help with lunch lugs

[nitrogen]

I am building my first rocket. Its about 45 cm(17,71inches) long including the nose and the tube is 3cm (1.18inches). The fin span width is about 15 cm (5.90 inches). I am planning to place two 5mm( 0.20inches) lunch lugs, one top of tube , other on the base of tube, or a 3cm (1.20inches). Which method is more approved ;one or two lugs? Please help. Thank you

 

[rocketsmith]

I always use 2 lugs, one about 3-4 cm from the base of the rocket and one about 1 diameter behind the center of gravity. I generally use about 15mm long lugs angle cut at 45 degrees at each end. Hope this helps.

 

[WillMarchant]

For smallish rockets people tend to put one launch lug near the center of gravity of the rocket. You want to make the lug long enough that it wont bind as it travels up the launch rod.
Best wishes,
Will

 

[Micromeister]

Personally: If I'm going to use Launch lugs I like to use 2 smaller ones rather then 1 larger one for esthetic reasons.

LL's are by and large on the ugly side. it is my intent always to minumize or hide these parts whenever possible.

Generally I'll bury one at the forward part of a fin fillet and the other somewhere around the Loaded model CG. If at all possible I'll use a wire ring "antenna" for the forward lug. to it sort of looks like it's supposed to be there If using standard lug materals the forward lug is usually minimized by reducing the length to somewhere around 1/8 -1/4" depending on the diameter and overall size of the model. Since we try to hide the LL's on most models not to many photos to show these effects

Heres one of the Antenna lugs made from .020" stainless or steel music wire wrapped around an old 1/8" launch rod bent with a pair of needle nose pliers.
They work really well on PMC models also as shown on the bottom of this X15's & other aircraft.

 

[shreadvector]

The advice above is good. Also, you should almost never place a launch lug in front of the center of gravity. I have seen simple very stable models become unstable because someone added a launch lug near the top of the body tube. The air hits it and creates a pretty good drag force and because the lug is so far forward it creates a large destabilizing torque (force of drag x moment arm which is the distance from the center of gravity). This is similar to why tube fins should never extend forward of the center of gravity.

Long rockets = good.

Fins at back = good.

Fins at front = bad.

Anything hanging in the airstream in the front = bad.

 

[nitrogen]

Well, Thank you very much all for the response. I though that common sense would be to place them aroung the CG. Wow, impressed by the prompt and thourough responses. I combine all the advice and see which one is the best for my model. Thank you again

 

[shreadvector]

For smallish rockets people tend to put one launch lug near the center of gravity of the rocket. You want to make the lug long enough that it wont bind as it travels up the launch rod.
Best wishes,
Will

Most good kit designers and expert builders/flyers will place the one launch lug on small rockets near the back of the model or at least just aft of the center of gravity. Never straddling the center of gravity where the front end of the lug will be in front of the c.g. and create a destabilizing force.

Examples are many and include:
Quest Starhawk
Estes Wizard
Estes Alpha
And remember the Farside and Farside-X lug is on the middle or second of three stages.
And a few hundred other examples.

 

[Solomoriah]

The advice above is good. Also, you should almost never place a launch lug in front of the center of gravity. I have seen simple very stable models become unstable because someone added a launch lug near the top of the body tube.

Well, I've seen rockets bind on the rod because the lug is in front of the CG. The rocket gets just a bit sideways, and the leverage of the engine over the lug binds it in place.

Just another good reason to set it back a bit.

 

[MarkII]

Well, I've seen rockets bind on the rod because the lug is in front of the CG. The rocket gets just a bit sideways, and the leverage of the engine over the lug binds it in place.

Just another good reason to set it back a bit.

Is it possible to set it too far back, though? And if only one lug is used, is there any kind of rule of thumb regarding the length? Unless specified otherwise, I have usually positioned mine to straddle the CG, but I will start placing them further back from now on. I have never had any problems caused by my placement of the launch lugs, though. I do have several clones that use two widely-spaced lugs (pretty much mandatory on long rockets), but I placed them like that per the designer's instructions. All of my Bic Stic MMX rockets also have pairs of short lugs placed at the top and bottom of the tubes, in keeping with Art Applewhite's design.

MarkII

 

[WillMarchant]

Tim van Milligan recommends straddling the CG in his book. Stine doesn't mention the placement issue.

 

[plano-doug]

Tim van Milligan recommends straddling the CG in his book. Stine doesn't mention the placement issue.

I prefer to straddle the CP. My rationale is that binding on the rod is more a function of crosswind than weight. A really good example of this is trying to launch a rocket glider with a breeze. Since the lug is all the way forward on the motor pod, only a tiny breeze is needed to twist the lug snug on the rod. Thus, I like to counter the effects of wind by spacing two lugs either side of the CP.

On the subject of binding on the rod, I always carry some fine sandpaper, ~220, with me to the pad, and clean the rod off. I frequently find myself wondering if a given flight I've observed was affected by impulse wasted on a cruddy rod.

A thought on mounting lugs...I take great pains to make sure the two lugs are perfectly aligned. I use a metal rod or a very straight dowel (wood), put a few wraps of tape on it to make it snug in the lugs, then tape it to the airframe to hold the two lugs in perfect alignment while the glue sets. It only takes a tiny bit of misalignment to have appreciable drag on the rod.

One upside to using two lugs is that you can use undersized rods with virtually no penalty, assuming the rod is still stiff enough. Since the lugs are spread well apart (fore and aft), the misalignment of the rocket on the rod (due to the over sized lugs) is on the order of a couple millimeters - hardly a factor in the liftoff or flight. Versus one lug, the extra diameter can make a big difference. For example, a 1" long, 1/4" diameter lug on an 1/8" rod will deflect several degrees either way off axis. But with two 1/4" lugs, 1/2" long and 8" apart, the deflection will be negligible.

Doug...short post gone long...

.

 

[plano-doug]

I generally use about 15mm long lugs angle cut at 45 degrees at each end. Hope this helps.

Bravo. That's a pet peeve of mine. Square cut lugs are...well, they strike me as neanderthal. They look slow and primitive.

Beveled lugs look much neater, much more aerodynamic, much faster. Whether they actually do any of that, unless you're competing, doesn't matter. But how your rocket looks does

Doug

.

 

[Solomoriah]

On the subject of binding on the rod, I always carry some fine sandpaper, ~220, with me to the pad, and clean the rod off. I frequently find myself wondering if a given flight I've observed was affected by impulse wasted on a cruddy rod.

I sand all four rods on my stand every time I set it up. Considering that we generally get in only 16 to 20 flights each outing, that seems good enough.

 

[MarkII]

Launch lugs don't really stand out to me, regardless of whether they are square-cut or angled, unless the lug is quite large in comparison to the size of the rocket (like in the old Estes Mini Brutes, for instance or in the Rock-A-Chute Mark II). My concern with angle-cutting the ends is that it effectively shortens the lug; if you use a single-lug set up, this gives the lug less "leverage" to keep the rocket going straight up the rod. So to compensate, you lengthen the lug, and then find yourself right back where you started. I agree that beveled lugs are more pleasing to look at, which is good because to the extent that they are noticeable, they do visually stand out more than do square-cut lugs. The latter seem to blend into the vertical line of the airframe better to me, and thus "disappear" more readily into the design (unless they are mounted on stand-offs). The micro launch lugs on FlisKits Micro To The Maxx kits are practically invisible, even when you examine the model closely.

MarkII

 

[MarkII]

Also, I have never seen a sport rocket veer during the ascent due to drag from a launch lug (although perhaps that can happen to high-performance competition models which, by their nature, tend to push the limits). There are plenty of other doodads that are routinely added to model rocket designs that produce much more drag than do the launch lugs. I have no doubt that the scenario that Fred describes can, in theory, happen, but I have never seen it in actual rocket flights when standard-sized lugs are used.

MarkII

 

[dave carver]

Ya know, I HATE it when the Lunch Lugs show up at lunch, always reaching into your food to take stuff and mainly your potato chips! Well, when that happens I take out my favorite lug wrench and start whacking away and they soon go away......


....What?.....this isn't about Lunch Lugs, you say?.....OHHHH, LAUNCH Lugs!!




...nevermind.....:blush:

 

[Micromeister]

Launch lugs are one of those love/hate items in rocketry. wind tunnel tests have shown quite clearly they are a MAJOR drag influence weither one sees it or not. Their elimination or at vary least minimization is as important as rounding the leading and trailing edges of the fins. These simple easy alterations can make a marked increase in flight performance in even the simplest of sport models. I've seen claims anywhere between 15 to 25%. personally I believe it is somewhere in between coupled with other factors but the overwhelming evidence in the data show high gains when these simple little steps are taken.

If one is going to use a single Lug it should be long enough not to bind on the rod for 1/8" and 3/16" rods that usually means somewhere around 1-1/4 to 1-1/2" long. mounting location very NEAR the models Liftoff mass CG ensures that the model motion on the rod remains forward and around the CG as with all freee flying objects. If I were using a single lug (which I almost NEVER do) it would be placed with about 2/3rd to the rear of the loaded CG.

As for shortening the lug by angle cutting; That is a good thing. Again minimizing these drag generators is almost always of some benifit. As mentioned before I'd always prefer to use as tiny a forward ring as possible with the slightly larger rear lug nested in a fin/body fillet near the forward edge of the fin. This reduces some of the drag by giving the disturbed air time in fin fillet to smooth out some before exiting the body. That said, yes we can put the rear lugs to far back creating even more disturbed air.

I've never seen the dark side of the moon or many things but that doesn't mean they are not real, exist or do not happen. I've also have had the unfortunate experience of seeing what should have been a fine simple model go flopping around in the sky by the addition of too far forward LL's. if the model starts out pretty neutral it doesn't take much.

Launch lugs are such a big drag issued there have been tons of schemes developed over the year to eliminate them from hi performance models. Towers, Pistons, Pop-lugs, drop lugs, rail buttons etc are all the result of wanting to eliminate or minimize the effect of these parasite nessessary but nusience parts.

 

[Solomoriah]

Re: Drag... I wonder. If you put a launch lug with a slanted cut tip next to a fin, then put a hinged flap over it, would that reduce the drag? The flap would fall over the end of the lug after the rocket came off the rod... the hinge wouldn't even need a spring since it would be pulled into place by the airflow.

 

[MarkII]

One can acknowledge that a phenomenon such as instability caused by a poorly-placed launch lug can, like the dark side of the moon, exist without needing to verify it through personal observation. One can also realize that placing the forward end of a 3" long launch lug one-sixteenth of an inch ahead of the CG of a 48" long rocket could, under the right circumstances, be the sole factor in causing the rocket to make loops in the sky when it is launched, while also realizing that the chances of actually witnessing such an event are extremely remote.

One can also compare the flight of an Estes Nomad that was launched on a C6-5 with that of a USS Andromeda flown on the same motor and reasonably infer, without performing wind tunnel tests, that the difference in flight performance was probably not due to drag from the launch lugs.

MarkII

 

[Conan4480]

I am more concerned with the rocket getting hung on the lauch rod than the drag caused by the launch lugs. I would much rather use two smaller launch lugs than one long lug. this prevenst binding on the rod and allow a smoother lauch.

 

[Micromeister]

Re: Drag... I wonder. If you put a launch lug with a slanted cut tip next to a fin, then put a hinged flap over it, would that reduce the drag? The flap would fall over the end of the lug after the rocket came off the rod... the hinge wouldn't even need a spring since it would be pulled into place by the airflow.

Good Question Solomoriah;
If we could closely match the forward opening of the lug and taper of the angle cut on the lug it might smooth some of the turburlance but I'm sure the time spent to devise and embed such a small flap might be more trouble and possibly create more unsmooth edges that would create greater disturbed areas then leaving the angled cut lug embeded in the fin body fillet alone. I think it'd be really hard to do on micros LOL!

 

[texas-bill]

Re: Drag... I wonder. If you put a launch lug with a slanted cut tip next to a fin, then put a hinged flap over it, would that reduce the drag? The flap would fall over the end of the lug after the rocket came off the rod... the hinge wouldn't even need a spring since it would be pulled into place by the airflow.

I recall reading that the drag of a launch lug was the same as that of a solid object of the same shape. So I doubt closing off the launch lug will reduce (or increase) the drag.

[edit]But burying the lug in the fin root might reduce drag (due to the lug).

 

[MarkII]

I'm not all that worried about aerodynamic drag from the launch lugs; perhaps that will start being a concern someday, but not right now. At the moment my big worry is having my rockets fly so high that I lose track of them and never see them again; that has happened a few too many times already.

MarkII

 

[Micromeister]

I'm not all that worried about aerodynamic drag from the launch lugs; perhaps that will start being a concern someday, but not right now. At the moment my big worry is having my rockets fly so high that I lose track of them and never see them again; that has happened a few too many times already.

MarkII

Fly the field mark, Fly the field; Just because we can stuff the model with a C, D, G or UP motor doesn't mean we always have to fly it with the largest thing we have on the field.

 

[MarkII]

Fly the field mark, Fly the field; Just because we can stuff the model with a C, D, G or UP motor doesn't mean we always have to fly it with the largest thing we have on the field.

I try to do just that, but sometimes new rockets surprise me. Our club's main field has a G limit, but I rarely fly anything there larger than a D. Even so, at my very first launch with the club at that field I lost a rocket on a D12 in its maiden flight. It just went up, up, up and we never saw it again. In that and other incidents like it, we didn't lose track of the rocket because it drifted out of the field during recovery -- we lost them during the ascent, when they climbed out of sight and were not observed again. The field is certainly large enough to afford good views in all directions (it is near an airport) but that doesn't mean that we can always track every rocket back to the ground. Often there are only two or three people who are watching any given flight, and depending upon how busy they are, not everyone watches them past apogee. Anyway, my point is that launch lug-induced drag hasn't emerged as an issue for me in the 10 years that I have been flying rockets. Perhaps it will someday down the road. I hope one day to be able to fly many of my rockets at their optimum performance level; when I get to that point I very well may start to pay more attention to limiting the drag caused by the lugs.

MarkII

 

[Micromeister]

I try to do just that, but sometimes new rockets surprise me. Our club's main field has a G limit, but I rarely fly anything there larger than a D. Even so, at my very first launch with the club at that field I lost a rocket on a D12 in its maiden flight. It just went up, up, up and we never saw it again. In that and other incidents like it, we didn't lose track of the rocket because it drifted out of the field during recovery -- we lost them during the ascent, when they climbed out of sight and were not observed again. MarkII

I can certainly co-miserate(sp) with you on those kinds of over powered flight loses. I've done similar out of sight flights on C and D powered models where we heard the pop at ejection but never saw a chute or streamer, and the models were never to be seen again. More recently I've been doing this with Micro models as well which has been even more distressing as everyone on the field KNOWS it couldn't have gone all that high. I've tried to explain to other modelers that Out of sight, is out of sight; be it 300, 3000 or 30,000feet, Once its gone it's generally not coming back unless someone stumbles over your model while out looking for another.

I think the only reason we even discuss such minor part influence has more to do with air stream flow management or searching for that elusive optium mass, throw weight, max coast time or whatever we want to call it then anything else. Makes us feel good thinking we're actually doing something positive to aid in the performance of our flying models.

 

[MarkII]

I can certainly co-miserate(sp) with you on those kinds of over powered flight loses. I've done similar out of sight flights on C and D powered models where we heard the pop at ejection but never saw a chute or streamer, and the models were never to be seen again. More recently I've been doing this with Micro models as well which has been even more distressing as everyone on the field KNOWS it couldn't have gone all that high. I've tried to explain to other modelers that Out of sight, is out of sight; be it 300, 3000 or 30,000feet, Once its gone it's generally not coming back unless someone stumbles over your model while out looking for another.

I think the only reason we even discuss such minor part influence has more to do with air stream flow management or searching for that elusive optium mass, throw weight, max coast time or whatever we want to call it then anything else. Makes us feel good thinking we're actually doing something positive to aid in the performance of our flying models.

Very true, John. I just wanted to get two points across. The first: in sport launching, we often fly rocket designs that have serious sub-optimum aerodynamics anyway. Many of the futuristic designs, as well as others, that we love so dearly are like that. But that's OK, because rocketry would not be nearly as much fun without those fantastic designs. But a focus on the drag caused by the lugs, when the rest of the rocket has so many other streamlining issues, seems a tad misplaced. If we are so willing to accept so many other aerodynamic compromises in many of our rockets for the sake of aesthetics, etc., which most of us are (and why not?), then the drag issue with the lugs shouldn't be all that big of a deal.

An additional point: this discussion is still enormously useful and valuable for people who are, say, building a TARC project. All of the information that was previously presented is extremely relevant to them. And anyone else who is seeking to optimize a rocket's design in order to maximize its performance needs to pay close attention to such things. After all, that's what real rocket scientists do.

That's all I wanted to say.

MarkII

P.S.: OK, one more thing. Even though I have been trying to minimize the issue in my posts here, the information that other people have posted has given me some things to think about as I build my rockets. It has also given me more things to notice and watch for when I observe them in flight. In the end, that may make me a better rocketeer. More knowledge is better than less. Thanks.

 

[Micromeister]

As mentioned in my 1st post to this thread, Launch lugs for the most part are just plain ugly. While the Data shows they are also a major drag influence the aero part for most of us really isn't as important as hinding the nasty looking things in one way or another.

If we can do so and accomplish a bit of Drag reduction in the process all the better Which is what we're doing by splitting the lug into two parts, burying it or at least one part of it in a fin/tube fillet and minimizing the forward portion as much as possible by angle cut, tiny length or "antenna ring" wire methods. All these tiny minor detail things just make for a better looking and little bit better aero model regardless of end use..sport or competition.

Glad to hear some of these thoughts and options are of help.

 

[BEC]

This is an interesting and informative discussion that will help me see the launch lugs on my rockets in a different light.

One comment though - the coefficient of drag for of a cylindrical object mounted perpendicular to the airflow is pretty high - so those "wire antenna" launch lugs, unless they're made of airfoil cross-section wire and the airfoil is oriented properly, are probably draggier than a short section of a tubular launch lug.

I have seen a very noticeable improvement in the glide performance of an RC airplane simply by adding a fairing to a round wire landing gear - a fairing which increased both the wetted area and the maximum thickness of the profile, but which very clearly reduced the drag.

 

[WillMarchant]

The NAR safety report at https://www.nar.org/pdf/launchsafe.pdf recommends a single lug spanning the CG and CP or a pair of lugs approximating that.