Building a 1/16 scale P39N Airacobra

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

  1. adensley

    adensley Member

    Good job so far Leif.

    As someone who has been involved in the restoration of a real P-39K in the past, I can appreciate some of the contours you are working with. With regards to the exhaust stubs, just be gratefull you are not doing a P-400 with 12 stubs per side..... :wink:


  2. Leif Oh

    Leif Oh Member

    10a. The aft fuselage sections

    For a month now I have been really looking forward to the aft section of the fuselage. When that comes on, and even more so when the vertical and horizontal stabs go on, the build will really start to look like an aircraft, and I will get some idea of the actual size of this beast.

    Also, it will be my first chance to try out the now classic manual by Ron on how to achieve a good fit. Ron's basic recipy, to reiterate, is:

    1) Join segments with joining strips before adding formers.
    2) Sand the former to fit the section joined.
    3) Fit the former in place at premarked pencilled-in position by fidgeing it with the help of a centre cut-out hole.
    4) Carefully apply glue to the joint between the former and the section sides without dislocating it.
    5) Start from the front and go back; similarly start from the aftmost section and go forwards (applicable in my case).

    That meant cutting out new big centre holes in the formers (there are no such holes marked in the kit, indicating that the designers had a different construction method in mind).

    Problem to keep in mind here is that once you've started from the tiny aftmost former and section, going forward, the alignment of the vertical and horizontal stab is likely to be determined (if you don't want to risk misalignment of the pattern caused by a correction that turns out to be necessary later on in the process).

    So be very careful to check 1) that the centre of what you've already built is correctly aligned; 2) that you get the first small aft former really correctly aligned in the aftmost section. This determines wheter your fin and horizontal stab is going to be straight or askew! (I have made some horrendous mistakes here in previous simpler card models; didn't want to see a replay of those for this build.)
  3. Leif Oh

    Leif Oh Member

    10b. Aft fuselage - doing it "the Ron way" (cont'd

    I have had a revelation. For the first time I have built a whole fuselage section "the Ron way", i.e. the right way - and from now on, for me, the only way! (See previous post for a quick outline of the method, then the images below.)

    I have never experienced anything like this in card modeling, since I read, first, Ron's article on edgecolouring, and then the one about making wheels, thereby getting wind of the Olfa circle cutter. This way of doing aircraft sections really ought to be made into a similar article as soon as possible, Ron, and I, for one, am very grateful!

    The only unexpected thing encountered along the way was the difficulty of manipulating formers, which at this scale ended up rather too far away to be able to reach with your fingers. Hence, the big holes I had cut in the formers had to be refitted with crossmembers, where two smaller holes were drilled to accomodate a pair of long tweezers.

    A large crochet needle with a hook, hitherto used solely for preforming, turned out to be the ideal tool for pushing and pulling the formers into their correct positions. Adjustments laterally were by tweezers in the holes, and "massaging" the skin. In the future I'll make several smaller holes (by hole punch) in formers, suitable for adjusting by tweezers and the crochet needle.

    I think it was Gil that pointed out one advantage of this method, namely that the exact position of the former longitudinally isn't really an issue, although you should of course strive for correct alignment.

    Once I got the hang of it, it was a beautiful experience, nothing like the inexact, smudgy, misaligned disasters I have always had to pretend I was satified with until now.

    This is another dimension of card modeling. Control was perfect all the time, never any rush or stress, since the glue wasn't applied until everything was perfectly aligned and in place. With this method, even a beginner will get good results, once he or she gets the principle behind it (see the three pictures below, plus the previous post).

    I do think it is worth underlining that there is nothing trivial about this method. However widely it may be used by better informed people, it is certainly not common knowledge (I never heard or read anything about it until having been a member here for more than a year). The common Maly kits are definitely not made for this. And not even the superb Halinski kit was actually made for it (since the need for holes in formers to be able to manipulate them was not anticipated), nor is it mentioned in the instructions.

  4. gera

    gera Member

    Hi Lief Oh:
    Your Aircobra sure looks GREAT!!!!!!!!, it looks perfect for an electric RC scale model.
    I have had good experiences using the former method you mention. And try to always ad a flap at the end for the connection with the other part in thinner paper, the fit is always better than without it. As you mention the fit of the former is not critical.
    Congratulations again on the P-39, will continue to follow your construction sequence and learn some new tricks from you. :p
  5. Jim Krauzlis

    Jim Krauzlis Active Member

    A superb write-up, Lief!!! :D

    You have very clearly illustrated the construction process you are using, and from one who has not attempted a build of this sort yet I am deeply greatful for the lesson! :D

    I am really enjoying this thread, and look forward to each update for your wonderful descriptions and outstanding photos.

    Oh, yeah...and your model is absolutely outstanding! :wink:


  6. Leif Oh

    Leif Oh Member

    11. Corrective tail surgery

    Checking the finished tail section from all angles, I was stunned to discover that the tail former, guiding the alignment of the fin, was disgustingly askew, in spite of all precautions. I really couldn't figure why this was so, since I had taken every precaution, I thought, and built it absolutely straight.

    One theory would be that the centre section part containing the back part of the canopy had gone on just a bit askew. This I was aware of (and you can even sort of see it in the pictures below). There was no way of compensating for this, however, since the pattern of the aft fuselage then would have been misaligned. Even so, it really shouldn't have resulted in by far that much of a lateral dislocation.

    It took me two days of nailbiting and pondering how to proceed, and whether I should try to keep this from the rest of you guys. I might have sort of glued on the fin a bit askew in the other direction, to the already askew tail former, hoping to get away with it, and making sure never to take photos that would reveal the mistake.

    But I would always know that it was there myself, and it really would have spoiled much of the joy for the rest of the build.

    So one sunny Sunday morning I took a scalpel to the offending tail former to get it out, followed by assorted power-drill abrasive tools to get at the glue joint (white glue really doesn't sand well at all!).

    After a good deal of holding my breath while hacking away, all glue remnants were finally gone, leaving the 0.25 mm skin miraculously intact, not torn up at all. Three cheers for the 3 euros (4 dollars) diamond-studded abrasive bit!

    Since the aft end of the fuselage now was open anyway, the alignment of the horizontal stab slots was checked. It wasn't too bad, but just to be sure I added an internal reinforcement strip to be able to cut new, slightly adjusted slots later on. Should have thought about that in the first place anyway.

    Dryfitting the freshly cut replacement aft former, it was found that a VERY small adjustment (not really visible in relation to centre marks and reference points on the skin parts, c.f. pictures below) would result in a great difference in alignment, and that there was virtually no way to get this right if you start with the tail former, having no reference to the rest of the aircraft.

    Lesson learned here: Follow procedure as per last installment, with the exception of the very first (aftmost in this case) former. Always glue in the aftmost tail former last, when the whole tail section is finished and actually joined to the rest of the aircraft, in order to be able to check the actual alignment.

    I now believe that you cannot achieve a really good alignment any other way.

    Sensitive viewers are cautioned about the images to follow. They are extremely graphic and display considerable violent and offending content.

  7. Leif Oh

    Leif Oh Member

    12a. The fin - preparing the parts

    At first the fin seemed a fairly straight-forward business, now that the tail former had been properly aligned. Not so! Things can get really complicated if you are set to do them as right as you possibly can figure out how to. Turns out it will take at least two installments to show how the fin got properly into place.

    To begin with, this Halinski kit did not have the rudder (nor the elevators to come later) separated from the fin (or horizontal stab). This was fixed by cutting them out, edge-colouring the two parts rather heavily along the split line, and then butt-glueing them back again, backed up with ordinary document tape on the inside (didn't want to use a card strip, since this might interfere with the fin framework).

    Second, there was a white smudgy spot marked on the fin. Couldn't understand what this supposed to be, until I checked a source book for the proper split line between rudder and fin. There it was, on the same page: A white navigation light, mounted in the fixed part of the fin.

    This was replicated by cutting out the nav light, edge-colouring the parts, backing up the nav-light parts with transparent overhead film, trimming those little parts, and then glueing them back in. The difference from just glueing on a bit of oval transparency is of course that you can get the nav lights flush with the surface, as they should be. Further, all lines (different panels, etc.) were scored or embossed (as on the rest of the model).

    As it turns out, this just marked the beginning of the most challenging part, how to mount the fin & rudder really flush and correctly aligned, first onto its own framework, then the whole assembly onto the fuselage. Had to pause and think for a while about the correct procedure here. Progress so far below.

  8. silverw

    silverw Member

    Hey Leif

    I really enjoy your detailed write-up and pics.

  9. adensley

    adensley Member

    Leif, just a heads up. The lights on the tail and the ones that you will find on the wing tips as well are not flush with the surface. They are actually raised teardrop shapes that come at least an inch ( 2.5 cm) proud of the surface in real life.
    These could be simply built up with layers of white glue as you did on some of the cockpit control buttons previously, using the marked outline to guide you on how large they should be.

    Not critiscizing your work which is stunning, just trying to indicate details that ould be apparant in 3D in this scale.


  10. Leif Oh

    Leif Oh Member

    Oops. So that'll be a lesson to next time. Couldn't make that out in the b/w photography, but should perhaps have guessed. Thanks! - Leif

    Ps. I'll contemplate adding a teardrop-shaped blob of white glue. Good idea!
  11. cecil_severs

    cecil_severs Member


    The nav light might be a good application for a clear "dimensional glue" like Judi Kins Diamond Glaze. It beads up real nice compared to regular white glue.

    Just a thought.

    BTW this is quite possibly the best construction write-up I've ever seen. Keep up the fabulous work.

  12. Leif Oh

    Leif Oh Member

    Cecil, Adensley - have done the little glue bulb. Will report as soon as I take fresh photos. Thanks! - Leif
  13. Leif Oh

    Leif Oh Member

    12b. Closing the fin (cont'd)

    Thinking about how to align, assemble, and mount the fin to the fuselage, I arrived at the following plan:

    1) Dryfit the framework onto the fuselage and mark areas that need trimming.
    2) Trim, and dryfit skin onto main spar. Mark & trim all other areas of the framework that needs trimming in order to be able to close the skin.
    3) Make pencil guide lines on the inside of the skin for glueing in the framework correctly aligned
    4) Glue the framework to the skin, first on side, then fold skin over and the other side. Apply glue only to the vertical spar in order to be able to "massage" the skin into correct alignment along edges.
    5) Apply glue from the bottom to the fin rib, much like when you glue in a former in the fuselage. Leave aft part of fin rib unglued (inaccessible by now).
    6) Close the skin from the outside, section by section. Make continuous sightings to avoid warps.


    This is how it turned out (and in the next installment, I promise, the fin will finally get into its proper place):
  14. barry

    barry Active Member

    Hi Leif

    Regarding glue a friend of mine put me onto to some strange stuff called "151 Clear Glue" and did actually get some for me and last night in desperation I tried it on the canopy and it seems to have worked well. It does other strange things like glueing metal to cardboard. On the tube it says not recommended for polythene, poloethylene, polypropalene or polysterene which probably means I should not have used it anyway.

    Address is

    151 Clear Glue
    S.B. Limited
    PO Box 457
    M60 3FB

    Hope this helps

  15. Leif Oh

    Leif Oh Member

    Thanks - I'll have to remember that. Now that I know it exists, I'll keep looking for it, or some similar stuff, in the shops here. - Leif
  16. Leif Oh

    Leif Oh Member

    12c. Finally mounting the fin! (cont'd)

    Learning experiences from the making of the fin are that the method seems to work, but that I'll have to be even more careful about making sightings to avoid warp. There is some (not much, but still) warp there, and you may well spot it in future pictures, so I might just as well owe up to it right here.

    I now think that I should have been more wary when I saw that there was a need for trimming. With a kit of this quality (generally exact fits), this is a sign of warped assembly by the builder.

    Generally, however, I think this is a good method to avoid stress and remain in control over the process. At no point was there any particular need to hurry. The most stressful part of the work was probably the closing of the skin from the outside (no pictures of that particular moment; unfortunately they came out blurred) - which is also where I should have paid greater attention and taken more time to achieve a perfect alignment.

  17. jrts

    jrts Active Member

    Hi Leif

    Wish I had problems that look like this 8)

    Looks better with every post, next please :roll:
    Demanding arn't we :lol:

  18. Leif Oh

    Leif Oh Member

    13a. The horizontal stab

    The stab was very much an affair of learning from the fin - twice over. Very good design, with a spar going through the fuselage. Each half of the stab can be finished separately, then pushed onto the spar, already aligned in the fuselage.

    Learning lessons here were: Emboss ribs (on the elevator) from the backside. Make pin-pricks where ribs begin and end, draw a pencil line on the backside. Not having any advanced embossing tools, I use a knitting needle with a dull point, at a very shallow angle; don't scrape the surface, twist it while applying hard pressure along a ruler, on a semi-soft surface like your cutting mat. Score panel lines from the front with your usual scoring tool, a dull knife or some such.

    Dryfit the framework onto the back of the skin very carefully. When you've decided what the correct position is (not self-evident, unfortunately), make any number of pencil guide-lines you need, the more the better.

    Trim and sand the framework; don't be afraid to make it slightly smaller than necessary. If there is empty space in front of the ribs, it's better than not being able to adjust it to a proper fit at the back (where it's closed). Glue the framework onto one side of the skin, along the guide-lines. Apply glue to the spar and the ribs, but not all the way out to the tips on any member. This will allow you to adjust the skin partially when closing.

    When closing it, apply glue on the other side of the framework in the same way. Stick to those pencil guiding lines. Close the edges afterwards, and make many checks for alignment to avoid warps. With a good design (such as this), no trimming should be necessary. You may still want to sand the edges slightly for smoothness, in that case followed by refreshed edge-colouring.
  19. Leif Oh

    Leif Oh Member

    13b) The horizontal stab (cont'd)

    Couldn't resist this little extra installment, to show that the stab and fin finally are in place. The Airacobra is starting to look like an aircraft now! Note the new bulbuous shape of the nav light on the fin, amended with blobs of white glue on each side - thanks Adensley and Cecil!

    The air intake will go onto the white spot behind the cabin, and the fin and stab fairings are still missing. Those will be the the final tasks on the fuselage before starting a whole new chapter - the wing.

  20. Jim Krauzlis

    Jim Krauzlis Active Member

    Great write up and superb photos, Lief! :D

    The detail on this build is amazing, my friend. And I'm learning a great deal along the way, thanks to your superb descriptions and excellent photos.

    I can't wait until the next update!

    Great going!!



Share This Page