THERMIK 70 by Frank Zaic

[ultralajt]

Hi!
Last weekend I was flying at local RC airfield, where I have oportunity to see my friends newest RC model sailplane. It was an vintage free flight sailplane designed by Frank Zaic in 1946. My friend took 70" wingspan plan and enlarge it to 100" (2,54m).

I manage to took a video of its flight at late evening:

https://www.youtube.com/watch?v=IlkK_MjFajM

Sorry for low quality. It was low light enviroment.

So, back to the theme!

I was always fascinated with some of the very cute vintage designs, and as I already knew Frank Zaic work (Model aeronautic year books) I decided to build one Thermic 70 model for myself last wekend. I use original Thermic 70 plans and start building the model.

I started with making bunch of balsa strips, mostly 3x3mm. They represent most of the fuselage structure.




Photo above: starting with basic structure. Two longerons and short struts between. Kind of "ladder".




Photo above: when first part was finished, I cover joints with plastic and started to glue second part just above the first in order to get two identical parts.




Photo above: I cut 1,5mm thick balsa fuselage formers, and start to assemble the fuselage.




Photo above: After the glue hardens I hang fuselage to its tail in order to stay straight till glue hardens fully. (I didnt use proper helling or assembly fixture, so this was one of the solutions..




Photo above: I add vertical fin basic structure, wing saddle, nose block and intermediate longerons.




Photo above: it was time to admire the nice lines of the fuselage.




Photo above: "fishy".. isnt it?




Photo above: I prepare CNC files for the wing parts and here I just tested (no gluing yet) how wing ribs fir into the slots of the trailing edge and wing spars.




Photo above: regardless that model is not finished yet and some wing parts holds just a masking tape together, it was time do the "photo session" of this beauty!

As my model will recieve 2 servos (elevator and rudder) I didnt follow the Frank Zaic`s plans. He draw double dihedrall wing as in these days of fre flight, model was stable with such wings. If such dihedral were used on RC model it will (over)react to any rudder input. So I took different pathe here and choose Gull wing configuration. It was modern shape back there on manned sailplanes.

So, this is my model as done this week. Now I must finish wing and tail. Then I must install servo, battery and RX tray. I need to build removable canopy for easy access to the RC equippment.

For the covering I will use Oracover transparent.

So much for now!

Regards!
Mitja

[Christian]

Superbe

Christian

[Frédéric]

Salut Mitja !

Superbe construction ! Travail très soigné, bravo !


Amicalement
Frédéric

[Orionis]

Oooh, une mouette!

Nice work!

[Paul_BB]

Nice bones ! Beautiful !

[ultralajt]

Today I sand the horizontal tail and now is ready for covering:













Bytheway, I accidentaly discover that wet elastic trampolin net is very interesting background for photohraphs, but one must be carefuil not to moist the balsa parts.

Mitja

[ultralajt]

I must say thank you for all nice comments of yours!
They are precious fuel for going fastforward with the build. :)

Mitja

[ultralajt]

I continue with model assembly.

I join the wing parts using a "book-alike wooden board". I can set dihedrall by means of threaded rods. Boards are hinged in the middle




Photo above: wing assembly board with adjustable dihedral.




Photo above: centroplane.




Photo above: outer part of the centroplane. I will install last two ribs at joining with outer wing panel.




Photo above: outer wing panel.




Photo above: "shaving" most of the material from the trailing edge strip with a razor plane.




Photo above: sanding the trailing edge. I leave about 1 milimeter of the edge.




Photo above: This is what I do with coarse sandpaper. From here forward I use fine sandpaper.




Photo above: finished trailing edge.




Photo above: joining gull wing dihedral joint.




Photo above: all wing parts joined in one unit.




Photo above: I discard clasical wing attachment with rubber bands and do plastic bolts attachment instead.




Photo above: I made cockpit "floor" (for battery and reciever) and servo tray for elevator and rudder servos.

[ultralajt]

When all parts were finished, I start to cover model.




Photo above: wings. First I cover the bottom side, and then the top surface.




Photo above: in order not to warp dihedral and twist, I heat covering with heat gun while firmly press it to the dihedrall board.




Photo above: for tailoring fuselage covering, I prepare paper templates. I take care that joints of covering overlaps at the fuselage longerons as much I can. Extra care was taken not to warp or bend fuselage while shrinking covering.

And finally the model was covered and prepared for the maiden flight!

































Mitja

[Paul_BB]

Very nice. I like very much your building board. :)
I would be interested to know the all-up weight?

For heaven's sake, please shoot a video.

-Paul

[ultralajt]

Sunny for one brief moment at last....

[AlainB]

Il est mignon tout plein ce planeur
Alain A+

[ultralajt]

[Anonyme]

Lovely Bluebird (despite the modern covering material ! )

[de plancke]

[Anonyme]

Du film d'emballage alimentaire alors...

[ultralajt]

I know that "plastic fantastic" is not much apropriate for such vintage model and that paper would be better in terms of authenticy. But this way, model is weatherproof more durable and even stronger.

[Anonyme]

Hi Mitja,
You are the builder so you choice according to your purpose...No problem
Enjoy your flights with it, a picture soon?

[ultralajt]

Yes, a picture in the air and maybe a short video would be nice. I will try to make some soon if weather permits. As I mentioned somwhere before, it is stormy weather here, and if rain stops, there is a strong wind.. not very apropriate for such fragile floater.

So, stay tuned and I hope to publish some more soon. (photos, videos and flight report hopefully

Mitja

[ultralajt]

Some photos of today short flights.





























Video will follow in about one hour as it is uploading now to youtube..

Regards!

Mitja

[ultralajt]

Video: https://www.youtube.com/watch?v=S-svn7qoO-Q

Mitja

[AlainB]

Qu ' est-ce qu ' il est jolie ce petit planeur . Beauté et grace au sol comme en vol . ; d' ou sort-il ? c 'est un plan perso ?
Bons vols .
Alain A+

[ultralajt]

[AlainB]

Merci Mitja

Alain

[Paul_BB]

Nice video, thanks.

[Anonyme]

Mitja, blue glider and blue sky , that's perfect !


pour Alain: le kit existe aussi chez les américains dont je t'ai parlé ! planeur en plusieurs tailles disponibles , je crois..

[AlainB]

Merci Marc , je vais y passer jeter un oeuil
Mitja : Marc a raison ; magnifique , bleu transparent dans un ciel bleu ....

A+

[Christian]

Christian

[ultralajt]

Short video of Hi-start with tiny bungee:

https://www.youtube.com/watch?v=vESjURovPgI

Mitja

[Paul_BB]

Hi Mitja,

How did you make your bungee?