The Razorback construction thread

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by rtfm, Jul 11, 2008.

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  1. Jan 1, 2014 #721

    Head in the clouds

    Head in the clouds

    Head in the clouds

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    I see where you're coming from Wizz but the setout of the mounts on a bed mounting isn't dependent on the existing bolt pattern on the engine. The rubber mounts cannot be directly bolted to the bolt holes of the engine in any case so the engine is first bolted to a plate or a pair of flat bars. If a plate is used the plate can be a large square if that is what is wanted, and the rubber mounts spaced well apart at the four corners of the square. However, the twisting moment from the torque of a small engine like this, in practice, is usually quite a bit less than the moment produced by the thrust since the thrustline is raised by the re-drive.

    So in this case I would bolt a pair of 10mm flat bars fore-and-aft to the existing engine mounting bolt pattern and then the rubber mounts would be positioned in front of, and behind, the existing pattern. That gives a rubber mount footprint of about 8-10 inches laterally and about 12 inches longitudinally and previously I've found that works well for that range of horsepower.

    Addition of a fifth mount tends to harden the whole arrangement up so much that unpleasant vibration gets transmitted back to the airframe.

    If it proved necessary you could always add another mount up high but I would not use a Universal mount for that, I would suggest a vulcanised mount (see pic) used in shear rather than tension or compression and they are readily available over here.

    All that said, there's nothing wrong with having a spare mount or two since the postage for the first one costs more than the rest of the order, and extra items add just a little to the postage cost.

    download.jpg
     
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  2. Jan 1, 2014 #722

    rtfm

    rtfm

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    Engine mounts

    Hi Wiz,
    Based on what Alan's said, I think I'll order a few more anyway.

    Cheers,
    Duncan
     
  3. Jan 1, 2014 #723

    Autodidact

    Autodidact

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    They say that if it looks good it'll fly good; if this flys as good as it looks it should be a very nice handling airplane. Cheers to you, Duncan.
     
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  4. Jan 2, 2014 #724

    Aircar

    Aircar

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    Greetings Alan, -looked up your link on Jabiru prop problems and concur on the soft wood issue --covering wooden props in a layer of fine glass at 45 degrees increases the stiffness and water proofing . We used Queensland maple for ultra light props and a rather unusual windmill (to reel in targets for NAVY target practice using Aztecs back in the early 80s ) with no problems --apart from the prop itself 'weaving' as shown by strobing at night (bordering on flutter) --glassing that prop removed any problem . That was a fairly large diameter prop on a rotary which gives hard power impulses and when there is some bending in the prop from the thrust this causes torsion and at some RPM will be resonant .

    Nice to see that Duncan has revised his mid wing to a low one and this should make the entry and exit, balance and view all better . An inverted gull will be harder to fabricate but will give better prop clearance and shorter landing gear --an inverted parasol will give the same advantages with less wing complexity and possibly lower drag --not a usual arrangement (except for some lower biplane wings) but worth considering .
     
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  5. Jan 10, 2014 #725

    rtfm

    rtfm

    rtfm

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    Razorback mold building - update

    Hi,
    Today I completed the seat. It was a little fiddly, and took a number of steps, but I now have a very ergonomical bucket seat made from spray foam. It fits in the mocked-up cockpit really nicely. Now all I have to do is to wait for the foam to settle down, vacuum bag the seat to "fix" it rigid, and then make molds from it. The front and the back will both act as structural bulkheads.

    Also committed myself to my wing layout today. Sitting in the cockpit I was able to accurately measure where my knees ended up. The optimum FWD sweep of the wing turns out to be 12 degrees, which puts the spar directly under my knees and safely out of the way. There is even a space for storage between the spar and the seat. So, that's half way between my initial 8 deg and the 18 degrees I was considering. I feel a lot more comfortable with 12 degrees. There is nothing like actually testing the setup as opposed to drawing it on the screen. So I glued up the platform on which I will build the central section of the wing. 22 degrees anhedral, 12 degrees fwd sweep. A little tricky, but it is all glued up and setting at the moment. I'll return this afternoon and check that the angles are correct. Then I'll take a few photos.

    Regards,
    Duncan
     
  6. Jan 10, 2014 #726

    Aircar

    Aircar

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    Dunc, do you have an articulated human body template? --you can download the layout and photocopy to your scale or buy one ready made (and also 'action man' or "GI Joe' 'poseable' figurines - usually one foot tall, allow to design your ergonomics without needing to build a full size fuselage first ;very useful things to have.

    If your spar is running 'under your knees' and the wing is swept FORWARD then you are a long way aft of the loaded centre of gravity -- you will need a long nose to balance and it will end up being very payload sensitive ( I detailed the fiasco with the Wickers Cobra some time ago that resulted from the same combination .

    I suggest you do a weight and balance and see if everything is OK.
     
  7. Jan 10, 2014 #727

    rtfm

    rtfm

    rtfm

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    Hi,
    You are correct on all three counts - I am quite far back from the CG. But I do have a long nose and yes a third time, the plane is going to be very payload sensitive.

    I constantly check the weight and balance as my estimates get more precise. My main concern is if someone else flies the plane, and they weigh more/less than I do. The plane is set up for an 180lb pilot. However, if a 220lb pilot jumps in, the CG is going to be way off. The only solution I have been able to come up with is to sacrifice 15lbs and put a 15lb weight in the tail. Removing it allows a 200lb pilot to fly with the same CG. Moving it to the front accommodates a 220lb pilot. Not particularly elegant, but since most people don't lend out their single seaters to a bunch of people (or anyway, not too often), once positioned correctly which should only take a few minutes, the CG should be fine.

    Here are some photos of the cockpit, with seat in place, and two views of the wing jig

    The seat is a work of art. Pure sculpture. First I made a glass/foam sandwich, and put it all in an airtight plastic bag. Then I put it on my car seat, and sat in it. I then drove home (anything to use up the time required for the seat to set). It got pretty warm (Brisbane was experiencing 38 deg heat at the time, so it was like a bloody sauna in there). But the seat conformed beautifully to my bum and my back. Only problem was that it wasn't "true". Either my body is asymmetrical, or the car seat is skew. Either way, it explains my back pain when I drive for a long time...

    So I then added tons of bog to the bottom of the seat to get it "true" and then replaced it in the cockpit on a layer of spray foam. I weighed the seat down with a great big "breeze block" (a cement building block) till it was all set. Once removed, the outline of a wonderful seat remained in the foam. Some minor sanding and tidy-up later, I now have a bum-hugging seat. If you have a big arse, you're out of luck I'm afraid... :)

    The awkward thing about the jig is that the central section has to turn through 12 degrees forwards, and 22 degrees downwards. First step was to get the sweep angle right, which is what you see in the photos. This wing jig will be covered with a single sheet of 3mm melamine board and bonded in place, curved over the gap in the middle. Then the 22 degree sweep of the anhedral will be drawn onto it. The central 24 inches is curved, of course.

    Regards,
    Duncan
    Cabin mockup - with seat 2.jpg Cabin mockup - side view with seat.jpg Central wing section jig 2.jpg Central wing section jig 1.jpg
     
    Last edited: Jan 10, 2014
  8. Jan 12, 2014 #728

    wizzardworks

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    UpperSurfaceFormers.jpg TopAndBottomFormers.jpg UpperSurfaceFormers2.jpg Duncan, Although I haven't built this yet I have a plan for moulding wing skins. Basically the formers for the upper wingskin are on one side of some 3/4 plywood and the formers for the lower wingskin are on the other side. Then a perimeter surrounds these pieces. There is to be a recessed edge for most of the formers to install a surface of 1/4 inch plywood sheets with screws and glue. The leading edge will be strip planking covered with bog. The top and bottom also have a 7 inch wide plywood surround influsion flange.
    wizzardworks.
     
  9. Jan 12, 2014 #729

    autoreply

    autoreply

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    Re: Razorback mold building - update

    That's a great place for some storage in closed pockets. Hinge them at the bottom and you can temporarily shove your leg to the side to put in/out stuff like maps.

    As for the C of G, I'd design a battery in the tail. Have the battery in the tail for very light pilots, in the nose (engine) for heavy pilots and use LiPo's if you're in-between.
     
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  10. Jan 13, 2014 #730

    ThadBeier

    ThadBeier

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    RTFM,

    With respect to forward swept wings, there might be one more thing that you want to look at.

    On any swept wing, the more forward parts of the wing induce changes in airflow for the parts of the wing further back. On traditional rearward swept wings, the outboard parts of the wing see an increased angle of attack. One of the reasons that swept flying wings have twist is for stability, but another very important reason is that otherwise the tips would be flying at an increased angle of attack, leading to the possibility of a tip stall. IIRC, the wings of the SWIFT are washed out 9(!) degrees.

    Forward swept wings have the same issue. The inboard part of the wing would see a higher angle of attack. If you don't twist the wing to compensate (that would be "washing-in" the wing) then the lift will be shifted inboard and rearward.

    I confess I don't know of good tools to calculate what this twist should be. I don't think AVL, for instance, takes sweep (and thus, spanward flow) into account. 12 degrees is not a huge amount of sweep, but it's enough that you should take this into account.
     
  11. Jan 13, 2014 #731

    rtfm

    rtfm

    rtfm

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    Hi,
    I must confess, this is the first I have heard of this. My understanding is that one of the desirable features of the FSW is exactly that the wing tips keep flying, and that the inner section of the wing stalls first, and propagates outwards. Just like a rectangular wing would do.

    I went looking for the SWIFT you mentioned. Exactly what plane is this? Those I found on the web didn't seem to be FSW aircraft?

    Regards,
    Duncan
     
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  12. Jan 13, 2014 #732

    ThadBeier

    ThadBeier

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    The SWIFT is the Brightstar SWIFT designed by Ilan Kroo of Stanford. I was just pointing out that swept wings need a lot of twist.

    The X-29 had a large amount of wash-in according to a Grumman test pilot (my father-in-law.) but it had a *far* greater sweep than you are talking about.

    Let me try to find some rules for twist vs sweep.

    You are right that it is likely that the root of an untwisted FSW would stall first. That's great in a unswept platform, you maintain lateral control. If the root stalls first in a FSW plane, then you could get substantial pitch-up, as the remaining lifting part of the wing is substantially forward of the CG.
     
  13. Jan 13, 2014 #733

    ThadBeier

    ThadBeier

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    The SWIFT is the Brightstar SWIFT designed by Ilan Kroo of Stanford. I was just pointing out that swept wings need a lot of twist.

    The X-29 had a large amount of wash-in according to a Grumman test pilot (my father-in-law.) but it had a *far* greater sweep than you are talking about.

    Let me try to find some rules for twist vs sweep.

    You are right that it is likely that the root of an untwisted FSW would stall first. That's great in a unswept platform, you maintain lateral control. If the root stalls first in a FSW plane, then you could get substantial pitch-up, as the remaining lifting part of the wing is substantially forward of the CG.

    You can check out

    http://www.rcsoaringdigest.com/OTW/on-the-wing4/162-HCP2.pdf
     
  14. Jan 13, 2014 #734

    rtfm

    rtfm

    rtfm

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    Hi,
    Thanks for this. Most interesting. I search all over the web for stuff like this and someone else always finds more than I do... :)

    Regards,
    Duncan
     
  15. Jan 13, 2014 #735

    Aircar

    Aircar

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    Check out the Cornelius fuel glider (X something) and Cornelius freewing --very highly forward swept and tailless to boot.
     
  16. Jan 14, 2014 #736

    ThadBeier

    ThadBeier

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  17. Jan 15, 2014 #737

    Aircar

    Aircar

    Aircar

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    Another highly forward swept wing - Cornelius fuel glider and Frewing/Mallard - Cornelius XFG-1 - Wikipedia, the free encyclopedia
    also see 'Cornelius fuel glider images'

    Weight and balance and aeroelastics are where angels fear to tread when conflicting and unusual features co exist.
     
  18. Feb 7, 2014 #738

    rtfm

    rtfm

    rtfm

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    Update on plug making

    A quick update...
    I've basically run out of materials, and while I wait fort a job to land, I'm messing around the fringes, making adjustments, fiddling with design configs etc.

    One thing I HAVE done is to almost complete the tail extension on the original single-seat design. I also sized the rudder, cut a template, and mounted it. I'll have to extend the tail somewhat. (See pic) Please excuse the background clutter - there is very little room to move in the shop. I need to mount the base on bigger wheels so that i can move the plug outside for some decent photos.

    So all I'm waiting for now is a cash injection, so I can stock up on supplies and get the plug ready for the molds.

    Duncan

    F1 plug with tail extension and rudder.jpg
     
  19. Feb 7, 2014 #739

    Aircar

    Aircar

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    I much prefer the aft swept fin Dunc, --having the horizontal tail aft of the fin is a good thing as regards spin recovery (see the Provost,Victa and Winjeel --the Tobago also does this --the idea being to have the 'blanketing ' of the horizontal tail miss the vertical tail . Still like to see you TRY a 'bubble' canopy to go with it though...
     
  20. Feb 7, 2014 #740

    rtfm

    rtfm

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    Hi Aircar,
    I HAVE tried to draw a bubble canopy, but I can't get it to look like anything. Always ends up looking really crap. I know my drawing skills aren't up to much, but I have tried. I'll give it another go.

    Duncan
     

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