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Old 03-26-2017, 02:11 PM
luke strawwalker's Avatar
luke strawwalker luke strawwalker is offline
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Join Date: Dec 2007
Location: Needville and Shiner, TX
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Cont'd...

SO, what can we gather from all this? What "best practice" can come out of it? Strictly from a "leverage and binding force" argument, having two smaller lugs placed as far apart as possible will give the least binding force on the rod due to wind effects on the fins with the rocket sitting on the pad, and due to the offset mass of the rocket (since the lugs are almost NEVER aligned vertically through the very center of the rocket with the vehicle's mass evenly spread around it (monocopters being an obvious and immediate example that breaks this general observation). Placing one of the lug AS FAR AFT AS POSSIBLE will maintain the rod's guidance to help overcome the wind's influence in trying to rotate the rocket into the wind as long as possible (until the last lug leaves the rod, at which point the rocket is in dynamic flight with "six degrees of freedom"). Placing the forward lug at the CG would place it in such a position that it would have a "1:1 leverage" with the forces exerted on the lug through the CG; placing the lug as far forward as possible would give it GREATER LEVERAGE against this force, meaning less force exerted on the rod due to leverage from the forward placed lug versus the CG placed lug. This would be the ideal placement of lugs with the lowest "binding force" exerted by the lug on the rod. If, however, for whatever reason one desired just a single lug, or if it were necessary to use just a single lug, then we can still deduce a better location than just "sticking it on the side at the CG" location... we know that the wind acts through the center of lateral area (rearmost 'cardboard cutout' CP location) sitting the pad, which will induce substantial torquing force on the rocket (binding force) between the lug and rod... Shifting the lug back towards this CP point would, therefore, minimize this torquing force on the lug caused by wind. Moving the lug away from the CG point will, however, cause the CG to then exert some binding force due to leverage between the CG location and the lug location-- so to minimize or at least balance these two forces, a SINGLE LUG should be placed BETWEEN the CG and lateral CP points.... the least binding force in high winds is to locate the lug closer to the lateral CP, but the lowest binding force in NO wind is to place the lug at the CG point... so if *I* were flying in an area with little/no wind, placing the lug at/near the CG point is fine... BUT if I were flying in an area that is frequently windy, especially with a rocket that tends to "weathercock" into the wind at liftoff or fly "low-n-slow", I'd place the single lug AT OR VERY NEAR the "lateral CP" center of pressure point to minimize wind induced binding force and "weathercocking" at liftoff...

Still, a PAIR of lugs has advantages over a single lug. Some people object to double lugs, thinking they "double the drag". This need not be the case. While launch lugs DO contribute considerable drag to an otherwise "aerodynamically clean" rocket, we CAN minimize launch lug drag... cutting a launch lug to a leading edge angle of about 45-60 degrees and a trailing lug edge angle of about 30 degrees can cut a lug's drag by an enormous amount. Placing lugs in fin root edge fillets also substantially reduces drag. Since we have to place one lug as far aft as realistically possible, and fin roots are typically at or very near the aft end of the rocket body tube, there is little reason NOT to install a double-lug with the rear lug embedded into a fin root fillet, after angling the front and rear edges of the lug appropriately. This will reduce drag to the minimum. The forward lug will of course have to be aligned with the rear lug, and placed as far forward as possible (or convenient) on the rocket's body, and as already shown, tapering the forward and trailing edges of this forward lug as well can reduce its drag tremendously. Such a tapered double-lug setup would have less drag than a single long untapered lug located between the CG and lateral CP points. So there is little "performance reason" to cling to a single lug design. In addition, when one considers performance gained through less friction between the rod and lugs on a double-lug setup spread far apart well away from the CG/CP location, versus a large single lug located at the CG point, the double-lug setup will have A LOT less friction and "binding force" for the motor to overcome to accelerate the rocket, therefore giving a performance boost to the double-lug equipped rocket, which should more than offset any disadvantage due to aerodynamic drag on two tapered lugs. It would certainly be "down in the noise" or a virtual toss-up from a performance perspective between drag of a double tapered short-lug setup versus a single tapered lug located between the CG/lateral CP...

For ABSOLUTE MAXIMUM performance, ie contest flying, one wouldn't want to use a launch lug anyway-- the proven advantages of a piston launcher or tower would clearly make them a superior choice to launch lugs in that application... but for "sport flyers", the benefits of the double-tapered twin lugs surpass those of the single CG located lug, or even a tapered lug placed between the CG and lateral CP.

Later! OL J R
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