Experimenting with kitbashing

[birder]

Good one!:D

[Leif Ohlsson]

So I did what Shrike suggested, went back to the drawing-board, and analyzed the Russian drawing of a sabre-shaped prop for the Sopwith Triplane. The result was two print sheets with 40 prop parts. More about how to go about drawing these in the next post. Here are photos of the result:

1. The bottom part is extremely thin, almost impossible to cut out, which is why two "flags" at the rear were kept when cutting it out. The additional (good!) idea was that this would ensure enough of stability for a proper alignment of all following parts. (Similar flags at the leading edge would have been very difficult to cut out, once additional layers are hiding the print lines.)

2. The outlines of the part to follow is printed on each part, to enable proper positioning of each layer. Seemed like a good idea, but it's not good enough. I have an idea for how to improve this. Note here, that at this stage of the assembly the bottom flags have been cut out. Note also my mistaken belief that, for once, I could be a bit liberal when applying white glue. Don't repeat that mistake - thick layers of white glue is terrible to sand; it just turns into some rubbery black goo.

3. The finished prop could be handled while the mass of white glue still was semi-dry. Here I am trying to align it by pressing down the rear edges. I have an idea for how to get away from this problem, which will also solve (I hope) the alignment problem of each individual part.

4. In the end I resorted to clamping the prop between two scrap wood parts, with additional scrap pieces pressing down the trailing edges.

5. The prop was left like this over night, awaiting sanding.

6. With the excess amount of white glue I had applied, sanding was not pleasant. Here, I've got to the stage where one layer of matt varnish (I am still sticking to the water soluble variety) has been sanded down completely. 180-grade dry/wet sanding paper is used at this stage. The varnish sucks into the paper and hardens exposed edges, which is very beneficial (as opposed to the gooey white glue; the thin black lines you see is white glue turned gooey black).

7-8. The varnishing-sanding procedure was repeated until the result was acceptable. I think I put on three coats, and sanded down the last one very slightly with 320-grade dry sanding paper. By now the trailing edge is very thin, and apt to knicking. The profile is much thinner than the first attempt, which is what I wanted. The overall feeling is one of slightly bendable plastic.

9-10. Final shots for good measure. I am pleased, since the prop is such a prominent part of the aircraft of this era, and it could make or break the final impression of the model. The curvature of the prop certainly is beautiful. Some imperfections at the backside rear edge remain.

[Leif Ohlsson]

As Shrike warned, there is no way to arrive at the proper shape of each part other than by sheer pencil work. The only difference is that I made it in Illustrator (other graphic programmes, of course, will do just as nicely - as long as you can make vector curves, and layer them properly).

Image one:

1. The pixel drawing of the Triplane prop from Airwar.ru was imported and enlarged so that the profile got a thickness 10 mm thick. This is for ease of layering. The final result will be reduced to proper diameter at the final stage.

2. Outlines in red are drawn for front, side, and profiles.

3. Guides for ten layers added. The side view is used for determining the top position of each layer and sublayer.

4. Within each layer (1 mm thick), guides for 4 sublayers are drawn. Vertical guides are positioned at each sublayer (1/4 mm thick), and the points on the baseline of the profile noted. This is where the points for each part should be. Here, the positions for sublayer 4-1, at profile no. 4 from the hub is illustrated.

Image two: This process is iterated for each of the five profiles. The result is one sublayer. Then the whole thing is started over for the next sublayer, and is iterated 40 times to get the proper shapes for 40 layers. As the layers add up, you start seeing the shape of the whole thing emerging. Each layer was adjusted manually slightly along the blade to get proper shape and sequencing distance, and more so close to the hub and at the top of the blade, where the sum of the outlines determine the final outline of the prop.

Finally, the contours of the layer above the one at hand were copied, and cropped. This is to provide guides for glueing on each part. Every such composite part was then copied, rotated 180 degrees, and positioned properly (not illustrated).

Then the whole drawing is copied and reduced to proper 1/16 scale diameter (which seems to be 159 mm, judging from several drawings).

Now, and only now, each part is copied and positioned on to the two parts sheets. The idea is to print every other set of four sublayers on different shades of brown to simulate layering.

The two parts sheets are shown only for illustration. If I can manage, I'll redesign them slightly to include tabs midway out in order to help alignment and solve the need to press down on the trailing edge. This will also eliminate the need for printed guidelines of the next layer on each part. These tabs will build up to thin columns, which should be relatively easy to cut/sand away, once assembled and dry.

The idea is that this will enable pressing down the assembly evenly along the blade while drying. Without such tabs pressing down is impossible, due to the curved side profile. (C.f. the side view; this flat back/curved front side profile is as faithfullly replicated as the sabre shaped front profile, the varying profiles along the blade, and the increasing angle of attack along the blade.)

In addition, "flags" at the rear end will be added for every second main layer (every eigth sublayer). The idea here is to glue 8 parts at a time, as subassemblies which can be pressed down while drying, for a final assembly of 5 such subassemblies. The "flags" will of course be cut away before this final assembly.

With a more sparing application of white glue, this should make for a much easier & more fail-safe build of this prop.

[pahorace]

Thanks again Leif,
it's a tutorial from master.
I had imagined that construction of the propeller but now I see for the first time the steps.
The propeller is quite extraordinary.
Thanks for sharing.

Regards
Orazio.

[B-Manic]

Great tutorial Leif and a fantastic result. This process would work with ships screws as well but would be very challenging given their relative size.

[Leif Ohlsson]

Thanks guys, I'm glad you like it too! And thanks Gil for the link to the Engine History site, and their section for rotaries. It was a gold mine. Marvellous to listen to the Gnome sound. Since the Gnome didn't have a throttle, this would be the actual sound in flight. Much like a two-stroke on idle, right?

The lack of a throttle would be the reason for the flying techniques of, for example, early Sopwith Pups with Gnome Monosaupape engines: Climb to ten thousand feet or something like that, then glide down; climb again, etc. Formation was held by adjusting mixture for small change of rpms (very much like un-throttled model engines, that!). And landing approaches were made by "blipping" the engine (switching the ignition on and off with the blip switch on the stick), much like the sound at the beginning of the sound file, I imagine.

But try "blipping" heavier rotaries... the gyroscopic effects must have been spectacular. Which of course is one of the reasons for the Camel killing so many fresh pilots while coming in for landing or even just generally flying about - despite the fact that the Clerget and other rotaries did have proper throttles.

I thought for a while that the reason for why the Gnome didn't have a throttle was that they had the intake valve being situated in the piston. But that isn't it; the reason is that the fuel was injected through the crankcase. The name "Monosaupape" ("single valve") does in fact refer to a later development (I'm reading from the piece about rotaries you can download at the Engine history site), in which they got rid of the piston valve (it created a great many maintenance problems), replacing it with intake openings in the crankcase instead, uncovered by the piston. The cycle is quite complicated, much more so than I understood earlier, and interested should read up on it in the Gnome Monosaupape fact sheet (pdf).

As for props, it turns out The Engine History site has a section for propellers, too! Have a look at these very instructive photos and note the clever tool for measuring angle of attack along the blade in the last photo. Also, I count nine layers. Stands to reason if you want a layer of hardwood top & bottom. Oh, well, close enough. Ten layers of four parts each does result in proper scale thickness, which was the main thing for me.

[birder]

The alternate colors make a realistic wood appearance, Leif, very pretty. Nice work on the design end as well!

[Gil]

Hi Leif,

The engine, cowl and prop are taking on the proportions of an ancient Norse Saga...,

I agree with you that something as visually obvious as the propeller requires more attention to its detail. The results of your efforts have captured that eye appeal. Nice to have achieved such a desirable effect. Congratulations!

+Gil

[CharlieC]

Something I tripped over - this thread has some quite nice detail images of the Gnome engine.

Gnome Rotary Engine run - The Aerodrome Forum

This engine was rated at 16 pints of oil per hour (9.6 liters) - no wonder the WW1 pilots had permanent diarrhea.

Regards,

Charlie

[shrike]

Another little tidbit on the Gnome. The cylinders are machined from single solid piece of steel, fins and all. The original billet weighed around 45kg, and the finished item around 4.