Building a 1:20 Scale Grunau Baby IIB

Discussion in 'Aircraft & Aviation' started by Gil, May 9, 2005.

  1. Gil

    Gil Active Member

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    I've begun the build of GPM's Grunau Baby IIB, Catalogue Number 222, Series A. The original glider was designed by Wolf Hirth for the Schneider Company in 1932. Over 5000 variants of this ship were produced under license Worldwide!

    The kit contains two models one at 1:33 and the other at 1:50. Why a glider? Several reasons. I wanted a fairly simple kit with a minimum of parts to simplify the build as a larger 1:16 flying version is planned (more on that at a later date but it should help further the cause of card modeling). Last but not least they're quite reasonable.

    The models 1:33 scale wingspan is 40 cm and it stands 4 cm high at the vertical stabilizer. The 1:20 scale model will span 66 cm and will be 6.6 cm high. The cockpit is fully detailed but could use a pair of rudder pedals. The use of graphic "embossing" to bring out detail works but is a bit overdone. The design should have used a lighter color to bring out the transparency of doped covering but this is a matter of tastes and is easier said than done.

    Overall this little bird grows on you to the point where you began considering the 10 or so others that GPM offers..., a whole bevy circling in a thermal mobile..., nice, very nice!

    The image below shows the cover of the model and the scanned, enlarged and rearranged parts on legal size paper (8.5 x 14 inches).

    Best regards, Gil

    [​IMG]
  2. Leif Oh

    Leif Oh Member

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    Excellent choice!

    Excellent choice of model! I will follow this closely. If and when I follow, it will be an exercise in recolouring to the Swedish version. I once made such a model in 1/6 scale for RC. Never flew it though - couldn't bear the thought of putting a dent in that beautiful plane. Subsequently lost it when the store to which I had borrowed it for display went belly up, after we moved from the area, so the owner apparently couldn't contact us. Oh, well, that's how it goes. All the more reason to enjoy your build of it.

    Good luck and happy flying the Grunau Baby, in due course!

    Leif
  3. Gil

    Gil Active Member

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    Leif,

    With over 5000 produced there has to be an incredible variety of paint schemes. The 1:16 scale has a kind of special twist to it and if it works out will be a real kick in the head so to speak. I'm currently working with a supplier to see if a supply channel can be set up to supply the demand. In fact you won't have to care about putting a ding in it with this method. It's as easy and care free as that. But first let me just finish this little jewel. Funny thing is that it will probably hang in a local hobby shop too...., maybe this should be reconsidered as I'd like to see the shop stay in business.

    Best regards, Gil

    P.S. What was the Swedish color scheme?
  4. Leif Oh

    Leif Oh Member

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    The Swedish Grunau Baby originated within the air force during the 1940s, who used it as a means of recruiting prospective pilots. Subsidized (I presume) planes were built by local glider clubs. The colour scheme, as supplied by the air force gliders, was all beige (kind of very light, bland, yellow, with black registration letters, SE-XXX). Nothing fancy, but elegant in its own way. Much lighter and blander than the yellow in your kit, and no bright stripe on the rudder or any other embellishment. And no seeing through, either. That could only occur on clear doped planes, and I doubt that there were any of those around.

    All-civil Grunau Babys were probably painted in a variety of schemes (red and blue likely), as were most gliders eventually (after their first re-covering), but I have no detailed information or examples.

    I'm sure the Baby jinx won't spread over time (more than fifteen years ago) and continents, so your hobby shop should be alright...

    Leif
  5. Gil

    Gil Active Member

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    Grunau Baby IIB Build Report

    Hello All,

    Went through nearly four internal wing spar box builds till the right combination for the scale was achieved. From heavy enough to use as a hammer handle to delaminating during drying over a radiative heater. Haven't met with this much discouragement in quite a while. Finally hit on the right combination of paper (0.52 mm) and things finally went together well (by now I'd had enough practice to be quite proficient).

    The design uses a built-up box spar and requires the usual attention to straight and precise cuts to guarantee proper alignment. The instructions recommend "Butapren" glue for the first stages and CA thereafter for model stiffness. I ignored the Butapren and went straight for the odorless CA.

    One area that needs comment is the way the box upper and lower skins are marked and labeled and caution about the spars. The upper and lower skins are nearly identically marked and the spar locations are indicated in Polish. The Polish indication is wrong. You have to match the skins up by the dotted spar lines to get two matching upper and lower pair of skins. In the image below I've marked the leading edges of all four skins and the upper skins have been glued. The upper skins need to be shaped to the airfoil before being glued to the spar structure.

    [​IMG]

    Begin by joining the forward and aft spars together. Insert and glue the spreaders, mid section ribs and mid section spreader. The center leading edge ribs and trailing edge ribs are added next along with their angle braces. Keeping alignment is key but is a little tricky. I used a ruler to set up a line of pins on a building board to align one spar as the other was positioned with the spreaders already glued on it. The second was then pinned into place and glued. Don't build one wing half and then the other as it's pretty easy to make two left spar sets and keeping alignment between the two is a real chore. The top skins are then aligned and glued. Make sure the but joint in the center is tight and that the right and left pairs are correctly paired with their corresponding spar sections. The bottom skins are then aligned and glued with extra attention paid to the the tips where the spars taper.

    The image below shows a trial sock style fit of the right wing skin. More on that in the next installment.

    Gil

    [​IMG]
  6. Leif Oh

    Leif Oh Member

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    Oh, I like this. A good, detailed building thread! Shouldn't really ask at this stage, but here goes anyway - does the kit provide for the quite substantial washout in the wings? Looks that way from your photo at least. That washout is what created stability good enough for a secondary trainer, although there was no V-shape at all in the wings of the Baby.

    I think the washout may come automatically with the elliptical wingtips if you don't press the trailing edge down but let it follow its natural curve upwards as it progresses upwards along the top outline of the increasingly shorter ribs towards the wing tip. The effect of course is that the bottom side of the wing will have an increasingly more negative incidence as compared to the straight section of the wing.

    The satin yellow finish looks very good!

    Leif
  7. Gil

    Gil Active Member

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    Leif,

    Knew you would like the detail! The washout isn't really explicitly provided for in either the instructions or in the the construction diagrams. The only dihedral is a small amount contributed by the underside taper of the spars toward the tip. Washout could be set into the wing when the outer skins are fixed to the spar box which I think I'll do for scale reasons. Any idea of how many degrees washout on the original? Normally it would be 2-3 degrees but could be more depending upon the desired flight characteristics of the overall wing planform and the need to maintain aileron control at high angles of attack. It will be hardly noticeable to the average viewer but the purist says that it must be done in the name of scale. The spar box came out amazingly straight and it's strength is robust enough that I'll include it as a standard design method for scaled up designs. Swinger's cupping technique was used on the wing tips. Works amazingly well! Thanks again Swinger! The satin yellow is the result of two layers of clear coat followed by a single coat of matte sealer. It seems to approximate the dope coated fabric covering very well and will be used for all external surfaces.

    Best regards, Gil
  8. Leif Oh

    Leif Oh Member

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    Then you're alright sofar. The dihedral should be exactly what you say, the underside of the wings sloping upwards, while the top side along the spar should be flat, right across both wings.

    Similarly, the washout should be an effect of the topside being kept flat looking along it from the tip or base, which would result in the bottom surface (and the profile as such) acquiring a steadily more negative incidence as you progress towards the tip.

    You seem to be doing just fine! - Leif
  9. Gil

    Gil Active Member

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    Leif,

    I was just going to edit that in when you replied. The washout results from the tip taper and the elliptical shape of the wing or as they say "it's built in".

    Gil