Improving a Sig Skyray 35

Sig Skyray 35

Improving the Sig Skyray 35

The Skyray 35 builds into a nice plane even stock. In spite of suffering from my mistakes in building, lack of experience, heavy finish, etc, in the words of one of my friends, "it flies beautifully." However, there are some things about the kit I felt could be improved, and since I already had a second one on hand, I decided to see if the Skyray could be made even better. Mostly, I wanted to decrease the wing loading and improve ground handling.

Fuselage

I thought the stock front end looks a little ugly from inside the circle, what with the exposed engine beams and blind nuts, so I decided to add a 3/8 balsa tripler on the inside and use threaded brass inserts instead of blind nuts to mount the engine. To make up for the extra weight I did a lot more sanding than I did on the original Skyray. In fact, when I was done the 3/8 tripler ended up about 3/16 thick. The other change I made was to the landing gear. I don't really like the stock, single wheel wire gear. It's hard to get the plane to take off from grass, it bends almost every time I land, and it doesn't look like it would work too great for launching from a stooge. So, I replaced it with a Dural gear from RSM Distribution. They sell two sizes, for this project I used the large one because I want enough clearance for 10" props. To mount the gear, I recessed the balsa tripler so the inboard gear would mount directly against the inboard ply doubler. I also positioned it so both bolts would go through the lower maple engine bearer. This turned out to be a mistake, as the final position of the gear ended up exactly where my FP 20's 2030 muffler wanted to be, so I guess I'll be running an LA 25 with tongue muffler. If I were to build another one, I would extend the bottom engine bearer back to the wing leading edge, allowing the gear to be mounted back out of the way of the muffler.

Wing

The wing on my first Skyray had a pretty wavy trailing edge due to the fact that the die cut ribs weren't all identical. Conventional wisdom is to trash the stock plywood ribs and subsitute balsa ribs - lighter and more accurate. Since I can't bear to throw anything away, I stacked the kit ribs and sanded them identical, then clamped them together and drilled some lightening holes. I'm going to have a straight strong wing, right? WRONG! I neglected to pay attention to the 3 center ribs, so while most of the wing is now wave-free, there is a nice bow along the center trailing edge. Also, because I was in a hurry, I drilled the lightening holes in the ribs by hand, 13 holes in each full rib and 4 holes in the sub ribs. If I had been patient and waited to use my friend's drill press, I could have easily gotten 27 holes in the full ribs and 10 in the sub ribs. One thing to keep in mind when drilling the ribs is that the wood is very brittle, so if they aren't clamped together very tightly, the drill bit will grab as it breaks through each rib and rip out a little (or not so little) chunk of wood - ask me how I know:)

The other change I made was to increase the wing area. I spaced all the rib stations (except the center ribs) an extra 1/8" gaining 2" in wing span, and added fixed flaps; 1-5/8 at the root tapering to 1/4 at the tip. I also left off the (ugly IMHO) stock tips and went with simple 3/16 end caps. I ended up with about 450 sq. in. vs. 396 for the stock wing.

Tail

An accepted ballpark figure for the horizontal tail on a stunt ship is 18 - 20% of the wing area. Since the tail on the Skyray is far smaller than this even with the stock wing, I decided I needed to enlarge the stab and elevator. Also, the 1/8 thickness of the stock tail seemed a little "iffy" so I went with 3/16" material. Finished area of the new tail is 81 sp. in., 40% in the elevator and 60% in the stab. Fin remains stock. One problem - my fancy new elevator and stab weigh a combined 10 lbs!!! Well, at least it seemed that way until I had a friend put them on a scale. Turns out they really are 2 oz. combinded, including hinges. Sure feels heavy though...

Finishing

I decided to go with Monokote on the wings with Rustoleum on the fuselage and tail. Just can't beat this combo for a quick and easy finish that looks good enough. I found the flat tips much easier to cover than the stock ones. My non-stock landing gear ended up interfering with the tank mount as well as the muffler, so that will required some clearancing in the future, but it's together now and I WANT TO FLY!!! All up weight ready to fly is 30 oz, which is better than I expected.

Flying

New Year's Day dawned bright and pretty, about 20 degrees warmer than the previous week, no wind, and no more excuses not to fly this thing. Balance and elevator nuetral checked out OK, and I was the first one up to fly. Took some pictures first just in case... I decided to replace the 9x4 prop on the LA 25 with a 10x4 in an attempt to simultaniously slow the plane down a little and increase the thrust. Engine started easily as always, went to the handle, double checked elevator nuetral and control movement, gave my friend the release signal, and...

Well, hung on for dear life until the engine quit, plane glides well (guess that extra wing area is good for something) and lands very nicely, unlike the flip overs that were normal with the stock gear. For the second flight, I went back to the 9x4 prop, richened the needle about half a turn, and moved the pushrod 2 holes down on the control horn. Smaller prop had no problem getting airborn and flying the plane. This flight was much more manageable (read less scary). It was still a little fast, but I was able to do some climbs and dives (remember, I'm just a beginner pilot here) to see how it responds to the controls. Flies nice, straight, level, and smooth. Second landing was also smooth. I don't think I'll be using single wheel landing gear any more... Decided not to push my luck and called it a day. Need to do some things before the next flight any way.

Closing Thoughts

I think this has real potential, and some of the improvements make this airplane much nicer to fly than the stock Skyray. The biggest improvement was the landing gear. This airplane is very easy for me to land well, something that was almost impossible for me to do with the stock landing gear (I was flipping the plane on almost every landing). Also, it doesn't get bent out of shape like the stock gear. I do wish I had paid more attention to the LG placement though, so I could run the stock OS muffler with an FP-20.

The added wing area also appears to have benefitted this airplane (maybe I'll change my mind after flying in the wind). It glides a lot better than my stock Skyray. I was worried that the extra wing area (drag) would make the 25 LA marginal in the power department, but so far that doesn't seem to be the case. If anything it pulls this airplane better than it pulls my stock Skyray.

The one modification that I think was a waste was enlarging the stab and elevator. My original flew very smooth, but this one is way over sensitive with the pushrod in the same hole. Putting it 2 holes down tamed it some, and I feel I can get the rest of the way by going 1 hole further out and/or narrowing the handle spacing. At best though, it will be exactly as smooth as the stock stab and elevator but with more weight from the added wood. When I build my next Skyray, I'll keep the stock stab and elevator. This has been a really fun project and I think I learned a thing or two. Can hardly wait to build my next Skyray...

Updated January 2, 2007

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