New build and a rare one at that. PB.3 Praha, Vintage 1932

[GunnyK.]

OK guys My next build will be this. A PB.3 Praha which was a 1932 design in Czechoslovakia. It incorporates my favorite feature, a constant cord wing. I chose an airfoil for it which is a SD7062 (14%). Once again I will build a small proof of concept in 1:12 scale and than go to a 1:6 scale model. I will enlarge the flight surfaces a bit on the small scale model. I did quite a bit of research on this aircraft and found that there can be some interesting and beneficial mods made to it from the original concept and design. The original had a span of 11.5m which means that the 1:6 model will be a petite 75.5" . If I lie this one I will than scale it up to 1/4 scale at 113" wit a 12" cord (aspect ratio 9.3). That's 9-1/2 ft2 of wing area. For me that is quite big.

[gunny11]

Hey Gunny, the 1/4 scale will fly soooo much better than the 1:6 scale. Size wise, You'll get used to the 1/4 scale very quickly. I know... I've been there with my Aviation Concepts 2-33.

[GunnyK.]

Of that I have no doubt but I will not get ahead of myself. The 1:12 concept model will come first. I hope I can knock that out by the end of march and than I will start the 1/6. Most likely I will be building some components in unison. I wish I knew of a place I could fly slope here in the Chicago area. Since I am new to gliders and have no such contacts I am helpless in that matter. Any opinions on my choice of airfoil? I use the U of I airfoil plotter. I am open to suggestions.
http://www.airfoiltools.com/search/list?page=s&no=7 I was also thinking of taking that airfoil and reducing it to 12% from the 14% or my other choice would be the SD7032 and increasing it to 12% or also the S4180 at 12% or 14%

[ARUP]

That is a good looking sailplane! If the foil is shown properly on the plan then just use an enlargement of that. All of my models have scale foils. On one, the scale foil on outer panels was transitioned to fully symmetrical to induce wash-out. The models fly fine as far as I'm concerned. I'm not that great of a pilot and still manage to get some decent flight times. These ain't contest ships!

[GunnyK.]

I doubt very much that the airfoil is represented correctly. It looks good but who knows.

[GunnyK.]

Well gents, I am doing it. I have the plans almost finished. I will have the preliminary design done by Monday and will send the wing ribs to the laser cutter tonight. I will do a "half" size verification model (1/12) and do a test build and fix any deficiencies in the plans before I enlarge it to the 1/6 scale which is still a diminutive 75.5". If there is any interest in the design I will than enlarge it to 1/4 scale 113"

[lenb]

This will be interesting. Looking forward to the build.

Thanks for sharing

[ARUP]

+1 Build your 'test' model then go for the 'gusto'! Build the 113" version second! The wings are in halves and if you make the stab/elevators and rudder removable that would ease transportation woes.

[GunnyK.]

My biggest problem is that I have nowhere to fly large glider models in the area I live in. I wish I did. I am sick and tired driving for hours just to fly a large model. The closest place would be my friends farm near Kankakee and there I would have to mow the field myself every time I go down there. He bought five archers to build a observatory on. The plot is east to west so it's ok since the prevailing winds are from the north west corner. The 1/6 models are the most convenient for me. It's bad enough that I have to drive almost an hour and half to go to the gun range when I want to go.

[ARUP]

Yessir...1/6 is a good size to store, fly and handle in general.

[GunnyK.]

Well, all the parts have been finalized and the layout was sent to the balsa cutter. I should have my parts by mid week and start assembling my model for parts fit verification. I will make all changes as needed and finalize the drawings accordingly. I hope to have parts verified for the double scale model and parts hopefully within a couple of weeks.

[GunnyK.]

Late last week I sent my parts layout to the laser cutter and yesterday I go them back. I bought two sets just in case. here is what they look like. I must say that I am totally satisfied.

[stihac]

I'm looking forward to this build. Keep it coming.Great subject by the way.

[GunnyK.]

Started putting together the fuselage yesterday. I got this far and already found places I can make improvements. This is why I like to do an "alpha" build before I commit to a larger model. Also figured out that the model will build the best up-side-down, with the lower half being built first to hold the superstructure. I will most likely start doing the CAD fixes this weekend.

PS. Dekuji Stihac. Jak se mate?

[stihac]

Ahoj Gunny, mam se dobre. Doufam ze vse je OK s tebou. Bohuzel nemam cas na letadla a tak alespon obcas koukam na forum.

[GunnyK.]

The lower half is ready to remove from the board once dry. I am using Ambrulose cement which is the replacement for Ambroid. It takes a couple of hours to dry all the way.

[GunnyK.]

Started the upper fuselage.

[GunnyK.]

This is how far I am at this point. I definitely discovered some changes that have to be made to the design for the full size model. Mostly to some of the wood sizes and the construction technique. Some of the wood sizes I am using right now will not bend in the way I imagined that it would. Some of the wood sizes I will make thinner and some will be replaced altogether with cut parts to prevent warpage which I am getting some at this point. I am reliving the stress on this build with Ammonia. I also discovered that some of my formers were not symmetrical, imagine that. I will work on those changes this weekend as I continue the build.