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Glenn Martin Flyer 'El Sonora', 1913, Scratch-Build in 1/72


Old Man
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I have been working on this for a while, and have recently got the last of the difficult sub-assemblies (the spoke wheels) in hand. This will be rather long, as I will be putting up the steps involved in making the motor and the wheels.

 

Below are two pictures of the machine which is the subject of the build. This aeroplane, under the grand title 'Air Fleet of the Northwest' provided important support to the faction of the Constitutionalists led by by Alvero Obregon during 1913 and 1914. The aeroplane, along with the services of a professional pilot and mechanic, was acquired from the Glenn Martin Co. in California for $5,000, and smuggled across the border with Arizona in circumstances attended with a more than usual ration of skullduggery.

 

I freely acknowledge that a good deal of what I am doing in this build is conjectural, as there are no drawings available, nor have I even seen stated dimensions for this particular aeroplane. There is a decent photographic record of 'El Sonora', though, and Mr. Martin's designs at this time were largely copies of Curtiss aeroplanes, albeit tweaked somewhat. I am operating on the courteous assumption that basic elements, such as aspect ratio, were not significantly altered by Mr. Martin and his workmen.

 

 

 

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The first thing I did was construct an early Curtiss V-8 motor. There is a photograph available from a Mexican museum site which gives a good look at the motor of 'El Sonora' : it is definitely earlier than an OX-5, and might even be an O.

 

I used 'flying jigs' to get the cylinder pieces uniform. The bits to be assembled were measured against, and in some instances attached to, stock strip pieces of known thickness, and sanded down to match these standard pieces.

 

The picture below shows the principle, though it is from an earlier run. From left to right: finished cylinder, dressed cylinder piece on the 'flying jig', raw cylinder piece on the 'flying jig', and raw cylinder assemblies. On this run, the upper step was 2.5 mm, and the lower 0.75 mm. This did not allow for the irreducible thickness of the base ring, and so on the finished item I reduced the lower step to 0.5 mm. This necessitated boring all the way through the lower piece, and fixing the wire pin in the upper piece.

 

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This is the cylinder mount. It is hollow, with a base piece of 15 thou card, 11mm long and 5mm wide, a spine piece 2.5mm high down the center, and side pieces tented in. Shaved discs of 2 mm rod are attached.

 

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Here are the cylinders mounted on a properly shaped 'early Curtiss' block, with detailing begun. A further disc of shaved 2 mm rod tops each cylinder, with a head piece of slightly thinned 2 mm rod atop this. Curtiss cylinders were held down by four long bolts and an 'X' fitting over the cap.

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Here is the motor, with much of its plumbing in, ready to be fitted to the machine when the bearers are ready on the lower wing.

 

 

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Next was producing spoke wheels. This was made more difficult by their small size, only about a three-fifths the size commonly employed later, in the Great War and Golden Age periods.

 

I don't mind confessing this step nearly bounced me off the project. But I have got enough of the bugs out to present a method which can produce acceptable results.

The method is derived from routines presented by the late Harry Woodman, master scratch-builder, whose book is available on-line, and highly recommended....

http://rclibrary.co.uk/files_titles/1216/ScaleModelAircraft_Woodman.pdf

But his method was meant for 1/48 scale work, and probably (though this is not clearly stated) employed waxed sewing thread for spokes. Adjustments have to be made for work at a smaller scale, and employing monofilament ('invisible thread') for the spokes.

The basics remain the same: two circles pressed together to make a wheel, sandwiching between them a spacer representing the axle, and a pattern of spokes attached separately to each circle and, in their centers, to each end of the spacer/axle. This crowns the spokes out as the unit is assembled.

 

Below are the jigs I worked up to facilitate making these,

 

The jig at the left is for weaving the spokes onto a wheel half. It finally occurred to me to have wife print out something with eight points. However, the eight points, probably owing to distortion in being blown-up from a small original, do not quite align correctly. On this, the notches at the points were adjusted till all the lines crossed precisely over the center dot before the central spacer, of 0.75mm rod, was placed.

The jig in the center is to hold the wheel halves aligned when they are attached to the axle/spacer. It began as a weaving jig, but failed in that because I had assumed the points were aligned truly, so that it produced off-center spokes. The four rods of 1mm rod around what was originally a centralizing collar hold the alignment. The center was cut out to give a good view of how the spokes were lining up.

The item at the right is the tool for fastening the wheel halves together. Its base is an earlier weaving jig ready to hand for the purpose. A ring off the tubing stock is in the center. Four long pieces of 1.5mm rod are attached and form a guide. The wheel halves, joined already to the axle/spacer, are placed inside. A length of tube then presses down, guided by the rods, and applies uniform pressure to both wheel halves, while the rods preserve external alignment. This item also has the secondary function of holding the wheel halves while their mating surfaces are marked for notching.

 

 

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Here are the pieces employed....

 

At the top are two raw wheel halves freshly cut from the tube. Beneath these, at the left is a wheel half tinned down and marked for notching, and at the right is a notched wheel half ready to receive spokes. I had tried notching in earlier experiments, but discarded it as it weakened the wheel halves and they bent in assembly, but that was in the early going, and the assembly tool now removes that difficulty.

On the near right is a finished axle/spacer, and on the far right is the 'raw' item. A hole is put in a square of 0.75mm sheet, a length of 0.75 rod is put through and fastened. The square is clipped and sanded down till round, and while of greater diameter than the rod, not greatly so. The rod is then trimmed down. It is important one end be longer. Both must be sanded to a bit of a point.

 

 

 

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In the pictures below, on the left is a notched wheel half with its spokes. The spokes, pressed into the notches, work to center the wheel half. These are glued in, being sure the spokes are deep in the notch, and the notch filled above them with CA gel. The notches are put in with a knife-edge needle file. Once the wheel half is cut off the weaving jig, an the excess trimmed, the mating surface can be sanded flat with a fine grit sanding stick. The notches fix the spokes firmly enough in place to allow this it very little risk of disturbance. On the right is a wheel half with spokes trimmed, and the wheel's axle/spacer affixed. The short end goes into the lower wheel half.

 

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Here are the joined wheel halves in the assembly tool. Glue (CA gel) is applied in a manner which avoids the rods. The length of tube is pressed down; it can be held in place between thumb and middle finger, or placed on the bench and pressed down. After a ten count I apply liquid patience, and continue to hold for a slow sixty-count. A Once the 'plunger' is removed, a knife-point will lift the wheel off the base, and it can be lifted out readily.

 

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Below, on the left is a wheel fresh out of the assembly tool; on the right is a finished wheel.

Finishing consists of clipping the excess off the axle/spacer, and dressing it down with a 'swizzle-stick' sanding stick. Examine the outer rim for un-glued seam, and apply a bit of CA gel smoothed down with a tooth-pick. Sand the outer rim down smooth. Then You can sand down the sides of the tire with a heavy grit sanding stick, keeping it tilted slightly away from the center and the spokes. Though you should not hold the wheel in a death grip, you can hold the center while you do this with reasonable safety. Sand down close to the 'points' of the notches (the dark color of the set CA gel will be visible). Finally, knock down the edges of the outer rim to give the thing a bit of rounding, and end with some smoothing from a fine-grit sanding stick.

 

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Over a weekend, I made seven of these. The picture below includes a wheel from an Eduard N.17 kit for comparison, and a penny for scale.

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At this point, I have begun to make the wings.

 

As indicated above, a good deal of what I am doing in this build is conjectural, as there are no drawings available, nor have I even seen stated dimensions for this particular aeroplane. There is a decent photographic record of the machine, though, and Mr. Martin's designs at this time were largely copies of Curtiss aeroplanes, albeit tweaked somewhat. I am operating on the courteous assumption that basic elements, such as aspect ratio, were not significantly altered by Mr. Martin and his workmen.

That said, after some study of photographs, and period drawings of various contemporary Curtiss machines, I settled on dimensions and pattern for the wings. In these old 'flyers' produced by early designers in the U.S., the wings are the main structural element. Aside from motor and associated plumbing, there is not much besides sticks and strings (though there are a lot of those, the strings particularly). A rib spacing of 4.5mm provided a wing-span of 180mm, with a little fudge at the tips, scaling up to about 12.9 meters, or about 42' 6", measuring flying surface tip to tip, which is within the range of long-span Curtiss 'flyers' Mr. Martin would have been copying (spans given for Curtiss machines, by the way, often give the distance from aileron tip to aileron tip, not the span of the actual flying surfaces). I used the five foot basic chord usually employed by Curtiss. The 'ear' wingtips are Mr. Martin's; Cutiss wingtips generally were squared and ended at the outer bay of interplane struts.

 

I managed to cut myself a little cutting these out, but am of the school that blood sacrifice to the modelling deities secures some favor in a build....

 




The wings started out as blanks of .030" styrene sheet, cut to measure and bent by hand. After bending, they were sanded to a smooth curve surface. On the concave undersurfaces, this was done with heavy grit sandpaper fastened to a large bottle (about 4" in diameter), with a such paper on a smaller bottle (about 1" in diameter) used for final shaping. The upper surfaces were done with heavy grit sanding sticks. Once the shape was in hand, I began laying out the ribs.

 

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Once the ribs were laid out, I 'broke the plane' between them with a narrow 'swizzle-stick' sanding stick, and the curved edge of a #10 blade used as a scraper perpendicular to the plastic. You don't need to go very deep, just enough to leave a 'ridge' readily apparent to your finger-tip brushed along the surface.

 

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Once this had got a couple of coats of white primer, and a couple of coats of Future, the next step was applying rib tapes, quite literally in this instance: I used 1/64" pin-stripe cut masking tape. Fiendishly useful stuff, and I am beginning to look for resupply (the company is 'Line o' Tape') as I am now down to about half of my last 240' roll of it.

 

Martin used a dark tape (possibly even blackened strips of wood or cane) to secure fabric to the ribs. I darkened the tapes with an ordinary #2 pencil. This then received two more coats of Future, which helps fix the tape down firmly, and seals the pencil marking. On the undersurface, it is only necessary to draw in the tapes, there is no need to 'break the plane'....

 

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After this, I have begun to paint the fabric

 

t will be clear from the photographs Martin used a pale fabric indeed. I have used a mix a light grey and tan, quite thinned, and brushed on between the ribs. It will get a couple more coats of this, but I will say the coverage looks more uniform to the eye than it does under flash and a macro lens.

 

Once painting is complete on upper and lower surfaces, I will pick out which will be the upper wing and which will be the lower, and begin constructing undercarriage and engine bearers and such to the latter....

 

 

 

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Proud Kraut

Over a weekend, I made seven of these. The picture below includes a wheel from an Eduard N.17 kit for comparison, and a penny for scale.

 

UNBELIEVABLE! I really admire your modeling skills and your patience! Thanks for sharing this outstanding project!

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Thank you, Gentlemen. I appreciate your interest and your kind words.

 

Spoke wheels are something I have wanted to get a handle on making for a long time, they are kind of a Holy Grail for models in the early periods I am most interested in.

 

I hope to have more progress to report next week. I am preparing sketches to try and set measurements for engine bearers and undercarriage (which interact at significant points, and also tie into the struts supporting the upper wing). At this stage it isn't like the sub-assembles: a stand-alone item like the motor can just be scrapped and replaced should it turn out wrong, no matter how much work went into it, but if I go off half-cocked on this stuff, which will require putting holes and notches in things, and affects the whole business including the gap between the wings and the eventual tail assembly, it would not be so easy just to start over....

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You're off on a great "quest", and an unusual subject. If nothing else it will be a neat conversation piece, kudos to you Sir!

I look forward to it's completion.

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Gulp! Whenever I see pics of white plastic in the modelling section my heart begins to beat a little faster. The engine and wheels are masterpieces. And in 1/72nd scale, no less. Phenomenal.

 

Tom

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