Plan enlargement problems.

[Tony Collins 1073]

At the moment I am building a 120 inch M5 Sparrowhawk and am working from a Phillip Kent drawing which was scaled up by 1.4.
The fuselage sides have been made and I placed them inverted on the plan ready for the cross pieces to go in. I then discovered that one fuselage side when laid on the plan view was shorter than the other. I have to add that the longerons were partly preshaped to take the curve of the fuselage and were exactly the same length. These sides were held in place by jig supports for the final fit. After much head scratching I took a cotton line secured at the front centre line plan view and then on to the rear of the centre line. At mid fuselage there was a deviatation of 10 millimetres
which would have resulted in a banana shaped fuselage .
I applied the same check to the original size drawing and it was accurate. I then went to a large format printer who digitally scanned this drawing and printed it out to the correct size. Thinking I was now good to go I laid the drawing out but decided to check the C/L just to be sure. Would you believe this was also curved almost as much. I decided to to try to change the shape of the drawing with the wifes iron and after an hour managed to get it straight.
Please can anyone offer me an explanation as I am completely at a loss.

[Andy Boylett]

Hi,
I have found that drawings which have been 'scaled' up can be bannana shaped because of the scanning equipment or process used to enlarge them. A drawing is fed through a scanner and if the rollers that the drawing is fed over are pulling the paper at just a tiny amount slower or faster at one end then the result is a bent scan/drawing. This can in part be due to the actual drawing being creased or the paper being 'slippy'. I found this because I used a local printer to scan in some very large drawings for me. However, before they were printed at a larger scale I asked the printed to send them to me electronically so I could make some changes. I uplaoded them into AutoCad and found the bend. The printer also advised me that the same can happen a little with the actual prints, although rare.

When scanning drawings I would advise checking the electronic files in a CAD package before printing.
regards, Andy

[Tony Collins 1073]

Thanks Andy, that seems to answer my query. However if the accuracy of printed drawings cannot be taken for granted there must be a serious flaw in the design/method used for so called modern equipment.
I can see the way to get over this is to put the digital files in CAD for correction, but not all of us have the neccessary skill to do this. I do have the CAD programme but have never really been able to get my head around it and at my rather advanced years I don't see this changing.
Still, the wifes iron has done the trick for now so at least I can jig the fus up knowing that it is not going to end up like a banana.
Thanks for getting back Andy.
Cheers Tony.