and glue in place
It gets a lot easier from here onwards.
|Oct 14, 2012, 01:11 PM|
Step 4: Add the front fuselage
This is where the technique gets a bit different from a standard correx build in that I experimented with using a pool noodle to create the round fuselage shape. However, I found that the pool noodle is not quite rigid enough to support the long nose overhang of the Vulcan, so I added a correx box that gives stiffness and also functions as a battery box.
The photos should be self explanatory - the thing to watch is to make sure the battery box is properly aligned with the centreline of the plane in both axes. Mine initially pointed up slightly, which resulted in the nose pointing slightly upwards; it did not look right, so I cut it out and reglued in the correct place.
Now for the novel part - shaping the pool noodle to form the fuselage. This is quite easy, but I suggest practicing on a bit of scrap until you have a feel for the technique. Use a very sharp knife with a long blade.
a) Mark and then cut out a section from the pool noodle that corresponds to the profile of the wing. It does not have to be 100% accurate - I just did a freehand/approx outline. The key thing is to make sure that you keep the knife parallel to the table top and don't rotate the pool noodle while cutting - else you end up with a twist in the cutout. Trial fit and trim off any areas where there are bumps in the outside profile (you may need to do step b first).
b) Cut away suffcient foam to make space for the battery box. I used a combination of a knife and a heated wire. The wire works best, but be carefull not to melt too much material, especially from the area in front of the battery box.
c) Now form the nose. The technique is to cut away material from the centre of the pool noodle and then pinch the noodle closed to give the desired shape on the outside - a bit counter-intuitive, but gets easier with practice. The pics may help more than words. The Vulcan nose needs two wedges cut out - one on the horizonal and one on the vertical plane. I cut and glued the horizontal plane first, and, based on the resulting profile, marked out and cut the second wedge in the vertical plane.
|Sep 22, 2013, 02:57 PM|
Joined Mar 2009
Hiya Extreme Sports I have been following this thread with interest recently and am thinking of doing this build but appreciate that the build thread is incomplete. Any chance that the build will continue, or am I missing something? I have become interested in building with correx and this looks like a challenging project.
|Apr 24, 2014, 03:11 AM|
Step 5: Completing the build
Apologies, Bogbrush10...there seemed to be no interest in this design, so I stopped posting.
I suppose its better late than never, but here are the last few build details and pictures...hopefully I can remember what I did:
Installing the motor and pushrods
In order for the plane to balance with a pusher prop configuration, the 2200mAh lipo needs to be as far forward as possible...which means extending the three wires from the ESC to the motor. Rather than solder the extensions to the ESC, I used gold banana plugs on the extension wires (made from normal 3-core wire IIRC) so that I can remove the ESC for use in other planes.
I cut the motor mount from cheap 6mm plywood and glued it onto the back of the wing with zero thrust angle. This seems to work fine. The motor I used is a CD style .10 size motor...I think it weighs about 70g, which is relatively light for this size motor. A heavier one will make balancing the model a real challenge. I did make a tail cone from a soda bottle to hide the motor, but I have never used it in flight...plane looks realistic enough as it is and I was worried about over-heating issues. I also never got round to adding dummy jet engines as I never found a suitable lightweight tube of the right diameter...but the model would look more realistic if both these were added.
This Vulcan has elevons only (no rudder), which are activated using torque rods. In order to get the CG in the right place, I put the servos as far forward as I could and connected them to the torque rods using push rods. One could use servos in the wing and conventional pushrods (which would be simpler), but I wanted a clean wing, so elected to keep all controls hidden in the fuselage.
Completing the Fuselage
I used correx to build a round rear section for the upper and lower sections of the fuselage...just remember to slit every second flute to get the correx to bend. IIRC, these two pieces of correx are not shown on the plans as you will need to cut them to suit the size of pool noodle that you have used and to fair nicely with the pool noodle...but its a fairly straightforward operation and the photos should be sufficient guidance. Tack into position using hot glue.
The pool noodle fuselage is then glued to the battery box constructed in the previous step, but I left the top 'flap' of the pool noodle unglued....this acts as a hatch that I can lift to insert the battery, and is kept in place by simply wedging it under the vertical tail.
Finally, glue on the vertical tail...I used a few bamboo skewers to give the joint some rigidity as well.
The model is now ready for covering. I used vinyl throughout as paint does not stick well to a pool noodle. This added a lot of weight (over 100g). It might be an idea to paint the correx sections and use vinyl only on the pool noodle. Note that some help from a heat gun (on a very low setting) is needed to get the vinyl to follow the curves on the nose.
The attached pictures should clarify much of the above.
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