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Old Oct 14, 2012, 08:56 AM
Always room for one more!
chawkins's Avatar
United States, MI, Midland
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Help!
Riser Grown Up!!!

I am planning a winter build to strech a Riser 100 up to 144". I will also be adding length to the fuse to get it to about 70 inches and enlarging the tail group. I plan both spoilers and flaps (flaps for landing) The wing I just plan to add a foot to each wing panel, my question is the main spar. I am still debating on size and length? Any help on this would be appricated!

Thanks.......Chris
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Old Oct 14, 2012, 02:46 PM
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How about some photos or drawings of how it's built?

How long is the existing fuselage?

The load at the root would go up approximately with the square of the span, so if you were making it out of the same material, and the chord was the same, you'd be in the ballpark if you made the spar cap a bit over twice as wide. You could make it thicker, but the material close to the center wouldn't be working as hard. The easy way would be to use a material that was twice as strong. Got any black locust lying around? Sugar maple might be 50 percent stronger, and douglas fir would be somewhat stronger. Plus there's always carbon fiber. Depending on what form of unidirectional carbon you're using, it would be maybe 20 to 40 times as strong for the cross sectional area. Maybe a bit less for a hand layup of tow, or quite a bit more for a really well supported, pultruded carbon spar cap. But buckling becomes a problem, so you'd probably want to make it thicker, especially on the top spar cap.

The load drops off quickly. At 22 inches out from the root, the existing spar cap size would be fine, and it could taper down after that. I think I'd be tempted to add something to increase torsional stiffness. You could make a d-tube by filling in between the turbulators, or you could do a bunch of diagonal half ribs in front. These could be trapezoids, they wouldn't necessarily have to touch the covering. Maybe you could make the inboard ribs and the diagonal ones a bit thicker than standard. I've also heard of using two layers of covering on the center section.

I've assumed we're making this of comparable strength to the original, i.e. could withstand a similar number of g's. I've also assumed the wing loading would be about the same. If these assumptions don't hold, you'd want to make it stronger. Especially for heavy footed winch launching. And stiffer in tension.
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Old Oct 14, 2012, 02:50 PM
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P.S. By tapering the caps, and maybe replacing the removed part with balsa, or using taller shear webs, you could keep the weight on these spars down.

P.S. If you're into exotic woods, I think Purpleheart is supposed to be considerably stronger than Black Locust. But it's also supposed to beat up on tools and be hard to glue. And it's pretty heavy. I forgot to mention that Black Locust is supposed to be almost twice as strong in compression as sitka spruce. And compression is the critical thing.
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Old Oct 14, 2012, 03:11 PM
planepainter
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Mt. Juliet, TN
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Hi Chris. I modified my Riser 100 out to 132" by adding a center panel and I also modified the original inboard wing sections to feature flaps. I was very pleased at how well the flaps worked and the plane was a wonderful, gentle floating sailplane with no bad habits. It is going through a repair right now after having had the wings blown off of it by a friend.
I should have the plane flying again in the very early spring as I have a move coming up and all of the holidays as well.
I have attached photos.

PP
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Old Oct 14, 2012, 03:23 PM
planepainter
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The main problem I ran into was not any aerodynamic problem with the plane. It was with the original build. The Riser 100 was the first sailplane I had ever built so I just built it according to the plans. Bad idea but I didn't know any better at the time. This would come back to haunt me later. The original build called for, I believe, the use of wedge shaped balsa braces inside the joiner rod section of the end rib and the length of the rod. I can't quite remember now but I believe they are epoxied in place but was not a solid block like my OLYIII. This created a very weak joiner box and was not able to take a winch well or even some of the stresses that other sailplanes take easily.
Now, that was my plane and does not speak of the success or failures others have had with this technique. Some may have had some good success with winching a stock Riser 100 as long as they tap it up. I, however had several joiner box failures on this plane.


PP
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Old Oct 14, 2012, 03:39 PM
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Lincoln;
The orginal fuse length is 51 1/2" so I would be adding about 20 inches or so. I do find one term your using abit confusing as I am new to this enlarging on this scale. The term "Spar Cap" I am guessing thats the you are refering to the top and bottom main spar at the dihedral joint? I am comfortable working with exotic woods as I do quite a bit of wood working and have worked with purpleheart in the past.
I also am wondering about the joiner rod, my thought was something on the line of what the Great Planes Spirit has, a lamanted unit running out close to a third of the inner panel?
Thanks for your input, it is a big help to me and my learning more about this whole process.
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Old Oct 14, 2012, 03:43 PM
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PP;
Your are right, the joiner section is wedges of balsa sourrounding a brass tude. In the past I have always loaded this section with epoxy then sealed it on both sides with 1/8 balsa. Never had any problems but only used a high start.
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Old Oct 14, 2012, 06:29 PM
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The Bubble Dancer uses joiner blocks like this:
http://charlesriverrc.org/articles/b...iner_build.pdf
http://charlesriverrc.org/articles/b.../joiner_V2.pdf
This is for a 3 piece wing, but it takes a far higher load than a Riser could without extensive modification. And it might make sense for you to do a 3 piece wing anyway. I prefer a steel joiner to a carbon one. More damage tolerant. Or perhaps a thick walled aluminum joiner.


Maybe the wedges in the Sig kit had the grain horizontal instead of vertical? I'm guessing if the grain is vertical and they were medium to heavy balsa, with any gaps filled with epoxy and microballoons and/or flox, Cabosil, etc. that they wouldn't have broken. I would use ply and not balsa for the sides.

I've uploaded a crude sketch of the cross section of a spar, to show what I mean by caps. Usually there is something glued or wrapped on the sides, but I've left that out. Carbon would go on the top and bottom of the whole thing, if it was added.

Of course, it's not THAT hard to use carbon fiber.

I wasn't entirely serious about purpleheart, but it IS supposed to be very strong in compression, perhaps even in proportion to its very heavy weight. So if you could get a decent glue joint... I think if I was going the exotic wood route, I wouldn't go any further than black locust. It's supposed to glue ok, isn't it?

A lot of times, a thicker section will do a better job than a thinner one. If you have a simple, solid beam, balsa will be far stronger for the weight than steel or spruce, just by virtue of how big it would be. In a thin wing, it might be hard to find enough room, and this is where the heavier materials may be justified.
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Old Oct 14, 2012, 08:07 PM
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Hey,
Good luck on the Bubble Dancer type joiner block construction. It is a great idea and ought to perform very well. But, there is no tolerance for error. The hole boring and cutting angles need to be exact as do the hole diameters relative to CF or brass tube reinforcement.

I tried the Bubble Dancer joiner block construction for a different plane this past summer and learned that one needs an excellent drill press and bits and a very precise vise set up to drill the holes (I don't have that). After a few tries, I eventually ended up with something that seems very good. But, I'll not do it again unless I am befriended by the right person with the right workshop.

The next set of joiner blocks for me will be the wedge shaped, dense, vertical grain balsa filled with epoxy and cabosil around a brass tube with ply shear webs on the sides (all wrapped with Kevlar).

Also, the weak point on the Riser fuse is right at the training edge of the wing. I recommend ply doublers on the sides and bottom extending an inch or so fore and two inches or so aft of this point.
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Old Oct 14, 2012, 08:26 PM
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You could use an oversized bit and add a bit of epoxy filler. One could also drill just the right sized hole at the approximate orientation, run a rod or tube through it, and use the rod or tube to jig the block to cut it correctly. But of course the wedge with filler is probably of similar strength and only a little more weight.
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Old Oct 16, 2012, 03:03 PM
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United States, IN, Fort Wayne
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Chris,

I scaled up an older 55" plane to 2M this spring and found this Sailplane Calc spreadsheet from Curtis's website really helpful for checking dihedral and tailplane sizes.:
http://www.tailwindgliders.com/Files.html#Files

-Chris
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