Stability with fixed vs freely pivoting forward mounted canards

Discussion in 'High Power Rocketry (HPR)' started by Wallace, Jun 20, 2019.

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  1. Jun 20, 2019 #1

    Wallace

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    Can anybody steer me towards any info on very forward mount (like way up on the n.c. forward) canard effect on stability? I'm basically looking at not having to add any more nose weight than absolutely necessary and before I get to far ahead of myself in creating parts (as in hopefully not having to make them more than onceo_O) need to determine weather or not it'd be worthwhile to design them with free pivots as far forward on the fins as feasible vs simply fixing them in place. If it turns out the effect is negligible I won't waste a ton of time and effort on it. Thanks in advance..Oh, Btw, I have been searching, searching my arse off and drawing blanks....
     
  2. Jun 20, 2019 #2

    Wallace

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    It can't possibly be as simple as simming it using only the "size" of the fin forward of the pivot can it? The section pivoting behind the pivot point has to have a fair amount of effect does it not?
     
  3. Jun 20, 2019 #3

    heada

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    If they pivot freely, then they'd have a near zero AoA into the airstream and thus would provide nearly no corrective force and would convert into just mass objects, no?
     
  4. Jun 20, 2019 #4

    Wallace

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    I did find some "general" info, but nothing that'd help as far as sims go....
    "I read an article about this awhile back, can't recall where ATM... maybe Sport Rocketry magazine?? At any rate, the idea is, that on "air-air missile" type rockets (like the Python and Sidewinder) that use forward fins (or TWO sets of fwd fins on Python) that makes the rocket VERY unstable... the further forward the fins are (closer to the nosecone) the more destabilizing they are for passive stability (typical model rocket) purposes. Rockets like AMRAAM with mid-mount forward fins are MUCH easier to make stable through simply shifting the CG with additional nose ballast and such, because the nose weight is further away from the fwd fins, and because the fwd fins, being further back on the body, don't bring the CP as far forward, meaning the CG doesn't have to be shifted as far forward to compensate. At any rate, the model in question used "pivots", essentially long dowel rods, to which the fins were attached. The forward fins, being essentially right triangles, were slotted for the dowel "axle" to be glued into the fin at about 1/4 to 1/3 of the root chord. Essentially, the further forward you can put the pivot, the better-- because basically all the fin area IN FRONT OF THE AXLE is STILL DESTABILIZING TO THE ROCKET. For fins shaped like a right triangle, with the axle at say the 1/3 root chord line, that puts only a small right triangle shape about 1/3 the size of the fin in front of the pivot point. IN flight, the airflow hitting this part of the fin tries to 'flip it over' the pivot point, but of course the larger area of the fin below the pivot point overpowers this tendency and keeps the fin pointing straight into the airstream. BUT, this area DOES generate forces that can be transmitted to the rocket, essentially moving the CP forward, only by a MUCH smaller amount than if the whole front fin were glued solid to the body tube. In flight, say the rocket experiences something that pushes it off course. The angle of attack to the surrounding airflow would, on a model with solid-glued forward fins, cause the same angle of attack to be presented by the forward fins, which would then generate lift at right angles to that angle of attack, tending to increase it, IE make the rocket go unstable. (This moves the CP forward with increasing angle of attack and makes the problem worse-- something that tends to cause extremely long rockets like super-roc models to go unstable in windy conditions). On the rocket with pivoting forward fins, when the rocket is perturbed from its path to an angle of attack to the airflow, the forward fin is presented the new angle of attack, but immediately pivots directly into the airflow, generating no (very little) lift, and therefore not inducing a destabilizing lift force into the airframe, or moving the CP forward. The fin pivots in relation to the rocket body by the same angle as the angle of attack (for all intents and purposes anyway). The main fins on the rear of the rocket can then create corrective forces to return the rocket to the proper flight path, and when the angle of attack goes away, the forward fins pivot back in line with the rocket body tube.

    The main thing the article stressed is that the fins MUST be able to move freely-- if they bind up, the rocket WILL go unstable and crash! IIRC they used a combination of wood doweling for the "axles" and brass tubing glued into the rocket body tube for the axles to pivot within. Also, the fin root edges need some clearance from the body tube (using a small poly washer or other low-friction solution) to ensure the fins don't 'drag' on the body tube and are thus prevented from freely turning the direction the need to go to negate any angles of attack in flight."
     
  5. Jun 20, 2019 #5

    Steve Shannon

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    Well, the pressure required to get them to rotate acts to try to turn the rocket around. Once they have pivoted, the pressure to keep them there continues to attempt to destabilize the rocket, but as you correctly say should be low. At the very least the area represented by thickness x length of the canards creates a destabilizing lift.
     
  6. Jun 20, 2019 #6

    boatgeek

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    This. There is definitely a drag component forward regardless of lift, which will move the CP forward.

    I think it's more important to have the axle turn absolutely freely than to have the axle as far forward as possible. You definitely want it to align to the airstream, but having the axle set so that 25% of the fin area is forward and 75% is aft of the axle would take care of that. Before adjusting the sim to reduce the effects of the forward fin, I would want to see that the fin turns freely enough that it will rotate to vertical under gravity alone. Even then, I'd worry a little bit.
     
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  7. Jun 20, 2019 #7

    jadebox

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    Last edited: Jun 20, 2019
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  8. Jun 20, 2019 #8

    Wallace

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    Was planning on using sealed bearings. Since she's gonna need nose weight anyway, might as well put it to work.
     
  9. Jun 20, 2019 #9

    Wallace

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    That's pretty cool. I'd have bet against that horse every time, does not look like it'd fly. I'm currently/simultaneously working on a scaled down test version myself. No better way to know for "sure"...
     
  10. Jun 20, 2019 #10

    jderimig

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    If the pivot was frictionless and the canards massless then there would be no AOA. But there is friction and mass so there will be some AOA. Good idea to test subscale. Interesting concept question.
     
  11. Jun 20, 2019 #11

    G_T

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    The neutral point on a symmetric airfoiled fin is at 25%MAC. The suggestion to have a quarter of the area in front and 3/4 behind is not a good suggestion. That is right around the neutral point. Plus, the neutral point shifts forwards as angle of attack increases. At 25% the results are unstable. You want more area to be behind than that just so the fin weathercocks to the freestream. Just to note, it isn't area in front or behind the pivot, it is pivot location compared to percentage of the MAC (mean aerodynamic chord) and you might want to bias away from some of the area right next to the rocket body as the airflow is different there.

    https://aviation.stackexchange.com/...tre-of-pressure-aerodynamic-centre-and-neutra

    As you get into the transonic region the neutral point will shift, and continue shifting as the rocket goes faster.

    Another issue which I think wasn't mentioned is everyone seems to be applying a statics analysis to a dynamics problem. Think about materials flex, time lag on response due to inertia and non-steady flow, and potential resonances and potential instabilities. You might find those pivoting forward fins at certain speeds can force the rocket into coning or other bad behavior.

    Gerald
     
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  12. Jun 20, 2019 #12

    Wallace

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    Good info. The fact that they'll actually be on the radiused section of an ogive nose cone should then also have an effect? Think.. 20190620_161809.jpg
     
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  13. Jun 21, 2019 #13

    Wallace

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    This where I'm currently at, want to decide before printing the front 1/2 of nose cone. Or do I just print it with bearing bores and plug 'em if I decide to solid mount? 20190621_102406.jpg
     
  14. Jun 24, 2019 #14

    Nytrunner

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    That's my vote. you could always print plugs for them so it's not just a wad of epoxy too.

    What's the motor for that thing?
     
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  15. Jun 24, 2019 #15

    Wallace

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    Motor for which one? The 6"er'll probably fly exclusively on DMS motors. Can't justify 75/98mm hardware in that price range, just wouldn't make sense since I can't afford to fly that often. I'd have to live to be somewhere around 234 years old to hit the break even point and that's barring anything going wrong anywhere in that time period.
     
  16. Jun 24, 2019 #16

    Nytrunner

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    Yep, The big Bulpup looking one Which DMS? M1350? O5280?
     
  17. Jun 24, 2019 #17

    Wallace

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    10,000' waiver at my home field and my thin wallet are definitely pointing towards 1350. Initial/rough sims keep it safely under the waiver and under 600mph. Just trying to avoid the 'ol 6lbs of lead in the nose cone thing if at all possible.
     
    Last edited: Jun 24, 2019
  18. Jun 24, 2019 #18

    Wallace

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    I used the shipping tube from my L2 motor and a core from a roll of aluminum foil for a 29mm motor mount for the scaled down/test version. Figure it'd be unwise to throw money at something I'm not 100% certain of..I'll just do the initial launches when there's no one else around in case it does not go as planned.
     
  19. Jun 25, 2019 #19

    Wallace

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    I've taken your advice, we'll have results in 36+ hours (hopefully). Undersized the bearing bores .05mm, taking historical shrinkage with this particular filament into 20190625_060657.jpg 20190625_060531.jpg account, I can ream to final size for press fit. Anyone tried LocTite stud and bearing mount on plastic? Might have to do some testing...
     
  20. Jun 25, 2019 #20

    Wallace

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  21. Jun 25, 2019 #21

    Wallace

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    $2.50 Bargain bin 6mm carbon arrow shaft..Just gotta see if it's feasible to cut "E" clip grooves in it for retention. 20190625_081930.jpg
     
  22. Jun 25, 2019 #22

    Wallace

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    20190625_112338.jpg Not sure why, but it didn't close in the tops of the bearing bores even though I had bridging set up? Not really important as far as retention but I suspect it's gonna make reaming to size less than funo_O
     
    Last edited: Jun 25, 2019
  23. Jun 27, 2019 #23

    Wallace

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    Ended up cancelling the first attempt a few hours in, failed to account for the extrusion multiplier's effect on bore size. I've found that keeping the temps up and over extruding a certain amount results in a much stronger part. The I.D. on attempt #1 ended up a hair under 9mm This is attempt #2, bearing bores ended up @ 9.95mm, I am very happy with it. Current thought is to move ahead with the pivoting canards and design a lockout/removeable weight system in case they end up being disallowed for a cert flight, that way they'll already be in place for future flying. If they flatout just don't work I can fillet 'em in, or even do a tip to tip job if necessary. Although it's not relevant in this case, the weight compares favorably to FWFG. Straight from Madcow's 6" page, and this is for a much longer (10.5") Von Karmen design.
    Von Karmen:
    Height: 34.5"
    Weight (w/ coupler): 5.5lbs
    Weight (w/o coupler): 4 lbs
    I'm sitting at right around 3 pounds without the tip right now according to slicer info. Just need to find time to get over to the scale... 20190627_064931.jpg
     
    Last edited: Jun 27, 2019
  24. Jun 27, 2019 #24

    Nytrunner

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    What's your TAP/L3cc saying?

    You could always fly it on a high L to compare. A baby M like the 1350 wont be much more problematic?
     
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  25. Jun 27, 2019 #25

    Wallace

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    That is the plan, start with an L flight and see how everything "works". Recovery is still in mind sim stage and although I have several ideas as to where and how to locate electronics, I have yet to decide. Just trying to keep momentum without having do overs due to lack of foresight at this point in time. As far as my TAP's opinion, don't yet know, sent him the info, just waiting for a reply. What's your opinion, not on the cert flight aspect, just the concept/execution. You are mechanical engineer no?
     
  26. Jun 27, 2019 #26

    Nytrunner

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    There's a piece of paper that says I'm a Mech E, but my job these days seems to be drawing diagrams and arguing with the government.
    I love the concept, and my heart wants to see it fly just to see what happens.

    Putting the heart away for a minute, lets talk technical.
    That rocket reviews article shows there's a Lo-power precedent for this kind of noseweight reduction scheme. He had successful flights simulating the area ahead of his pivots.
    But you aren't building a lo-power park flier, you're building a Level 3 rocket. See G_T's post above, I tend to agree with him. At higher than LPR speeds, the canard is going to wiggle back and forth in the high velocity airstream and that wiggle will impart an unknown amount of force right at your nosecone. This could add additional stresses to the airframe and aft fins.
    It's those dynamics that will be difficult to analyze, and a subscale flight of similar speed and duration may be the best way to find out.

    Do you have a side image showing the shape of the canard, as well as an end image showing the cross section? (airfoil, flat plate, double wedge, etc...)

    Worst case scenario is the canards destabilize the flight. Not as bad case, the canards shred off and the flight proceeds as a normal rocket. Best case, everything works great!

    Good slick bearings should help against binding with torque as they rotate, but there's an off chance the rod itself gets snapped off by the force on the canards.
    6mm carbon shaft yes? What's the inner diameter (if they aren't solid) and how far will the canards be offset from the nose? (<1mm?)
     
  27. Jun 27, 2019 #27

    Wallace

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    Although I do have myriad options, I've yet to decide on material for canards. The size/shape is predetermined since it is intended to be an accurate scale model of an existing missile. Profile will be flat plate w/bevels, I'm leaving myself "some" leeway in that area as far as looks and performance go.. .As far as clearance, again I'll need to do some tests to determine what will be necessary. And, yes the 6mm o.d. tube does have me concerned enough that Ive been researching ways to reinforce it internally. Can even scale it up if need be since I have found 8 x 10mm bearings, so I didn't print myself into a corner, at least in that regard... Assuming shear is gonna be the major potential failure point, I plan on doing some testing before proceeding in that direction. Even if I do not follow through with the pivots on the 6" version I will still build and test the theory on the 2.5"er because, well now I just HAVE to know.. I just don't have enough information at this time to know weather or not results will scale up that amount. Of course if the small scale fails miserably everything else is moot.
     
    Last edited: Jun 27, 2019
  28. Jun 27, 2019 #28

    Wallace

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    So basically, if I can make the axles stiff enough to resist bending the next potential failure mode would be shear, followed by???
     
  29. Jun 27, 2019 #29

    Nytrunner

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    If you're making the canards out of hardwood, probably ripping the pivots out of your nose cone or snapping the fin on landing.

    This is all assuming the pivot and canard doesn't destabilize the flight of course.
     
  30. Jun 27, 2019 #30

    heada

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    Carbonfiber tubes from McMaster-Carr

    Part 2153T117 ($6.37 for a 12" length, 6mm diameter)

    Wall Thickness 0.02"
    Tolerance Rating Standard
    OD 0.236"
    OD Tolerance -0.012" to 0.012"
    ID 0.196"
    ID Tolerance -0.006" to 0.006"
    Length 12"
    Length Tolerance -1/8" to 1/8"
    Hardness Not Rated
    Hardness Rating Not Rated
    For Use Outdoors No
    Minimum Temperature Not Rated
    Maximum Temperature 180° F
    Impact Strength Not Rated
    Impact Strength Rating Not Rated
    Tensile Strength 120,000-175,000 psi
    Tensile Strength Rating Excellent
    Straightness Tolerance Not Rated
    Compressive Strength 75,000-128,000 psi
    Flexural Strength 89,000-174,000 psi
    Flexibility Rigid

    I would imagine that the bearing mounts would fail long before the shaft sheared.

    I'm not any kind of engineer (train or otherwise) but I think the failure modes would be 1) bearing mounts, 2) fins (delaminating, ripped from shaft, etc.) 3) bearing seizing due to lateral force and then 4) shaft shearing.
     
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