The Razorback construction thread

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

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  1. Dec 21, 2008 #81

    AVI

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    Roger that!
    There's a lot to be said about having the balls to get started without all the minuscule details worked out. Good for you, Duncan! However, don't forget the parachute on the test flight.

    Seeing that there have been so many vast improvements to the KR2 design of late, such as new airfoils, stretched fuselage, etc., it makes one wonder how much detailed engineering went into the original KR2.

    How much was eyeball?
     
  2. Dec 21, 2008 #82

    AVI

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    Hey, looks a bit like a Donkervoort!
     
  3. Dec 21, 2008 #83

    Mac790

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    Yes Donkevoort was my favourite Lotus Seven version, but now I have even something better than that in my head/mind:gig:, it's time to put it into 3D and next into a real thing. I'm not ready to build an airplane yet.

    Seb
     
  4. Dec 21, 2008 #84

    Topaz

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    Not a lot. Back when Ken Rand did the KR-1 and KR-2, people regularly accepted that most homebuilts were going to be rather 'twitchy' to fly. It was pretty much normal practice (in the H-B community) to gain a few knots with an undersized tail or a smaller-than-really-practical cockpit. The first Glasair was the same way. Not to mention that Ken Rand himself as a fairly small guy, and more or less designed the airplane around himself. It could just fit two people of his size - two people of average size back then would've found it a very 'cozy' fit. The average person is even bigger now (especially in girth, sorry to say) and so the KR-2 needs some substantial alteration to make it a practical airplane on today's terms.

    What surprises me is that Rand-Robinson hasn't consolidated the most beneficial builder-initiated changes into a 'KR-3'. Even if they had to pay a licensing fee for that new wing airfoil, it seems to me that it would be worth it. The airplane, with all the 'regular' modifications done (KR-2S with additional fuselage stretch, new airfoil, wider cockpit, bigger horizontal tail, Dragonfly canopy, etc.), is a really nice bird by all accounts. Still a tad small inside by, say, Bonanza or Rockwell standards, but it'd be a lot better for most of the public and might be a truly viable contender in the plans-built market again.

    I guess Ken's wife and whoever else is in the company are content to simply live off whatever plans sales continue to come in on the current model. They don't even seem to have a 'brick and mortar' location anymore. I used to work in Huntington Beach and the mailing address given on the website is one of those "Post Office Etc." sort of places.
     
  5. Dec 21, 2008 #85

    Topaz

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    I'll second that. I could go out and buy a nice used SGS 1-26 for about $6000. Maybe $8k for a really nice one. My design won't have much more performance than that 1-26. Granted, it'll probably cost half of what the 1-26 will, but I could get the 1-26 right now, instead of the time it takes to build my own.

    But it's my own creation. A challenge for myself. A dream I've had for years. And it'll fit in my garage, whereas I'd have to rent a tie-down space at my airport and leave a 1-26 out in the weather all the time.

    That last is just a worthy rationalization - the challenge and the "doing it myself" is the real motivation. :gig:
     
  6. Dec 22, 2008 #86

    gschuld

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    Duncan,

    Thanks for the reply and again, my intention is not to offend anyone. I've designed and built custom boats and boat parts, hardware, iceboats, all kinds of things over the years. It's my love as well. I would love to design my own plane from scratch. I have spent may hour thinking about it. I suppose I just can't get over the concerns of missing an important detail and underdesigning/building a critical part on an airplane. Basically, I'm a sissy:lick:. For those of you that have designed and built your own planes, I have a tremendous amount of respect for you and I am indeed jealous. I'm looking into a KR-2s with the typical modest modifications that have become popular to suit my sissy concerns and paranoia.
    I too am very interested in your build and wish you the best of luck with it. Please keep us informed on the progress and I'd be happy to help if I can.

    George
     
  7. Dec 22, 2008 #87

    Mac790

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    In summary of this discussion, It seems that Duncan came a long way since summer, just compare these two attachments. Of course still need to learn a lot, but I believe he's going a right way. Maybe not a simplest one but we have different paths. Wish you luck, and keep us informed.

    Seb
     

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  8. Dec 22, 2008 #88

    BBerson

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    A load test is something to consider. Even Boeing does load testing to confirm the engineering calculations. Boeing had to redesign an under strength wing on the 787.
    The only way to know for sure is by testing.

    Of course, testing can damage the part. So maybe just testing to limit load is the designers choice. I tested my wing to ultimate and found some beefup was needed. Glad I did the test.
    BB
     
  9. Dec 23, 2008 #89

    rtfm

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    Hi BB,
    Yes, full load testing of the one-piece 28ft wing will be undertaken! I'm not building two wings which need to be joined in the middle. I'm bypassing all that messy stuff, and building a single wing which I will bolt directly into the bulkheads. Jim Marske has a very comprehensive set of calculations for working out exactly how many graphlite rods per spar cap (etc) are required. It's going to be a nervous day, but I'll do a full load test to 9g

    My question earlier about how to glass the vertical tail perhaps needs some more detail:
    I am glassing the Airex foam with a single layer of 300gm (8.8oz) glass cloth, 8H Satin weave. I'll split the newly glassed foam shell, and complete the inside/outside glassing via vacuum bagging with further layers. The fuse sandwich will be 1/2" thick. So far, so good. But the tail presents a problem, because it will NOT be constructed with 10mm Airex foam. It will be a solid blue foam piece. Glassing the outside will be easy. But what about the inside? I guess I just leave the foam where it is, and close it up with cloth. Guess so. Have to make sure I leave space in there for the controls. Importantly, the tail will not be an add-on piece, but integral with the rest of the fuse. In fact, I'll be running glass fibre rods embedded into the foam from the sides of the fuse up to the top of the tail (flush with the surface) to help anchor the tail and add to its rigidity.

    Some of the things I still have to find out: (anyone able to help with drawings, pictures, explanations)?

    1. How to create an anti-servo tab
    2. How to create a variably sensitive elevator
    These are the things I am thinking of at the moment...

    Regards,
    Duncan
    PS Picture of yours truly celebrating with a beer after completing phase 1 of the planking. I'm very pleased with the results. Next phase: add the tail planking, and fit the tail.
     

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  10. Dec 23, 2008 #90

    addaon

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    Is there a reason you're particularly worried about elevator feel? Most small designs seem to do fine without (anti-)servo tabs.
     
  11. Dec 23, 2008 #91

    rtfm

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    Hi,
    I learned to fly in a Robin R2120 which had an all-flying h-stab, with anti-servo tab. It felt OK, but was still quite pitch sensitive. My instructor could fly S&L, but I struggled. I finally got it, and thought that I was a WWII flying ace. But then I flew another aircraft (can't remember the type, unfortunately) with an all-flying h-stab and NO anti-servo tab. I was all over the place. I really struggled to look cool, rather than a lovesick dolphin. I just couldn't feel anything through the stick.

    So because the Razorback is marginally short-coupled, and has an all-flying tail, I thought it wise to design in an anti-servo tab. Especially since I'm expecting to cruise at about 150kts.

    Regards,
    Duncan
     
  12. Dec 23, 2008 #92

    addaon

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    You can get more or less the same effect (higher stick forces) by putting the hinge forward a bit more; that is, being slightly less aerodynamically balanced on the surface. But you might find that you grow to like lower stick forces; most people I've spoken to do.
     
  13. Dec 23, 2008 #93

    Rom

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    No problem. That is why the pic was posted.

    I have both sides glassed. The hot glue was fast and easy. The holes left from knocking the forms off of the fuselage were simply filled with micro.
    I'll be posting some new photos of what what it looks like so far.
     
  14. Dec 24, 2008 #94

    rtfm

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    Gluing the foam planks together

    Hi,
    I was out at Hangar F2 today to figure out how to glue the foam planks together. Mmmm not so easy. I have cut and fitted the planks so closely that although there are some gaps (inevitably), most of the planks lie very close to their neighbours. So close, in fact, that I can't get anything (glue or foam) between them to hold everything firm while I shape the surface.

    So I think what I might do is to take my Dremmel tool and grind a groove between the planks. This will make a convenient channel into which I can smear the two-pot foam mix. That ought to do it.

    Duncan
     
  15. Dec 24, 2008 #95

    flyoz

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    My 2 cents worth ...
    Keep foam strips equal in width maybe 50 mm allows for some twist and bending and a slight angle on the edge keeps the gaps to a min . There will be areas where you have to "slice " the strips to fit them in but it reduces the amount of cutting . The strips can be pre made instead of individually.Dont make the side angle too sharp ( say 5 deg ) otherwise when you sand them down you get gaps .
    Flyoz
     

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  16. Dec 24, 2008 #96

    rtfm

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    Flyoz,
    Hi. Merry Christmas.

    Yes, I did what you suggest. I cut all four sheets of foam into 50mm planks, and laid these over the formers. There was some cutting and shaping involved, but it is mostly done now. My main problem at the moment is figuring out how to glue them all together so I can sand the foam smooth.

    I tried 2-pot foam, but it has insufficient strength to hold the planks in place. Next step is to try something stronger. I've been reading similar threads where the guys use "micro". Is that resin and micro-baloons? I've got plenty of micro-baloons. Maybe I'll try that - not neccessarily along the whole join, but possibly at strategic places sufficient to firm up the planks. Then I can fill with the 2-pot foam and shape the lot. Just a bit worried about sanding the micro and the foam together - micro will be a lot harder to sand than the foam, and I'll get uneaven lines. "Spot" gluing with Micro and filling the rest with 2-pot foam might be the way to go.

    Regards, and hope you have an enjoyable day with the family...

    Duncan
     
  17. Dec 25, 2008 #97

    flyoz

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    Hi Duncan
    A merry Xmas to you too
    I think you are right - filling the foam gaps with micro and resin just makes level sanding almost impossible - not sure i have the answer - polyurathane foam glue seems an option but its messy .
    Flyoz
     
  18. Dec 25, 2008 #98

    rtfm

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    Figured out how to do the tail

    Hi,
    I've figured out how to do the tail. I've tried explaining the technique a few times, but it will be FAR simpler to show it in a series of photos with captions, so that's what I'll do. I'm super frustrated at the moment, because I have to earn my Christmas Brownie points at home, so there is no chance of nipping off to Hangar F2 to try my solution.

    In a couple of days, photos to follow...

    Duncan
     
  19. Dec 27, 2008 #99

    Rom

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    After hot gluing the strips together, one of two dabs of glue between the foam and forms and about every 4 inches apart along the length of the strips, the inside was microed then glassed. The outside was easily sanded to shape after removal from the female form, due to the fact that the whole structure was already reinforced on the inside.This is the main reason I chose to use a female form. After sanding with a coarse sandpaper using a virbatory sander, all of the joints were microed right before glassing the outside.

    If the structure is stiff enough do your sanding befor any micro is applied, sand first.
     
  20. Dec 28, 2008 #100

    Inverted Vantage

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    Hey rtfm, just sent you a PM :)
     

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