Canard difficulty

Discussion in 'Rocket Boosted Gliders' started by Rktman, Dec 28, 2018.

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  1. Dec 31, 2018 #31

    Rktman

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    Eric Noguchi

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    I used an online canard CG calculator (https://rcplanes.online/cg_canard.htm) that says it falls 6" from the aft end of the fuselage. My trimming tosses put it within 1/8" of that and results in a nice glide. The calculator result is only an estimate since its formulas/equations assume squared and not rounded tip chords (as you can see mine has rounded tip chords).
    kestral CG.jpg
     
  2. Dec 31, 2018 #32

    georgegassaway

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    I think lack of airfoil on the main wing, and airfoiling on the canard is part of the problem.

    As I said, canard designs need for the canard to stall first, and main wing stall last. I think your is having the main wing stall before the canard sometimes, yet perhaps the canard stall other times depending on the angle or attack and airspeed. No airfoiling always stalls before an airfoil does. Oddly enough, if your canard was say half (or 2/3) of its current area but twice (or 1.5 times) as much angle of incidence, it would tend to stall the canard more easily due to the higher angle of attack (incidence). While the higher angle of incidence would make the canard create more lift. Compare the canard size/area of a couple of well known old B/G's, the Mini-Dactyl and the Delta-Katt. Pretty small canards with a lot of incidence.

    Delta Katt: note fuselage angle to wing, canard is straight (zero incidence) to the fuselage, the pop-pod was mounted parallel to the wing. This pic from a 2.9X scale-up article: https://www.rocketreviews.com/scratch-groovy-katt-by-geof-givens.html
    [​IMG]

    Mini-Dactyl: it also had the same sort of fuselage set-up as the Delta-Katt, though the pod was parallel to the fuselage not the wing. If this was scaled up for C size or lager, it might have some problems with pitching up on boost unless it was built to roll (easy way is to angle the pod "crooked" in yaw about 3 degrees, causing a roll on boost without messing up glide trim like rudder trim for boost would.
    [​IMG]
    PLANS from JimZ's site (The above is from part of the instructions): http://www.spacemodeling.org/jimz/km-6.htm

    BTW - Aerodynamic scaling related to "Reynolds Number", is that the smaller an aerodynamic surface is, the less effective it is (smaller = lower Reynolds Number). Because air molecules are a constant size, if air was instead small particles of rock, full size planes would fly thru sand while smaller models fly thru gravel and smaller thru pea gravel, and really tiny models fly thru "boulders", in a relative sense of size. That is a wild oversimplification of course (no plane could "fly" thru sand) but a way to get across the idea of air molecules of constant size vs how the airflow behaves with smaller and smaller models . Suffice to say that the smaller size of a canard will tend for it to stall sooner than a larger one, everything else being equal. https://en.wikipedia.org/wiki/Reynolds_number

    Also, don't give the canard an airfoil, round it only. And try to do what you can to sand some airfoiling into the main wing. Even if it's pretty much rounding the leading edge and sanding a crude taper into the UPPER part of the trailing edge (don't taper the underside of the trailing edge).

    Another thing to try would be to add a turbulator strip at 25% of the main wing chord (on top of the wing, not bottom). That "trips" the boundary layer early, causing it to remain attached longer at higher angles of attack than without it. I have used turbulator strips n some of my R/C sailplanes and a few R/C rocket boosted gliders to reduce/solve tip-stall problems (wingtip on one side stalling, causing the glider to veer to one side and begin a dive rather than stall straight ahead like a well behaved R/C model should. This alone might solve your problem right now. Vinyl tape about 3/32" - 1/8" across would be about right. I say vinyl, like electrical tape or repair tape, since that would be about the right thickness, thinner tape would not work as well. Needs to be thick enough to have a significant "bump" to trip the boundary layer. The image below is for a more advanced full size airfoil and shows the turbulator farther back than 25%, but 25% generally works well for our size of models.

    [​IMG]
     
    Last edited: Jan 1, 2019
  3. Jan 1, 2019 #33

    Rktman

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    Eric Noguchi

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    Appreciate your analysis and diagnosis of the problemm George, especially the detailed explanation and pics/examples to clarify things. It pointed out things I never noticed or was aware of in various previous BG designs...things that were incorporated to make the gliders fly stably and successfully in the first place.

    BTW should the turbulator strip follow the shape of the wing's curved leading edge?

    Based on the feedback I've gotten so far, my first step will be to remove the existing canard wing and replace it with one approx 30% smaller, and with rounded edges only, NO airfoiling and with a degree more incidence to compensate.

    Step 2 wil be to add a turbulator strip to the main wing. (Airfoiling it at this point would be difficult without destroying it since it's been tissued/doped and because of the dihedral angle). Those are the least drastic changes, so if those two steps don't correct the problem I'll scrap this model and build a new one with the aforementioned chages built in from the start. A shame, but an extremely valuable learning experience and design lesson I can carry forward.
     
  4. Jan 1, 2019 #34

    Crawf56

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  5. Jan 1, 2019 #35

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    Eric Noguchi

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    Good to know! I've read about them before but never really understood how effective they can be till now. Really helps to have a real world example.
     
  6. Jan 2, 2019 #36

    Rktman

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    Eric Noguchi

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    George should the turbulator strip run the whole length of the wing, or just a section of it?
     
    Last edited: Jan 2, 2019
  7. Jan 2, 2019 #37

    Rktman

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    Eric Noguchi

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    Thought further about what amount of incidence to set the canard wing at. Since I'm not sure what amount will be needed I'm thinking of adding flaps to the canard rather than just guessing and gluing it in at a set angle. Maybe pin vise a small hole through the fuselage and canard and thread a small leftover 2mm bolt through both and use a nut and locknut to adjust the angle.

    If anyone has any thoughts/advice/suggestions please feel free to jump in.
     
  8. Jan 3, 2019 #38

    georgegassaway

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    Yes, follow the curve, proportionally (25%).

    And do it full span, since the issue being tackled here is not tip-stalling, but apparently the whole main (rear) wing stalling before the canard is (or main sometimes stalling or sometimes not, inconsistently, making trimming very hard).
    So, you want the whole wing to be able to handle a higher angle of attack before stalling than it seems to be doing now.

    Since the LE and TE are square.... i just occurred to me you might need to make the turbulator strips extra thick. So if you do not see much improvement in trim stability with one layer of vinyl tape, try 2 to 3 layers. On my R/C gliders I had 2 to 3 layers (laid atop each other onto plastic the cut to width with a ruler and model knife), but those were bigger wings with larger chords.

    Another thing I'll note on my experience with Canards, is that sometime they need to be more nose-heavy than I would trim normal gliders. Sometimes a "fine line" between having a good pitch trim that does not want to stall, but glides level, versus being too nose-heavy where it isn't actively trying to "dive" into the ground. BUT it is definitely in a bit of a shallow dive - if it sped up then the canard would give enough lift to pull the nose up, from diving, but at the steady-state glide velocity it's more nose-down than a normal glider would be. Sorta hard to explain it until you experience it. One of several reasons why I have rarely done canard models, other than proven good designs/kits (Like Edmonds, and the Mini-Dactyl. Even the Delta-Katt was a bit tricky).

    I did make up a Canard design a few years ago for an easy-build glider for club members. Some info on on it in this post:
    https://www.rocketryforum.com/threads/canard-glider-plans.142817/#post-1737448

    And the plans. Not exactly the coolest glider design, but it worked. The Canard angle was "TLAR" (That Looks About Right), 1/8" over 2" which only now did I calculate to discover it's 3.58 degrees. Had the prototype seemed to have needed more or less than that after glide trim throws, I would have changed the angle, but it was in the ballpark. Of course that angle is all relative to the canard area, wing area, and distance between them for this design which is OK with it, so do not presume 3.58 degrees is some magic angle. :) Probably woulda been OK at 3.0 (less noseweight) or 4.0 degrees (more noseweight)

    [​IMG]

    [​IMG]
     
    Last edited: Jan 3, 2019
  9. Jan 3, 2019 #39

    Rktman

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    Eric Noguchi

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    Appreciate all your input George. Your real-world experience is far more valuable than the "how-to" canard design blogs and articles I've read (where most of the "rules" are based on generalized one-size-fits-all formulas or hypothesis, and usually for the much larger RC or DLG type gliders, not the smaller versions we fly). Not to say that those articles aren't a good starting point for sure, but I haven't found much that address the exception-to-the-rules stuff that happens in certain situations. Sure would be nice to have design articles with a "troubleshooting" guide, a kind of "what to do if this does or doesn't happen".

    That's why I value your knowledge and expertise based on years of experience. Thanks for the help and advice to someone still moving up the learning curve and prone to making such rookie mistakes like airfoiling things I shouldn't and not airfoiling things I should, etc.

    Will make the changes you advise. Thanks again for all the valuable insight.
     
  10. Jan 4, 2019 #40

    Rktman

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    Eric Noguchi

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    Before I go making another clueless mistake, is putting flaps on the downsized canard to try different incidence angles okay, or just adding needless complexity? Not sure about the size (1/2 - 1/3 of the canard wing) or whether a smaller control surface like that would help or hinder things? Any guidance greatly appreciated.
     
  11. Jan 4, 2019 #41

    georgegassaway

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    "Flaps" on an airplane wing allows the wing to fly slower without stalling. So my concern is that adding an adjustable flap flap on the canard will mean the canard may not stall until reaching an even higher angle of attack than it is reaching now before it stalls. Which means the main wing will be more likely to stall first. So, I'd keep it flat, with the rounded edges, no flap.

    I'd suggest "tack gluing" the canard on with a two drops of CA, one front and one rear of the canard. And if you think it needs more angle then use a razor blade to slice the glue joint under the front, then put a shim of 1/64 or 1/32 plywood under the front, another drop of CA, and try again. And so on. Once you reach an angle that seems right, then permanently glue.

    Now, I know that full size aircraft have control surfaces on the canard, but they are designed very well (often have turbulators) and have much better Reynolds numbers (flying thru "sand", "not rocks"). The Astro Blaster I had......always flew sorta funky, and it did have a canard with a "elevator" (pitch control) surface on it. But also it was just 1/8" balsa with rounded edges, while the main wing had a flat bottom airfoil, so it didn't exactly have a stalling problem but just handled different than the other R/C models I've flown. I even converted it to electric R/C a few years ago and while it flew.... again it wasn't a comfortable "fun" model to fly.

    As in contrast to some flying wing models I have flown, also unconventional, that flew very honestly and consistently. BTW - do not take that as as suggestion to switch to a flying wing, as those can be just as troublesome to design from scratch. The ones I speak of R/C wise were mostly highly popular Flying Wing kits or ARF's that the designers nailed just right (the ones that suck usually don't become popular. Usually). And one designed by a Mr Vern Estes ad John Shulz in 1961, the Astron Space Plane kit (not R/C of course). I scaled that up 4X as an R/C RBG and the glide handling was SO sweet once trimmed that a beginner could fly it in glide.

    [​IMG]

    Oh, I recalled a canard Boost Glider I did long ago that had a canard flap because it moved from flat on boost to down for glide. Bruce Blackistone's "Disaster 17B Valkyrie". It had very flaky trim. Like I said designs usually do not become popular unless they fly well - "usually". This didn't exactly become "popular", but it was a plan in Model Rocketry Magazine. In those days there were not a lot of glider plans, and this stood out as a "different" glider when seen at some big contests (where IIRC he never had much success in scoring well with it). Blackistone made several big versions of the Valkyrie, using engine choices (not many options in the early to mid 1970's) that were often at the heart of why they failed before even having chance to glide. F67 Shred. F100 shred. Three D12 cluster Shred. D20 staged to F7, D20 boosted it to 50 feet, ignited the 7 second burn low thrust F7 which slowly pitched over and gravity-turned into the ground to crash become burnout.

    An article here about a scaled up big (2X) one that Bob Koenn built. http://archive.rocketreviews.com/reviews/all/scratch_valkyrie.shtml

    [​IMG]
     
    Last edited: Jan 4, 2019
  12. Jan 4, 2019 #42

    Rktman

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    Eric Noguchi

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    Thanks George, will stick with your suggestion of a 2/3 resized canard set at a fixed 4.5 - 6 degree incidence.
     
  13. Jan 26, 2019 #43

    Rktman

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    Eric Noguchi

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    Final Flight Report
    It lives!

    Following George’s advice on adding a turbulator strip to the main wings made an enormous difference. It provided the needed lift to keep the glider airborne so the new canard could do its thing. I ended up using a (non-airfoiled) size roughly 15% - 20% of the main wing area and increased the incidence angle from 3° to 4.5°. The glider still required a scant few grams of nose weight, and interestingly the CG ended up exactly where the online CG calculator estimated it should be (I was 1/8” aft originally).

    Had a chance to test fly it last weekend and—success! The changes (though they were far from extreme) made an enormous difference. Following a straight launch the glider transitioned immediately into a shallow glide, circling in a tight approx. 25’ radius turn. My takeaway is that even seemingly small changes can have a big impact on the size gliders we fly.

    In retrospect, I now know that the causes of the malfunction were: A): as George pointed out, no airfoil on the main wing and; B): an airfoil on the canard when it shouldn’t have had one; C): wrong incidence (decalage) angle on the canard. All this probably caused the main wing to “sag” from insufficient lift so that the nose pointed up. This was made worse by the airfoiling on the canard, which provided too much lift so that the glider swooped upward, decreasing its speed to the point where it couldn’t maintain a stable glide and nosedived. Once it picked up speed headed downward, the whole cycle repeated itself.

    Since I mentioned that this was a first design attempt and therefore a learning exercise, I feel I’ve gained quite a bit from this experience, so it’s all good. Much thanks to everyone who provided guidance!
     
  14. Jan 26, 2019 #44

    Richard K.

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    Gotta love it when a plan comes together! This is a great forum, lots good info from people willing to help out!
     
  15. Jan 26, 2019 #45

    Rktman

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    Eric Noguchi

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    Absolutely!
     

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