How To/Model with CAD

WHAT YOU NEED TO KNOW TO DRAW DIG­I­TAL PLANS

Model Airplane News - - CONTENTS - By Gerry Yar­rish

What you need to know to draw dig­i­tal plans

Com­puter-aided de­sign (CAD) con­tin­ues to in­crease in pop­u­lar­ity, and over the years we have re­ceived plenty of reader mail ask­ing about what’s in­volved in its use for de­sign­ing RC mod­els. Die-cut kits are all but ex­tinct now, and many RC air­plane kit man­u­fac­tur­ers are now us­ing CAD to pro­duce con­struc­tion plans and kit parts. Laser cut­ters, CNC routers and milling ma­chines, and even high-pres­sure wa­ter cut­ters (for metal parts) start with CAD pro­grams to pro­duce their tool path files. Here are some of the basics for de­vel­op­ing a set of model plans us­ing a CAD program.

WHERE TO START

First of all, you can’t just buy a CAD program and ex­pect to start draw­ing plans overnight. Check out the var­i­ous CAD pro­grams avail­able on­line, and pick one that matches your bud­get and com­puter re­quire­ments. There are many pro­grams to choose from, and there are even some free down­load­able CAD pro­grams you can play with to get the hang of dig­i­tal draw­ing. Once you’ve in­stalled it on your PC, ex­plore the fea­tures it has and get fa­mil­iar with it to the point where you can draw ba­sic shapes and line types. From there, you’ll learn how to de­velop var­i­ous geo­met­ric shapes. Most pro­grams to­day come with de­tailed man­u­als and help­ful tu­to­ri­als for the ba­sic func­tions of the program. If you re­ally want to get into CAD, you can also go to a lo­cal tech­ni­cal high school or com­mu­nity col­lege and en­roll in a ba­sic CAD class or two to speed your learn­ing process. An­other great place to check out the use of CAD pro­grams is on YouTube, where com­pa­nies like Ash­lar-Vel­lum and Au­toCAD have up­loaded tu­to­rial videos to help you learn their var­i­ous pro­grams and func­tions.

After you’ve mas­tered the basics, you can then start de­vel­op­ing your own model air­plane plans. For this, there are two direc­tions you can take:

You can de­sign your own mod­els and de­velop unique sport flier planes, or you can de­velop and en­large scale 3-view draw­ings and re­verse-engi­neer them into work­able model plans.

BE A COPYCAT

Be­fore you start us­ing the CAD program, learn about ba­sic air­plane con­struc­tion by first study­ing other peo­ple’s plans. See how they laid their plans out and how they solved ba­sic en­gi­neer­ing and struc­tural is­sues. This will help you un­der­stand the proper wood sizes to use for spe­cific pur­poses. Also, no­tice the place­ment and spac­ing used be­tween fuse­lage for­m­ers and wing ribs and other such struc­tures. Re­fer to draft­ing and draw­ing books so that you can de­velop a good sense of how things should look when drawn in top, side, and front views. A work­ing knowl­edge of draft­ing is a ba­sic re­quire­ment for us­ing CAD, so don’t put your cart be­fore the horse. Ba­sic Draft­ing: A Man­ual for Be­gin­ning Drafters by Le­land Scott (avail­able from Ama­zon) is an ex­cel­lent ref­er­ence.

Next, find a good-qual­ity, de­tailed 3-view draw­ing and scan it into a file that can be im­ported into your CAD program. Try to find draw­ings that also have a few cross­sec­tions and air­foils shown. With PC-based pro­grams, this would be a bit­map (.bmp) for­mat; for Mac users, a .pict file will do. You could use other for­mats, but these are the most popular. With the new Graphite plat­form, you can im­port sev­eral file for­mats, in­clud­ing text files (.txt), EPS, Metafiles, PNG, Claris CAD, .JPG, DWG (Au­toCad), DXF, and IGES files. After your im­age is im­ported into your draw­ing file, you can be­gin trac­ing it with the var­i­ous draw­ing tools your program makes avail­able to you. And here is where many wannabe CAD users run into dif­fi­culty.

When I first started draw­ing plans, I be­gan with sim­ple air­plane de­signs. Old WW I bi­planes and home-builts have fewer curved lines and are rel­a­tively less com­pli­cated to draw than more mod­ern air­craft. Start with the sim­ple air­planes like the Cur­tiss Jenny, Fokker Ein­decker, or Pi­eten­pol Air Camper, then work up to curvier de­signs like Spit­fires, Mus­tangs, and Warhawks. That be­ing said, here are some ba­sic rules to get the most out of your CAD program:

USE LAY­ERS. Im­port your im­age and as­sign it to a spe­cific layer. With my Graphite program, I as­sign the im­age to Layer 1 and I re­name it “3-view.” Place all the other draw­ing and de­tails you add on their own sep­a­rate lay­ers. Think of lay­ers as sheets of trac­ing paper laid on top of your im­ported 3-view. I typ­i­cally trace the 3-view and then place struc­ture (for­m­ers, ribs, and out­lines

of other parts) on a sep­a­rate layer named “Plan.” De­tails such as the en­gine, ser­vos, and other ra­dio gear go on the “Hard­ware” layer, and so on. I even add my di­men­sion lines, ar­rows, and in­for­ma­tion on their own lay­ers. In this way, you can look at spe­cific items by turn­ing lay­ers on or off, or you can look at ev­ery­thing all at once. This helps keeps things straight in your head.

USE REF­ER­ENCES. Start all plans by draw­ing a da­tum line, cen­ter­line, or other ref­er­ence line on the fuse­lage side and top views. From this, you can en­sure that things like ribs and for­m­ers are drawn square with (or par­al­lel to) one an­other. Ver­ti­cal and hor­i­zon­tal ref­er­ence lines also are im­por­tant when you de­velop fuse­lage cross-sec­tions and former shapes.

THINK SYMMETRICALLY. When it comes to things like wings and fuse­lages (in top view), draw only one half and then copy and paste a mir­ror im­age of it to com­plete the draw­ing. Do all your work on one side of the cen­ter­line, then du­pli­cate it and flip it over to pro­duce the other half. This en­sures ex­act sym­me­try and cuts your draw­ing ef­forts in half!

SAVE DE­TAILS. One clear ben­e­fit of CAD is that you won’t ever need to draw any­thing twice. Once you draw some­thing like an en­gine, elec­tric mo­tor, servo, re­ceiver, or con­trol horn, save it to a mas­ter “Hard­ware” file. After you have saved them, you can then copy and paste them into new plan draw­ings. You can also en­large, shrink, or mod­ify them to make new mas­ter de­tails. (Draw­ing them in top, front, and side views also is a good way to hone your over­all draw­ing skills.) USE THE TOOLS. The Tool pal­ettes in­cluded with all CAD pro­grams are a col­lec­tion of sev­eral use­ful draw­ing tools and func­tions. It is al­ways eas­ier to use these tools than try­ing to draw free­hand over your im­ported 3-view draw­ing. Geo­met­ric draw­ing tools for cir­cles, squares, el­lipses, and arcs are all easy to use, and us­ing them will make your draw­ings look cleaner and more pre­cise. Wingtips, en­gine cowls, and other parts are eas­ily re­pro­duced by com­bin­ing seg­ments of el­lipses, curves, and straight lines.

WORK IN­WARD. After you’ve drawn your ref­er­ence and cen­ter­lines, draw the out­line of a wing half top view, fuse­lage side view, tail sur­face top views, and half of the fuse­lage top view. From here, you then es­tab­lish the locations of the main for­m­ers, dou­blers, land­ing-gear mounts, wing spars, ribs, and so on.

AIR­FOILS. In­ves­ti­gate the many sources of down­load­able air­foil plots, or con­sider get­ting an air­foil generator program. These will save you hours of te­dious air­foil lay­out and loft­ing work. Above all else, re­mem­ber this is a hobby, so us­ing CAD should be— above all else—fun!

DRAW FULL SCALE. Once you have your 3-view traced and cleaned up, en­large the draw­ing to the size air­plane you want to build be­fore you add any in­ter­nal struc­tures or de­tails. This way, when you draw an 1/8-inch-thick

former or 3/32-inch ribs with 1/2 x 1/4-inch spar notches, they will be the cor­rect size. If you were to add all the de­tails and then size the draw­ing for a spe­cific wing­span, your de­tails may not end up as com­mon sizes.

BOT­TOM LINE

Learn­ing to use CAD for ba­sic model­ing is not all that dif­fi­cult, and many ex­pe­ri­enced mod­el­ers have added this skill to their model­ing ré­sumés. For many, draw­ing plans with CAD can turn into an­other ma­jor part of the hobby. Be­fore you know it, you’ll have a whole col­lec­tion of CAD 3-views and draw­ings to show your friends. The hard part will be pick­ing which ones you want to build and fly!

This is my new­est CAD model: the 1/4-scale Sop­with Camel, which was pub­lished in the De­cem­ber 2015 is­sue of MAN.

These 3-view draw­ings were the ba­sis of my plans. They are Wy­lam draw­ings that I cleaned up to elim­i­nate un­wanted de­tails.

TYP­I­CAL RC HARD­WARE I think the best part of draw­ing with CAD is that it elim­i­nates the need to draw any­thing twice. Once you’ve saved your files, you never have to draw those items again! Here are just a few RC hard­ware de­tails I’ve drawn that I use over and over.

For some air­planes, like this Pi­eten­pol Air Camper, you may not be able to find a good set of 3-views. This plane was based on one from years ago at the Old Rhinebeck Aero­drome. To pro­duce my own 3-views, I mea­sured the ac­tual air­plane and took a lot of photos.

This is an early ver­sion of sheet 1 of my Sop­with Camel plans (X1215A). As you can see, it in­cludes sev­eral hard­ware de­tails (in blue) that I had drawn ear­lier and sim­ply copied and pasted into the draw­ing.

This scale 3-view of the Pi­eten­pol Air Camper was the very first CAD project I drew, in 1998. I en­larged these draw­ings to 1/3 scale (116-inch span) and then de­vel­oped the plans I used to build the model.

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