JK Thermic double build

[EdSoars]

As the April winds showed signs of letting up, the old thermal bug caught Chris and me. He'd heard of a couple of good slope sites, and they not only face the prevailing westerlies, but have landing zones that are crunchie-tolerant (not friendly, just tolerant). So what to build? These were the specs:

1) Two identical ships... test the pilots, not the gear.
2) Good L/D... with windy conditions and long slopes to back up the thermals, a floater won't do. With a long ridge behind us, long-range visibility and decent speed are good things... we fly between 7000 and 9500 feet which increases airspeed: good for flying, tricky for landing. It's a compromise on the side of speed.
3) Light enough for a high-start, fast enough for a bungee launch. Bungees are much more tolerant of brush, especially when attached to an SUV roof rack.
4) Sturdy enough to encounter brush and an occasional rock with minimal damage.
5) Simple for now: spoilerons for slope aerobatics at times, but no flaps, just loads of up-travel.

We chose the ArtHobby JKThermic. I'd had a 2m Boar in the past and enjoyed the slippery HN1033 airfoil. That one was set up with full-span flaperons.

Here are the modifications I came up with:

1. 5-cells, 1400 mah 2/3 AA battery pack if it fits and balances. It does.
2. Ailerons lengthened to extend from wing panel joint to 1" inboard of the tips, and the chord increased to 30%.
3. Spoileron movement to be as close to 90 degrees up as possible, for safe long-distance thermal bailouts.
4. Spoileron servos are HS125's installed in the center panels at the joint, protruding out about 1/4" into the tip section. Like the AeroMod Mirage, the only thing to disconnect is the clevis from the spoileron control horn.
5. Ballast tube for up to 9 oz. of 1" lead slugs, inserted through the wing saddle. It does get windy here...lots.
6. Removable Vee-tail to allow compact travel and fine-tuning the decalage by trimming the platform.
7. Stab section thickened and airfoiled instead of flat slabs. Reduced null-zone lets the CG be set a little further rearward.
8. Tail pushrod wire housings are glued to a 1/8" diameter CF rod to stabilize the wires under compression.
9. Dihedral increased from 3 degrees to 5 degrees...more relaxing at long distances, and better roll-rudder coupling with spoilerons up.
10. Instead of glassing the wing joint surfaces, the foam was melted away and filled with epoxy and microballoons. It's thin most places, but 1/2" deep next to the joiner rod blocks and behind the LE.

My transmitter is an Eclipse 7 ch., Chris's is a Futaba 9C. My receiver is an FMA 5-channel, which just barely fits between the Dymond D60 tail servos and the front of the ballast tube. The ballast slugs and the batteries are cushioned at the front.

With no finish on the wood, the assembled plane with all gear installed weighs 28 oz. With the 5-cell battery, it balances as close to perfect as can be seen without flying. The wing area is 4.33 sq ft, so if the covering adds 6 oz, the wing loading will be 7.8 oz. Not a floater, as intended. With 9 oz of ballast, it will be nearly 10 oz / sq ft.

I will finish the wood with a coat of shellac to seal the pores, then varnish to attach a layer of chiffon, or polyester fabric, on the wings, not the tail. This is essentially the same as a heat-shrink fabric like Coverite, but without the heavy sealing and adhesive film. It weighs 1/2 oz per sq. yard, so adds little weight. It should fill with 4 coats of varnish. This will increase the poplar veneer's resistance to splitting.

[Skysailer]

I too have a JK Thermic that I fly in a TD mode. I have found that the fuselage is very weak in the wing saddle area and less than perfect landings would result in severe fractures. I have added a few extra layers of glass in that area and it's pretty strong now. At a high wing loading you will probably increase your landing speed somewhat, which could cause some issues if you mess up the landing. I have also got the Drela shaped airfoils on the tail - that improves the handling. I'm certainly interested in the method you are using to drive the ailerons - it looks like a good idea.

[EdSoars]

Thanks for the heads-up on the fuse. That's what I found on my Boar. The ballast tube is bedded in epoxy+balloons, so that stiffens the area somewhat. I'm also thinking about using a nylon bolt for the front "pin" instead of the steel pin in the kit.

It's a real challenge to design a kit that has all the easy, cheap bits break first, but that's what the full-size designers do. The trick for us amateurs is to know just how strong those nylon bolts are in tension, in shear, etc. I'm using two 6-32 bolts for the stab, and an 8-32 for the rear wing hold-down, maybe an 8-32 for the front pin.

If there's any interest, I'll post detail photos of the build...wing servos, etc.

ed

[chris country]

I'm taking a slightly more conventional building approach on the Thermic compared to Ed, mainly due to time constraints. It will be interesting to see how his mods affect the performance though. We'll be flying side by side much of the time so we'll have to post our observations on things.
It is amazing how lightly the fuse is laid up though. I'm definately laying down some extra glass or carbon in there.
Chris

[Forest Flyer]

....If there's any interest, I'll post detail photos of the build...wing servos, etc.....
I am watching and interested in more details. I have never seen so many mods done at once to an AH glider!

I also think that it is generally a good idea to add more dihedral to any Thermic (or Castor). I added more dihedral to my Castor (essentially an e-Thermic) than in the standard plan, and I am very pleased with the result, which is great hands-off tracking and improved turning.

If you fly your Thermic in any kind of strong wind, you are going to want all 9 oz of ballast and more. I have found that my "e-Thermic" will fly nicely well into the 40 oz range, and at 40 oz it still seems to need more weight for high wind situations. And BTW, it will also thermal pretty nicely at 40 oz.

Many of your mods make a great deal of sense given the slope where you intend to fly the Thermic. However, I would like to stress to those unfamiliar with Art Hobby planes that the wing veneer needs no strengthening for regular "friendly" field use. The wing saddle is indeed a relatively weak spot in hard landings and will require some attention for your purposes.

Thanks for the thread!

FF

[Batmanwpg]

I too have a JK Thermic that I fly in a TD mode. I have found that the fuselage is very weak in the wing saddle area and less than perfect landings would result in severe fractures. I have added a few extra layers of glass in that area and it's pretty strong now. At a high wing loading you will probably increase your landing speed somewhat, which could cause some issues if you mess up the landing. I have also got the Drela shaped airfoils on the tail - that improves the handling. I'm certainly interested in the method you are using to drive the ailerons - it looks like a good idea.



This sounds familar! ;)
Along with these recommendations:
Instead of shellac I would use water based Polyurethane. Light and great finish in only 3 coats. I would glass the centre section as it would give more strength. I used 3 tapering layers, 1-6oz, 1-3oz and one of finishing cloth. They extend about 10" either side of center. I used 2 nylon bolts for wing attachment. 1/4-20 for the front and 10-32 for the rear.

[EdSoars]

ForestFlyer and Batman: good suggestions and confirmations. The shellac, BTW, is only for the first sealing coat and to bond the polyester to the wood. As soon as I test for compatibility, I'll finish off with water-based polyurethane varnish and film trim. I'm hoping the Thermic will finish out a little on the heavy side, given FF's statements about ballast. If windy day penetration seems to be a problem, I may add flaps to the center panels. The HN1033A airfoil looks like it will respond well to 30% flap reflex, and then I'll have the slow-down for landings as well. One step at a time...you know; fly the plane- fix the plane.

Batman: A 1/4-20 bolt in front seems like overkill; do you think a 10-32 would have enough strength, and still reduce the force on the saddle in a rough landing? Did you add all that FG on the inside or the outside?

The ruddervators turned out nice and thin but very flexible. I covered them with 0.2 oz CF mat. BIG increase in stiffness.

[Batmanwpg]

ForestFlyer and Batman: good suggestions and confirmations. The shellac, BTW, is only for the first sealing coat and to bond the polyester to the wood. As soon as I test for compatibility, I'll finish off with water-based polyurethane varnish and film trim. I'm hoping the Thermic will finish out a little on the heavy side, given FF's statements about ballast. If windy day penetration seems to be a problem, I may add flaps to the center panels. The HN1033A airfoil looks like it will respond well to 30% flap reflex, and then I'll have the slow-down for landings as well. One step at a time...you know; fly the plane- fix the plane.

Batman: A 1/4-20 bolt in front seems like overkill; do you think a 10-32 would have enough strength, and still reduce the force on the saddle in a rough landing? Did you add all that FG on the inside or the outside?

The ruddervators turned out nice and thin but very flexible. I covered them with 0.2 oz CF mat. BIG increase in stiffness.

The 1/4-20 is probably too much. I added fibreglass to the inside of the pod before I assembled it. You could still do it after but it gets harder to do.

[EdSoars]

I finished up the Thermic last night...the paint in the polyester covering came out heavy, adding two oz to my 35 oz estimate. The fabric still isn't completely filled, so we'll see how it flies with a matte surface. At 37 oz, the wing loading is 8 1/2 oz/sq ft, so it's still not exactly a "lead sled", so more finishing is in store.

What was interesting is that unpainted, the model balanced with no added nose weight. With paint, (including some CF mat to stiffen the ruddervators, I had to add 3 oz of lead sheet around the batteries up front.

Chris is still building his more "stock" Thermic, but we're headed off to fly today, on a good slope+thermal site. Results will be posted ASAP.

ed

[EdSoars]

And the flying results were...fantastic! The maiden flight needed a few clicks of elevator trim and lasted about 50 minutes. After landing, the aileron throw was increased. Elevator compensation with full up spoilerons was reduced.

The only thing left to do back in the shop was to adjust some of the programming mixes. With three flights and about 2 hours air time, we moved to another site. Again, superb lift, both thermal and slope. The Thermic penetrated much better than expected. No ballast was needed in gusty, 20+mph winds. My only compliant was the flexibility of the outer panel joiners. They are strong enough, but have a lot of slop. If the CF rods were 2 inches longer, they would be much more solid.


On what was intended to be the last flight of the day, I walked down the hill to land at the base...about 100 m down. A glance down to check my footing, and the Thermic vanished from sight. Chris was watching at the same time and he lost it too. A gust of wind must have blown it back over the ridge into the rotor behind.

The photos tell most of the story.

Conclusions:

The polyester covering cloth added much to the wing's strength. After a very high-speed impact in hard ground, only one small part of one wing panel's LE and veneer was damaged. The ailerons, inner and outer panels, were all scattered separately over the ground. The ailerons were hinged with silicon glue, which separated completely. The only broken bits were the wing joiners. The wing will be rebuilt as a one-piece wing. Only 3 of the 4 wing joiner CF rods were recovered.


Although the inner fuselage nose and nose cone were broken from the main body of the fuse, only the part around the inner cone was broken. The tailboom broke off the fuselage stub and is repairable. There was no damage to the vee tail. No servos were damaged, although one output arm was broken.

This was a sorry end to a fantastic day of soaring, but the Thermic's basic design, with the modifications, proved a VERY airworthy sailplane, in some pretty trying conditions. The only real criticism I have is the wing panel joiner design. It could be easily improved by lengthening the CF rods and changing the rods' mounting blocks from balsa to hardwood.

It will be rebuilt! In the meantime, enjoy the photos. I'm the grey-haired guy with the dog (Joe the Border Collie).

Ed

[tw126a]

Sorry about your loss, Ed. Sounds like it flew real well, have one new in the box that I need to get to.
Tom

[chris country]

I'm getting close to wrapping mine up, taking the sticks on Ed's yesterday is making me want to hurry it up. His ship flew so nice, really responding well to the larger ailerons and penetrating surprisingly well considering the wind speeds. Really is a shame that we lost it in the sky and mind blowing how quickly it happened,but we're lucky we even found it. It will be back!
Chris

[Forest Flyer]

Ed,

Well, that is an unhappy ending to the maiden flights. No doubt about the fact that the maiden flights are the most dangerous for any plane, but it looks like the Thermic flew well. I am happy to see that it did well in the wind without ballast. I agree that carbon joiners could be longer (along with longer wooden blocks), but I don’t think that the balsa blocks are too weak given the glass sleeves that actually hold the carbon rods.

From the photos, it does look like the wing was fairly little damaged given the nature of the impact. I had a similar crash with my Castor about in Feb. 2007, when some sort of radio lock-out occurred and the plane eventually winged-over at about 150 feet and went straight into the rock-like frozen ground at a good clip. There was a lot of damage to the leading edge of about 50 % of the inner wing panels, but I managed to fix it pretty well. The fuse pod and motor were toast, but the remainder of the plane was okay.

There is not much on fixing Art Hobby wings here on RC Groups, so if you’d care to share your ideas about how to fix the damage, I’d like to hear them. By the way, the most difficult problem that I had was fixing the balsa blocks that hold the carbon rods, which were pretty badly torn in places. How does that look in your wing?

FF

P.S. That is one beautiful spot for a flying slope!