Just bought a Balsa USA Thunderbug 120. It's an old kit but everything seems to be in great shape. I've always wanted to build something like the 4*120 or Stinger 120. This one is very similar and was such a good deal I couldn't refuse.

Has anyone built or flown one of these?

Construction-wise, it looks to be very similar to the 4*120, except without all the laser cutting and tab-lock fast-building. It also doesn't have the always excellent Sig instruction manual. First thing I'll do is convert it to a taildragger.

I agree with changing the gear. I wonder if it would benefit from clipping the wings 1 rib like the 4-Stars. You might also look at enlarging the rudder and maybe the elevator.

I have a Sting Ray 40, borrowed from a buddy to test engines. It is the Mid-Star clone, whereas the Thunderbug is a 4-Star clone. The gear style is reversed from their Sig counterparts. It, like the 4-Star, flies much better with the wings clipped.

I didn't build the plane, but I like the Balsa USA ones because they have sheeted leading edges. The Sig planes, with multi-stringer leading edges, have a tendency to pick up warps and are less twist resistant than planes with a sheeted LE.

I built one last year for a friend of mine. I agree the directions leave alot to be desired. I pretty much tossed them and went from plans only. I found one piece int he kit that wasn't right. I believe it was one of the formers behind the canopy that was the wrong size and had to be cut from the size of the one on the plans. I have pictures of my build that I did for him located at the link below. My other site crashed and has not been recovered but this remained. Hope it helps answer some of your questions.


http://douglas.johnston.home.attbi.com/JimDorsey.htm

Thanks for the replies.

Doug, very nice build sequence - much better than the included instructions. Looking at your pictures answered a lot of my questions already. I will definitely use it as a reference when I get to building it.

I agree with sheeting the wing out to cover the main gear mount. I was thinking exactly that when I studied the plans last night. Since I will mount the main gear on the chin block for the taildragger conversion, I will probably replace those ply ribs with hard balsa ones. I also like the tip on using the 6' level. I have one and never thought of using it.

BTW, what engine is that? Did you get a chance to fly it? I have a TT Pro120 rear exhaust with a tuned pipe that just doesn't fit any of the aerobatic planes very well. I'm thinking this plane will be perfect for it.

Ed, you know, I never did try your tip on wing tip plates (how's that for a tongue-twister ) on my Super Stunts. This one comes stock with them! I might try clipping the wings, if for no other reason than to fit my pickup bed better. Then again, if I do that, then it won't be much bigger than the 4*60 I'm building, and I was really wanting something larger. In any case, I will for sure enlarge and reshape the control surfaces.

I built and flew one several years ago when they first came out with kit. One thing you really want to do if they haven't already recommended this is use flying wires on the tail feathers. Mine met its untimely demise due to the lack of them. Like I said before they may have already corrected this.

The engine that was on there was a Magnum 108 I believe. Jim the guy I built it for said it flies great. I personally have never seen it fly yet. He loves it. I did add the flying wires on the tail also. I have more pics on my pc at home I believe so if you're interested I can dig them out for you. Just let me know.

Doug, I would appreciate that very much. Take your time though, as I likely won't be able to start this kit any time soon. I already have a 4*60 kit in progress and several other project in the works.

The kit plan and instructions I have do say to install tail braces. Because of the lightweight construction, this plane really needs them. I thought about just sheeting the tail feathers instead of flying wires, but the mounting platform for the horizontal tail doesn't look big enough to grab a firm hold of the feathers.

By the way I used an ASP 108 and it flew just great.

I built one of these planes last year and really liked it, but I made quite a few changes. I shortened the wing by one rib bay on each end, I made it into a canopy style cockpit instead of open, and I closed in the engine compartment similar to a sportster. For power, I used an OS 160 mounted upright with a standard muffler. This was one sweet flying airplane. Now for the bad news. The tail section disintegrated on flight #6 and resulted in a total loss. And yes, I was using flying wires made from 4-40 rods. I do plan to build another one, but I will use solid sheet balsa on all the tail feathers instead of the built up ones. It should'nt matter as far as weight goes as I had to add several oz. of lead to the tail for balance anyway..........RS

I had a Stingray 120, which is pretty much the same plane, except midwing instead of low. Mine was covered in koverall and nitrate dope. The color coats were Black Baron Epoxy. An ASP 1.08 made for a good match. I sold it after 2 yrs to fund other projects.

The stock landing gear is terrible and would drag in grass. You'd be much better off substituting any suitable aluminum blank. Excellent elevator response, but the ailerons were slow to my liking, even with lots of throw. If I did it again I would clip a bay off each wing.

Regards,
Kelly

Yes, it looks like the Stingray has the exact same wing with a little less dihedral. I think BalsaUSA might have simply flipped the fusealge upside down.

Funny everybody seems to like the Magnum/ASP 1.08 on this plane. I have the Magnum version and like it very much. I had it earmarked for my full-wing 4*60, which BTW is only a bit smaller than a clipped wing Thunderbug. Unfortunately, a friend of mine needs it more than I do for the GP PT-19 I'm putting together for him. In any case, if everybody is happy with the 1.08 on this plane, the TT 1.20 should be a blast.

I looked at the kit lumber more closely again. All the sticks are straight and of appropriate weight. The diecutting, while not as nice as laser-cutting, is very crisp and far superior to the die-crunch of the last couple of GP kits I had.

Russmall, sorry about your loss. Hmmm... the tail feather seems to be a weak point of this plane. Can you tell if the tail feathers just broke free of the fuselage, or did the open trusses just gave up? Now I'm thinking I might sheet over the built-up tail feathers and use flying wires.

bluther, what about yours? Did you have enough plane left to do a postmortem on how it failed?

If I had it to do over I would sheet the entir empenage. It looked like the stick structure was not stout enough for high stress. I had about 15 flights on her and I was coming out of a pretty steep loop when they departed. Needless to say it was a total loss.

Volfy, I was never able to find the tail pieces so I really don't know what failed. But, it was not the glue joints at the fuse as they were still intact. The conclusion that most of us reached was that the weak tail structure began to flutter and simply blew apart. I would strongly advise you to use solid balsa sheet for all tail surfaces along with good strong tail braces. It won't be that much of a weight factor...........Russmall

My version of tail feathers for a Stingray 120 converted to low wing. I cut cores from white foam and sheeted them with 1/16" balsa.
This was an old Stingray 120 kit that I never got around to building.

vertical stab, before cutting out rudder.

Horizontal stab cores.

After sheeting

contrasted to the plans.

Elevators

Since the tail is now a flying airfoil instead of a plank,
a saddle is required to fit the horizontal stab.

I`ve have one in a box I picked up at a fly in,dirt cheap..I have a couple of ST engines..One is a 2000 the other is a 3250. I need to try one of those on something..Compared to an ACE R/C Big
Bingo the Thunderbug 120 is kinda homely..

Does anyone know the CG to this plane

Give me a day or so and I`ll dig out the plans and look for you..Usually if you`re balanced on the main spar,then your close enough.