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  #1  
Old 03-25-2017, 08:29 PM
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Default Semroc Maple Seed question.

I'm finishing a Semroc Maple Seed #FA-35. I can't find ANYWHERE in the directions where to place the launch lug.

Anyone know?

Thanks!
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  #2  
Old 03-25-2017, 08:41 PM
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Don't ask don't tell.
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  #3  
Old 03-25-2017, 09:21 PM
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Quote:
Originally Posted by Jerry Irvine
Don't ask don't tell.

????????
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  #4  
Old 03-26-2017, 03:17 AM
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Last step of the instructions "step 30: Insert an unused D12-3. Take your balanced Maple Seeds and hang them on their hooks. Using a piece of cardstock or ruler, balance your Maple Seed rocket and make a mark on the launch lug line at the balance point. Remove the seeds from the hooks. Mark the center of the launch lug. Line up the mark on the launch lug with the mark you made on the launch lug line and glue it into place.

This completes the assembly of your Semroc Maple Seed. We hope you have a lot of fun flying it."

Such detailed instructions for locating the launch lug makes me think they experienced some rod binding on the prototypes.
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Last edited by Skp : 03-26-2017 at 03:18 AM. Reason: spelling
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Old 03-26-2017, 05:18 AM
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Ah, so!

I see it now!

Thanks.

(Duh)
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  #6  
Old 03-26-2017, 06:55 AM
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Technically on any rocket you want one short lug at the CG and one short lug as far back as possible. My prior comment notwithstanding.
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  #7  
Old 03-26-2017, 02:11 PM
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Quote:
Originally Posted by Jerry Irvine
Technically on any rocket you want one short lug at the CG and one short lug as far back as possible. My prior comment notwithstanding.


I agree with the principle of two short lugs... but I put them about as far apart as I realistically can. One does NOT need to be at the CG with two lugs-- they will experience the least about of "leverage" (aka "binding" force) on the launch rod when placed as far from each other as possible. It IS best to have the second one as far back on the rocket as possible, to provide guidance for the rocket all the way to the end of the rod.

I would SERIOUSLY argue against placing a single launch lug at the CG. There's an argument for it, in that since ALL the mass force of the rocket is concentrated and acts through the Center of Gravity point, that this point will exert the least "torquing leverage" (again, read "binding force" on the launch lug and rod when the rocket is sitting inert on the pad. This is, to an extent, true (by that I mean that a single launch lug does indeed "bind" to some extent irregardless of placement, due to the fact that the rocket will, to some extent or other, tend to rotate slightly and take up the excess clearance between the lug and launch rod, so that one side of the rod is against the top edge of the lug on one side, and usually the bottom edge of the lug has the rod touching the opposite side of the lug. In effect, the lug is acting like two short lugs placed very close together, with the upper lug hanging off the rod to one side, and the lower lug pushing against the rod to the other side. By being very close together, and concentrated closely with the center of mass (CG), the 'leverage' forces on the lug are minimized (for the single lug case). Still, two SHORT lugs placed as far apart as possible will exert less sideways force on the rod than a single lug, and the contact area between the lug and rod will be virtually the entire length of the lugs, if two lugs are used, because they are far from the CG, giving them a longer "arm of inertia" (more leverage against the force exerted from the mass at the CG point).

NOW, all that above applies to a rocket sitting on the pad WITH NO WIND. When the wind is blowing, it is exerting force on the rocket. We've all seen a rocket "spin on the rod" and turn in the wind, usually shorting the ignitor clips to the rod, deflector, or each other, or pulling them off entirely, and having to make another trip out to the pad to correct the problem. When a rocket is sitting still on the pad in the wind, the fins are STILL exerting force on the rocket's body. Since the rocket has no VERTICAL velocity sitting on the pad, (since it's not yet in flight), and since the wind is coming from some direction (N, S, E, W, or some direction in between thereof), the force of the wind is coming from a 90 degree angle of attack (horizontal) to the rockets static pointed direction (vertical). Therefore, the wind is acting through the rocket's center of lateral area, (aka the "cardboard cutout" CP location), which is as far AFT as the CP can theoretically move during flight (the definition of the "center of lateral area" method of determining CP assumes the airflow acting on the rocket at a 90 degree angle of attack, which is precisely the way the wind is acting on the stationary vertical rocket on the pad, since the wind moves horizontally for all intents and purposes at Earth's surface (ignoring ground effects and turbulence from nearby buildings or other objects disrupting the wind's airflow and creating vortices/drafts). ANYWAY, by having a SINGLE lug at the CENTER OF GRAVITY POINT on the rocket, the "binding force" created on the lug can be MUCH GREATER due to the leverage distance between the lug's location (assumed to be straddling the CG) and the actual "center of lateral area" (rearmost Center of Pressure) due to wind torquing the rocket around this CP. The stronger the wind blows, the more pronounced this effect... So therefore the "best" location for the lug, especially in windy conditions, would 'apparently' be at this "center of lateral area" CP point, rather than the actual CG point. I say "apparently' because it really isn't, but I'll discuss why shortly...

Now, as the rocket ignites and lifts off, the "apparent wind direction" changes... this concept can be difficult to visualize, but think of a car sitting at a red light in the rain... if it's windy and the rain is coming from behind the car, the windshield may stay almost dry, because most of the rain is hitting the back glass and roof of the car, and on the hood, due to the angle of the rain falling in the wind coming from behind the car. The light changes and the car accelerates away, the angle of the raindrops changes... at a point where the car's forward speed matches the wind and falling rain, the rain will appear to fall "straight down" onto the car, and as the car accelerates further, the rain will appear to be falling "toward" the windshield, because the car is now driving forward through the rain, which to a stationary observer is STILL falling from behind the car to toward the front... As the car accelerates more and more, the angle of the raindrops hitting the windshield gets flatter and flatter to the ground (more and more "horizontal") as the car is 'racing' through the rain... (this effect is more pronounced and easier to see driving through falling snow...) The same is true with a rocket and the wind... As the rocket sits on the pad, the wind's "apparent" direction is 90 degrees to the vertical rocket, thus the rearmost CP location. As the rocket accelerates, at the point at which it is moving forward at the exact same velocity as the windspeed, the apparent angle of the wind has shallowed to 45 degrees from the vertical direction of flight (actually the angle of the launch rod, but we'll assume "vertical" for simplicity). As the speed increases, this apparent wind angle shallows more and more, proportionally, but never actually approaches "Zero" (except in "zero wind" conditions). So, the maximum "torque force" a single lug at the CG of the rocket places on the rod is at zero velocity. As speed increases and the "apparent angle of the wind" decreases, the CP moves FORWARD TOWARD the CG. Of course this is ALSO proportional to the forward speed compared to the wind speed, which is why rockets experience "weathercocking" in windy conditions-- the rocket does not achieve maximum speed until usually about a second or two (sometimes more depending on the rocket and motor combination) or usually 40-~200 or so feet in the air, and the "minimum apparent wind direction" off the rocket's flight axis does not occur until then, (assuming a rocket traveling exactly along the path directed by the rod, which it does not do in actuality once it leaves the rod). The rocket, once free of the rod's restraining influence, will rotate about the CG due to the aerodynamic force of the wind acting on the fins through the apparent wind direction exerting force on the fins through the CP, turning the rocket until the "apparent" wind direction of air moving past the rocket is aligned with the rocket's velocity vector, IE, at "zero" angle of attack... (of course we know that due to inertia it actually continues PAST the "zero point" until force builds to arrest this rotation and reverse it back towards the zero point AGAIN, creating an "oscillation".) Since the rocket IS in flight at this point and velocity is increasing, the apparent direction of the wind is aligning more and more with the flight direction as velocity increases in proportion to the wind speed, and the rocket turns "into the wind" around the CG due to the rotational force exerted on it by the fins through the CP, this is a highly dynamic process...

TBC...

OL J R
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Old 03-26-2017, 02:11 PM
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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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  #9  
Old 03-26-2017, 02:30 PM
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Cutting a single launch lug from a kit into two tapered launch lugs is ridiculously easy, requiring only three easy cuts with a sharp hobby knife...

See below.

Later! OL J R
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  #10  
Old 03-26-2017, 05:13 PM
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Holy crap! That's a lot of launch lug info to absorb. I'm sticking with rail buttons ...
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