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Old Jul 21, 2014, 10:54 PM
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Build Log
PURES 2-meter Sailplane

For about six months I've been enjoying a 2-meter RES sailplane (called the PURES) which I obtained from a gentleman in Germany, Mr. Josef Gergetz. The airplane is built to a new class for competition. I haven't seen any formal rules. I understand the class features these constraints: airplanes are limited to 2-meter span, controls are limited to RES, and structural materials are limited to wood (except for particular structural members such as spars or tail booms). Here in the States, I have found the airplane ideal for a local competition we call "Hiss and Boink", which is a sort of thermal duration affair. And, it looks way cool and flies really great.

The rules or constraints result in airplanes that emphasize minimum weight, and lots of parts. The Pures I built met the design weight (490 grams) with a lighter battery than the designer specified, and some nose ballast. It was fun to build, featured excellent fit and finish, was very complete as a kit and had excellent build instructions. However, I made some boo-boos and discovered some vulnerabilities to our local flying conditions (here in New Mexico, grass fields are not common; rocks more so). I wanted to try again, and get it really right this time.

The first picture shows my first example, ready to fly.

After I got some experience on the airplane, I had a good time communicating with Josef. Based on feedback from DLG design, he decided to attempt a built-up tail for reduced drag and weight.
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Old Jul 21, 2014, 11:14 PM
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The first step in the Pures build are the tail feathers. The new tail is extremely light and uses extremely light weight balsa. I found the parts to be very difficult to work with - until I figured out how to use some of the supplied jigs.

There are two "combs", used to align and space apart the wing ribs. The spacing is also correct for the tail ribs.

I aligned one comb on the tail plan, so that the bottoms of the slots were in line with the rear spar of the tail. See the first picture. I pinned it down, and added a piece of 3mm scrap to hold the rear spar precisely at the correct height for the ribs.

I threaded the ribs onto the two carbon spars, and fitted the rear ends into the slots in the rear spar, leaving off the tip rib. The rear stand-offs on each rib set the proper spacing. I lifted the front of the ribs up by the carbon spar, and slid in the other comb, thus setting everything square. A dab of glue at each contact point of rib and spar, and let it set. I used medium CA. (I did one tail with thin CA, and it glued together not only the ribs and spars, but also the comb and spacer.) I used a file as a weight to hold the tail steady while I applied medium CA with a stick.

The surfaces will mount on the boom, with the carbon tube spars fitting over steel pins. Easily removed for travel or maintenance.

Once set, I lifted the front again, and inserted the tip rib, using two pieces of 3mm balsa to hold it properly. After that, the leading edge (a piece of 1.5mm carbon tube) was started at the tip end, then worked to the root, gluing as it went.

The CA set (no accelerator was used) and the tail was lifted from the board. I used nippers to rough-cut the stand-offs, then carefully sanded the remaining nubs. The balsa ribs are extremely soft, and it took only a few careful swipes with 120 grit paper on a sanding block to get everything the right shape.

I used a Dremel with cutting wheel to remove excess spar material at the root.

The rudder-vators are simple sheet, but must be tapered for best weight and drag. They took about twenty minutes of sanding to complete.

Next up: the fuselage pod.
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Old Jul 22, 2014, 08:56 AM
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Nice idea on the tail.
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Old Jul 22, 2014, 05:51 PM
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Ohhhhhhhhhhh, Now I gotta watch this one, ... considering one will be here in November.

Carry On, ...

Regards
Bill
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Old Jul 23, 2014, 07:32 PM
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Sorry for the delay. I realized that with the tail ribs being so soft, I'd better cover the tails right away. It took some doing, but here is the first one, covered.

The final technique went this way:

Coat all edges with a water-based, heat-activated adhesive (I used "Cover Grip" from Deluxe Materials, found at the LHS. I cut a piece to fit one side, and sealed it along the rear spar, then carefully pulled it as tight as possible over the leading edge. I sealed all edges, being VERY CAREFUL to not shrink the covering at all. I then carefully trimmed all edges as close as possible.

I repeated the process on the other side, once again getting all edges down with out shrinking the covering. When all the edges were down, only then did I shrink the covering. Thank Heavens for UltraCote.

The elevator went on top side first, with the elevator and fixed portion bottom-to-bottom with 3X5 note cards as spacers. Then I flattened it out, and did the bottom. The result is a nice skin hinge, very flexible.
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Old Jul 24, 2014, 02:39 PM
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Hi Greg,

Boy, ... she look really SWEET!

I to find myself reaching for the same old colors. What do the tails weigh?

Regards
Bill
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Old Jul 24, 2014, 11:26 PM
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I promise there will be more build pictures tomorrow. Today, Spent cutting out and preparing the basic fuselage pod parts.

Here are the two types of tails for comparison. The built-up tails weigh 18g for a pair; the slab tails weigh 26g for a pair. That's 8g savings, at the single most critical place on the airplane. That means at least 16g or maybe as much 24g out of the nose, compared to my first Pures. I'm happy.

Besides that, the built-up tails have a profile, while the slab tails are flat plates. Flat plates have good drag figures, but only at zero angle of attack. Otherwise, a flat plate sucks. At least that's what I remember from those aeronautical engineering classes from soooo long ago. My own experience with DLGs, as tails evolved from flat or tapered balsa sheets to truly shaped tails, makes me really enthusiastic for a shaped tail surface.

The green/black color scheme is growing on me. My wife suggested the name "Green Hornet", but I've got friends who used to fly with them in Vietnam. So, the name "Horny Green-it" has been hereby bestowed.

Like I said: pictures of the fuselage tomorrow. Really.

Yours, Greg
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Old Jul 25, 2014, 11:10 AM
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The fuselage pod: I followed the instructions pretty much as written. The big issue with the pod is to avoid making two right sides, or two left sides. The way I avoid that is to layout all the parts for each side, top-to-top.

I thinned some aliphatic wood glue with a bit of water (just drops!), then spread it on the plywood with a throw-away paste brush. Like Josef says, use fuselage bulkheads to line things up, then weight down and let set for at least twenty minutes. Repeat for the other side.

In the pictures, you can see the tools I used to separate the parts from the mother boards. A box cutter works well for plywood, but I use a single edge razor for any balsa. I clean the attach points with sandpaper (new, 3M 120-grit works well) or (for plywood) a small file.
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Old Jul 25, 2014, 01:47 PM
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If your kit is like mine, there is a minor mismatch between the balsa sides and the fretted plywood liner. What I did: I made sure the wing saddles (plywood) lined up with the balsa sides, then elongated the slots in the fret to allow the rear bulkheads to line up with the top of the wing saddle.

I then clamped the sides together, and sanded down the longerons, so both sides are perfectly matched. Took about twenty minutes altogether.

Next will be setting up the servo tray, then joining the sides.
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Old Jul 25, 2014, 06:23 PM
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At this stage, it is best to assemble the whole control system, and install it on the servo tray. Saves a lot of pushing and pulling later.

I used Hitec HS65 servos because I had a pair, and they fit. I've used them in my other Pures, and in other applications with no problems. The servo tray will also take MKS6100s, JR-285s, Futaba S3114s and many others of that size. The load is not high - there are many that will work.

I cut off the unnecessary servo horns, and will be using the inner-most hole. I already know they work the correct direction, based on my previous Pures.

The pictures show the installation. Josef recommends a NmHd battery, but the LiPo I've shown has much more capacity (even though it is lighter in weight). It will fit, there's an identical battery in my first Pures. I've installed a Hitec Minima-6. An Optima-6 Lite will fit easily; a regular Optima-6 is a bit tighter. I used an old Daniels regulator, which also beeps and flashes lights to show me the battery voltage. The receiver is glued to the servo tray, and the regulator to the receiver.

The last picture shows it all fitting.

The extra servo wire is for the spoiler servo.

HUGE WARNING! Do NOT repeat NOT glue the sides together at this time. There is one more critical step to take.
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Old Jul 26, 2014, 01:02 AM
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Pures

Could we get Mr. Gergetz's e-mail? That is a very good looking sailplane.
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Old Jul 26, 2014, 04:53 AM
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Quote:
Originally Posted by PDARR View Post
Could we get Mr. Gergetz's e-mail? That is a very good looking sailplane.

Take a look at this thread:

http://www.rcgroups.com/forums/showthread.php?t=1874549
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Old Jul 26, 2014, 10:37 AM
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Quote:
Originally Posted by PDARR View Post
Could we get Mr. Gergetz's e-mail? That is a very good looking sailplane.
Josef.Gergetz@gmail.com
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Old Jul 26, 2014, 11:02 AM
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Greg,

Nice build so far can't wait to see it finished. Just ordered 2 of the kits today and as a "Green Hornet" myself the scheme would be cool.

Rich


Quote:
Originally Posted by glidermang View Post
I promise there will be more build pictures tomorrow. Today, Spent cutting out and preparing the basic fuselage pod parts.

Here are the two types of tails for comparison. The built-up tails weigh 18g for a pair; the slab tails weigh 26g for a pair. That's 8g savings, at the single most critical place on the airplane. That means at least 16g or maybe as much 24g out of the nose, compared to my first Pures. I'm happy.

Besides that, the built-up tails have a profile, while the slab tails are flat plates. Flat plates have good drag figures, but only at zero angle of attack. Otherwise, a flat plate sucks. At least that's what I remember from those aeronautical engineering classes from soooo long ago. My own experience with DLGs, as tails evolved from flat or tapered balsa sheets to truly shaped tails, makes me really enthusiastic for a shaped tail surface.

The green/black color scheme is growing on me. My wife suggested the name "Green Hornet", but I've got friends who used to fly with them in Vietnam. So, the name "Horny Green-it" has been hereby bestowed.

Like I said: pictures of the fuselage tomorrow. Really.

Yours, Greg
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Old Jul 26, 2014, 04:09 PM
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Man, that is sooo cool! You didn't know Mike Douglas, by any chance?

Yours, Greg
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