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Thread: Sig Kobra


  1. #1

    Sig Kobra

    • Wing Area - 390 sq. in.
    • Wing Span - 45 in.
    • Rec engine - .19-.35
    • Engine used - Norvel BigMig.25
    • Finished weight - 3 1/4 pounds

    Hits:
    • Well Engineered
    • Excellent Instructions
    • Wood choice
    • Hardware
    • Fantastic Die Cut Ribs
    • Flying Charcteristics

    Misses:
    • Old Style Building (cut some of your own parts)

    My sons have been nagging me to build and fly a jet. To satisfy them and use existing engine and radio we built a Sig Kobra. The Kit is good but old style. Die cut balsa and ply parts with fuselage and tail feather shapes printed onto balsa for the modeler to cut out. The engineering is simply outstanding. The wood in the kit is all top quality and the right densities for the application.

    Wing Construction. The Ribs are all tabbed to produce washout, the instructions stress this and solid time proven building techniques to insure a true wing with the correct amount of washout. Everything is standard construction using almost exclusively 1/16 or 3/32 balsa. The bottom spar is pinned to the plans, the ribs are attached and pinned down to insure the wing is true then glued to the spar, Top spar, trailing edge, are added, trailing edge is tapered to match air foil, and top part of trailing edge sheeting is added while it's all pinned down. The leading edge is built up from several layers to produce a very light D tube with nearly flawless joints. The instructions seem vague but the plans are clear and once you start building you get the general idea. The spars are trimmed using the dihedral gage and then the wing is removed from the plans and the bottom leading edge and trailing edge sheeting are installed. Repeat for other wing half, join the wing halves, then root rib is glued in place, add landing gear blocks, 1/16 ply supports and sheet the center section and outside wing bay (only about 1" wide). Then add cap strips. Glue on the blocks on the ends and cut away everything that doesn't look like a wing tip. The very light 1" X 1/16" balsa sheets used to create the D-tube leading and Trailing edges is very low density and during construction is easily cracked so be careful. That said now that the wing halves are built and joined the wing appears and to be very strong and is very light. Torque tubes are of typical installation.

    The fuselage is built by first gluing 1/4" tri-stock on the printed lines where the top and bottom of the fuse will be. The fuse sides are made of 3/32-balsa sheet. Then attach a 1/32-plywood double on the front section of both halves. A few 3/16 square balsa stiffeners on the back sections then cut out the sides. They are then lined up and sanded so they will be exactly the same size. It is basically a light box fuse. The sides are then matched up to formers over the plans and glued in place. Cross grain sheeting is then applied to the bottom. The entire fuselage from the firewall back is very simple. This is a typical box fuse like an ugly stick, quickie 500 or similar. The differences come in when you add the turtle deck and cowl in the engine. The engine is mounted with it's spinner and blocks are glued to the nose and the spinner is traced to the end of the blocks. The engine is removed and you then carve away the nose until it matches the spinner outline. When done you have a very stream lined fuse that fits your engine exactly and looks great. Cutting out area for the nose steering is a little tricky.

    The Tail feathers are printed onto balsa sheets, you have the choice of cutting out a T-38(F5) or an F-16 style tale. Because the rudder area is larger in the T-38, they were cut out and assembled.

    The wing was mounted to the fuse per instructions. The stabilizer was test fit to the fuse and the fuse sanded until the stabilizer was square to the wing. The stabilizer is aligned to the fuse and glued in place. The fin is squared up and to the stabilizer and fuse and glued in place. The turtle deck is one piece ABS plastic that is attached to the top of the fuse, the jet-like canopy fits the same way. Not having a pilot of appropriate size, I simply painted the inside of the canopy with silver paint. Then cut and sanded the fin fillets to shape. They match the turtle deck and reinforce the rudder mount.

    Since this was a project for both my son Spencer and myself the color scheme was a joint project. The Kit comes with Thunderbird decals and although nice they weren't the color scheme my son wanted. So Blue Angeles it is.... The entire top of the fuse and the fillets were painted with TopFlight LustreKote Insignia Blue paint. This way I didn't have to figure out how to cover the ABS turtle deck. The rest of the plane was covered in Corsair Blue UltraKote. To turn our dark blue Kobra into a U.S. Navy Blue Angles demonstration team jet we needed the Blue angles logo, shield, big US Navy on the bottom of both wings, etc. Lots of cutting and fitting of Ultrakote to create custom lettering, logos etc. (Not so cool), I was dreading this part. Bull Fulmer came to the rescue http://www.flightbox.org/customcut. I emailed him the wing outline and after a phone call he cut a set of Blue Angles Logo's, and tapered US NAVY for the bottom of each wing panel. We got a great surprise in the package. Bill knew that no Blue Angles Jet would be complete without the famous Blue Angles Shield so there were two appropriately sized, included. He did a great job. They were easy to apply and really set off the model. All I needed was a yellow dart on top of the turtle deck and yellow wing tips to be complete.

    There is plenty of room for standard servo's, RX, and battery. With the light Norvel .25 a 700mah nicad battery pack was placed between the ruder and elevator servos to balance the plane with no additional weight. The completed weight with a 1/2 full fuel tank was 3 1/4 pounds, about 3 pounds dry. When you have it in your hand it feels heavier but it's a light model. The Kobra has a finished wing loading of 18oz/sq.ft.

    I took the Kobra to our club fun fly hoping to get a test flight or two before the fun-fly. The Norvel BigMig.25 hand started easily, and was adjusted about 1/8 turn rich from max RPM with a MAS 9-4 prop. After taxiing it around the field a couple times I pointed it into the wind and advanced the throttle. It accelerated briskly and stuck to the runway like it was glued down, when it had way more than enough air speed I added a touch of elevator and it jumped into the sky, climbing like a lovesick angle. Three clicks of down and a click of right aileron and it flew hands off. Flying the Kobra just confirmed what I'd read about it previously. It was very smooth and precise. After a couple circuits of the field I did some mild aerobatics. Changes in speed did not effect pitch. Rolls were a little slow for my tastes but very axial, and lot's more throw can be added. Rudder input had zero coupling. I had more than the recommended amount of elevator so high speed full deflection loops lead to a small high speed stall. Knife edge was incredible, rolled on it's side than give it full deflection of the rudder and it before I knew it, it did a very 1/2 knife edge loop, surprised I released the rudder and leveled out. Knife edge only requires a little rudder. Snaps were fast and stop instantly. I did a few split S's and Cuban 8's. Spins are fast and stop instantly. Flat spins are simply a matter of cross controlling aileron once a regular spin starts. I was very surprised at how well it flew aerobatics. This plane will make anybody look good. Landing is absurdly easy, line it up to the run way and cut the power about 500 feet out, when it's a foot or so off the runway gently pull the nose up and it will settle in a nose high landing on the mains, once the front wheel touches it's glued to the runway. It's thick airfoil and light weight limit it's glide so I wouldn't try and stretch out a dead-stick landing. On our clubs rough grass field it didn't bounce once in it's first day of flights.

    After the test flight we started our clubs fall fun fly. I was going to fly my trusty old Four Star 40 but since the 1st event was a taxi race through a slalom course and the Kobra performed so well on the ground I decided to fly it in the fun fly. The events were a taxi race through pylon cones, take-off do three loops three rolls and land, Spot landing, two-minute flight, and most loops in a minute. I won 3 of the 5 events outright and took a 4th and a 3rd so won the fun-fly overall, not bad for the first flights with a new plane. During some of the events I was able to check out the Kobra. Testing the power of the little Norvel I pulled vertical after a long run down the runway, it has unlimited vertical by the time i stopped climbing it was a very tiny dot. I slowed it down, and even at 1/4 throttle settings it was capable of rolls and loops. It could maintain level flight at less than 1/4 throttle and the controls remain crisp and responsive through the entire speed envelope. Towards the end of the contest the wind picked up quite a bit. The little 3-pound Kobra penetrated easily and handled very well in the wind. At slower speeds gusts would bounce it a little but not nearly as much as you'd expect from a 3 pound plane.

    This is an old style kit built over the non-cad designed plans with no tabs or fancy notches or laser cutting. It is very well engineered and the parts fit very well and it built a light sturdy airframe. Compared to the other Sig Kit's I've built in the past two years the Sig LT25, and Something Extra it's more work but the final results are worth it. Its flying characteristics are outstanding, that's probably why after decades Sig is still making the Kobra. From first flights the Kobra need not apologize for being a small .25 size plane and can hold it's own against it's bigger brothers.

    Tom Rhodes
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