Hi, Lloyd-
'sorry to be so late to this party, but here's my $.02 worth. The Sterling Yak-9 was my first "big" model, which I built around 1960-61 from a kit, with no wood substitution and only the information given in the plan instructions. I used the movable flaps, just as shown in the instructions, and I'm pretty sure I used a 3" aluminum Perfect bellcrank. There never was an original full-sized plan, and I have found aspects of later plans and kits that did not conform to what I found in my original box.
As you'll see from the attached plan (from the instruction sheet), the horns were die-cut plywood. I used them and attached everything with Ambroid. You will also see from plans and picture that the flaps are not as shown in later plans, and I find that a nice kit prepared for me by a great kit producer were larger and of different shape. My original flaps seem to be between the extremes and, to me, look better.The photo is of the actual kit flap from a Sterling kit I bought later in high school. I replaced it with better wood, when I completed the plane forty years later. My kit flap connector was a very stiff wire with only a single bend in the center. I have since learned that using two bends really decreases the twisting of the flaps, due to the swept forward hinge line. In fact, I have seen a couple with pinned hinges, whose flaps have no centering torque and actually drop of their own weight, when the model is stationary.
I built mine strictly according to what was printed and powered it with a $5.98 McCoy Red-Head .35. I probably got lucky ('didn't use a jig and knew nothing about balsa being too heavy), because it flew very well on 52' lines. I knew nothing of any "stunt pattern" back then and only did lazy eights and wing-overs. Surprisingly, it turned fairly tight. The plane met it's end when I crashed it for the second time trying to do lazy-eight vertical eights with maximum height of only 60-70 degrees and the outside loop at the bottom. The wild part for me was that I actually accomplished a couple of those. My point is that despite being built from a stock kit with no replacement wood, it flew well. I may have lucked out on the wood.
The drawback of the design is that it is short-coupled; that is, it has a short tail arm. The negative flap pitching moment fights the elevator, which needs to be on a longer arm. So this is not ideal for a flapped model, and I'm guessing that it would fly just as well with the flat flaps held stationary, which BTW were shown by the NACA in one report to be superior in maximum lift coefficient to airfoiled flaps (contained in wing contour) at neutral. They needed better drag figures, of course, and those data favored the streamlined shape. As far as control ratios are concerned, why not just set it up with a bit more elevator than flap deflection and play with the handle spacing and other ratios to get it to fly to your own preferences? I doubt that anything "normal" will be dangerous to the plane.
Anyway, I really loved my Yak-9 for fun days at the flying circle. The silver one below is the one that took 40 years to complete. The wing tips are modified (heavier and smooth), although they are the same in plan view.
Topic: Sterling Yak 9 controls (Read 3364 times)