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125. Hoop Hitches

05/08/2014

The gap between the petrol tank and the cockpit is covered with 2mm ply, and at each end the ply is fixed to a nearly semi-circular hoop. The picture shows one fitted on the back of the petrol tank, and the other on the front of the instrument panel. Both of them are sloping, but by different amounts, which means that the outside surface is at a strange angle to the front and back faces.

The photo shows the back of the petrol tank at the right, with one of the ash hoops fitted to it. The other one is in front of the instrument panel between the cabane struts. The ply is attached to these two, and to the longerons in between.

The photo shows the back of the petrol tank at the right, with one of the ash hoops fitted to it. The other one is in front of the instrument panel between the cabane struts. The ply is attached to these two, and to the longerons in between.

The drawing requires them to be 15mm thick and 30mm wide, made up from 3 laminations of 5mm thick ash. Theo tried this, and it doesn’t work. Of course, if you cut strips of ash 30mm wide and 5mm thick, they will form an upright hoop, but not a sloping one – and in any case, 5mm is far too thick to bend round the radius required.

He did manage it, eventually, by means of several short pieces of ash all sort of feathered in to each other, but there were some pretty thick glue joints and by the time we’d sanded off the front hoop to give a straight front face, there was less than 30mm width.

The rear one is less critical and we were able to use this, but in order to make the front one acceptably, we used 1.5mm ply cut into arcs (which we defined by rolling a template of the final shape of the hoop over the sheet of ply).

The arcs of ply cut round a curve determined by rolling a template of the final shape of the hoop set at the correct angle. For a 15mm final thickness we needed 9 laminations of 1.6mm ply.

The arcs of ply cut round a curve determined by rolling a template of the final shape of the hoop set at the correct angle. For a 15mm final thickness we needed 9 laminations of 1.6mm ply.

Then we glued them together using parts of the the same template. It was the one used to make the templates for the petrol tank, so we are hopeful the one will fit accurately to the other!

The result was pretty satisfactory, and when sanded down fitted the required dimensions more or less perfectly.

Next stage is to make the ply cover, which is REALLY tricky!

2014-07-29 Ply hoop being laminated

 

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From → Building, Technical

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