Belly Canard Flap on STOL airplane

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by PagoBay, Dec 15, 2019.

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  1. Dec 15, 2019 #1

    PagoBay

    PagoBay

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  2. Dec 15, 2019 #2

    BJC

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    What is the question?


    BJC
     
  3. Dec 15, 2019 #3

    pictsidhe

    pictsidhe

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    Whereas anyone can view almost all of HBA, your link: "You must be a member of this group to view and participate."
     
  4. Dec 16, 2019 #4

    PagoBay

    PagoBay

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    I see. As a member, I did not know membership is required for simply viewing a post.
    Thanks
    ===
    Hello Zenith 701 drivers,



    Time for comments and questions on my proposed "winter weather" airplane project. First, let's get the obvious issue out of the way. This program is neither necessary or nor very practical. I already fly an airplane highly optimized for short take-off and landing and slow speed flight. This is about gilding the proverbial lily and taking it yet a step further. I am looking for a fun wet season project and thought this might address an issue with my Zenith CH 701 that is just part of the baseline design. Nothing that needs to be "fixed" really, just a chance to experiment and play some....you know, that is what homebuilts are all about.

    All inputs welcomed...regardless of how well informed you are. Bonus points for real aerodynamicists or folks with STOL or canard design experience. If you have them, questions are welcomed.

    Details in the PowerPoint file link following. Ready, set, go

    EDIT: Link is now Updated.

    https://storage.ning.com/topology/rest/1.0/file/get/3770698713?profile=original

    XXXCanard%20Flap%20Overview.pptxXXX



    [​IMG]
     
    Last edited: Dec 16, 2019
  5. Dec 16, 2019 #5

    cluttonfred

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    I can't view the PPTX file, perhaps you could just explain what it is you are trying to do and why you think this "belly canard flap" is the solution. As I understand it, the issue is the control force needed to maintain slow flight with full flaperon deflection. I would think that a larger elevator trim tab and/or adjustable spring bias on the elevator circuit at the stick or on the cable would accomplish that goal. Cheers, Matthew
     
  6. Dec 16, 2019 #6

    PagoBay

    PagoBay

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    I am not the OP on this issue. There are a few HBA members who are in the Zenith Forum membership.
    Just networking for the benefit of encouraging interesting ideas and discussion. There are great resources on HBA beyond those on the Zenith forum.
    Hoping for some interaction. Good for everyone on the Zenith forum to have an introduction to HBA members and the wealth of knowledge available. The Zenith members would welcome the input.
    RMM-Guam

    PS: Updated the download link to the PPTX file. Thanks
     
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  7. Dec 17, 2019 #7

    bmcj

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    If you have not already, research the Robertson STOL kit for the Cessna 172... same principal. I can’t quote you any figures off the top of my head, which is why I say to ‘research’ it.

     
    Last edited: Dec 17, 2019
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  8. Dec 17, 2019 #8

    pictsidhe

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    Adding a canard will move the neutral point forward. That will reduce the stability margin. This is NOT a suck it and see mod.
     
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  9. Dec 17, 2019 #9

    Topaz

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    Essentially this, plus the proposed mod isn't really going to improve anything. I get what's being proposed, but unloading the tail on an airplane like that isn't going to really help the takeoff and climb performance. For one, you're adding weight, which definitely will increase the takeoff run by some small amount, simply because the thrust isn't changing and the airplane now weighs more. Slower acceleration up to takeoff speed. Secondly, even at these low speeds, you're adding drag. Which is directly subtracted from engine thrust. Simply less available thrust to accelerate and climb out.

    Then you have the issue that pictsidhe mentions, which is that the canard surface is destabilizing in pitch. Either the horizontal tail needs to get bigger to compensate (more weight, more drag) or stability may be compromised to the point where the flying characteristics are unsafe.

    All in all, this mod is unlikely to obtain significant gains for this design, and better results could be had by increasing power, reducing weight, or both. Adding span would help the climb rate, too.
     
  10. Dec 17, 2019 #10

    trimtab

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    I flew the Wren 460 a few times in 1987. The basis of the design choice is pretty solid with clear plusses and minuses:

    1. Lift. The position of the canard behind the prop is important for a few reasons. The speed of the prop wash is large, on the order of 85 knots static and of course higher once underway. My own pitot measurements in my own 182 show 85 at rest at full throttle and close to 190 knots at 130 knot cruise power (all in TAS).

    This does a few things at a.low maneuvering speed. First, the small canard produced nearly 400 lbs of lift at full power and full aft elevator. The potential was much higher, but structural considerations and stability considerations limited this. The canard was essentially stall proof with even moderate power levels, and this presented the stability issues. But unloading the wings of perhaps 200 lbs of weight from the canard lift plus perhaps 50-70 lbs of reduced tail force required while flying at 2400 lbs for a 2800 max gross 182 meant a stall speed reduction of a few knots at best at idle, and easily several knots at full throttle flares that were demonstrated to me and which I got to try for myself once. Not world changing, but still significant. You don't have to worry about stability as much while flaring, and the flare could be effected by keeping the wheel still while applying power almost alone.

    2) the canard demonstrably increased net thrust at low speed. The static tests increased by 5%,.probably from reducing vorticity of the prop wash and from the angle of attack on the left canard. The designer wanted to make the canard angle of incidence optimized on each side for a takeoff distance criterion. He never did as far as I knew.

    3) the mods were heavy, and the canard was a big part of that. I also never saw more than 124 knots on the Loran, ever, at full throttle (80% power at the time). Two people, perhaps half fuel. The climb rate at Vy seemed pokey, while the climb angle at Vx seemed like a large improvement over stock.

    Seeing the special airspeed indicator go down to 32 knots at touchdown, and rollouts measured in a dozen or wingspans or so with minimal braking was pretty neat though for a teenager at the time.

    If you want to give up a lot to be able to fly slow, and can work out the stability issues (reduce effectiveness and potential of the canard to preserve positive stability even at full throttle), perhaps it might be worthy of consideration.
     
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  11. Dec 17, 2019 #11

    Victor Bravo

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    I'll add one small consideration to the Robertson / Katmai style "third wing" configuration. Can you imagine what the FAA put them through during spin testing? Anything whose job it is to push the nose upward in flight is also potentially creating a "flat spin" situation that's a lot harder to recover from, or perhaps not possible.

    Which means that IMHO it is definitely not a "suck it and see" kind of thing.

    The belly flap proposed in that photo (looks like a cowl flap unless I'm seeing it wrong) seems like it would be somewhat inefficient. Half of it seems to be buried in the "dead zone" area where there is no good airflow under the cowl. It would have to be a much longer span than what he shows in the picture.

    So I'll postulate that the way to accomplish this safely on an experimental is to have some sort of a switchblade or swing wing mechanism that allows two small canard wings to rotate outward from the cowl or from under the fuselage, that can be retracted instantly with a lever after takeoff or in case you want to get rid of that nose up force in a hurry.
     
    Last edited: Dec 17, 2019
  12. Dec 17, 2019 #12

    flyrite

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    I think what you’re interested in is what I mounted years ago and works great!

     
  13. Dec 18, 2019 #13

    ZenithDriver

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    Hello all,

    I am the original poster of this proposal on the Zenith Builders site. Thanks to R. Michael Moore for moving the discussion over to this site and thanks for all your comments. Some are definitely spot on and some give me something to think about. As I said initially --- This program is neither necessary or nor very practical. I already fly an airplane highly optimized for short take-off and landing and slow speed flight. This is about gilding the proverbial lily and taking it yet a step further. I am looking for a fun wet season project and thought this might address an issue with my Zenith CH 701 that is just part of the baseline design. Nothing that needs to be "fixed" really, just a chance to experiment and play some....you know, that is what homebuilts are all about. I may not even leave it in place if it works, but proves to be finicky or challenging to deal with.

    Some general comments in response to input above:

    I am looking to take the download off the tail and have the airplane in general more balanced at high AoA. All that down force has to be carried by the wing. Numerous comments are valid about increased tail down moment and others have done things to the tail to give it even more authority. It is already huge for the plane and has an inverted airfoil and lower surface vortex generators (one of Chris Heinz signature configurations). Some others have increased the size, some have lowered the front of the stab, and others have increased the trim tab size. All of these increase the tail down force. My approach was to reduce the tail down force by adding some nose up pitch moment. This also adds the benefit of reduce load to be carried by the wing.

    I am definitely not looking for shortened take off run and certainly not increased climb rate (though both are fun). These light weight, high lift optimized Zeniths with full span slats and Junkers full span flaperons will already climb at a shocking and sometime disconcerting climb angle.

    I could definitely add more wing and fly slower / climb faster. There is an extended wing design for the CH701, but new wings are a bit more of a project than I want to take on now.

    Someone commented on it looking like a "cowl flap" actually it is a fairly high aspect ratio airfoil section with a chord of ~9.5 inches and a span of 72 inches. Around 20 inches of the span is under the fairly narrow cowling and ~2 feet extends on either side in the prop stream and free air.

    For complexity sake, I doubt I will be doing a swing wing canard :) ...though the idea is cool.

    Weight wise this surface, which is essentially half of a wing flaperon, is about ten lbs. I know weight is the enemy, but that is fairly negligible. It is attached with the hinge pivot points connecting to tabs welded onto the motor mount where float struts would mount. Therefore, no new structure added and plenty strong.

    In my researching, I had already run across the Swamp Monster and the Wren 260 and King Katmai. Both of these are actually somewhat different, but related cases. The Swamp Monster belly flap seems to be right between the gear and therefore around the CG. I would guess, great at increasing drag and very little pitch effect. The wren 260 is of course a much more complex true canard that is slaved to the yoke and rear elevator and provides adjustable pitch authority. I appreciate some of the subtleties mentioned in this design...much more complex than it appears initially. My thought was more of a front flap that would be deployed in conjunction with the wing flaps to provide a more balanced high lift configuration without the need for extreme tail down force.

    Again, thanks to ll for your comments and feel free to weigh in if you have anything else to offer.

    I have uploaded the detailed PowerPoint in case you could not access it before
     

    Attached Files:

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  14. Dec 18, 2019 #14

    Topaz

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    Welcome to HBA! If you move forward with testing this mod, I'm sure we'd all be interested in seeing your build and the ultimate results.
     
  15. Dec 18, 2019 #15

    stanislavz

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    As far as i have researched this topic too (tri surface) - with certain configuration - canard wing stall first, it have autorecovery from spin under climbing with sudden power off, and not stallable, then level flight.
     
  16. Dec 18, 2019 #16

    BJC

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    Interesting concept. Welcome to HBA.

    Some numbers might be interesting:

    Current minimum steady level flight speed, landing and takeoff speeds;

    Projected new flap Reynolds numbers, lift and drag coefficients, lift and drag, from the new surface, and what that will do to minimum steady level flight speed, stall or minimum controllable speeds both power on and power off.

    Have you taken a look at the airflow where the new surface will be, at the full range of power settings, AoAs and airspeeds? A tufted section of streamlined tube mounted in the proposed new surface location might work for that.

    I would approach the flight testing carefully to ensure that the airplane is controllable should the actuator fail in (the worst) position.

    Rockiedog2 is the resident Zenith expert, and he has written much about various mods to improve STOL. I would provide a link, but the search feature found nothing. You might try Google.

    Please consider having a build log, with photos, so we can follow your progress.


    BJC
     
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  17. Dec 19, 2019 #17

    trimtab

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    Keep mind the issue I mentioned above...the AoA for the canard right behind the prop is power dependent. It can stay flying even well beyond the point at which the wing or tail stalls. Even if you design a canard to reach critical angle of attack way before the others, the prop wash largely determines the AoA.

    It's a thing. The prop wash allows a large amount of lift in a small surface to be generated (asymmetrically), but also adds a twist to the general stability strategy for the stall regime that needs to be addressed.

    The Wren 460 canard elevator travel was very limited for this and other reasons. The things that are usually extolled on regarding canard designs are totally irrelevant for this case.
     
  18. Dec 23, 2019 #18

    Sockmonkey

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    Well, I did have this idea.
    [​IMG]
    Turn the gear struts into mini-delta wings with a symmetrical airfoil set at a neutral AOA.
    Minimal drag during cruise, but generating vortex lift when landing at a high AOA.
     
  19. Dec 24, 2019 #19

    jedi

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    Interesting but several issues emerge.

    First let me say a big plus for the canard is that those looking for great STOL performance start with installing a bigger (as in heavier) engine. The resulting forward CG makes the elevator download greater which reduces both performance and control. The canard helps for both of those issues. As previously posted by others, if the CG is to far aft the canard will not help. Hopefully there is a happy middle ground.

    Second, with regard to the delta canard. You definitely do not want the main wing to stall and the delta to go into vortex lift unless the delta 1/2 cord wing is aligned with or behind the main wing 1/4 cord station. Also the canard will probably need variable incidence to account for the high angle of attack required for the canard vortex lift.
     
  20. Dec 24, 2019 #20

    Sockmonkey

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    Both solvable I think. Placing them far enough back is easy enough, as the main gear have to be aft of the CG anyhow.
    With the deltas having a sharp leading edge with a lot of sweep, the vortex lift will kick in while still at a fairly low AOA.
    The purpose would be to provide a non-moving structure to supplement the STOL ability of planes with other systems like flaps and slats.
    It could also grant a safety margin for planes without those systems landing at a higher AOA than they normally would due to stall risk.
     

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