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Jul 13, 2020, 04:29 PM
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Build Log

PURITO! New F3RES Sailplane

I was recently made aware of the Purito, a new F3RES airplane from some of the guys originally responsible for the PuRES and Slite - both of super-fond memory to me. The same team is working on the STS, which I saw at the German F3RES Open in September, but that airplane is still in development. This airplane is sort of like an interim airplane, and builds on the PuRES and Slite. John Kennedy (one of my American teammates at the German Open) and I are apparently the first two to get the Purito, here in the US.

At first look, the Purito appears similar to many previous F3RES designs, but there has been a lot of original thought applied. For one thing, there is no conventional drawing at all, of the total airplane. There are plans for the tail surfaces (which are quite clearly from the Slite conventional tail), but no overall picture of the finished airplane. Instead, there are jigs and an instruction booklet. Of course, the instructions are in German, but not to worry! There are lots of excellent photos. More than that, the photos include a lot of pairs of photos: one showing the parts required, and the next showing what they look like, assembled. QED.

Of particular interest is the wing jig. Instead of the PuRES jig of balsa sheets, or the Depron sheets in the Slite, there is a single sheet of light ply. It appears confusing, but there is a simple scheme for marking which slots take which ribs, and once taped down, you can build the center section, left intermediate panel and left outer panel in turn. Turn the sheet over, and build the right intermediate and outer panels.

The basic build will be:

1. Fuselage and hatch.
2. Boom and tail surfaces, mated to the fuselage.
3. Wing panels in sequence.

Well, now that New Mexico has re-instituted lock-down, I had to cancel out of going to the Nats. I was wondering what I was going to do, but the answer has arrived! I'm going to build another airplane!

By the way, here is full disclosure: I actually have two (2) of these kits. The way I do this, is I build a portion of one, then I build the second example of the same portion, but correctly! Those pictures you see thus have a much better chance of being correct! Not always, but pretty often.
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Jul 13, 2020, 04:52 PM
Afflicted with DDD
Soarhead00's Avatar
Jul 13, 2020, 11:10 PM
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Jul 14, 2020, 06:30 PM
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Further Information

I realized I neglected to include some really important information - like, how to find one of these airplanes.

This is not Setatech, that did the PuRES and Slite. Instead, the Schweigers (Stefan and his father Josef) have a F-book site. Look up Holzmodellbau Schweiger on F-book, and there is a lot of information: pictures, video clips, all kinds of stuff. Sorry to put it that way, but I'm not allowed to fully spell out F-book.

I met Josef Schweiger when he and his wife Petra competed at our first F3RES contest several years ago. They stayed at our house with the Josef Gergetz and his family, and we had a really good time with them all. I don't know what it is about aviation, but all those silly boundary issues seem to go away. Josef Gergetz and Josef Schweiger compete as a team often, and Petra is a strong competitor all by herself - as I found out in Germany last year. I only met Stefan last year in Germany, and he is certainly a comer.

The Purito is CNC-routed. I don't know what other feelings are out there, but my experience is this: the designer pays his money, and takes his choice. Whether laser-cut or CNC-routed, I figure that I (as a consumer of airplanes) will be the sure winner with a kit that is far, far away better than the old die-cut stuff of years past. In fact, I have experience with a single design that has been available both as a CNC-routed kit, and a Laser-cut kit: the Slite. The Slite v2 is a product of Gruner, who is a large-scale manufacturer, and even though most of the kits from Gruner I have seen are laser-cut, he does have a CNC-router and uses it with the Slite v2 for certain parts, notably those with 3D shaping.

Blah, blah, blah. Somebody stop me - I can't quit typing.
Jul 14, 2020, 07:12 PM
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Fuselage Pod

Today, I assembled the fuselage pod.

I can sure detect the PuRES and Slite foundation, here. The basic elements are plywood sheer webs coupled with carefully shaped longerons and formers in strategic locations. What is different this time around are the choice of materials, and the neat way the nose block is tied into the rest of the fuselage. The fuselage has little wetted area, but is straight, stiff and very strong. The upper longerons are plywood, to resist compression in a dork landing. The lower longerons will only need to be strong in tension, so are light balsa. There is no taper until the aft-most fuselage former, so it is easy to get straight.

I started with gluing a few key parts into subassemblies.

I used 99% wood glue, for all wood-to-wood joints. If you want to use super glue, go right ahead. I have an alergy to CA, so I avoid it as much as possible. I used a tiny dab of 30-minute epoxy to glue the nots into the wing hold-down mounts, but otherwise, nothing but glue. I prefer Titebond original aliphatic resin, but anything similar will work. Avoid epoxy, which is heavy.

I first gathered all the necessary parts. I usually use a box cutter to remove plywood parts from sheets, no matter the thickness. I use either a razor blade or scalpel to remove balsa parts. I used a sort of nailfile my wife found to quickly sand off all the little connector nubs. Do sand those off. Parts fit better without bumps, and those bumps that don't interfere are still excess weight.

The plywood doublers or sheer webs are very thin, and curl up like ribbons. Not to worry - they will lie flat with appropriate weight. MAKE DARN SURE TO MAKE A LEFT SIDE AND A RIGHT SIDE!! I thinned some Titebond with water, and painted it onto the plywood, then carefully lined up each doubler and applied weight. I got it right this time, but with one more kit to go, there is still chance to screw it up.

I glued the forward former to the servo tray, and clamped that to a 90-degree block. There is both a pre-cut tray, and a blank tray. I am going to use Dymond D47 servos, and the pre-cut tray is fine for me.

Finally, that wee, little bit of epoxy and the nuts were glued into the hold-downs. I went off the have lunch.

After lunch, everything looked pretty good.

Look carefully at the bottom longerons. They are soft, but have two, little marks that should be on the bottom of the fuselage, and they will locate the tow hook mount. So, the little marks should be near the wing saddle. I spread on glue, and laid in the longerons.

The top longerons are plywood, and also have a front end and a back end. They should fit into the plywood doubler exactly. More wood glue, here.

The wing saddle pieces are applied directly to the plywood doubler. Be sure to get good contact, and use the formers to assure proper location.

Now, I checked the tail boom against the fuselage formers. Believe it or not, the boom can be inserted into the forward ofthe the two formers from behind. The aft former is a VERY tight fit, and I could not get to its proper position on the boom at all, so I will have to ream it out a bit. But, the effort was not wasted. I will simply leave the aft former off until later.

Having said that. I glued all the formers into their respective slots in one side. They all fit very, very closely and were easy to square up. I glued the nose block pieces (three of them) together, and glued that to the same side. The other side fit easily down on them all, including the wing hold downs. Don't forget the hold downs - but, do NOT glue them into place. They slide a bit, to allow proper wing positioning. So, the second side came down on the assembly, and all looks good.

I examined the fuselage at this point, and made sure all the longerons were flush with the edges of the sides. It didn't take much - just a bit of sanding with a block. I used a square, and everything was straight and true. The next piece in was the tow hook mount, located between the little index marks on the bottom longerons. A perfect fit, I might add.

I added the bottom sheeting, starting with a piece immediately aft of the tail hook mount. I cut it short, just short of the aft former. The fuselage aft of that former must taper to meet the final former, so that is why it is not yet covered. The, I covered forward, all the way to the nose block. I checked again with the square, and was very happy. It also lies down flat on either side, on the table top. Cool!

I sanded little bits and bumps off the sides, and will stop now. Tomorrow, I will prepare and add the tail boom. Maybe, make the tail surfaces, too, although everything can be squared up without them.
Jul 14, 2020, 08:38 PM
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Larry Jolly's Avatar
Great to see you have survived COVID and are going strong. Did you do a comparison between this wing profile, and the one used on the GS. Hard for me to compare with the pictures so far.
Keep up the good work..
Jul 15, 2020, 03:06 PM
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I hope you are doing OK, too. Vicki and I are fine, but my son, grandson and daughter-in-law all had the virus, back in January. My son's in-laws came down to Hannover to help, and also came down with it (no surprise - it wasn't even diagnosed at the time). Meanwhile, my daughter and her husband in Seattle came down with it, too. And just a while ago, she let us know that her own in-laws also have it now. It's hitting all around me, and here I stand.

Corky uses AG35-AG38 airfoils - I got that from his web site. I have an email into Stefan, asking what the Purito uses. He is in Austria, so now we wait.

I am embarrassed to admit that for an aeronautical engineer (retired as well as dated), I care little for airfoils. To me, the classifications are "thin" and "thick". I will compare the ribs on the Purito to my Yellow Jacket and get back to you.

Yours, Greg


Well, I got the real word back from Stefan: AG35-AG38, depending on the wing station. He sez:

Part A (center section): AG35
Part B (intermediate sections): AG35, transitioning to AG37
Part C (outer sections): AG37, transitioning to AG38.
Total area is 38.6dm I think that's decameters, or tenths of a square meter. Some one correct me, please.
Last edited by glidermang; Jul 15, 2020 at 05:02 PM.
Jul 15, 2020, 05:38 PM
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Hatch (Not, as promised, the Tail Boom)

I happened to look at the instructions, and the boom is NOT repeat NOT next. The hatch is. And it is such a cute construction I decided to do what I should do anyway, and so here is the hatch.

I assembled the pieces. Again, look at the pictures in the instructions, and check the part maps for all the pieces. This hatch is almost exactly what you might get in a Slite kit, so I find it easy. It certainly works in the field: close fitting, easy to secure on the airplane, reliable, light.

The plywood ribs go on first. I just applied Titebond, and held them on for a few seconds.. The latch holder fits exactly between them. By the way, the ribs are ambidextrous: either rib on left or right, and there is no front or back. The holder does indeed fit exactly between them, so no problem centering, either. And finally, the ribs fir exactly in the opening, so the hatch will always align properly. How good is that?

I checked the fit of the fore deck, hatch and aft deck, and no trimming was required for the fit I wanted. I just glued the foredeck down on the longerons and flush with the nose block.

I glued the latch piece under the aft deck, making sure it centered. Again, the aft deck fits exactly centered on the longerons, but do NOT glue it on yet. It is time to construct a latch.

Bundled with the tubing are two pairs of what must be the German Standard push rods: white plastic tubes containing steel wire. Pick one. I found that the two slots in the latch pieces are each 13mm long, so I cut two pieces of the white tubing, to that length. They press easily into the wide slots. No glue yet!

Now, the wire: I bent about 5mm up, then all the way over to 180 degrees. That, then , I further bent to 90 degrees from the rest of the wire. This is the "handle" for our latch. Measuring the tube and slot in the hatch, it seems that 20mm of wire will extend to engage the aft deck, but still retract to completely withdraw into the white tube. So, cut there. I put the wire into the white tube, and pressed the tube into the slot in the hatch. The handle sticks up out of the top of the hatch. I applied a wee drop of medium CA to the tube, being careful to not get any near the ends, so the wire latch slides freely back and forth.

Now, experience rears its ugly head. I cut a small piece of this ply, and glued that over the wire and tube, for security. I have found that pressing heavily on the latch will force the tube out of the balsa.

I out the other piece of tube in the aft deck piece, and applied another small drop of CA. And, followed up with another small piece of plywood.

Now we can glue on the aft deck. A quick check and the hatch goes in place, the wire slides back and forth, and we are done with this until time to shape the fuselage.
Jul 15, 2020, 08:00 PM
Scale Aircraft = Scale Crators
mikejr83's Avatar
Is there an ARF or ARC?
Jul 15, 2020, 08:42 PM
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gliderguide's Avatar
Unlikely on an ARF. Should be a fairly logical build though. Having built multiples of both the pures and Slite, plus various other 2m RES birds, they are very logical and build out well
Jul 16, 2020, 04:21 AM
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Originally Posted by glidermang
Total area is 38.6dm I think that's decameters, or tenths of a square meter. Some one correct me, please.
square decimeters ( a decimeter is a tenth of a meter, a square decimetre thus is is 1/100 th of a square metre...)
a decametre (dam) is 10m ...
Jul 16, 2020, 12:32 PM
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I'm afraid not.

The class is designed to teach beginners all about RC soaring - including building, which is also a way to keep costs down. There are a few ARF/ARC models available, completely legal for F3RES, but only one that I can name right now. Now that I said that, the name has flown out of my head! I will have to get back to you.

And, I can hear the next question coming, so here is the answer: No, I will not build one for you. I do build for other people, but always within special circumstances of need (such as disabilities).

But, you can build, too. I know that for a fact. I have walked neophytes through complete builds, and they have so far left the shop with good, straight, working airplanes that fly well. It takes roughly 14-16 2-hours sessions, start to finish. Each post in this thread represents (amazing!) about two hours of leisurely, simple work.

Yours, Greg
Jul 16, 2020, 12:35 PM
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Thank you!

I sure wish my native land (America) would finally go metric. I was OK in the Army using klicks and meters. There, I said it!

Yours, Greg
Jul 16, 2020, 12:36 PM
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Originally Posted by mikejr83
Is there an ARF or ARC?
Looks like a few hours of building and it will be ARC. Many of these newer kits are very well engineered and are surprisingly easy to build, you should give it a shot.
Jul 16, 2020, 03:52 PM
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Tail Surfaces

Once again, I'm out of sequence with the manual. But, I'm waiting on word from Stefan Schweiger about the tail pylon, and that doesn't affect what comes next. This stage is strictly old-school model-building: cork board, pins and outlines. There is an excellent tutorial on their home page.

I did the vertical tail, the rudder, then the horizontal tail and finally the elevator. The fit of parts is excellent and the quality of wood is also, so you can expect everything to be straight and true when you are done. I recently built another kit, and managed to make the rudder post curved, when looking from the side. I was able to recover from that misadventure gracefully, but this time, I will show you a trick that my Uncle Buzz showed me, once when you guys were not around.

The trick is this: lay down a straight-edge ( a steel ruler in my case) along the rudder post, and back every thing else up to it. Works a champ. By the way, for the Purito, get that straight piece down, and all the other parts key off it, and the fit will be superb.

So: wax paper down on the drawing of the vertical tail, and the straight edge down on the drawing. My sequence was to lay in each rib, and then follow up with the leading edge. Notice I use "map pins" to hold the straight edge, and "grip pins" to hold wood to the straight edge, and other parts. I do not pin through the wood if I can help it. Note that parts S4, S5 and S6 come out of a thick piece of balsa, and are "sideways" compared to the rest of the parts. To get them to lay down flat will require you sand them (lightly) smooth. Also, clean out the vertical holes with a 2mm drill bit NOW, before gluing them in place. I use a twist bit, in my fingers, not in a drill or pin vice.

The sequence for the rudder is slightly different: lay down. the straight edge, sure, but then put down the entire perimeter. Place each diagonal rib, and all will fit perfectly with only slight pushing and pinning. I would have shown several pictures, but my camera battery went flat, and I had to work while the glue pot was out.

The horizontal tail goes just like the rudder: outline first, then all the diagonals. My preference is the center H2 first, and work the diagonals from the center to the tips. I took the ribs out of the carrier as I needed each one, being sure to lightly sand off any nubs (which are small-but-unwanted weight). I found no need at all to trim or adjust the ends, which fit neatly with each other and the perimeter. The horizontal is finished by fetching the light ply "plug" from the small parts bag. Clean the hole with a 2mm drill, and it inserts with a close fit. Some good penetrating glue works for this.

And, the elevator, which is tougher only because the parts get smaller.

I am not going to sand anything to profile right now. First, it is too hot and muggy today. Second,

If you are having any difficulties at all, I recommend you go to the Holzmodellbau Schweiger home site, and view the video tutorial.

When everything was off the board (with Titebond, allowing the parts to stay pinned for at least twenty minutes is fine - then they can be pulled off), I put them with the boom. The tail pylon is a simple stack of pieces on 2mm rods. The vertical tail mount is just two, longer 2mm rods. Without any glue at all, I was able to assemble the tail surfaces, boom and fuselage and check fit.

I noticed that my vertical tail had a very light bend to port. Well, this is easy: I have some 1/32X1/4 basswood, and I flattened the vertical tail on my board, and glued a piece of that basswood along the rudder post. You should not expect to do this with your kit. I used 1/32X1/4 because I live in America, and metric wood is not readily had in my local hobby store (which is Hobby Proz, by the way).

ADDENDUM: I went back once more and looked at the tutorial, to make sure I didn't leave something out. The tutorial shows Josef pinning through the wood: that is perfectly fine. I'm a snob for suggesting otherwise. Also, Josef uses superglue. If you can use superglue or CA, it makes things much faster and is just as strong. I cannot use superglue, except with special precautions. That is my problem, and certainly not yours. If you can use superglue, do so with a free and happy heart!
Last edited by glidermang; Jul 16, 2020 at 06:49 PM.

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