Art hobby's Sierra or J.K Thermic ???

[Masterpiece]

HI

I am looking for a new thermal plane and I have been looking at Art Hobby's web site and I am interested in the Sierra 2.5m or the J.K Thermic 2.5m. I'd like to hear from people who have flown one or both of these models to get some feedback to which would be better for thermal flight. Both look very well made and I have looked at a few threads already about the Sierra model.

In the specs for the J.K Thermic it says that it has a 1090mm fuselage length. Is this correct, as it seems a little short for a 2.5m wing??? Whereas the Sierra has a 1230mm fuselage length, which sounds better in length for a 2.5m wing.

Any feed back would be GREAT

Cheers,
Glen

[Ollie]

The lighter wing loading and higher aspect ratio of the J.K. Thermic should give it a performance edge for thermal soaring.

[tclark]

I have a Thermic, and a friend of mine has a Sierra. The Thermic is slower and I think a nicer flying thermal plane. The Sierra is quite a bit heavier and lands a bit hotter. The kit quality of either plane is excellent.

-Tracy

[Masterpiece]

Thanks guys for the replies,

Tracy; with your Thermic, is the fuselage length only 1090mm? Does this affect the pitch of the plane with the fuselage being short? I thought that a 50 inch fuselage would be better.

Oh ... and also what is the weight of your plane too, as I am interested in the AUW with radio and bits and pieces.

I have a JR 3810 radio to use and I was hoping to install flaps as well as ailerons. Do you know of anybody who has done this operation or maybe you may have :) ? If you have or know of somebody who has done it, is it worth while to do it? I was reading a few threads about the HN1033 foil and some commented that this airfoil has a low L/D ratio, and benefits with flaps.

I have had a look at the HN 1033 foil and I like it. The Thermic looks like a great plane, it's just the fuselage length that concerns me .:confused: Apart from that it's priced very well for such a well made kit and from what I have read it is rated very high with flyers in build quality.

Later
Glen

[tclark]

Glen-
Sorry it took a while to respond, connections are a bit spotty when travelling for the holidays.
1090 mm is correct for the fuselage length. It does look a bit short relative to the wingspan, but seems to fly OK. Pitch stability is fine. The hardest part of flying it is coordinating the ailerons with rudder in a thermal turn - this is my first sailplane with ailerons so this is what I'm trying to learn. If I were to build it again, I would probably use more dihedral, as has been written up by Garwood and often mentioned favorably by our local aero guru Mark Drela. My Thermic (per plans) has just over 3 degrees of dihedral under each wing. I don't think the plane needs flaps for sport flying, as landing it on the fiels with spoilerons deployed is fine. For competitions you might want flaps to hit that tiny 100 point circle.
My finished weight is 31 ounces, I think. Pretty light. It flies well at this weight, penetration is pretty good. I can cover so much more sky than my poly floater, I like it alot. I am using 4 HS81 servos and a 600AE pack, and needed a bit less than 2 ounces of lead in the nose to balance. Do everything to keep the tail light.

-Tracy

[Masterpiece]

Hi Tracy,

I know what it is like when you are travelling (hard to get to a cafe or connections can be very hard at times).

Anyway ... I have spoken to Andre at Arthobby about the fuselage length. I am very impressed with the first rate service I have been getting with Andre at Arthobby (email responses so far).

At 31 oz. your sailplane sounds nice and light (great for light lift). I hope my plane comes close to that weight. I have a 1100maH battery for the receiver that I am going to re-configurate from a flat pack to a square pack, and get some (more) Hitec HS81mg for the flaps (if installed) and 2 more HS81 / 85's (plain gearing) for the v tail, and 2 very thin (9mm) Topaz 9g servos for the ailerons with 1.8kg torque. I'll probably stay with the flattish dihedral as I prefer this configuration wing for good thermal response. Did you do anything with the tailplane to beef it up such as a very light fibreglass overlay?

Later
G.

[danny_isr]

i was considering myself buying it , but i heard that it wont take anything except weak high start - is that true ?

My freind at the feild got Sierra - VERY happy with it.
Till he broke the wing on high start (well he kind of over ...high start it)

Danny

[Masterpiece]

Hi Danny,

From what I've been told, it can take a normal high start BUT it's the winch start that has to be weak to moderate. I can't talk from experience (as you'll understand I still have to get the plane) with this plane, but I have done extensive research on the Thermic, and I am really looking forward to getting it now.

Later
G.

[tclark]

Glen-
I didn't reinforce the tail, to keep it light. I just used the water-based poly to seal the balsa tail. Keep it light and don't crash it, rather than make it heavy enough to survive crashes but not fly well?

The fuselage is pretty sleek and tight. Try to put everything as far forward as you can. My 600AE pack fit nicely in the nose, I'll see if I can dig up a picture and figure out how to post it.

The Thermic wing won't take a full-pedal launch on a heavy-duty winch, nor will it take a huge zoom. At least I don't think so, I haven't broken mine to determine it's limits! I have launched it off of as stiff of a high-start as I cared to pull back, the wings were fine. I can't use the small up-elevator lauch preset with high-starts, the plane tends to pop off if I do. When I launch with the winch I am providing more tension and speed than the high-start.

-Tracy

[Ollie]

Tracy,

There are two things you could do to prevent popoffs when you use a little up elevator preset with the histart. If the tow hook is robust enough, you could file a shallow notch in the tow hook for the launching ring to sit in. About 1/32 of an inch depth of the notch or a little less is all you need. The other thing you could do is to remove the tow hook and put the threaded end in a vise up close to the bend and hammer a little more bend into the tow hook. Two or three more degrees of bend is all you need. Either measure should suffice.

Once you have modified the tow hook to prevent popoffs you can then adjust the tow hook location to preclude the need for elevator preset during launch by moving the tow hook back just a bit.

Fine tuning the tow hook is the way to better launch height. Just make sure the changes are small so that the launch remains controllable and stop when the plane shows the slightest tendency to veer on launch.

[Masterpiece]

I'd love to see pictures of your plane. And thank you Ollie for the following suggestions with the tow hook (I will try them too when I get my plane). What is the best position for the tow hook anyway? I have always gone with 40 degrees forward of the CG (but I have realised that this does have quick short comings, with different fuselages, tail moments, tail areas .... etc). I have heard of people positioning them right under the CG with their F3B / and F3J planes for certain types of launching, such as flatter speed launches. Does the manual for the Thermic detail where the tow hook is best positioned?

Happy flying to all,
Glen

[tclark]

Glen-
Here's a pic of my Thermic, sorry for the dubious quality - but I'm no photographer. The 600AE battery is way up in the nose, followed by the HS81 servos for the V-tail. It's hard to see it in the picture, but they are mounted at an angle for better clearance with the nose cone. (It'd be easier to see this angle from a top view.) The power switch is behind the servos, so it is easy to get at by sliding the nosecone off just a bit. I used two bolts to attach the wing rather than 1 bolt plus a dowel.

Ollie-
I noticed that my towhook angle is a bit obtuse, so I think I will put a bit more bend in it. I'm still not sure why it pops off on the high start and not the winch. On the high-start it seems to 'mush' through the air with a higher angle of attack, but on the winch it looks good. Maybe it's good to have something to work on, all the more fun. First we need to melt all this snow...

-Tracy

[Masterpiece]

Hi Tracy,

I am amazed at how the fuselage is not very tall (which is fine because it has less drag area). I estimated this from the fact that you are using 600AE's (I have a few handy) and so I was able to see for myself how tall the fuselage must be. Where abouts is the receiever? I intend to use HS85 BB's for the v-tail and HS81 MG's for flaps and maybe HS81's for the ailerons or Topaz 9g's (as they are very thin and still give 1.8kg torque)

Only 3 more days til I get mine :) :) :) .

Any tips with the building that you suggest? I will probably use the 2 bolt option too.

Later,
Glen

[tclark]

Glen-
The fuselage is very sleek, I like it because it looks cool that way but it did take a while and a few trials to fit everything. The receiver is right behind where the nose cone ends, it butts up against the switch and plywood tray used to mount the switch and servos. I had to plug in the servo/battery connections, wrap with foam, and then stuff in through the opening under the wing. This was a bit tedious. My best advice is to think light thoughts while building the tail, you need weight as far forward as possible. Also, do ALL of the work on the servo/switch and battery mounting before gluing the inner nose cone to the fuselage, access makes this much easier. The instructions are not so hot, so I had to stare at the pictures and the actual pieces quite a bit to figure out exactly what I wanted - and I'm quite happy with the end result.

-Tracy

[Ollie]

Installing a radio in a minimum fuselage is a one time price to pay for slightly better performance on almost every flight because the fuselage drag is a bit less. For that reason, I dislike fat fuselages. There isn't much effect on slow speed flight but the reduced parasitic drag improves the ability to penetrate into the wind.

[Masterpiece]

Hey Tracy,

Just a quick note to let you know that I have just received my Thermic this morning. Great plane, and the workmanship is fantastic. Super straight and nice thin trailing edges. Everything fits like a glove. I can fit the HS 85 BB's in the fuselage but I can see how thin it is. Tell me is your V tail removeable or permently fitted? Also I take it that you have to buy tail bolts and wing bolts (if more than one is used on the wing).

Can't wait to get this plane into the air now.

Later,
Glen

[tclark]

Glen-
I agree on their workmanship, I only wish I could build a wing like that. My V-tail is removable, but I have never removed it. It bolts into two clips that are glued to the boom. I seem to recall that they supplied bolts for the tail on mine. I don't typically use the metric hardware that often comes with a kit since it is easier and cheaper to get SAE harware here in the states (maybe someday we will catch up with the rest of the world on the metric system). So I re-drill and/or tap as necessary, I think I used 8-32 nylon bolts on the wing and 4-40 nylon bolts on the V-tail.

-Tracy

[Masterpiece]

Hey Tracy

I have a good look at the instructions but I have come across a few differences from the 3D plan to the A4 plan instructions.

Firstly did you build the wings with 100mm dihedral or 140mm?

Did you use the control horns in the kit? They seem a little flexible and so I was just wondering. The v-tail horns I will use though.

Did you do anything to cover the gap in the v-tail?

Did you use the C of G from the plans or did you experiment with it? If so what C of G are you using?

Did you make your own tow hook from what wire is provided in the kit, or did you use another one?

How did you secure the push rod cable in the boom?

Did you install your servos upon the inner glassfibre (the non coloured one) pod or just beneath the fibreglass? I am thinking that I will might have to sink them down as when the outer pod sleeve is slid over, it may interfer with the servo arms' play.

And finally when you installed the radio did you run the aerial through the carbon boom or out of it before you get to the boom (it's just that I have heard that you can get interferance with carbon booms)?


I have had a look at putting N 600 SCR's in as the battery pack. They fit width wise but length wise they get a bit long and I will have trouble installing the servos then ...... so I will settle with the KR 600 AE's instead.

I have capped the outer wing panels (where the two wings meet at the carbon rods) tonight and will sand them down flush tomorrow, as well as shape the wing tips too.

Later,
Glen

[tclark]

Glen-
And the fun begins...
-I used 140 mm dihedral (70 under each tip). As I mentioned earlier, I think more would be better for me.
-I used the control horns in the kit. They have been fine and the 'offset' v-tail ones helped installation.
-I did not cover the gap on the V-tail hinges, as I was concerned with binding. If the hinges start to look worn out (they have no such signs yet), I plan on replacing with tape.
-I started with the plan CG, but I think I subsequently removed some nose weight. I don't have the plane here to check right now.
-I made my own tow-hook out of 3mm steel rod. I bent into an L, threaded the end, and glued a plywood plate with T-nut into the fuselage that the hook screws into. My hook is about 3 or 4 mm aft of the plan location, and I may tweek it back just a bit more.
-I glued the outer pushrod tubes to a thin foam board (used as a rafter vent at HomeDepot) which was cut to fit inside the boom. Then I slid the tubes/foam into the boom and glued in place.
-The servos are too high if mounted to the fiberglass 'shelf'. They mount on a plywood tray that is glued into the nose piece. The plans do show this, but it took me a while to see what they were showing me. Both the tops and bottom of the servos will barely clear the nose cone, so you need to be pretty careful with this mounting.
-I ultimately ran my antenna through the CF boom. I was concerned, so I range checked the system with the antenna both in and out of the boom - and there was no difference with my system.

-Tracy

[CactusJackSlade]

I have the Art Hobby Bore (with ailerons and added flaps ) which I am sure has at least the same size if not smaller nose pod.

I fit the two HS-55's in like this and it allowed me to use a Hi-tech slimline 8 ch (w/o case and shrink wrapped) in front of the servos and a 4 "AAA" 700 NiMh battery pack in the nose.

All this and I still had to add a couple oz. to the nose. YES keep everything FORWARD! My Colibri was the same way, I wish the nose pods were just a tad longer, I guess you could shorten the boom, but I don't know how that would affect the handling...

My 2 cents

[Masterpiece]

Hey,

Neat installation of the servos there Cactusjackslade. I will have to install my servos below the inner fibreglass pod though as I am going to use HS85 BB's and their servo arm will be interferred with by the outer pod. Very tidy though.

Also Tracy .... I was looking at the outer wing panels and I thought the HS 81's would be too thick for installing into this wing. What have you used??? I don't really want to sacrifice torque with a smaller servo.

Cheers
Glen

[Masterpiece]

Hey Tracy,

Just a quick question about your thermic ..... in the instructions it says to use about 15mm throw for the ailerons both ways but I will be using differential throws with my model to avoid adverse yaw. What aileron throws did you use for your model? Also did you mix aileron throws with the ruddervators too?

Cheers,
Glen

[tclark]

Glen-
I used HS81's on the ailerons, I did not want to go with anything thinner/lighter for fear of weaker gears and stripping. They fit pretty well, with just the slightest bump below the wing contour - I don't think it is enough to create a problem. I used differential throw; twice as much up as down. I do have the aileron stick mixed 50% onto the rudder function, but in hind-sight I don't think that it very useful as the two functions are really quite sepatate in a thermal turn. I usually need to cross-couple the aileron stick - i.e. a left-circling thermal turn requires left rudder stick and right aileron stick. The mixing of aileron-rudder sort of gets in the way here, I think. I think I set my 'up' aileron throw for 15mm, and 'down' throw was half of that, but I'd have to check to be sure. Spoileron offset is about 45 degrees, and still allows up travel on top.

-Tracy

[emersunn]

Glen,

I just ordered a JKT, and would like to hear how yours is flying, any building tips, etc . . .

[Masterpiece]

Hey emersunn,

My Thermic is still in the building stage (I know shame, shame, but it is not very often that I get to work on it until I happen to be at my folks place {and also at a computer too}).

It is a great plane though and I have finally fitted the flap servos too as well as the ailerons. I had to re-inforce the area where the flap servos went because the servo hole cut through the spar, so I had to add some 6mm ply just in front of the servo. A job I kept putting off but I finished it last week and am VERY happy with the result too.

Some tips I would give are:

*Where the wings sections join (the main section and the wing tips), use some 3mm ply instead of fibreglass cloth, as discribed in the manual, as you can get a perfect joint this way.

*Don't cut out the hole where the ailerons servo leads come out before gluing the main wing section together and then bandage (the reason is quite obvious but the manual says to do it in this order). Instead mark out where the hole is, glue with 24 hour epoxy and then bandage (I used carbon bandage for looks and strength). Finally cut out the small hole and enlarge as necessary.

*Personally I have gone with the 2 bolt wing attachment option to attach the wing rather than using the steel pin in the front of the wing with the 5mm bolt at the trailing edge.

*I'm planning to put gap seal on all the control surfaces too (v tail, flaps and ailerons).

Thats all I can think of at the moment but if I think of anymore I'll let you know. I really haven't looked at the instructions lately.

Later
G.

[emersunn]

Not finished? Shame on you ;) I guess I'll let you know how it flys ;)

Thanks for all the tips.

[bsnrjones]

How is the tailboom connected to the fuselage? I am planning on ordering just the pod for Aegea II, but would like to see how it was done on the Thermic.

Could anyone possibly post a pic?

Burke

[Masterpiece]

Originally posted by bsnrjones
How is the tailboom connected to the fuselage? I am planning on ordering just the pod for Aegea II, but would like to see how it was done on the Thermic.

Could anyone possibly post a pic?

Burke

Hi Burke,

NO pics, but it is attached by the boom sliding over an inner tube (about 5cm long) molded into the pod as part of the fuselage. Quite secure attachment.
I'll try to get a photo of this for you soon.

Later
G