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Oct 12, 2021, 08:47 PM
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Glider flap project

Maybe I should do a blog post on my latest exploration? Just so I don't blame it on anyone else!

I had asked others input about possibly adding flaps to a glider wing.. thinking that I could do something simple and easy! Uh.. no! In part because half had suggested full deploy flaps, and half had suggested full up TE spoilers! Well.. that.. makes it more interesting, to have both options. This is more of a test bed for several things, than a functional part.. but hopefully it will do that too!

Constraints: Must add to both sides, and there is no room at the TE center section, to pass a "elevator joiner" type wire to control both together. And worse, no space there to add any kind of control horns.. and even worse.. what kind of control horn does a full 180?? None that I know of! Much like fresh epoxy, the plot thickens. I only wanted to use one servo, but had a limited amount of space in the center section. For some time I've thought about the better mousetrap of a pull-pull cable system, using Kevlar fishing line, and genuine pulleys! I couldn't envision how two horns could reside on the hinge centerline, to do 90 either side of neutral. So I came up with this:

I'll make 2 small pulleys. They will be attached to the TE with a axle made from music wire, bent into a U shape, and the pointy ends will stick into the TE. So there is a free standing pulley. The trailing edge will need a slot cut into it, ahead of the hinge line (and a reinforcement in front of that, for no loss of strength). I want the cord to always have full efficiency, such that it pulls from a tangent on the pulley. And in theory, if driven by another round pulley, should not have any slop. Famous last words! To connect to a flap that will be hinged to the TE.. I'll add two thin pieces of AC ply on either side of the pulley, and both pieces will have slots cut in them, and the flap will fit into that slot. They too will ride on the pivot axle. To connect it to a servo, I'll use Teflon tubing, housing Kevlar fish line. Inside the center section will be a servo mounted above a larger pulley. I didn't have any 180 servos, so the drive pulley will have to be 150% of the flap pulley diameter, and to allow for some overrun, so the end positions can be trimmed with TX end point adjustments. A shroud around the drive pulley will allow the Teflon tubing a place to be attached.. and there will be a loop of line for each flap, going off the pulley in opposite directions. To make both go the same way, one set of Teflon tubes will be crossed up.. so that the servo rotating in one direction will pull both cables and flaps, either up or down. To keep it simple, (funny guy).. there will be no adjustments in the lines.. but the plywood horns, will be bonded to the TE pulley, with the servo at 0 and both flaps held at 0, as the only and final position adjustment.

I sometimes get carried away at the lathe.. obviously I did on this first part shown. I didn't want any cords to be overlapping, or binding, so each cord has a separate small groove. I still have to drill a few more holes in the drive pulley.. but you get the idea.
Last edited by hobiepilot; Oct 12, 2021 at 10:11 PM.
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Oct 18, 2021, 10:40 AM
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Servo mounting, with lines attached

I made a support for the servo, which has a partial surround, to keep the lines for each flap, in place, once it is all tensioned up. With servo, pulley, the bracket, and 4 lengths of Teflon tubing, and Kevlar line, it all weighs about 1 oz. I suppose it could be made lighter. With an effective diameter of the pulley at 1.140" and driving the servo 120% both directions, the total line pull length is 1.25". To verify the required driven pulley diameter, I double the 1.25" (full circle) and divide by pi. Answer is .79" dia. So, as planned, a .75" diameter of the flap pulleys, will have the servo operating within a normal range of rotation, with a small amount of end point adjustment available, for a full 180 of movement.

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