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  1. #1

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    how to analyze scratch build

    Hi,
    I was planning to build this 3d airframe from free plan I found. I completed the cad build & now only thing left is to extract the formers for laser cutting. Before I invest in that, I wanted to analyze how my airplane will fly. Are there any software available for this? Will they be able to analyze only straight flight or also high alpha?

    Ameyam
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    Preflight check is optional , picking up the pieces is consequential

  2. #2
    Moderator BMatthews's Avatar
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    I'm not sure what you expect to find out from any sort of computer analysis. Models of this sort if built light enough and provided they have enough power are typically like flying a computer screen mouse in that they go where you tell them and turn on a dot if you give the controls enough throw. And the closer to 70 degrees either side of neutral you can manage the better.
    Witty saying to be plagarized shortly.....

  3. #3

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    I need to know whether I have the wing & tail incidences correct for straight & level flight at least. If I need to add to much trim, that will mess up the controls in other attitudes

    Ameyam
    Preflight check is optional , picking up the pieces is consequential

  4. #4
    Moderator BMatthews's Avatar
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    For a model of this sort which spends so little time upright the normal setup is to put the thrust line, wing and tail all at 0 degrees to the center line. You then run the design's numbers through one of the CG calculators and set the stability margin to zero or at most 3%. From there you adjust the CG in small amounts to zero out the stability so the model handles dead on neutral. Once you get it trimmed in that manner there is no elevator trim to mess you up when inverted.

    For online CG calculators you can find links in the sticky thread at the top of this forum or you can simply do a search for "online cg calculators" and get a few hits to work with.

    That help out?

    If you then move on to adjusting things to avoid pulling to the canopy or pushing towards the belly while in knife edge flight that's a whole other matter. For that sort of stuff there's a variety of tricks that you can learn about for the asking over in the pattern flying forum.
    Witty saying to be plagarized shortly.....

  5. #5

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    Well, I did put this through XFLR but there are some limitations. The airfoil is a NACA0014 but not full- the ailerons are barndoor type. The tailplane is flat, not builtup and XFLR doesnt have a shape for that. Also, I am not adept at XFLR, just starting and my occupation doesnt give me any time to pick it up as yet. I am not muchexperience in building either- exactly why I decided to re-engineer the plan to make it out of interlocking ply. A cad build means I can get exact holes and slots for the laser so that build adjustments are minimum

    But I still got some output from it- there doesnt seem to be a moment about the CG at 0 Deg Alpha and the moment at a higher alpha is corrective. XFLR doesnt seem to give me results at negative alpha, but I havent gotten into figuring out why thats happening. XFLR lets me model the tailplane as well with a generic foil. But nothing beyond that.

    As stock, the model has 0 Deg Incidence on both stab and wing but there is a difference in the centreline of the wing and the prop- that may lead to a moment about the wing (I may need to trim down if there is not tailplane incidence). Let me check up on these issues and try my hand on XFLR a bit more before I revert

    Ameyam
    Preflight check is optional , picking up the pieces is consequential

  6. #6
    Moderator BMatthews's Avatar
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    I'm not surprised that there is no moment about the pitch axis at 0 degrees Angle of Attack (AoA). It's simply that sort of model.

    Clearly you do not have a lot of flying time with this sort of model either. This is not a model that cruises around looking good. It's intended to spend much of the flight trying to turn itself inside out. Flying such a model straight and level where you worry about some of the factors you mentioned is simply not what this sort of model is designed to do. And since XFLR5 is intended to analyze FIXED conditions I'd suggest that what you are doing is totally missing the point about building and flying this sort of model.
    Witty saying to be plagarized shortly.....

  7. #7

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    Actually, I have had the Phoenix Topstar 60 (almost the same as this model) since 2009. However, unrelaible engines prevented me from really enjoying it. In 2011, I resurrected the airplane with the FS91SII and it flew well and helped me return to confident flying. By this time it was a old airplane passed on by my mentor and was repaired several times, so had gained weight. It still flew like a balloon, not too precise and I didnt have a good setup (I realised that later). I was more comfortable on the Reactor 46 then and I moved on to that. I tried converting the Phoenix Topstar to dle20 but it became overweight and I passed it on. This year the Reactor got dunked from a deadstick in a hover and the landing gear rotated in the fuselage. Its still fine for dunking into tall grass for a landing in the monsoon time but I wont be able to fly it subsequently as I cant install a landing gear

    I am hoping this scratch build given me a more aerobatic flier that I can fly as a cheap hack around / sunday flier / to do all kinds of carefree tricks. I dont need it to fly straight and level but it would help if it flew that way in the first place. Thats why I want to test it before I build. But I see that you cant test the airplane unless you build it

    Ameyam
    Preflight check is optional , picking up the pieces is consequential

  8. #8
    Moderator BMatthews's Avatar
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    No one says you have to take a heap of risks. There's still the proven online CG calculators that you can use. Set the model up initially with the information from these calculators for about 5 to 8% Stability Margin. That'll get you through the first flight. From there you can play around with the CG location until the model is flying as near to neutrally stable as you want.

    In terms of "flying level" a model of this sort never really does fly level. As mentioned highly aerobatic models are typically, and correctly set up with a 0-0-0 thrust-wing-tail setup so they can do well with the near neutral CG location that you'll end up using once you trim it back to that point. But because the wing uses a symmetrical airfoil the model has to fly a degree or two nose high upright and inverted. So it's really never level at all even when flying level. If you DID set the wing and tail to let the fuselage fly dead on level it would only work for one flying speed and it would only work when the model was upright. Change speed or flip over to inverted and the fuselage is then at some other angle. So the widely accepted feeling is "why bother?" and the fuselage is set up so it's zero as well as the thrust line, wing and tail to keep things easy.

    In the air you simply do not see this angle.
    Witty saying to be plagarized shortly.....

  9. #9

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    Hi,

    actually I moved on from that UCD type plan because I wanted something a bit more aerobatic. Based on this Funtana 60 plans, I did a cad build attached. Still requires cleaning up. XFLR number seem ok (no pitching moment, everything is set at 0 degs. Just wanted to know whether I should persist with this and build it or stop and call it a day here

    AmeyamClick image for larger version. 

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    Preflight check is optional , picking up the pieces is consequential

  10. #10
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    If you are worried about the incidence angles, you could build the stab and fin as a bolt on unit type, then you could take it off and adjust the incidence after test flying until you find the sweet spot.Building a removable tail will be slightly heavier of course..


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