Quote:
Originally Posted by brinesharks
However they also say that Spruce is difficult to bend (in one book the author says it's the hardest to bend successfully) and another says that soft woods should not be steam bent rather soaked in water and then held to shape until dry.
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Hi Bryan,
When I was working on the DH9 we steamed the rear longerons so that they could be bent and placed in a jig, to replicate the original curve. The DH9 was originally restored with the original longerons but on the Queen's Birthday weekend in 1987 a drunk Army member - I think he was a cook or a chef - crashed through the shed and shattered the DH9 into three sections, so we had to rebuild the rear fuselage.
He took the corner at 45 degrees, rather than the traditional 90 degrees
and obviously had no control but managed to avoid two narrowly spaced trees, and two very solid upright steel sections used to construct the building we carried the work out in. It was either a large shed or a hanger. The chances of missing those trees and the steel upright members must be negligible, let alone then proceeding to destroy a priceless First World War aeroplane. The Army of course protected him and moved him within a matter of days.
The leaders of the restoration group wanted to make a series of cuts in the back of the timber, so that the longerons could be bent to the correct shape. After a great deal of debate and heated discussion - reminiscent of some of the "discussions" which go on here - I finally convinced the team to give steaming a go. There is too much shoddy restoration work which goes on, and I was very happy to make a key contribution to some quality restoration work. I honestly believe that any restoration should be carried out as authentically as possible. Often the relevant information is available, or at the very least easily obtainable, but restoration teams often don't know any better, think it is too difficult or time consuming, or just don't believe they have the capability to carry the work out correctly.
The Spruce we used was not the best quality, and certainly fell far short of the timber used in the original longerons. The grain was relatively wavy, and the timber in general was much more solid and appeared to be more dense than the original Spruce. I have had some discussions with Ellis Walker, who is very experienced in the selection of Spruce and other timbers used in the construction of timber aeroplanes. In the near future I will conduct an interview with him, and record the results of our discussion. Having talked with him it is obvious to me that there are very many factors which need to be considered when selecting Spruce which is suitable for the construction of aeroplanes. The real experts (and I am not one of them yet) can spend a day to select one flitch, and sometimes they will not select any timber at all, even after a complete day of searching through a pile of timber, and conducting various tests.
The steaming apparatus was very simple. We used 2" galvanised water pipe, which of course we covered the longerons with, put rags in the end to prevent too much steam from escaping, and supplied steam at the other end with my Mum's kettle, which we constantly filled, to replace the water lost as steam. The steaming process took a very long time - mostly due I suspect to the relatively poor quality of the newer Spruce. It's very interesting to hear on this thread that soft woods are more difficult to steam than hard woods, but it makes perfect sense because the grain of hard woods is much more open, as mentioned. We simply nailed timber blocks onto a piece of chipboard, which I had drawn the shape of the longerons on. When I last worked at the AWM as a contractor, this jig was being used as a work bench top. I was very happy to see that they still have it, although a lot of staff there must have a hard time working out what it is. We did not allow for any over bending, which with hindsight would have been a very good idea, but the results were excellent. We were all very surprised at how well the replacement longerons worked.
Regards,
David.