FreeCAD News

Hang glid­ers are mes­meris­ing when you see them in the sky. Silent­ly cut­ting through the sky smooth­ly, often spot­ted in fine weath­er and in beau­ti­ful loca­tions there’s a lot to be said for them. Over on fos­stodon we’ve seen a UK based hang glide com­pa­ny Avian Hang Glid­ers men­tion­ing how they use FreeCAD to devel­op their newest gen­er­a­tion of glid­ers using clever sim­u­la­tion and advanced man­u­fac­tur­ing tech­niques. We caught up with the own­er of Avian, Tim Swait to chat about their process.

Tim explains that he took over the com­pa­ny some years ago and they still sell a range of glid­ers from before his tenure, and have added to the ros­ter of glid­ers with a new­er design “RioT”. The old­er designs had some com­po­nents designed in CAD but this was lim­it­ed to indi­vid­ual com­po­nents, usu­al­ly those that need­ed CAD for Com­put­er Aid­ed Machin­ing (CAM) and the pro­pri­etary CAD used for this was lim­it­ed in that it didn’t have any type of analy­sis tools and no real approach that could be used to help design the wing sail pan­els crit­i­cal to the per­for­mance of a glider.

It’s fair to say that hang glid­ing is a fair­ly niche busi­ness and as such a com­plete suite of pro­pri­etary CAD tools wouldn’t be cost effec­tive for Avian and, even if it was, wouldn’t offer the lev­el of flex­i­bil­i­ty and func­tion­al­i­ty that Avian have devel­oped using open source tools. Part of the inspi­ra­tion came from see­ing @looo’s Glid­er work­bench which, to the lay per­son with regard to human sus­pend­ed glid­ing might look like a per­fect solu­tion, but is in fact built for paraglid­ing rather than hang glid­ing. How­ev­er there are parts of that code that are of great use and inter­est to Avian, par­tic­u­lar­ly the func­tion­al­i­ty where that a fab­ric wing design, full of com­plex curves and seams, can be flat­tened to cre­ate a dxf export.

Work­ing with Julian Todd who has con­tributed to FreeCAD in many ways, work began on cre­at­ing tools to help design the next gen­er­a­tion of hang glid­ers. This hasn’t tak­en the form of a ful­ly fledged work­bench but rather is a set of macros and tools that can speed up the design iter­a­tion process. Tim describes a fas­ci­nat­ing work­flow that bounces between FreeCAD and anoth­er open source tool from NASA, Open­VSP. Open­VSP stands for Open Vehi­cle Sketch Pad and is a tool to essen­tial­ly per­form aero­dy­nam­ic and flight analy­sis on any air­craft geom­e­try. Open­VSP is para­met­ric and so much of Avian’s design work cen­tres around a file Tim refers to as a dictionary.

A dic­tio­nary file is a para­met­ric list of a desired wing sails prop­er­ties and clev­er­ly this file can be processed by the FreeCAD macros they’ve devel­oped to cre­ate com­pat­i­ble geome­tries in both FreeCAD and Open­VSP. This means that it becomes pret­ty triv­ial to cre­ate a design idea and test it’s flight char­ac­ter­is­tics in Open­VSP, tweaks can be made in Open­VSP and this can be pulled back into the dic­tio­nary file to be then pushed into FreeCAD to update the geom­e­try there and also to use tools like the excel­lent Cfd-OF (Open­FOAM) work­bench. Open­VSP uses a vor­tex lat­tice method (VLM) of analy­sis, which in sim­ple terms means that it analy­ses air­flow just on the sur­face of the geom­e­try. Com­pared to oth­er forms of analy­sis that may analyse a large 3D vol­ume, this VLM tech­nique, with it’s more focused sur­face approach, means that analy­sis can be quick, often reduced to a few sec­onds. There­fore it’s pret­ty easy for Avian to run thou­sands of sim­u­la­tions in a pret­ty small amount of time.

When a wing design is ready for real world test­ing Tim describes how it’s rea­son­ably triv­ial to cre­ate a flat­tened pat­tern in FreeCAD which can be sent to indus­tri­al sail cut­ters who use huge laser cut­ters to cut the com­pos­ite fab­ric pan­els to incred­i­bly high tol­er­ances. Avian then have a pre­ferred sail mak­er part­ner who does a fan­tas­tic job sewing the pan­els togeth­er. Tim describes that over the entire length of a wing a seam drift­ing just a cou­ple of mil­lime­tres can make a wing turn in an unde­sir­able way and whilst this can be tuned/trimmed out they pride them­selves on wings that are so accu­rate­ly made they fly straight on assembly.

They say the “proof of the pud­ding is in the eat­ing” and Tim proud­ly speaks about how the Avian Puma wing has helped him win the Sport Pilot class in the 2025 Sport Class Pre-World com­pe­ti­tion, a prac­tice event for the 2025 world hang glid­ing com­pe­ti­tion. It’s great to think that FreeCAD and open source tools have helped cre­ate a world class hang glid­er system!

Final­ly Tim has an excel­lent Youtube chan­nel for if you are inter­est­ed in learn­ing some of the sci­ence of flight relat­ing to hang glid­ers, the chan­nel also con­tains lots of inter­est­ing infor­ma­tion about FreeCAD and Open­VSP and def­i­nite­ly is a fas­ci­nat­ing watch. Tim also cur­rent­ly is using Patre­on to fund raise for the devel­op­ment of a new wing and you can find his Patre­on page here.

Many thanks to Tim for tak­ing time out to chat to us.