Building a 1/16 scale P39N Airacobra

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

  1. barry

    barry Active Member

    Happy birthday Leif

  2. rkelterer

    rkelterer Member

    happy birthday to you, leif

    with the best wishes
  3. cardfan

    cardfan Member

    I'm with everyone else! Happy Birthday and best wishes!
  4. bfam4t6

    bfam4t6 Member

    Same as the rest....Happy Birthday Leif!
  5. tausugAir

    tausugAir Member

    Leif ,

    Better late than never : HAPPY BIRTHDAY..... from me who had some fruitful exchanges with sometime in the past..

  6. Leif Oh

    Leif Oh Member

    17. Assembling the big wing frame

    17. Assembling the big wing frame

    So here we go again. The first proper building for a long time. The big wing frame took a lot of jigsawing in 2 mm card, and some inventing of tools for cutting those pesky slot ends (see previous posts in this thread).

    Assembling it was a bit like a Chinese cube; you know the kind where everything sticks together and you have no idea where to start assembling or taking it apart. My main worry was that the center part wouldn't fit into the slots and frames already built into the fuselage - they might have turned out to be just slightly too thick or wide in places.

    So a lot of thought and measuring went into where exactly ribs would have to be moved outwards just a fraction of a millimeter in order not to stick - and where to make the corresponding reductions of adjoining parts, since I didn't want the changing of center rib positions to make every other part move as well.

    Notably, there was but one part of the whole framework that couldn't be worked into an ordinary A4 paper, even when building the kit at double its design size, which simplified things considerably. The big main spar had to be divided into two parts, with a joint at the center. This joint governed the alignment of the whole wing, so it had to come out right.

    I took two precautions: The reinforcements of the centre joint was carefully dryfitted into the slot in the fuselage. And the main spar was joined on top of an extra copy of the centre section of the original undivided spar (from the scan).

    After dryfitting every part of the big jigsaw puzzel, and dryfitting the unglued assembly into the fuselage everything was deemed ready for final glueing, which actually went rather quickly and painlessly. Learning experiences were:

    • Do not make tabs the full 2 mm deep (in my case). Many of them will join up with a flat surface of another part on the back side. If they protrude, the fit will be bad.

    • Do not believe that you have understood the assembly completely, even with careful preparations. In my case there were a number of surprises:

    • The main spar is actually not straight. At about half the wingspan it is markedly swept backwards, and the bend has to be accomplished while glueing the parts together.

    • Also, all the horizontal reinforcement pieces are not perpendicular (that is "normal" to you, Gil!) to the spar or ribs. The only way to get the angle correct is to follow the marks on other parts.

    This last point is a bit problematic, since you really want to make as many subassemblies as you can (you have to decide which, and how many, for yourself), before finally assembling the whole wing. If the glue of the subassemblies have dried completely, you may have problems in adjusting angles of individual reinforcement pieces. You should be alright if you make the wing in one session, though.

    A final remark is that some pieces clearly are incorrectly marked. If you take really good care in dryfitting and check everything twice or more, however, you will easily spot them, and there is no real problem here.

    This, I hope and think, will be the last big batch of 2 mm card for this build. I really look forward to some straight paper cutting from now on!

  7. jrts

    jrts Active Member

    Hi Leif

    Another good read with some very good tips.
    Its realy is looking the part now with the wings started, a smart bird indeed 8)

    More when you can, this is just what the troops need on a dark and rainy day.



    PS I do love the photos, so clear and crisp
  8. Jim Krauzlis

    Jim Krauzlis Active Member

    Superb post, Leif! :D

    I almost feel like I'm standing there, looking over your shoulder as you build this beauty...a lot to learn from your build and, thanks to the superb narrative, some of it might even have sunk in. :lol:

    Excellent photos, as usual. :D


  9. barry

    barry Active Member

    Way to go Leif

  10. Gil

    Gil Active Member


    1:16 is a very nice scale. Now be a good chap and show us how the wing skins go on...,

    Impatient, Gil
  11. Leif Oh

    Leif Oh Member

    18. Details to go inside the wing

    18. Details to go inside the wing

    I love it when you guys are trying to rush me to get the wing skins on, but it's just not going to happen, at least not this installment. Reason is that the design calls for a number of details to be constructed first, which all are to go inside the wing.

    And here is where the extra little effort to write in for an English translation of the instructions really paid off. Turns out that all these details are to be glued to the inside of the wing skin, and not to the framework, before the skin goes on. That I would have never got by myself - thanks Halinski for providing an English translation! (And anybody else interested can download it from the Parts bin on this site.)

    Learning experiences from the interior details were:

    • When you have all those little details that are supposed to be doubled up before cutting them out because there are patterns on both sides, it pays to leave a tab where the part number is printed. Sometimes that is crucial, for example when there's a difference between left and right side details.

    • On all other little parts, be careful to write the part number with pencil on the back side, however small the part is.

    Here are some pictures of this weekend's work on details.

  12. Leif Oh

    Leif Oh Member

    19A. Preparing the centre section

    19A. Preparing the centre section

    Glueing in the different parts that were supposed to go inside the centre section I was gradually filled with more and more awe at the design. What an accomplishment to design all these little parts, true to the original, and make them fit inside the model with VERY small tolerances.

    As the parts went on to the skin (scary business, glueing subassemblies to nothing but the skin, having to believe that they really would fit inside the framework later on), I began to think about how absolutely crammed the inside of these aircrafts must have been with gear. Not visible from the outside you have coolers, voluminous air ducts for cooling air, and wheel wells - plus miles of connecting wires, hydraulics, etc, not modelled here.

    And then I'm starting to think that the design effort of this model must have been almost as extensive as for the original aircraft - considering that the design of the model in all likelihood was done by just one person, while there must have been a huge team on the original aircraft.

    Anyway, some more lessons learned:

    • It pays to pencil in some positioning marks on the backside of the skin for details that are to go on absolutely flush with apertures - much easier to get it right.

    • If you are the least bit hesitant about which way is which when handling parts like the skin upside-down, back-to-front, pencil in "L" and "P" (corresponding to part no:s; Polish for left and right) to minimize the risk of making idiotic mistakes. (Now, why do I feel this is good advice - three guesses!)

    And joyous tidings - it will fit! Dryfitting revealed that even the front air ducts now by some miracle will end up butt to main spar, where they can be glued. Did not have the faintest idea that this would be so, until I actually bent the skin with attached parts in place for the first time. Goes to show how much (or little, in this case) you actually can visualize. There's nothing like hands-on experience.

  13. jrts

    jrts Active Member

    Hi Leif

    If thats messy then Ive got big problems :lol:

    Love the blow by blow account and Iam learning a lot from this on how to do the big stuff.
    Great thread and look forward to the next set of class notes 8)

    More soon I hope :roll:

  14. Leif Oh

    Leif Oh Member

    19B. Closing the centre section

    19B. Closing the centre section

    The centre section has been closed. It is not perfect, but I am still happy that it is done. The wings are on their way now!

    The trouble is that these sheets are so big, and you've got to be everywhere at once, pressing them down and making sure that they go on well aligned. I did it in two-and-a-half installments - bottoms first; then adjusting the airducts and glueing them to the spar; then stretching the tops on and backwards - but it was still stressful.

    There's still some trimming to do in the gap which is to fit into the fuselage, but I'll wait with that. The cooling air outlets likewise will have to wait until the landing gear and all the rest of the external details are to go on.

    The really big stuff is due next - the wing panels.

  15. barry

    barry Active Member

    Brilliant bit of engineering Leif your cutting is so accurate I can't see you having trouble with the big panels the ones you've done are the worst bits to undertake.

  16. Leif Oh

    Leif Oh Member

    20A. Correcting the wing skins

    20A. Correcting the wing skins

    I knew there would be trouble with the wingpanels. These sheets are just too large, for discrepancies not to creep in, one way or the other. Remember, they are glued together from two print sheets, each a full A4 width, and extra length to boot. So I decided to regard the first attempt as nothing but a trial, which would need corrections, probably in the computer.

    Therefore, I cut out one set of wing panels and joined them with the method described earlier for making separations between rudder and fin, elevator and horizontal stabilizer. Luckily, there was a panel line situated right where I wished to make the separation, well forward on the bottom part, in the flat section of the wing profile, so the joint is pretty invisible.

    No problems so far. The joint (butt glued, with document tape on the inside) was flimsy, but that would straighten out once glued to spars, etc. No recesses (undercarriage and air intakes) were made, and no embossing, since this in all probablility just was a trial run.

    Which was just as well. No way the panels would fit snug with the center section. There were pretty huge gaps at both front and aft ends, and they were too short at the centre section end to go all the way round. This just had to be amended. But how would I find out by how much, and where?

    My solution so far has been to tape the wing panel together with masking tape, with enough slack at the centre-section back end to allow them to cover the entire centre section. The entire section then was slid over the framework, and coaxed into the correct position for the landing gear recess to be exactly at the correct position (this is the critical part that had to fit).

    From there on I just measured how many millimeters that would have to be added, front, back and lengthwise. Back to the computer and creating new retouch layers for all the big parts.

    First of all the top section was distorted to be around 5 mm longer at the centre-section back end, while keeping the wing-tip end at its original size. New outlines at the centre section were drawn, sampling the colour of the lines from existing lines, and the empty space filled in seamlessly with the clone-stamp tool. Finally, the bottom-part front section was distorted to fit in with the broader, filled out top section.

    And now I'm just waiting to print the new sheets. Three big ones, but what the heck, let's get this thing right. As others have found out before me, funny things happen when you upscale, and I have more than doubled the scale.

    So it's waiting time. Just wanted to keep you updated.

  17. Leif Oh

    Leif Oh Member

    20B. Rosie the Riveter at work

    20B. Rosie the Riveter at work

    The wingpanels seem to be jinxed. After having printed a new set of sheets, I sprayed them with my usual matt acrylic, from a freshly purchased spray canister. The first picture below shows what they looked like after a few minutes of drying. Disaster strikes again!

    I have since test-sprayed scrap pieces, after shaking the can vigorously, and for a long time, in the belief that the gray spots must be matting agent clogging up at the bottom of the can and not properly stirred into the varnish. No change. Tomorrow I will of course go to the artists' shop and get the can exchanged. But what if the new can is the same; what if it's the whole consignment?

    Does anybody have an idea what this might be? The varnish I am using (and have used without any problems, albeit a previous can) is Winsor & Newton "All Purpose Varnish For General Purpose, Arts & Crafts, Matt". High-quality producer, should be no problem, but there you are.

    Meanwhile, I've printed a new set of wing panels (third set), wisely not spraying them, and played Rosie the Riveter. The idea was to emulgate Swinger's beautiful rivets on his Ju-88. His tip was to use a cut-off syringe, sharpened like a hole punch. So that's what I copied.

    First learning experience was that you must not turn the needle while pressing down. In that case you sort of uncap the top layer of the paper and you end up with a white spot, instead of just a circular incision to mark the rivet. Just press down.

    Second learning experience is that you really, really need a good handle for doing this kind of work. I couldn't stand making more than a dozen rivets, until the pain in the fingers (and arm for that matter) became unbearable.

    When making handles for this kind of small tools, it pays to have saved old brushes. Cut of a suitable length for a handle, drill a hole the same size as your tool, and just insert.

    A note here: Do not drill all the way through the handle. I did that first, because I thought I would need a clear path through the needle, in order to clean it from paper residue. Big mistake. You cannot fasten the needle in a handle drilled all the way through, not even with super glue, and expect it to stand up to the kind of pressure you regularly subject it to, pressing down hundreds of times for a single wing panel.

    Also, make sure you make a long enough handle. That is more important than finding a wide handle, since you will hold the tool more like a pencil anyway.

    Third learning lesson is to use a steel ruler for lining up the row of rivets your are making. Makes it much easier. Sometimes the print is not to clear, and it helps to just go along the ruler, rather than trying to find every little printed rivet by chance.

    Finally, I made a couple of rows of rivets with an earlier tool, which is just the shaft for an old abrasive bit. You may have noticed the rivets on the centre section (previous posts) which where made with this tool. They are much too coarse, which I didn't notice until i saw the photographs myself. The new tool is much better, and gives very realistic rivets. Thanks again Swinger, for giving the rest of us something to strive for, and for another good tip!

  18. Gil

    Gil Active Member


    Suggest that you warm the can of spray to 80 degrees Fahrenheit (27 degrees Celsius). This might fix the problem although spraying outside at this time of year in the northern lattitudes is pretty "iffy".

    Best regards, Gil
  19. Bowdenja

    Bowdenja Active Member

    I agree with Gil.

    I tried your suggestion of spraying before cutting and the first time it worked great. The next time it did not. Same can of spray, only difference was the temperature. It was colder and the spray gave a speckled appreance when dried. AND it never changed.

    I also think humidity may effect it too. Too humid equals speckled finish also.
  20. Leif Oh

    Leif Oh Member

    Thanks guys. I'll try to think of a way to spray in warmer temperatures, although I will also go back to the shop.

    I sprayed in the stairwell, which isn't exactly outside, but still cooler than the apartment. I just can't spray inside, too smelly by far. Even the dried out varnish smells slightly too much.

    May have to give up pre-spraying until summer. I'll give it a last try down in the basement, though. Seem to remember that it's considerably warmer there.

    There's always the final varnishing with water-based matt acrylic. Doesn't smell, but can't be applied until parts are mounted, since it deforms them otherwise.


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