Lexan strut fairing how to

[C5Engineer 1170]

Stole this from another site. Seems like a PIA to do a big strut but it might come in handy for someone.

 

LEXAN STRUT FAIRINGS

I experimented with different ways to streamline my struts and other exposed tubing for years. I finally found a way that works well, looks good, and is light and inexpensive, using polycarbonate (Lexan) film. They come out lighter and cleaner looking than any other strut fairing I've seen.

The film (which is what they call anything thinner than 0.060") is available from:
Plexiglass Sheet, Acrylic Sheet, PLEXIGLASS ONLINE STORE, UHMW, Industrial Plastic Sheet, Rod or McMaster-Carr

They stock the polycarbonate film in 2' by 4' sheets, and roll it for shipping. It looks best if you do the entire strut in one piece. But making a segment more than 4' long would be quite difficult. Plan on wasting a couple full sheets for practicing and goofs.

I found that 0.015" thick film is ideal for tubing 1" diameter or less. 0.020" is best for larger diameters, but will also work on smaller tubing for planes that fly a little faster than U/L speeds. Streamline any tubing that's near perpendicular to the air flow.

Expect to gain about 1 mph in cruise speed for every 4' of tubing you streamline. I put about 28' of it on my experimental U/L, and gained 7 mph in cruise and top speed... which is huge. And the plane climbs and glides a lot better. I don't recommend this streamlining method for faster experimentals. It's also not suitable on tubing that vibrates a lot.

A 4 to 1 chord to camber ratio is standard for strut fairings on faster planes. But Lexan fairings with that much chord won't look right nor hold their shape well. At U/L speeds, 3.5 to 1 is just as effective, and looks and works well. Even 3 to 1 is a reasonable compromise, especially for large diameters.

CUTTING

The film comes with a protective plastic on both sides. One side is translucent white, and the other is clear. Leave both on for now.

Cut strips of film about 7" wide for 1" struts, 9" wide for 1.25" struts, and proportionately wider for larger struts. Measure it accurately, mark it with an ordinary pen on the white side, and cut it very carefully with sharp scissors. Once you get the touch, you can just push the scissors through in a continuous motion. Cut the end of the strip to the length of your strut. Or you can leave it full length and cut off whichever end looks worse after heat forming.

HEAT FORMING

Heat forming the leading edge is the touchiest step. Using spring clamps every few inches to clamp the trailing edges together, carefully bend the film into a "U", without creasing it much, with the white side out, and the trailing edges perfectly aligned. Stand it up on the spring clamp handles with the leading edge up.

Using a heat gun set on low, pointed straight down very close to the leading edge of the film, begin heating the leading edge using side to side strokes several inches long, starting at one end. The film will get a sharper curve on its leading edge when it reaches the right temperature.

But the melting point of the film is just a few degrees hotter than the heat forming temperature. So it is extremely easy to overheat a spot which creates a dip in the leading edge. You have to work slow, using a smooth, continuous motion with the heat gun.

It takes practice to get just the right curvature and no melted spots. And the longer the piece, the greater your chances of melting a spot. A melted spot will look like a dent in the leading edge, and is impossible to remove.

I heat formed the one in the second picture a little too much. Yours should come out a little wider.

The camber of the airfoil will decrease as you heat form it. Heat form it just enough so the camber of the completed fairing is a little less than the diameter of the strut. In other words, the strut will hold it apart a tad. If you heat form it too much or too little, the airfoil won't be as efficient. Heat form it evenly over the whole length.

For larger strut diameters, you can make a larger radius leading edge by angling the heat gun and making two passes, about 1/2" to each side of the center of the leading edge.

When you're done, remove the spring clamps and the protective plastic from both sides of the film.

TRAILING EDGE ALIGNMENT

Now another touchy step. The trailing edges need to be lined up with each other both chord-wise and span-wise. A span-wise misalignment will create a twist in the completed fairing that will look terrible and be less aerodynamic. Even if you cut the ends of the film carefully, you can't judge span-wise alignment by the ends.

Clamp the trailing edges together on a straight 3/4" by 3/4" or 1" by 1" board, at least as long as your fairing, with spring clamps, with at least 1/2" of the trailing edges hanging past the wood, and the trailing edges aligned chord-wise.

Support it by the wood only, above your workbench, with the leading edge facing horizontal, out towards you. Stand back several feet, and see how the bottom of the leading edge is parallel to the bottom of the board, comparing the ends. Be super critical.

The forth picture is way twisted.

You can work the top film to one side, by pushing hard with your thumb between the clamps, starting at one end and pushing towards the end you want to lower, momentarily releasing each clamp as you go. Keep working it until the ends of the leading edge are perfectly even, relative to the wood, and the trailing edges are lined up. It's okay if the middle bows a little. But if it has a big bow, remove all the clamps, flip the fairing over, and try again.

The fifth picture is straight, and ready to be taped.

TAPING

Buy a roll of 1" wide, 3M brand, clear, 8672, urethane (leading edge) tape from an aircraft supply store. (Aircraft Spruce part number 09-00841). It costs about $57 for a 36 yard roll, plus shipping.

Tape the trailing edges together in segments between the clamps. For practice pieces, you can just use masking tape instead of the urethane. Then remove the clamps and tape those areas. Handle the urethane tape with freshly washed hands. Firmly rub the tape with a fingernail or something to fully bond it, until it turns clear.

Slip the completed fairing over a piece of strut tubing. With the trailing edge towards you, stand back, and make sure it's not twisted. If it's twisted, you'll have to clamp it back onto the wood, get it straight, and tape it again. Do not reuse the urethane tape.

INSTALLATION

Install the fairings on your struts, and clamp them in place as hard as you can with 2 bar clamps. Stand behind the plane and carefully align the trailing edges to the wind. Then attach the fairings with small, aluminum, protruding head, pop rivets, spaced about every foot. Be real careful the drill doesn't slip and ruin your nice new fairing. It helps to pilot the rivet holes with a tiny drill bit.

You can slash cut the ends of the fairing parallel to the wind on struts that angle. Be sure to allow extra length for that when you make the fairing. And you can melt holes in the fairings with a soldering iron for hardware and things that protrude.

[lostman 198]

I might use this to do the horizontal stabilizer struts. I've been thinking they need to be faired a bit. Looks like by the writers stats I should pick up about 2 MPH by doing that, it would sure be worth it.

[C5Engineer 1170]

If those struts are the same diameter as the other Avids Rans sells some premade ones that will slide right over and they are pretty cheap.