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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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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.
Jul 22, 2014, 08:56 AM
DLG Bug Bit Me
Tim Harbour's Avatar
Nice idea on the tail.
Jul 22, 2014, 05:51 PM
3D Toy Designer
MicroRotors's Avatar
Ohhhhhhhhhhh, Now I gotta watch this one, ... considering one will be here in November.

Carry On, ...

Regards
Bill
Latest blog entry: My New Toy!
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.
Jul 24, 2014, 02:39 PM
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MicroRotors's Avatar
Hi Greg,

Boy, ... she look really SWEET!

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

Regards
Bill
Latest blog entry: My New Toy!
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
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.
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.
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.
Last edited by glidermang; Jul 25, 2014 at 07:36 PM.
Jul 26, 2014, 01:02 AM
Registered User

Pures


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

Take a look at this thread:

https://www.rcgroups.com/forums/show....php?t=1874549
Jul 26, 2014, 10:37 AM
3D Toy Designer
MicroRotors's Avatar
Quote:
Originally Posted by PDARR
Could we get Mr. Gergetz's e-mail? That is a very good looking sailplane.
Josef.Gergetz@gmail.com
Latest blog entry: My New Toy!
Jul 26, 2014, 11:02 AM
Registered User
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
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
Jul 26, 2014, 04:09 PM
Registered User
Man, that is sooo cool! You didn't know Mike Douglas, by any chance?

Yours, Greg
Last edited by glidermang; Jul 26, 2014 at 04:31 PM.
Jul 26, 2014, 04:29 PM
Registered User
Here's how the fuselage pod went.

I inserted the servo try (with two bulkheads) into one side, along with the third intermediate bulkhead. The two wing hold-downs also went in, but with out any adhesive - just loose.
I joined the other side, and added the end bulkheads.

The cross-grained top and bottom sheeting went on next, then the nose block. I rough-sanded (80-grit paper on a block) the edges, and did a quick trip to the scroll saw to provide the initial taper to the nose. After that, I used a razor plane, big block and small block to remove anything that didn't look like a streamlined sailplane.

I did a better job on this one than on my first.

The fuselage pod is now ready to join to the boom. After that, I'll cover it with glass cloth and epoxy. I want a smooth finish on this one, and intend to shoot it with satin-finish black.

And by the way: that battery pack in the last picture? It still fits into the nose bay.
Jul 26, 2014, 06:06 PM
Registered User
Thanks for taking the time to do this build thread, Greg. I hate doing build threads because I don't feel that my effort is being appreciated. But as you know I still have my PURES in kit form (I'm lazy and I prefer to fly during the summer rather than build planes) and I will eventually get to it. So thanks again for your efforts. This thread will be helpful when I finally get around to my kit.

Can't wait to see the wing being built. That should be fun!
Jul 26, 2014, 07:25 PM
Registered User
Well, Drew... here are some more fuselage pictures.

I assembled the boom crutches - they fit together perfectly without any adhesive at all. They are square (important a little later) and are labeled: "V" stands for a German word that means "put me at the forward end"; "H" stands for a German word that means "put me at the aft end".

I juggled things a bit until the boom was centered in the rear fuselage bulkheads. There is play there, so take your time. I convinced myself my bench was both flat and level. There are seams in the bench top where the boards are joined, so I lined up the nose, the notch in the V-stand and the notch in the H-stand, to make sure I have a left/right/straight arrangement.

Using a spirit level and a square allowed me to get things straight in a lateral/directional sense.

Having lined it all up, I then took it all apart, buttered up the bulkheads with epoxy (with glass filings added) and assembled it once more. With 30-minute epoxy, I get about twenty minutes to fuss and bother things. It worked last time, it should work this time, too. Once cured, I'll wiggle the rear block up to the bulkhead, then sand it to a nice shape.
Jul 26, 2014, 07:29 PM
Registered User
Josef certainly did an amazing job with the alignment jigs and laser-cut parts. This kit is a builder's dream.

Keep up the good work, Greg.
Jul 26, 2014, 08:16 PM
Registered User
Name doesnít sound familiar, Iíll ask around the local area.
Rich

Quote:
Originally Posted by glidermang
Man, that is sooo cool! You didn't know Mike Douglas, by any chance?

Yours, Greg
Jul 26, 2014, 08:21 PM
Registered User
Greg,

What servo's are you using?

Rich
Jul 26, 2014, 11:58 PM
Registered User
Rich:

For primary control, I'm using Hitec HS-65s. They (1) fit; (2) I had them on hand, so no additional expense.

Other servos that will fit and can do rudder/elevator duty: MKS6100 (massive overkill), JR-285, JR-188 (might require some work, they're pretty small), Futaba S3114 (seriously reliable for me, only $17). There is also a blank, un-cut servo tray if you're excited about some other servo.

Josef supplies a tiny Turnigy servo for the spoiler. It is seriously small; I'd never use it in a DLG, but the one in my first Pures soldiers on like a champ. The deal is, the center ribs are machined to hold the spoiler servo, so the one in the kit is what I'm using.

Yours, Greg
Jul 27, 2014, 12:03 AM
Registered User
DrewV:

The kit is CNC-routed, NOT laser cut. That might make a difference to some folks.

Being routed, there is no ash on the edges. The disadvantage is that the little pieces that hold parts to the mother board have to be sanded off. The advantage is that the parts never fallout of the mother board, but are safely held until needed.

The precision is exquisite. I've never used so much thin CA as I have with this kit: fit is usually perfect. It's possible to fit parts together, then just lay on a drop of thin CA and adhesion is complete.

Yours, Greg
Jul 27, 2014, 06:42 AM
DLG Bug Bit Me
Tim Harbour's Avatar
Loving the construction pics.
Man you built fast.
Do you work?
Jul 27, 2014, 12:03 PM
3D Toy Designer
MicroRotors's Avatar
Looks Fantastic Greg, and your cruising along at a pretty fast pace ...

What does the pod weigh? Also, is their a new PDF of the plan as the older PDF plan is for V1.

At the rate your chugging along, ... should be doing a maiden in a week or so.

Regards
Bill
Latest blog entry: My New Toy!
Jul 27, 2014, 03:46 PM
Registered User
TLHover:

I worked until October of 2013, then got laid off. However, after reviewing our financial situation, I decided that I was retired, instead. Big mistake - there is SO MUCH to DO! Every day!

And I'm enjoying it, too, you betcha.

Yours, Greg

Bill:

Sorry, I haven't weighed much, before and after. We'll have to wait for the final weight. The hope is a final all up gross weight of rough 16-17 ounces. I think that's reasonable and doable. Josef is a very good designer - my original Pures is flying at the design weight, and at the design CG. This one should be roughly 20-30g (an ounce or so) lighter, at the same CG.

Right now, the glass on the fuselage is curing. I had to really get down and concentrate to get the epoxy laid on (and soaked off) without it cooking in the cup, so no pictures until it cures. There will be an additional single strip of glass on the bottom, then lots of sanding and priming for the final paint (Black!).

Yours, Greg
Jul 28, 2014, 10:11 PM
Detail Freak
target's Avatar
Greg-
Try the transparent white covering. That stuff is the bomb. Up at altitude, it looks like there is a light inside of it. Its amazing. Great on wing tips!

R,
Target
Jul 29, 2014, 02:53 PM
Registered User
The pod is finished and painted, and curing now. I saw no point in posting pictures of me sanding.

One word of advice for those considering epoxy and glass: don't bother. I finally achieved a nice finish, but it would have been so much easier doing glass with water-based polyurethane.

Now on to the wing. The easiest to start with is the center section. and it is built from the center out. My technique is to cut parts as required, not before. It makes for a slow process (cut out a rib, lightly sand, place on spar - repeat until crazy) but at least the parts do not get mixed up. There are some ribs that appear almost identical - but they are NOT!

For this session (interrupted for lunch), I show the process of putting the spar. Later today, I will have all but the end ribs on the spar, then apply adhesive (mostly super glue for this part). The end ribs require use of angle templates, then there will be the center section sheet and the trailing edges.
Jul 29, 2014, 03:26 PM
3D Toy Designer
MicroRotors's Avatar
Quote:
Originally Posted by glidermang
One word of advice for those considering epoxy and glass: don't bother. I finally achieved a nice finish, but it would have been so much easier doing glass with water-based polyurethane.
With the exception of time.

Water based polyurethanes take to long to dry and they need lot's of coats. Glass and epoxy, ... Need two coats, One to get the glass on and another to fill the weave.

Bill
Latest blog entry: My New Toy!
Jul 29, 2014, 04:03 PM
Registered User
I agree about the WBPU. Takes a LOT of coats (at least 6-8 to fill the weave) and takes forever to completely dry. And if you don't wait long enough before painting (lacquer, for example), the lacquer will tend to blister and everything will bubble up. I think a solid epoxy coating is easier/faster/better looking.

Thanks for the pictures, Greg. Keep 'em coming!


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