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Old 08-30-2005, 03:36 PM   #1
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Help on designing

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I recently downloaded a 3 view drawing of an airplane that I would like to build. I have put this drawing into modelcad. I was wondering if this is a way to design and build an airplane. Next, how do you determine where and what size of formers/ stringers are neccessary. For those of you who are familliar with modelcad how do I adjust this 3 view for the scale that I want.
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Old 08-31-2005, 05:53 AM   #2
Ed Smith

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Most CAD programs have a "Scale" command. This enables the user to scale drawings up or down. I am assuming you can now edit the three view in Modelcad. So, decide what size you want the finished drawing, wingspan for example, multiply the three view by whatever factor will achieve the desired finished size.

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Old 08-31-2005, 06:52 AM   #3
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I'm doing this now but, with a different CAD.

If your CAD program supports layers, it's easier if you place the drawing on it's own layer. Draw some lines to length that you desire and resize/drag the drawing until it lines up correctly.

Be sure the original is correct scale, the scan is not deformed by the scanner, and that you don't stretch when resizing.

Placing formers and stringers is a combination of art and science. You not only need the strength but, you need to consider the flow of materials, including covering. sharp corners and compound curves require special attention. Mounting locations (like wing, tailfeathers, firewall and landing gear) too, as these will need extra strenght. I start with formers at curve "break" points like at the leading and trailing edge of the wing and the firewall and work out from there to ensure I can support the general shape, then I begin supporting the structure thinking of torsional and sompression & expansion forces. Some math but a lot of TLAR (that looks about right) as well.

There are several basic designs, and it helps if you've built kits of these designs. When it comes to the fuselage, If you're familiar with keel, double keel (my favourite), box construction etc. you're well on your way.

It often helps to build a smaller version of your intended scale, just to be sure what works on paper will work in real life. It can save you a lot of money, time and frustration. If you're really unsure, mock up your design with some scrap cardboard or cheap thin plywood. At worse these can serve as templates for your final design.

Hope this helps.
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Si fractum non sit, noli id reficere - (If it ain't broke, don't fix it).
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Old 08-31-2005, 07:38 PM   #4
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ModelCAD has a "SCALE" command. (at least in ModelCAD 97)

Select the object to be "scaled" and then using the shortcut key "S" enter the multiplication factor into the x and y boxes and then press "Enter".

Make sure that both the x and y boxes are the same numbers or the drawing will be stretched. This will also resize all componenents of the drawing thus changing spar sizes, rib cutouts etc.

Good luck

Fred M.
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Old 08-31-2005, 08:52 PM   #5
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Thanks for the help

Thanks, That should get me started at least, maybe a smaller scale is the better way to go at first, The airplane I'm thinking of building is the 767-300 (nice airplane) the wingspan would be 11ft powered with ducted fans(hopefully) as turbines are too expensive but would be nice. I would imagine that it will take alot of tinkering to gey it just right.

thanks for the help, will probably have more questions along the way.
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