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Launch Lug Position
#11
I went with stainless rods a long while back, and have not run into any issues with them, even with prolonged storage (15 plus years).
They are a good one time investment.
Greg Young - L3
TRA 00234
NAR 42065
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#12
(08-15-2015, 06:49 PM)Greg Young Wrote: I went with stainless rods a long while back, and have not run into any issues with them...

Stainless steel rules!
Living life dangerously...launching C's on a B field.
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#13
I have used the huge drinking straws sized for thick milkshakes (which have to be a larger diameter to allow the thick fluid to flow) for launch lugs even on 1/8 rods with no problems whatsoever. I don't think I'd use a 1/2 inch diameter "lug" on a 1/8 rod, but a 1/4 inch lug would work okay... especially if you use two of them. I typically use two.

I cut single lugs in half at a 45 degree angle, and then trim the back ends of the two halves on about a 20-30 degree angle (eyeball it) so there's a wide side to go against the tube, and a narrow side out away from the tube. It's been proven that aerodynamically, drag of the launch lug(s) can be reduced by about 3/4 by cutting the leading and trailing edges of the lug at an angle instead of keeping it flat (cut at 90 degrees as they come out of the box). This also has the advantage of making the launch lug look like it "belongs" on the rocket, as it blends in with other things we typically see on "real" rockets, like cable trays, propellant lines, APU's, ullage or retro-rockets, which are typically covered at least at the front end by an aerodynamic fairing to reduce drag and prevent excess airflow and pressure from building up in/around/on/under them. In short, it "camouflages" the lug and makes it look like a fairing or something that actually belongs on the rocket, even for 'sport' rockets that normally have no other such protuberances.

Having one lug near the center of the rocket (or at the CG or CP if one so desires) and the other near the rear end of the rocket minimizes the effects of "torquing" of the rocket on the rod that is typical of rockets with a single long lug placed "amidships" on the rocket... This "torquing" is produced by the fact that sitting on the pad, the airflow around the rocket from the wind/breeze is 90 degrees to the direction of flight (since it's obviously sitting still while on the pad) and this therefore produces a large force from the fins pushing the bottom of the rocket downwind, and trying to tip the rocket over into the breeze-- motion which is arrested by the launch lug twisting on the rod, pulling against one side of the rod at the bottom and the other side of the rod at the top. This can cause binding, especially on a dirty/corroded rod, and especially with a snug lug on such a rod. Oversize lugs minimize the area of contact and have more "wiggle room" and therefore have less tendency to bind, in my experience. As the rocket lifts off, the "perceived airflow" around the rocket changes from 90 degrees from the direction the nosecone is pointed to more or less straight back along the body of the rocket (as a proportion of the wind speed to the flight speed of the rocket at any given moment) which reduces the "torquing" effect as the rocket gains speed up the rod. Unfortunately, sometimes on an especially dirty or corroded rod, or with a particularly tight lug (paint or crud inside the lug itself) it can totally bind up and prevent the rocket from lifting off at all, causing it to bind on the rod for the entire motor burn.

I find that splitting the lug on an angle and gluing half to the midsection of the rocket and the other half to the rearward end of the rocket (usually in a fin fillet, and aligning the front lug with it) effectively reduces any torquing problems and rod binding due to even high winds at launch pushing against the fins as the rocket sits on the pad. In essence, the small but widely spaced lugs act like a single VERY long launch lug running from the location of the front lug to the location of the rear lug and spreads the forces out over such a long distance that binding is virtually eliminated. Trimming the lug to an 45 degree front angle and 30 degree back angle makes the part of the lug exposed to the most airflow the thinnest, and the 'swept' leading and trailing edges of the lug cut drag by a huge amount over a regular old plain "90 degree square cut" lug, in addition to aesthetically making the lug blend in and look more like it truly belongs on the rocket...

Works for me! Best of luck! OL JR Smile
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#14
Ahhh lug positioning...when you look at the physics, optimal lug position is more complicated than it seems.

For the case of a vertical launcher with side wind (where gravitational torque can be ignored), the zero-binding-torque position for a single lug is at the (90 deg angle of attack) CP, regardless of wind speed.

For non-vertical launcher with no wind (not sure why you'd do that except for impact zone control), the zero torque location is at the CG.

With both wind and non-vertical rod, the zero-torque position will lie at some intermediate position, whose exact location depends on the wind speed and mass distribution of the model (zero torque location is where the moment due to wind cancels the moment due to gravity). With any significant wind, the aerodynamic moment will dominate, keeping the ideal location closer to the CP. However the CP for alpha=90 is well forward of the CP for alpha=0 and often ends up close to or in front of the CG.

If you're going with a single lug, it really needs to be pretty near the ideal location range to avoid binding in crosswind. In most cases the CG is a decent approximation of this location. Unfortunately there are lots of commercial kits that have the location dreadfully wrong. The dual lug solution that brackets the CP and CG works well since both the wind and gravity moments are being applied at locations *between* the two lugs, keeping the binding torque very small.

Another bit of physics to remember when using dual lugs is that after the fore lug clears the rod/rail, the potential torque on the aft lug increases drastically. This is countered somewhat by the fact that velocity reduces the angle of attack. But bear in mind that if you are flying in a 10mph breeze and the velocity at the end of launcher guidance is 40mph, the angle of attack is still pretty large at 14 degrees. This torque is reduced quite a bit if you locate the aft lug a little behind the CP, not all the way at the rear of the model. I see a lot of HPR models with the aft rail button at the very back, which is structurally convenient but a definitely suboptimal location. Usually this doesn't cause real life trouble since most HPR rockets don't get launched in winds strong enough to cause problems.

So based on this, for a 2-lug model I usually try to set the forward lug 1-2 calibers in front of the CG and the aft lug a similar distance behind the CP. Personally I make them out of short (1/4 to 3/8" long) lengths of brass tube having an ID 1/16 larger than the rod...or use rail buttons. If the model fits in a tower I avoid all of this and leave off the lugs altogether Smile
Dave Cook 
NAR 21953 L2    TRA 1108
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