Thursday, 5 January 2017

Rebuilding the Duigan Biplane

This post will be added to as the model takes shape.

A brief description of the historical importance of the Duigan Biplane is at an earlier post here.

Duigan prepared some sets of plans for the Biplane 'Pusher' (a pusher locates the propeller at the rear of the plane, spinning in a different direction to a conventional pull propellor setup). He also prepared a set for a prototype. The prototype (the plan give imperial dimensions) and final version (which does not) are similar. There are a some differences, based on flight experience, the main variance being the omission of a second back elevator, streamlined rudder and more complex stressed back frame assembly, in favor of a simplified arrangement. The wheel arrangement in the final version also different. Some photographs were taken of the plane/s at different stages.

Duigan Pusher: Page 2/2

A couple of full-size reconstructions have been attempted, and some historical model reconstructions have been prepared for display in Victorian museums.

Nevertheless, there are still difficulties with any rebuilding. Firstly, there is some doubt about the actual wing length and the final length of the airplane, and some questions about the actual dimensions. Secondly, photographs of the flight show some features not included in the plans, and the plans themselves include some elements not found in the photo. Finally, some of the more sophisticated detail, such as the multi-component spar construction, will be obscured by wing covering.

Faced with these challenges, any reconstruction must make a couple of best guesses at different points. I have also chosen to build for flight, and this has prompted a couple of other slight departures.

The next question is one about size of the model. A matter for individual choice, i plan to eventually hang this on a ceiling, so am comfortable with a relatively large model. In this case, i have chosen a 1:12 scale, allowing the direct translation of 1' to 1" (and a final model size of 24 by 30 inches).

1. Plans

The first plan shows the layout of the wing and other covered elements on a 1" grid.

The length of the main wings is the subject of some doubt. Having tried a couple of alternatives,  and compared to the photographs, i have elected to use the prototype length of 24'. Note that the width is only partly represented on the plan (the midpoint of the wings is marked in red). I have staggered the rib arrangement at the extremities.

The wing 'extantions' are not given measurements in the original plans, and i have estimated them to be 3'6" in length with the upper pair being 1'6" across and the lower 1'. Because these could not materially add to lift, and forward speed was too low for a great contribution to glide, Duigan may have reworked these between tests, and they may have been shorter than might appear from other reconstructions.

The position of ribs in the rudder is guesswork, based on the need to keep the element rigid to resist collapse.

2. Wings and elevators

The curve of the spars lends themselves to being cut sequentially on the same piece of wood. Unfortunately, there are three slightly different spar forms, which is curious, given the added lift had the main wing spar length of 3'6" been made standard throughout and the advantages of a single size. Instead 26 of the 3'6" spars, 7 of the 3' spars and 7 of the 2'6" spars will need to be cut. As these will not be visible, I have elected to cut them from sheet ply to add to structural integrity.
In other situations, these might be manufactured using a 3D printer to replicate the multiple wood and curves of the original (a similar approach was taken in the museum model of the Flyer by Hasegawa, where the spars were constructed in polystyrene).

Two same-size wing frames, the front elevator, and the back elevator are constructed. I have used a slow think superglue gel and a kicker to weld the wood into position. I tried a number of approaches - settling for a strongly reinforced wing structure (above).

Each frame is then covered by cloth or tissue and coated with dope to seal and tighten the covering. I practiced with a couple of different covers before opting for tissue. A set of test doped linen wings (below) gave a nice but inconsistent coverage (i may eventually use the linen elements for a backlight model where i want the final look to be translucent).

The sealant stage cannot be skipped or foreshortened. Four coats of dope for tissue and wood sealant for cloth are applied allowing each coat to dry for about 12 hours. Between the first two coats, fine misting water is applied to add to the tightening process. Each coat results in a tighter covering and greater translucency.

Side frames are then constructed.

The main elements are then assembled (below is the tissue variant, which is being made to fly, at wind tunnel point). After testing in a makeshift wind tunnel, i recast the wings slightly wider than the plans would indicate, to reduce turbidity and increase lift. Those involved in flying a full scale replica noted that the design has problems with excessive air resistance. I also opted to use > scale spars and structural elements to assist rigidity in flight.

I have given the canvass surfaces two coats of a matt acrylic coat (Buff on top, Deck Wood below) and all wood a dark wood stain. This high contrast treatment unifies the structure and the slight difference in canvass colour slightly compensates for underwing shadows. Doped linen can have a significant yellow cast but I have opted not to try to replicate that.

Spars, undercarriage configuration and engine block placement/balancing varied from flight to flight. I tried a couple of different configurations and wheels before settling for a mixed metal/wood combination.

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