Building a 1/16 scale P39N Airacobra

Discussion in 'Aircraft & Aviation' started by Leif Oh, Sep 21, 2004.

  1. Lief, just out of curiousity. What kind of bit did you use to shoot the hole through the tail for the stabilizer. The bits I have at home are no where near sharp enough for card models if a hole needs to be punched by this way and I have no milling machine at home unfortunatly (Not to say you do)
  2. Bowdenja

    Bowdenja Active Member

    I'm speechless!! :shock:
    I just had to say something to let you know that I'm watching this AMAZING build too and eagerly await each update!
    And also to say thanks for giving freely these wonderful, very educational tips and pictures!

    Thanks Leif! :D
  3. Leif Oh

    Leif Oh Member

    Thanks, guys, for watching and rooting. Remember, there is not a single detail in this model which isn't there in the original 1/33 Halinski kit. Just building it in a size that shows them off.

    The abrasive bit is a diamond-studded one (about 3 euros, 4 dollars). Does a fair job on slots, but a lot of fluff and flock has to be cleaned up by scalpel when applying it to thick cardboard (not so in this particular case, though, just cutting through two layers of ordinary 0.25 mm paper). Reason I can use it at all is the scale again. In 1/16 your ordinary slot is around 2.2 mm instead of just 1.1 mm.

  4. Leif Oh

    Leif Oh Member

    14. The tail fairings

    Compound curves are scary things and the tail fairings were full of them. Took me a good while to just figuring out what was heads and tails of them, and how they were supposed to meet up.

    In many previous models I have sort of preshaped fairings like these roughly, smeared them with glue, and pressed them on, hoping for the best. None of that now - I really wanted to be sure that they would fit before glueing anything.

    After preshaping the parts separately, they were dryfitted and further preshaped in place with the help of the knitting needle, which is rapidly turning into on of my most used tools. Cut into the length of an ordinary pencil, its blunt point works fine for embossing ribs and such from the backside, and the tapered end makes it ideal for shaping small parts to different diameters.

    When I finally dared apply glue, the knitting needle came to the fore again, shaping that last difficult bit of the now wet and soft parts where the upper and lower fairings are supposed to meet up in front of the stab. Rolling the needle on the part really booth shapes it and applies pressure, while not chafing the surface.

    The only really stressful part was the little dorsal frontal fin fairing, since its two sides had to be glued on simultaneously and pressed from both sides, while checking that they were booth straight and joined up well in front. This joint required some small amount of sanding afterwards.

    The tail fairings are not perfect, but I'm well satisfied with having managed them at all, particularly since the mother of all compound curves lies ahead - the dorsal air intake. I really haven't a clue how to accomplish that one, but there is no more getting away from it now.

  5. Gil

    Gil Active Member


    Beautiful outcome...., and now for the Ram Air Scoop.

    Best regards, Gil
  6. Huey

    Huey Member

    Leif Oh

    You make this look so easy.
  7. jrts

    jrts Active Member

    Leif strikes again

    Fantastic thread with lots great tips to learn from 8)

    Cheers and keep at it

  8. Jim Krauzlis

    Jim Krauzlis Active Member

    Spectacular, Leif! :D

    I am looking forward to your post on the airscoop.

    An amazingly accurate model with outstanding photos and writeups!


  9. Leif Oh

    Leif Oh Member

    15a. Figuring out heads and tails of the air scoop

    I really and truly did not have but the faintest of notions of how to assemble the five simple parts provided in the kit into something as complicated as the structure representing the defining characteristic feature of the P39 Airacobra - the dorsal ram air scoop. It was not until I actually started to glue up the most obvious joints - in a sort of no-regrets tactic - that the logic of the design started to get clearer.

    It took many days of limited time to finish it. The process is detailed in a series of photos (which will take two installments to get through). The first learning experience is that the technique of making butt-joints by taping them with ordinary office tape on the back side is invaluable, and I'm very glad to have stumbled on to it. There isn't a single part of the air scoop that did not require at least one such joint, and sometimes several. (See previous installment on the exhaust stubs for a detailed review of the technique.)

    The small front bottom fairing part unexpectedly turned out to be the most problematic, since it required a number of cuts both top and bottom to be able to shape into a compound curve. This, in turn, threatened to make it come apart at the slightest tug. Its centre section therefore was backed-up with office tape too (not the top & bottom sections, since shaping then would have been impossible). I now think that it would have been impossible to handle it at all otherwise, and I strongly recommend this technique wherever it might come in handy.

    Now for the initial batch of pictures.

  10. Gil

    Gil Active Member


    Impressive compound curves! The knitting needle has become invaluable. I suggest that you also look into a boxwood clay sculpting tool set. They're low in cost and have all the right shapes for burnishing paper into the right shape. I also use them for formng tooling foil.

    Best regards, Gil
  11. Jim Krauzlis

    Jim Krauzlis Active Member

    Another great lesson on building this beautiful plane, Leif!

    The set of photos wonderfully illustrates your build of the scoop, and the deft manner in which you built those hair-pulling compound curves is an absolute pleasure to follow. I always take a valuable tidbit of information from your posts, and this was a prime example, my friend.

    I've got to check out that boxwood sculpting tool, Gil...great idea!
    I use a white composite plastic burnishing tool for some jobs, but it's too broad for those tight curves Leif is making in this last post.

    Just curious what the advantage is in using a knitting needle as opposed to using a bamboo skewer, which is what I normally use for some of the tighter curves. One thing does come to mind, and it's the smooth, hard surface. I imagine that provides some advantages in curling the paper around the shaft.

    Outstanding work, Leif!


  12. Leif Oh

    Leif Oh Member

    15b. Getting the air scoop into place (cont'd)

    One of the learning experience from making the air scoop has been recounted earlier, but is worth reiterating - edgeglueing with white glue and/or matt clear acrylic varnish mixed with water colours (whether in pencil or palette form) really is a most worthwhile enterprise. You can hide cracks, cover up sanded (and therefore white) glue joints, and at least soften the transition between a glued-on detail and the underlying surface.

    Use water-coloured glue when you want to fill in cracks and joints; and water-coloured matt acrylic for covering sanded spots. (If you try plain water colour, there's a risk that it'll turn out much darker than you would wish for, since the exposed sanded area sucks up a lot of colour; mixed with acrylic this effect is negligible, which is a boon I haven't discovered until lately). I recommend the technique whole-heartedly; you keep learning more of it each time you employ it.

    Do experiment with it, and try not to get too shocked the first time your coloured glue sticks out much more brightly than expected. First, it will darken as it dries; second, you can always tone down remaining disasters by one or several layers of matt acrylic varnish mixed with water colour (I've actually saved subassemblies of this build at least twice already doing that, including what's shown in the photos below); and third, the gloss of the glue will go away with a final coat of clear, matt acrylic varnish.

    The overall learning experience of the dorsal air scoop is that paper is such a wonderful and malleable medium when you step by step learn to treat it right. Shaping the doubly rounded outer shell with the help of just a knitting needle truly was a great and boosting experience. It is astonishing how you can create even complicated curves just by massaging the paper part round a suitable simple tool.

    The exuberant feeling at this stage was something best described as "give me a good design and a knitting needle, and I'll be ready now to take on any compound curve you'll throw at me". I suppose it will fade rather quickly with the next disaster, but for the moment it left a good feeling. Thanks, Halinski, for the good design - even though I may not have been able to get every sharp corner out, I never really believed the air scoop was going to work out this well (and I am no end surprised that I actually didn't have to print and glue it over and over again)!

    This means saying goodbye to the fuselage for a good while, in order to confront the huge wings. Lots of thick cardboard cutting (or sawing, rather) to do, and a lot of thinking about how to divvy up the huge wing parts so they'll fit into standard width paper.

  13. silverw

    silverw Member

    Hi Leif

    This is great information you are providing here.

    I'm having a few problems of my own, getting a smooth fusilage transition from one section to the next, but your methods and tips are a great source of possible solutions for me.

    Keep up the fantastic stuff! :lol:

  14. barry

    barry Active Member

    Well it's worth gloating over Leif I think you have hopped onto another level with this model.

  15. Leif Oh

    Leif Oh Member

    16. Intermission - making a tool for cutting slots

    You may remember that I've advocated using an abrasive bit in a power drill for making those pesky slots in ribs and frameworks. I now think that is bad advice. It leaves too much flock and fluff, and it is imprecise. Cutting the framework and ribs for the wings of the Airacobra I tried out a couple of different approaches and finally hit upon a solution with the help of suggestions by others here.

    The worst part is cutting the small end of slots; cutting the long sides with an ordinary knife is no problem. I first tried drilling holes at the end of the slots, and then remembered the nice solution suggested on this site to grind down straight-edged knife blades to make a mini-chisel. Only problem was I couldn't get hold of such blades in quantity (or at all) where I live.

    So I started looking around the tool-boxes at home, and found an old set of cheap instrument screwdrivers (most of them ruined by putting them to too hard tasks). These screwdrivers are very sharp to start with, and it took only a little grinding with the powerdrill and an abrasive disk to get a reasonable edge to them.

    And it worked perfectly! They now easily cut through my 2 mm laminated card. So I treated myself to a new set of such screwdrivers at the reasonable price of 2 euros (less than 3 dollars). The four flat screwdrivers included come in widths of 1.4, 2.0, 2.4, and 3.0 mm, which is perfect. For my 2 mm slits I intend to grind the 2.4 mm one, in order to get a really clean-cut end to slots in the future.

    One more little irritant taken care of - thanks to whoever it was on the site (and please respond, in order to take a well-deserved bow) who suggested the idea of making small chisels!

  16. Gil

    Gil Active Member


    Very nice solution..., I like the idea of having a set. Use a buffing pad on your dremel tool with polishing compound to strap a razor sharp edge on your chisels and other edges required to be razor sharp.

    Best regards, Gil
  17. jrts

    jrts Active Member

    Hi Leif


    Yet another good idea, now I know what to do with all those bust up screwdrivers I never toss out. Got any ideas for a use for rusty nuts and bolts, I've got millions of those. I've never got round to clearing out the workshop :lol: :roll:

    Have a great day and loads of fun building for your next up date :roll:

  18. silverw

    silverw Member

    Happy Birthday Leif. :D

    Your slot cutting would be a good job for a "square" hole punch, if you can find one that small.
  19. Jim Krauzlis

    Jim Krauzlis Active Member


    Hope you are able to enjoy the day doing what you like! :wink:

    Great idea for recycling those little screwdrivers! And the price paid for getting this new tool isn't bad, either. :lol:

    Have a super day!! :D

    Best wishes,
  20. Leif Oh

    Leif Oh Member

    Birthday present

    Thanks guys! Enjoying it very much in fact - look what I got as a birthday present from my dear old son. Makes me want to go looking for cardmodels of flying boats and old airliners! Those cabins are something else. Anybody got a good tip, with a quality, and preferably interiors too, that would stand scaling...? - L.

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