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Servo Leads and Science Fair

Posted by Tom Benedict on 10/04/2013

Last night and this morning I soldered the new servo leads onto the wing servos for my Raptor Advance 2000 sailplane. I still need to test them before they go in the wings since the installation is semi-permanent. But I should be able to do that tonight and do the installation tomorrow evening.

Meanwhile I decided to take on a fun project. I want to know what wing covering offers the most stiffness to weight for a foam plane. I have two foam planes coming in, a Le Fish and a Zagi, and I want to be able to make a good educated decision about wing covering for them. In the past, builds for both planes have involved spackling the foam with some sort of RTV rubber, placing strategic strips of fiber-reinforced packing tape along high stress areas, and then covering the whole thing with clear packing tape. Since then, new tapes have come out, new laminate coverings have come out, and a number of people have chosen to skip the fiber reinforced packing tape altogether in favor of carbon fiber reinforcement. This is one of those instances where you can’t necessarily trust what you read online because a build log from even one year ago may be missing a new technique that is stiffer, lighter, and offers more durability overall.

So I cut ten 1″x4″ strips of some unspecified foam that I’m going to cover on both sides with a variety of coverings. I’m pretty sure the foam is Depron, but I didn’t buy it myself so I really can’t say. Four of the strips will be covered on both sides with CP laminate of 1.7mil, 3.0mil, 5.0mil, and 10.0mil thickness. Another four will be covered with four different kinds of tape: Scotch packaging tape, Scotch extreme packaging tape (aka “biax”), Scotch clear duct tape, and Bunker hurricane tape. The last two strips will be left unmodified.

There are two parts of the test. The first is to use one of our scales from work to calculate a weight per unit area for each covering. I’ll do this by cutting squares of tape and laminate, and weighing them. In the case of the tapes, I’ll also work out how much they weigh per unit length, since tape is typically used in a single strip.

The second is to test each of the foam sandwiches for flex. The scale at work came with a 100g calibration weight. I plan to clamp (gently!) each of the strips with 3″ of cantilevered overhang, then place the 100g weight 2.5″ out from the clamp point. Then I’ll measure the overall flex using a machinist’s scale and a digital camera with a reasonably long lens to minimize parallax. No, this isn’t the most scientific test in the world. And no, I won’t get any “real” engineering values out of this, such as an effective Young’s modulus. But I’ll get enough information to say, “This one is stiffer than that one, and that other one over there is the stiffest of all.” In the end, that’s the value that’s the most useful for my application. After all, I’m just trying to choose a wing covering.

In case you haven’t made the connection between this test and the title of this article, I’ll help you out: Essentially, this is a science fair project. Except for borrowing a 20USD scale from work – well within the budget of most high school or even middle school science fair projects – I’m not using anything that a typical middle school or high school student wouldn’t have available to them. The complete assembly time will be less than a day. The time to take all of the measurements on all ten samples will also be less than a day. It’s nothing that couldn’t be done in a single weekend by a dedicated student. You’ll get to see the writeup here on this blog. From experience I can say that it’ll take me less than a week of evenings to write, edit, and post. This is not a year-long project, and it’s not rocket science.

But it is science – material science. And it’s good enough for a science fair. I doubt it would win any awards, but it’s a solid project. Even better, it has an immediate real-world application: providing me with a plan for covering the wings of two RC airplanes. That’s motivation enough for me to take it on.

So here’s my bit about science fairs: If you’re a student who’s planning to enter a science fair, keep in mind what science is all about. It’s a systematic way of investigating questions you want the answers to. Nothing more. You can look through a book of “100 Awesome Science Fair Projects!” or do a Google search for “Winning Science Fair Projects”, and you might even find one that’ll work for you. But it’s far more gratifying to use it as an opportunity to answer a question you want answered. You’ll be more motivated to finish the project, and you get the added bonus of having your question answered at the end. Besides, judges like to see self-motivated science fair projects. Trust me, they can tell.

– Tom


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