Hi Everyone,

I have realized I really like working on the planes in the shop and have purchased a SIG 4star 120 for my first kit to build. I would like to build this plane as light as possible, the engine on this will be a Siato 150. I also plan on covering the plane in transparent and solid ultra cote. The directions call for full balsa sheeting of the wings, instead I would like to put caps on the wing ribs to save weight, support the covering and to give a build view through the transparent ultra cote I would cover the wings with. My question is will the caps work and be strong enough so the wings wont break or do I have to use the sheeting to get the strength needed at the additional weight cost ? Also are there any other lightning mods anyone has successfully used with this plane?
If you feel you would like some would like some background on me, my abilities and skill level to properly answere my questions you may read on. I was a Journeyman tool and die maker for many years but currently work as a tooling engineer. I am very accurate with my work and am use to working with .0001 inch tolerances. In the past year and a half I have properly assembled /modified & improved 9 ARFS including trainers, low wing secondary type trainers, a 100" cub, a DP 330 Extra, an AW 33% 540 Edge, a 120 Kange Cap 232 and my last project which was the conversion of a new 120 Ultra stick to the flapped Low wing inverted Jim Feldman design in RC magazine. ( Ialso increased the wing span to 80" and added additional size and counterbalance to rudder and increased the elevator by 1" Elevator and rudder servoes moved to the rear with carbon fiber linkages, throttle servo and radio gear moved far forward and fuel tank on the CG. This helped lighten the plane and balance cam out spot on with no additional balance weight needed. This was a great project and the plane rolls much more axial and knife edges beautifully. I guess I used ARFS to learn about different types of construction and general RC plane knowledge. I realize the directions on ARFS always say 20 to 30 hours to build but I find it usually takes me well over 100 hours to properly go over them and correct building flaws, check/correct incidences,install top line light weight hardware and in general make them correct. Many of our club members have flown my planes and commented that they are light, strong and fly wonderful and I have not had a single failure of any airframe component. I'm sorry this got long winded but would like to say Thank You in advance to all who take the time and effort to reply.

Wow, that's a lot info there, Jim. Kinda makes me feel bad if I don't offer some suggestion.

With the 4*120, you couldn't have picked a better first kit based on your experience. I'm surprised that it calls for full wing sheeting, since the 4*60 and 4*40 are both open rib designs. Depending on your style of flying, you could eaily get away with just sheeting the D-tube (i.e. forward of the main spar). For added strength, you could laminate carbon fibre strips between the main spars and sheeting. At the bare minimum, you should sheet at least the top of the D-tube. A completely open bay wing will not have much torsional rigidity, so even it the wings don't fail they'll prbably twist in flight and cause the airplane to fly weird.

Cap strips don't add anything to the over strength of the structure. They act merely as "fillers" to be even with the sheeting thickness and, at most, stiffen up the ribs laterally.

Personally, I much prefer to save weight by replacing heavier balsa with lighter contest grade stock, instead of changing the structural design and skimping on construction. Built per the instructions, the 4*120 is already very lightweight. With a plane this big, the few Oz. you'll save will hardly be noticeable.

BTW, I agree about ARFs' overly-optimistic estimate of the commonly advertised 20-30 hrs or 90% finished, for those of us who habitually fuss over details. Just bear in mind that for a lot of folks who just want to slap an airplane together and go fly, 20-30hrs is probably about right.

Acording to my directions you only sheet 4" on each wing, not the whole wing. I joined the wings flat on the board, and sheeted across the wing joint for more strength using one piece of sheeting. A foam turtle deck might be lighter than stringers, and much can be done to lighten the tail if one desires. Clipping the wings two bays will also save some weight.

I just finished a 4*120 several weeks ago ,nice sport plane,
and flys well. A nice change from the higher wing loaded stuff.

The wings are not fully sheeted, only several bays in the
center area. I suggest that you take another look at the instruction booklet .

The construction is identical to 60 and 40 versions

Saw a guy flying one of these out at the field last weekend. His only had sheeting on the D-Tube. The ribs were exposed and it had transparent covering which made it look very good.

Flew excellent. He had a Saito 180 with loads of reserve. The 150 should be great.

Mike

Yeah, that's what I thought. I remember seeing Marty Hammersmith's Ryobi 31 powered 4*120 on his website. I went back to take a look, sure enough the construction is identical to that of the 40 and 60: open bay with turbulators on the D-tube.

Geeezzzzz --think I'll change my name to Red faced who flies RC4fun :stupid: . Guess I did a lousy job of my quick over look of the instructions. Thanks everyone for pointing out my mistake and helping me get it right.
I have read a couple of threads where people built the wings with no dihedral and i'm guessing here that it was to improve on coupling needed for rolls. knife edge, etc. Is this correct or is there some other reason ? Has anyone flown both a stock and no dihedral 120 and can tell me the difference in handling?
I appreciate all your ideas especially like lighter contest grade wood and carbon fiber strips for strength.

RC universe has the best group of knowledgeable and helpful individuals of any website I have ever worked with. Thank you all for taking the time and effort to enlighten us less knowledgeable about this great sport.

Jim

Jim,
I have built a 4*-60 with only half of the dihedralthat the plans calls for and it flies a lot better (i, myself think), than another at the field with none... just my 2 cents
Chris

I bought a used Ultra Stick 120 for my son to train on, however I think I need to replace the fire wall and was wondering if anyone could tell me the location of the thrust line and CG.

Hello, if you referring to the Big Bingo 4-120, cg is right on the main spar., 4 1/2" from the leading edge

Michel