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Stability of asymmetrical designs

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neil_w

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I've been wondering how you evaluate the stability of asymmetric designs at the design stage. Airplane-ish designs seem to be the most likely subjects, with larger vertical stabilizers on the top side than the bottom. Or some might have pods below and not above, or all sorts of variations to that effect. I'm trying to get an idea how, before you start building, you gain some confidence that the asymmetry will not cause problems. Are there rules of thumb to follow here? OR is not much help in this area, since many of the design elements that would result in asymmetry are not modeled.

I've sort of convinced myself (although John Boren hasn't specifically confirmed) that the four booster pods on the underside of the new Estes Shuttle are there at least partially to offset the drag of the orbiter on the top side. But I would have no idea how to plan this sort of thing in advance, and I won't be doing a bunch of iterations to perfect a design; just want to put together a scratcher and have some confidence it'll fly straight.

Any thoughts here?
 

EXPjawa

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I think the standard answer is: you just need more initial thrust/longer rail to make sure its going faster when it leaves the guide. I mean, that applies to everything, right?

In reality, I'd like to know about this too. There are number of designs that would be fun to upscale, but have asymmetric pods or fins. I built an Estes Hawkeye, in BT60 tube. I even built in the wingtop "turbine" pods so that they had a core tube and vanes fore and aft. Looks cool, but I've sure that it makes for a bit more drag on one side than the other. Or, say, the Estes Orbital Interceptor. Like Neil alluded to, its airplane-like and has a rudder with "T" tailplane. The only compensation is a bit droop in the main fin wings. Are these designs functional if they enough initial thrust? Does it even matter? How well do the sim programs account for this sort of thing? I've figured out how to model all sorts of things like this. And the sim shows it flying straight if there's no wind. But is that realistic?
 

Daddyisabar

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Easy. Just follow basic model rocketry rules of construction and flying. Ask yourself what is the drag around your thrust center line? Is it balanced side to side? Where is your CP and LCP and is the CG far enough ahead? Do you have enough thrust? If you don't like the answer your design is giving you, then get creative and make it right. Use all the tools that work for you. Rocsim, swing testing, cardboard cut outs, mini models, NAR Guidelines, Handbooks and conferring with others are all acceptable methods; I think they were listed in a NAR news letter on odd rocs a few years back.

I really don't know much about LCP and I don't use computers, card board cut outs or mini models. My handbook is still pristine as it did not help a whole lot as to what I wanted to build, but the pictures were cool. Can't swing test a train. I do sometimes confer a bit with an old timer and will discuss my upcoming plans with any one willing to listen at the launch, at least until they get too bored. Look on the forum and read reviews. Look at what flew and crashed and ask yourself what went wrong? Sometimes you have to think backwards on silly airplane rockets, but don't tell anyone or they will think yer crazy! Oh, there I go again, don't listen to that silly mindsim stuff, for that is NOT on the recommended list. The hardest thing is walking up with your silly airplane rocket and facing the cold reality of the RSO table with out a litany of accepted methods. How far can you push the envelope? Start small and simple and build your way up. No substitute for experience and street cred with your homies. But you only get one chance and if your oddroc doesn't work, well, just think how popular you would have been flying awesome 4FNC made of exotic materials, using sophisticated electronics and big booty motors!

Here you go. These will fly good:

Smaug Launch 1.jpgD V pad 2.jpgSquid Set up.jpgB 58 1.jpgPict launch 1.jpgTrain 7.jpgMega Tapeworm 5.jpgBeech Satggerwing 2 motors 2.JPGJU 87 Four.jpgP5030228.jpg
 
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Daddyisabar

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These will fly good:

P9170002.jpgPA050004.jpgHS132 takeoff tif.gifShinden takeoff tif.gifP1190089.jpgDO 217 2.jpg169.jpgP4190203.jpg


This will NOT:

Egg 3.jpg

It is very symmetrical. One chance only and if it doesn't work: :blush::(
 

rstaff3

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tarkus ii dot i 002 (2) (640x238).jpg21st_f104_06.jpg

20160627_190554.jpg


I generally start with a mindsim and then put something into RockSim as best I can, looking at the CP and radial stability plot. I also ask questions here and apply other methods like the cardboard cutout, neutral point calculations, and anything else I can think of. Back to the mindsim to see if the CG points determined by more rigorous methods look OK, and I often add a tad more weight. I add rail buttons if I can and put a large lug if not. Then I look back at Rocksim to pick a motor. Finally, I mindsim which direction the drag will take it and orient the rail/rod accordingly. Always fly on far away pads that are used for HPR motors!
 

Daddyisabar

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There is no comfort in flying these sort of rockets, no warm bosom to give you confidence. What is in store for you, odd roc flyer? Just the harsh, cold reality of a hard knocks life. That is why sooooo many folks just jump at the chance to build and fly them!
 

Daddyisabar

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View attachment 296110View attachment 296111

View attachment 296112


I generally start with a mindsim and then put something into RockSim as best I can, looking at the CP and radial stability plot. I also ask questions here and apply other methods like the cardboard cutout, neutral point calculations, and anything else I can think of. Back to the mindsim to see if the CG points determined by more rigorous methods look OK, and I often add a tad more weight. I add rail buttons if I can and put a large lug if not. Then I look back at Rocksim to pick a motor. Finally, I mindsim which direction the drag will take it and orient the rail/rod accordingly. Always fly on far away pads that are used for HPR motors!
What is that last photo?:eyepop::y::shock::jaw: Do you have an EKRANOPLAN?
 

rstaff3

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What is that last photo?:eyepop::y::shock::jaw: Do you have an EKRANOPLAN?
Hehe, and this one was flown based on the stability analysis of another rocket that has yet to fly. Mindsimbyproxy. It's a disease and it might spread! :dark:
 

rstaff3

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rstaff3

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C'mon there have to be other unsymmetric rocketeers here on TRF that have good examples and other approaches....
 

KenECoyote

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I think it just takes a somewhat unbalanced mind to make a asymmetrical rocket fly. :wink:
 

Daddyisabar

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Yes! Radial Stability Plot and neutral point calculations! I forgot about those. I am already feeling warmer inside. Thanks rstaff3! Confidence is KEY!

On the calm days our RSO likes the rods titled just 5 degrees away from the crowd. If I can sneak a little more tilt then great, but it could also hurt you if you silly odd roc spins a bit on the thick rod. Then why don't I just dump the thick rod and use a decent rail?
 

KenECoyote

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Then again, Shrox designs are fairly asymmetrical; however they seem more futuristic than unbalanced. :wink:





I'm sure Estes has others, but I can't think of any right now. :confused:

On a side note, one of my favorites is the Fat Cats Banshee (OOP and uses a stability probe like the Estes star wars tie fighter):

 

Daddyisabar

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C'mon there have to be other unsymmetric rocketeers here on TRF that have good examples and other approaches....
Yes. They do work in the right circumstances and can instill more confidence. They are all tools and should be used as such, tailored to each individual and situation. But they are not absolutely necessary or required. The more complex, the more likely hood of garbage in, garbage out, or they become too impractical. All it comes down to is confidence that it will have a decent chance to fly and that is a personal decision that should then be reviewed by your peers. I have seen so much confidence instilled by these methods and then disaster. You can not rely on them and then fail to think it through on a complex project. Then there is the human factor. At some point you are going to mess up. So in the end the only thing I can really say about confidence is that the flight must be done in a safe environment. You can only have confidence if you are prepared for the worst case flight scenario and that is also hard to quantify.

OMG, time to build the BUNKER!
 

Daddyisabar

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Then again, Shrox designs are fairly asymmetrical; however they seem more futuristic than unbalanced. :wink:





I'm sure Estes has others, but I can't think of any right now. :confused:

On a side note, one of my favorites is the Fat Cats Banshee (OOP and uses a stability probe like the Estes star wars tie fighter):

It is all about the looks. The Fire Fox is actually quite nicely balanced. Lexx Jet, not so much. But with all the best tools and intentions, a Lexx Jet flight I once saw was really scary, cool, but scary. And I still have no clue why it flew that way.
 

rstaff3

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Winged rockets can fly squirrelly, even when they have flow well many times. The Estes SS1 comes to mind. Regular kit, not the foam one. But that probably holds for it too.
 

Daddyisabar

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Winged rockets can fly squirrelly, even when they have flow well many times. The Estes SS1 comes to mind. Regular kit, not the foam one. But that probably holds for it too.
The foamy SS1 is almost guaranteed to flutter if unmodified, and needs at least the E12 motor, an F12 is the best.
 

Daddyisabar

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I've been wondering how you evaluate the stability of asymmetric designs at the design stage. Airplane-ish designs seem to be the most likely subjects, with larger vertical stabilizers on the top side than the bottom. Or some might have pods below and not above, or all sorts of variations to that effect. I'm trying to get an idea how, before you start building, you gain some confidence that the asymmetry will not cause problems. Are there rules of thumb to follow here? OR is not much help in this area, since many of the design elements that would result in asymmetry are not modeled.

I've sort of convinced myself (although John Boren hasn't specifically confirmed) that the four booster pods on the underside of the new Estes Shuttle are there at least partially to offset the drag of the orbiter on the top side. But I would have no idea how to plan this sort of thing in advance, and I won't be doing a bunch of iterations to perfect a design; just want to put together a scratcher and have some confidence it'll fly straight.

Any thoughts here?
Whatcha thinkin' of buildin'? An airplane with pods?
 

KenECoyote

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I've always wanted to build a lightning bolt rocket...still can't quite mindsim it stable yet. :)
 

rstaff3

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One thing I know is that you don't want a single large foam glider on the side of a rocket that doesn't have enough power and or isn't waaay longer than the glider. After seeing one go awry with baaaad consequences, I will always make my foam glider rockets symmetric. Kinda like hating tequila after tasting it for the 2nd time.
 

neil_w

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Whatcha thinkin' of buildin'? An airplane with pods?
Not sure yet. Since my workbench is now out of commission for a while, it leaves me free to focus on tinker with new designs (sometimes a long and drawn-out process, to say the least). Thought I should try something more sci-fi style for my next, and it sort of triggered the thinking about asymmetry.

This one came up before, though in it's current state it's actually symmetrical on each axis:
new_ring3.png
I've been fiddling with this one recently, still needs a lot of work:
newtube3.png

This variation in particular was giving me grief, trying to figure out how to add some fin area to the bottom, and not having any idea how much I actually needed.
newtube.png

OR does not make this easy, which is frustrating because I find it really helpful for visualization, totally apart from the CG/CP and flight sim stuff.
 

Daddyisabar

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Those are cool and should not take a ton of nose weight. Symmetry is good, nice and long so some modeling clay in the nose should do it. Be conservative on the first flight then scrape out the modeling clay as needed on subsequent flights until CG/CP perfection is achieved!
 

rstaff3

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+ 1 on that. Cool futuristic design with lots of inherent stability!
 

Daddyisabar

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+ 1 on that. Cool futuristic design with lots of inherent stability!
So is an EKRANOPLAN a cool retro design with lots of inherent instability offset by the weight of many, many tractor motors?

If you guys really want to see how it is done, click on The Original Rocket Dungeon link above (in Dick's signature line) and check out the Chesapeake Sea Monster (AN EKRANOPLAN).
 

rstaff3

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The little one flew well but the big one might test its ground effect heritage. LOL
 

Daddyisabar

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The little one flew well but the big one might test its ground effect heritage. LOL
If it did end up doing the ground effect it would be totally kewl. You could just say "I meant to do that for extra scale flight points!" Flying something as a rocket that was initially designed to suck itself to the ground of water surface is. . . well. . . a bit on the odd side.
 

rstaff3

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I predict a pucker factor of 3 on a scale of ten.
 

dhbarr

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I've always wanted to build a lightning bolt rocket...still can't quite mindsim it stable yet. :)
The Spitfire shows us the way. Just rectangular tubes instead of round.
 
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