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Flat Top Pistons, Three Deuces and a Three Quarter Cam

Remember when the hot setup for competition meant an audio variometer, a final glide calculator and a coat of wax? The price of admission is a little higher today. Active competitors pursue the ultimate l/d point with pricey solutions like wing wipers and microcomputer based flight computers. But they don't have a monopoly on useful speed mods. There are technological improvements today that are every bit as useful to the weekend pilot as they are to a world class hotshoe.

What would you pay to increase your polar speeds, reduce control effort, increase control power and improve the stall characteristics of your plane? The way we usually make a big change like that in our sailplane is to trade it in on another glider. An approach only slightly less extreme is to sand and profile the wings. There is no getting around having the right plane or the right wing. But if the old bus is just feeling a bit creaky compared to the latest glass slipper, and a heart transplant or major rebuild are not in the offing, then perhaps a less drastic approach would make more sense: Control Seals.

"Control Seals? Right. Look Peter, I've already sealed the control rods in the wing roots with airtight tents, closed the gap between the wings and the fuselage with flexible tape and trimmed off my homemade tip skids. What are control seals?" The airflow around a sailplane wing is as complex as it is subtle. As much as we'd like to think that our wings are near perfect, they usually are not. The good news is that there are things that we can do to improve that airflow. Let's go look at your wings.

Nice, huh? Smooth, more or less laminar and shaped to seduce and cajole the airflow to lift the plane, and you, into the air. But what's that moveable thing on hinges at the trailing edge? An aileron. It changes the shape of the airfoil when we move it up and down with the controls in the cockpit. That changes the lift over that part of the wing, and that is what changes the direction in which the plane will point next. Right stick, aileron up on the right wing, aileron down on the left wing: Lower lift on the right wing, higher lift on the left wing and the plane leans to the right.

But the aileron is only one of the moveable surfaces on the plane. There are hinged things hanging on the back of the vertical fin and the horizontal stabilizer as well. The whole plane works on moveable surfaces that change the shape of the airfoil they are attached to. Even the rudder works on the principle of changing the pressure differential between one side of the vertical fin and the other to cause the back of the fuselage to move toward the low pressure side. If you have a 15 meter class bird, flaps change the airfoil for different reasons and add another moveable surface that puts hinges along the whole back edge of the wing. There are a lot of things wiggling around on the trailing edge of sailplane airfoils. When you consider how sensitive the wing is to minor variations in shape and surface condition, the changes the controls make are aerodynamically pretty crude. At best they change the airfoil shape enough to move the plane, and in the process create drag, leak pressure and generally do things we wish they wouldn't. This is where control seals can help.

When you deflect a moveable surface, you want to move it as little as possible for the maximum pressure differential with the least drag. Aircraft designers have done as much as they can to minimize the drag from control deflection by moving ailerons up more than they move them down to generate the same amount of drag on each wing. There is nothing more that we can do about this kind of drag. It's the price of having controls. But a related problem occurs when the higher pressure on the bottom of the wing leaks through the gap between the moveable surface and the wing itself. This cancels out some of the pressure differential and causes the pilot to add more control deflection to make up for the leakage. Add that to the drag of the variable size gap caused by assymetrical hinges and you pay a substantial price every time you move a control. Control seals can do something about this kind of drag.

Control Seals streamline airflow across gaps, and block airflow between moveable surfaces and the airfoil they are attached to. Newer aircraft handle this in a number of ways: Flexible fiberglass on the top surface of Speed Astir flaps and ailerons, foam rub strips in ASW-19 elevators and S-shaped fabric strips inside LS-3 flaps to name three. Center point hinges help too. Newer aircraft are generally sealed better by design. Their gaps tend to be small and symmetrical. Many have internal seals already. Older aircraft generally hinge the movable surfaces on one side, usually the top, and leave a variable size gap on the other. It is on older aircraft that control seals can make a real difference.

"Oh, those control seals. I have them already. 3M tape along each hinge line." Yup. I had them too for a while. I put it on with the moveable surface deflected full down so there was excess tape to take care of control movement and contraction from age and cold. It cracked where it passed over the hinges and stood up a little but it looked like it was doing the job. It was too narrow for the rudder gap so I used something that looked like white duct tape and left the hinges exposed. The tape approach didn't work as well as I'd hoped. Cracks and gaps over the hinges let pressure bleed through just as it would through a puncture in a tire and defeated much of the purpose of the tape. The size of the hole didn't matter as much as the fact that there was a hole. Tape on top of the wing didn't do much to streamline the gap on the bottom of the wing and stabilizer either. But I thought I was doing the right thing at the time. Then I discovered Mylar.

Mylar. Racer stuff right? Custom gap seals in routed channels, available from secret German labs and installable only by certified, factory approved elves. Wrong. Mylar is available from your favorite source of glider stuff. Knauff and Grove carry it in 16.5 meter rolls. Eastern Sailplanes will cut it to any length you need. Look for it in their catalogs under accessories. Perhaps the toughest thing about buying it is figuring out what widths you need and how you apply it. That's where this article comes in.

Think of Mylar as a three part system: Mylar, Adhesive and Finishing Tape. Mylar is an off-white, cambered strip of semi-rigid plastic that comes in standard widths: 22 mm, 30 mm and 38 mm. It comes in wider chunks than that too. I used 30 mm. If you finish the top surfaces only, 16.5 meters is enough for two Standard Class Birds. Double surfaces on an Open Cirrus take a roll and a half. If you seal double surfaces on a 15 meter bird you need a little more than two rolls. In addition you need Transfer Tape. This isn't really tape as much as it is adhesive in roll form. You lay it down like tape, burnish it onto the surface and peel off the backing. Just get it straight the first time. There is no removing this stuff intact, and irregularities will affect the fit of the Mylar.

The Mylar itself is easy to install when you get the hang of it. Start with the top of the wing. Lay the Mylar down with the concave face up in front of the adhesive and butt the trailing edge against the forward edge of the adhesive. Then flip it over onto the adhesive at the center and press it out to the ends. Burnish it from the middle out. The bottom of the wing takes a slightly different technique. You can take off your wings and flip them over, or you can leave them on and hang the Mylar on the leading edge of the adhesive like an open hinge and then close the hinge. The 12 mm White Plastic Finishing Tape is then put down to seal the mylar strip to the wing surface and keep anything from getting under the leading edge. There is a fourth component that you might consider using: A layer of teflon tape on the moveable surface under the edge of the Mylar. It provides a bearing surface for the Mylar to slide on. It was used a lot to cut friction when Mylar was first introduced but isn't as prevalent now. Routing channels to make the Mylar flush with the wing was popular for a while. That's fallen out of favor now too. The thickness of a layer of Mylar is generally well within the boundary layer at the trailing edge of the wing.

There are a couple of caveats in all of this. The real danger with 3M tape is that it will stiffen and contract, limiting control movement. Mylar is semi-rigid and has its own peculiarities. Particularly with asymmetrical hinges, gaps on lower surfaces can be quite large. It is possible that a mylar strip that is too narrow, or placed too far forward on the airfoil surface could slip into the gap on full control deflection and jam the control. That could get your attention in a hurry if it happened in the air. Remember that the seal can come under considerable air pressure. When you put semi-rigid gap seals on a plane the Mylar must solidly bridge control gaps at full control deflection. The first flight after applying this procedure should be preceded by a very thorough preflight with full extension of all surfaces and should be flown as if you were the test pilot on the first flight of a new aircraft. Wear a chute in case you discover something you didn't find on the preflight.

Is it worth it? Yes, in ways I hadn't even thought of. I haven't done anything like the Dick Johnson Flight Test procedure on my Open Cirrus. But I can draw several subjective conclusions immediately. Control pressures are reduced. The ailerons were always heavier than the elevator and rudder. Now they are better balanced. All controls are more sensitive. I don't have to move them as far for the same result. Sink rate at speed is reduced. Air noise is lower. With the vents closed the noise level at 80 knots is about what it used to be at 60. The stall has changed. It was always straightforward. The plane would stall at a little below 40 knots and drop through straight ahead with no wing drop. The stall speed hasn't changed, but now there's no drop. With the stick full back the plane will stall, the nose will sink through the horizon until the speed rises to 45 knots and the nose comes up until the cycle repeats itself. The ailerons are functional throughout the cycle.

Mylar is expensive when you compare it to a roll of hardware store tape. A 16.5 meter roll of 30 mm Mylar is about $66.00. A 50 meter roll of 12 mm adhesive Transfer Tape is about $12.50. A 55 mm roll of 12 mm White Plastic Finishing Tape is about $4.50. When you consider how a hundred dollar bill and a rainy day in the hangar can effect the performance of your plane, it could be the most cost effective project you can tackle.

Any silver lining has its cloud. I stripped the area around the movable surfaces with Sears Epoxy and Lacquer Thinner to ensure a good adhesive bond. It worked but the surface was whiter than the rest of the wing. My wing wax had discolored over time. Not much, but enough so that it was noticeable. Rather than just strip the wax I decided to polish the wings before I applied new wax. Have you ever hand polished a 17.5 meter span? One thing always leads to another. It sure flies nice though. Shiny wings and smooth gap seals look good too. It's a keeper.

Posted: 2/23/1996 By: Peter King


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