New Products Flash Sale
Thread Tools
Old Jul 18, 2013, 12:51 PM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Build Log
Lightweight electric Corro-Cub

Here are some plans for a lightweight, stable correx plane suitable for flying at your local park. Useful for those of us living in countries where depron is unavailable/ costs more than gold but where correx is dirt cheap and readily available.

Hopefully it can also help challenge the conventional wisdom that SPAD/correx planes have to be ugly, heavy and not suitable for electric power and park flying!

Specifications:
Span: ~1m (~40")
AUW: <450g (the prototype weighed 450g exactly, but I see I got the print scaling wrong and it has a 1.1m wingspan)

Construction is primarily 2mm straight flute correx. Fuselage formers are scrap 6mm depron and 1mm ply (firewall and landing gear attachments) while I used 4mm balsa for the wing ribs and a 4mm CF strip for the spar. The wing struts and pushrods are 3mm dowel from the local hardware store. If you can only get 3mm correx, I'd suggest scaling up the plans to >120%.

Power: I've been doing the initial flights with a cheap E-max 2812 and it has more than enough power on both a 3S 800mAh and 2S 1300mAh LiPo (with appropriate prop combinations of course). I expect that the slightly smaller Blue Wonder motors popular with the Depron models will work fine.

The aerofoil is a simple curved plate. This keeps the plane light and suitable for slow flying. The downside is that ailerons don't work very well, so I have (for now) kept the plane to Rudder-Elevator-Throttle only. I may experiment with flaperons at some stage. The curved plate also tends to balloon easily, so the design has lots of down-thrust to counter this - but expect to do some experimenting to get the thrust angle right.

CENTER OF GRAVITY: This is not marked on the plans - simply because I never bothered to calculate it - instead I used the 25% of wing chord rule of thumb and have not needed to change it so far!

As with anything SPADwise, you can build this simply and quickly, or add more scale detail, which will add weight and time to the build.

Plans were drawn in Google Sketchup and then printed to pdf. For some reason that process added lots of blank pages in the tiled version - suggest just not printing the blank pages.

CAVEAT ON THE PLANS: I drew the plans, built the prototype, then corrected any errors on the plans. I have not built a second model to confirm that the plans are 100% correct. Expect to do some trimming here and there where measurements are not perfect.

Prototype will take off from the short grass at the local park in ~10m and lands at a reasonable speed. Flight is stable and docile, but it does have a definite nose drop in the stall (easy to catch with the loss of only a few feet of height). Loops and wingovers are simple, but obviously decent rolls are not within its repertoire!. It still has a slight balloon on application of power (but then again, that's exactly what most full scale planes do...)

Should be a nice project for someone looking for an easy to fly, 'scale' correx park flyer. I will add build details if there is interest.
Extreme Sports is offline Find More Posts by Extreme Sports
Last edited by Extreme Sports; Jul 18, 2013 at 01:15 PM.
Reply With Quote
Sign up now
to remove ads between posts
Old Jul 19, 2013, 09:59 PM
draftman1 is offline
Find More Posts by draftman1
Registered User
draftman1's Avatar
central CA
Joined Aug 2002
361 Posts
very cool! I got one printed off, I think I will use clear packaging tape to make a the wing flat on the bottom
draftman1 is offline Find More Posts by draftman1
Reply With Quote
Old Jul 20, 2013, 02:54 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Quote:
Originally Posted by draftman1 View Post
very cool! I got one printed off, I think I will use clear packaging tape to make a the wing flat on the bottom
Great, hope it works out well. I would, however, suggest that you change the aerofoil to suit a flat bottom profile. IIRC, the curved plate I used was 7% deep at 40% of chord - for a flat bottomed one you may want to make it slightly deeper and move the thickest part forward a bit (e.g. 12% at 33%). Note too that I inserted a 2.5mm GRP rod into the LE flute to get a nicer LE profile.

Please post pictures of your wing as you go - I've been experimenting with adding clear plastic sheet to get a flat bottom (the type school kids use for covering books), but decided against this technique on this model as the plastic underside and the tape on the top of the wing would not look neat once painted. I'd be interested to see how your tape only technique turns out.

BTW, just got back from some early morning flying. Once trimmed out, this really is a gentle flyer. I've settled on the 1300mAh 2S and 9x6 combo. It can get airborne in ~2m off grass and the large prop and slow speed make it a great plane for landing practice.
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Jul 21, 2013, 06:41 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Some details on the tail surfaces

Realised I forgot to make some changes to the plan to add tabs to the vertical fin where it slots into the elevator and between the fuselage sides. This second tab is important as it provides the strength to support the tail skid or tailwheel.

The attached pictures should be self explanatory, and also show how the two sides of the elevator are joined in the normal way with a U-bend of wire. Note my cut-outs were not 100% correct when the photo was taken, but I have added text to show what to do.

The horizontal stabiliser is strong enough to not need any bracing rods in the flutes. However you might want to add external bracing as per the real cub to get greater scale accuracy (I have not added these braces, but 1mm or 2mm CF rod should do the trick - see any 3-views or Cub photographs for the correct location)
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Jul 21, 2013, 06:52 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Some notes on glue

In order to keep the plane light, I used the hot glue stick very sparingly. IIRC, I used it for the following joints only:
1) Vertical fin to horizontal stabiliser
2) Empenage to fuselage
3) Wing dowels to fuselage
4) Additional strength for the firewall to fuselage joint
5) Front fuselage braces (the V-shaped ones that you see in the windshield when looking from the front - just a dab to hold them in place as they are purely cosmetic)
6) Landing gear fairings (just a dab to stop them swivelling)

All other joints used contact adhesive. I used Uhu-Por for all joints where foam was involved and normal foam-eating CA for everything else (the majority). Sand the correx lightly to get a decent adhesion of the CA.

Sellotape was used extensively to make temporary joints (e.g holding the fuse together while I checked that it was nice and straight) and to cover up the edges and joints in the correx before painting.
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Jul 22, 2013, 06:50 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Wing

The wing construction is pretty conventional for a curved plate correx wing. Six balsa ribs are used to form the correct shape and there is no dihedral at all. This means that the wing can simply be cut as a single piece of correx and no joint is needed (the template is just for 1 half of the wing). I used a 4mm CF strip for the spar, but a CF tube of about the same size would no doubt work just as well. Finally, the LE flute has a 2.0 or 2.5mm GRP or CF rod inserted to eliminate the 'waviness' that happens between wing ribs.

Broad steps:
1) Cut out the wing template and trace onto the correx sheet. Flip the template over and trace out the second half of the wing. Take care to make sure that the LE aligns 100% with the front flute. Note that the extra correx 'flap' that projects from the front of the wing in the centre will eventually fold underneath the centre wing ribs to give a solid base to the wing.
2) Cut out the wing. Take care to make sure that the LE flute remains intact so that the CF rod can be inserted. After cutting out the wing, I carefully trimmed excess material from the LE to achieve a neater profile.
3) Optional: Look at the wing profile and slit the underside flutes to enable a gentle curve. I suggest slitting every second flute in the areas of greater curvature and maybe every 3rd or 4th flute in less curved areas. Keep the front two flutes intact; also there should be no need to slit any flutes near the TE. I did not slit any flutes when I built the prototype, but if you go this route, you run a higher risk of bending the correx and getting a kink in the wing profile (evident in the attached pictures). Also, the LE will tend to lift up from the ribs if the glue joint is not 100% solid.
4) Insert the 2.5mm CF rod (or closest match that you have) into the LE flute.
5) Cut out the 6 ribs (the profile can be got from the side view in the plans). Sand them to ensure that they are all approximately the same shape. Now cut a slot for the spar in the TOP of each rib. If using a 4mm CF strip, you can simply cut the slot using a junior hacksaw to the full depth of the blade; if using a rod, drill a suitably sized hole. Lastly trim 2mm off the bottom of the centre two ribs (this allows the correx 'flap' mentioned in step 1 to fold under the ribs.
6) Glue the spar to the ribs at the approximate locations shown on the plans. The centre two ribs should be positioned in line with the edges of the correx 'tab'
7) Lightly sand the underside of the wing skin at the positions of the ribs and glue the ribs to the wing skin using Contact Adhesive. Be careful not to build any twists into the wing.

EDIT 08/11/13: Add dihedral at this point. See post #26.

8) Now fold the centre correx 'tab' into place and trim to size. Pull tight to get a nice LE profile, and once happy with the fit, glue in place with CA.
9) Insert a short length (~10cm) of CF rod into the centre of the TE. (Insert from 1 end and push into place with a length of piano wire or similar). This prevents the rubber bands from crushing the correx and is neater than using duct tape.
9) Lastly, glue on the fixtures for the wing struts. These are simply offcuts of correx that are glued to the underside of the wing with the flutes running chordwise . See the attached picture of how the struts will eventually locate into these fixtures (the details of the struts will be covered later).
Extreme Sports is offline Find More Posts by Extreme Sports
Last edited by Extreme Sports; Nov 08, 2013 at 01:58 AM.
Reply With Quote
Old Jul 23, 2013, 07:42 PM
draftman1 is offline
Find More Posts by draftman1
Registered User
draftman1's Avatar
central CA
Joined Aug 2002
361 Posts
it will be a while before i build

yellow packaging tape!!!!
draftman1 is offline Find More Posts by draftman1
Reply With Quote
Old Jul 24, 2013, 11:41 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Yellow packing tape will definitely do the trick - unfortunately we only get 2mm correx in white and black here, and I have never seen coloured packing tape (but we do get very cheap sheets of signwriter's vinyl, which is probably as good, if a little heavier)

Please post when you get round to building - you will save me a few experiments with techniques for flat bottomed wings.

Regardless, I will continue posting build details before I forget them - the fuselage construction in particular uses some techniques that are not shown on the template plan.
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Jul 26, 2013, 03:53 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Preparing the fuselage

1) When tracing out the sides onto the correx, be sure to align the wing seat, the fold line for the top of the fuselage behind the wing and the horizontal stabiliser mounting cut-out with the flute direction. The CorroCub wing has no incidence relative to the stabiliser. If you are building the more complex version with the windows cut out, cut them now. Handle the window area with care until you reinforce it late in the build process.

2) To make the folds between the fuselage sides, top and bottom, carefully cut the outside skin of the correx on the fold lines and fold inwards. Do not worry about the resulting 2mm gap – this will be covered later using sellotape (Scotch Tape/ Magic Tape). If you want to avoid using sellotape, you can cut and “V” the inside flute, but you may find that you have a 2mm gap where the top and bottom join – so bear this in mind when cutting out the sides.

3) Use scraps of correx as doublers for the joints in the fuselage top and bottom. Glue to one of the sides before joining the fuselage – see attached pictures.

4) Similarly, the firewall and landing gear former are held firmly in place by scraps of correx on either side. Glue these scraps in place. Note the downthrust built into the firewall. The prototype had no downthrust built into the firewall, but needed 3 washers of downthrust, so I have updated the plans to minimise the number of washers needed.

5) Trim the amount of correx in front of the firewall to suit the motor you are using and the degree of scale accuracy you want for the motor cowl. The plans as drawn will suit an E-Max CF28xx motor and leave space for a ~6mm thick foam cowl. To be more scale-like, the cowl should be much thicker – this will require trimming the correx closer to the firewall and a carving a more complex cowl.

4) I suggest drilling the holes for the motor mount before gluing the firewall into the fuselage. Given that you will probably add at least 1 washer to create right thrust, it makes sense to offset the holes slightly to the left of the centre line (CL) – this way the prop attachment point will be on the CL (as opposed to right of centre if you drill the holes on the CL). Also, remember to drill some larger holes/ cut a slot for the motor cables.

5) I attached the landing gear to the former using thin copper wire and epoxy. Drill out the holes for the copper wire before mounting the former. I also suggest that you wire the landing gear in at this point. Note that when you bend the landing gear (2.5 mm piano wire), slide on the correx fairings before you make the last two bends on either side (the piano wire slides neatly into the front flute of each fairing).

6) Glue in a scrap of plywood for the backing plate for the wing strut attachment screws – see photograph.
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Jul 28, 2013, 02:55 PM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Joining the fuselage

1) Glue the formers to one of the fuselage sides (UHU-Por works well). Make sure the fuselage top and bottom tabs fold over the formers properly. Leave F2 out - only glue it in once the fuselage has been joined. Note that in the picture the landing gear has not been wired to the former. This required some dextrous wiring later, which can be avoided if you wire it on before joining fuselage.

2) Glue one the second side, being careful to get the fit right.

3) Fold over the top and bottom tabs and just tape in place initially. Check the following and adjust before gluing:
  • The fuselage is straight and not twisted
  • The stabiliser mounting area is properly aligned and level
  • The top and bottom tabs join neatly - if necessary trim the tabs so that there is
    no overlap or excessive gap (a small gap is OK as you will cover the joint with tape).

4) Once you are happy with the alignment, glue the bottom tabs in place. Leave either of the rear top or bottom just taped for now – you need to fit the pushrods before finally gluing it. Add extra doublers on the inside wherever you feel the need to strengthen the joints.

5) Glue a former made of scrap foam behind the firewall. This protects the battery from the motor mounting screws and provides some cushioning in a crash.

6) Glue in F2 and fit the front top cover of the fuselage – it will probably require some trimming before it fits neatly. Glue in place using scrap correx as doublers. Note that the cover in the photos is not the same shape as in the plans (the plans are more up to date).
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Aug 01, 2013, 07:46 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Strengthening the wing mounting area – window cut out option

This step is only needed if you are using cut out windows.
1) Make up two shaped reinforcing rods from 4mm CF tube and 2.5mm wire. Wood dowel should also work, but you will need to carefully drill out holes for the wire or devise some other means for a strong joint at the front of the cabin area. The rods should run from the floor of the fuselage, up the front of the cabin sides and along the wing seat to behind F4.

2) Glue in place using contact adhesive. You may find that the glued joint is not strong enough to withstand the force of pinching the top the fuselage back into alignment – if so strengthen the joint with hot glue.

3) Cut and install the wing dowels. I used 4mm GRP rod, but a wood dowel or aluminium rod will work just as well. Hot glue in place, using the dowels to hold the top of the fuselage sides the right distance apart (you will find that the two sides are too far apart at the front of the wing mounting area and need to be ‘pinched’ together).

4) Cut and glue in the V shaped bracing bars in the windshield area. I used 2mm GRP rods and hot glue (Optional – purely for looks).
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Aug 01, 2013, 07:47 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Strengthening the wing mounting area – Tape/ painted window option

If you have not cut out the windows, this step is far simpler. Just glue a 2mm correx doubler to the inside of each side of the fuselage to strengthen the wing seat area and provide a more solid mounting for the wing dowels. I suggest making the doubler 30 – 40mm high and running the full length of the wing seat. Then cut holes for the dowels and glue the dowels in place (melting the holes with a heated rod of the correct diameter is the neatest and easiest option).
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Aug 05, 2013, 01:23 PM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Nose cowl

The simplest option is to just leave the nose open – this will also aid motor cooling.

A more scale like cowl can be made from scrap depron and foamboard. First, cut a piece of 5mm foamboard/depron to fit tightly inside the nose, flush with the front edge of the correx. Then glue some scrap depron to the front side of the foamboard and carve and sand to the desired shape. If the foamboard pops out while carving, simply glue it in with UHU-Por – you should be able to break the joint later tom take the cowl off for motor fitting. Note that you may have to trim back the correx to accommodate the large cowl.

Lastly, drill/cut/ gouge out the inside of the cowl to make space for the motor. Note that adding more layers of foam leads to a better shaped cowl. However, the more layers of foam added, the more gouging out required for the motor to fit, and the more the correx has to be trimmed back.

Paint the foam with WBPU/ acrylic and sand smooth. The WBPU provides a smoother, tougher surface and also protects the foam if you use spray paint later.
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Aug 08, 2013, 01:27 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Empennage, tail skid, servo mounting and pushrods

Glue on empennage using the minimum amount of hot glue. As usual, take care to make sure it is horizontal and square.

I decided to use a tail skid rather than make a more complicated tailwheel (since I operate off grass, which will just foul a tailwheel). The tail skid is simply bent from piano wire and glued into the first flute behind the hinge flute of the rudder – squeeze some hot glue into the flute and immediately push the skid into the flute. Be careful not to melt the correx.

I found that the servos needed to go directly behind F4 (the landing gear former) to get the right CoG with an E-Max CF2812 motor and an 80g Lipo pushed as far forward as possible. I use 5mm foamboard for the mount and simply screw the servos into the foamboard – seems to be strong enough for these lightweight builds. I would not suggest mounting the servos near the tail as the CoG will be too far aft.

I used 3mm wood dowels and piano wire for the pushrods, but any other method can be used. Just fit the pushrods before you finally close the rear fuselage else you may struggle to make the required holes through the formers for the pushrods to run freely. Also, install pushrods before gluing on the windscreen or you may struggle to slide them in (the windscreen is best left until after painting anyway).
Extreme Sports is offline Find More Posts by Extreme Sports
Reply With Quote
Old Aug 09, 2013, 02:05 AM
Extreme Sports is offline
Find More Posts by Extreme Sports
Registered User
Extreme Sports's Avatar
South Africa, GP, Johannesburg
Joined May 2012
753 Posts
Wing struts

Use 3mm wood dowels or 2/2.5mm CF or GRP rod for the struts. I attached an electrical crimp on fitting to each inboard end, which were then fastened to the fuselage with a screw. The outboard end slides into a scrap of correx that is glued to the underside of the wing. To do this, I attached an ‘L’ shaped piece of piano wire to the dowel using the brass part of an electrical connector block. To get a snug fit of the wire in the correx I covered the wire with thin shrink tube and made a slight bend in the base of the ‘L’ so that it hooks slightly on the outside of the correx flute.

The struts add considerable strength to the wing and allow you to easily adjust the washout. Set the initial strut length to have a flat wing, or at most a small degree of washout. You can then add more washout by simply inserting the rear strut on each side into a more outboard flute of the correx holder.

The struts are easily disconnected when you need to remove the wing. However, remember to check the washout every time you fly as the wing can shift a bit and the elastic band attachment method means that the position may not be the same every time – though I find that I used the same correx flutes almost every time.
Extreme Sports is offline Find More Posts by Extreme Sports
Last edited by Extreme Sports; Aug 09, 2013 at 02:12 AM.
Reply With Quote


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Build Log Super Cub DIY lightweight Tundra Tires - v0.5 Phaeton Scale Kit/Scratch Built 90 Oct 09, 2015 11:05 AM
Sold Dubro 1/4 Scale Lightweight J-3 Cub Wheels NIP $25.00 FREE SHIPPED!!! stevster Aircraft - General - Miscellaneous (FS/W) 1 May 06, 2013 09:38 AM
Gallery SIG Lightweight Piper-cub Erik v. Schaik Your Plane Photos 0 Feb 09, 2004 03:49 PM
Gallery SIG Lightweight Piper-cub Erik v. Schaik Your Plane Photos 0 Feb 09, 2004 03:47 PM
Gallery SIG Lightweight Piper-cub Erik v. Schaik Your Plane Photos 0 Feb 09, 2004 03:45 PM