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Jun 07, 2021, 06:16 PM
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Electric sailplane questions

Hey guys,

I'm planning on building a D-box composite electric sailplane heavily optimized for weight, and I was wondering about how to best arrange and build everything. More specifically, it will be a 3-piece-wing w/ v-tail. The D-box will be made of bidirectional 45 degree bias carbon fiber weave. The wing have a vertical-grain balsa shear web with unidirectional carbon caps, all wrapped with kevlar tow. In between sections of the balsa web will be 3/32" balsa ribs. In front of the spar (in the D-box) will be half ribs for extra support. The wing will be have a center ~1.2m section and two ~0.85m tips, all with a ~17cm chord (can't be tapered because I'm mounting something on the wing), and an AG35 airfoil. The tail will be attached via a tapered carbon boom. The tips and tail sections will have pultruded carbon rods slide into kelvar-lined holes (the proper polyhedral angle drilled into balsa blocks that the kevlar & carbon are attached in). The fuselage will be fiberglass, probably with carbon supports. The center wing will attach to the fuselage via two 1/4x20 nylon bolts.

My main question was about D-box geometry. Does the D-box go over the spar caps, or end before them? Does it need a carbon web on the back of the shear web (for "closure")? (see attached image)

I was also wondering about trailing edge construction. I think it needs a smaller pure balsa spar in the back going through the ribs, as an attachment point for ailerons and to change the twisting moment. However, I'm not sure if it's needed (or if there is a better option). I'm also wondering if I should use some 3/32" balsa for the trailing edge itself (what I think is best), or uni carbon, or what.

The wing itself is constant chord. However, the carbon spar caps will be tapered in thickness. I've read that you can also taper the balsa shear webs, by dividing it into two sections and tapering those down the length of the wing. Is this a good idea, or does it compromise the rigidity & strength of the wing?

I've read about people putting diagonal ribs in between the main ribs for torsional rigidity, and I was wondering if that was worth it, or if the D-box should do all the work in that department. I'm also curious as to whether cutting holes in each rib is worth the weight savings, or if it compromises the structural integrity too much or just isn't worth the effort.

Is outdoor vinyl tape or other similar tape enough to keep a wing on? This plane will be a slow flyer, and shouldn't experience much sideways force.

With the center wing piece being fairly large, I'm not sure it'd be easy to get a D-box that could fit the whole length. Would it hurt the rigidity much if the D-box was in two sections?

Would a tail made with a carbon fiber tube spar work fine, or would it be worth it to construct a similar spar to the wing for it?

Lastly, I'm not quite sure about the construction of fiberglass fuselages. I believe you'd need a carbon (or something similarly stiff) frame for the electronics & motor to mount to. How hefty does this need to be? Can fiberglass itself support the pull of the motor? Would carbon interfere with radio too much?

Thank y'all for answering questions on a somewhat ambitious project for a newbie. Again, the main thing I'm optimizing for here is weight.
Last edited by undrosaram; Jun 08, 2021 at 01:21 AM.
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Jun 07, 2021, 09:31 PM
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You are correct that a D-box without closure won't do diddly to torsion strength. But (in my experience) the balsa shear web alone is enough for closure, no need for carbon backing if weight is critical. D-box not over spar caps looks iffy, not enough bonding area for my taste. I'd recommend the D-box over spar caps, but without carbon backing, as per the topmost illustration on your picture.

I'd be weary of pultruded rods splitting under bending load. People are using them and seem to get away with it, but to me that just looks wrong, ymmv. I have made joiner rods by laminating unidirectional carbon on thin glass cloth, then wrapping it around a round balsa dovel while the epoxy is still wet.

You didn't mention the material of the D-box itself? If it is thick enough balsa to stay in shape without half-ribs, I believe that would be most weight-efficient full-balsa construction. If it is composite, remember to have the fibers at 45/135 bias, otherwise you will be wasting lots of material properties (in a 0/90 degree laminate the shear stress of D-box skin, and thus the torsional stiffness of the whole structure, comes mostly from the matrix resin and the fibers are there only for the ride).

You definitely need a sub-spar for aileron and flap hinges. Construction details are not critical, At typical component thicknesses just cutting the rear piece of the ribs off and glueing the sub-sper directly onto the ribs seems to work fine. I believe the most critical factor in dimensioning the trailing edge is resistance to ground handling damage.

Diagonal ribs: depends of the (lack of) stiffness of your covering material. Stretchy ones like Solarfilm would benefit from them, but for Monokote and similar ones they are not necessary. I'm not sure if there is significant weight difference between flimsy covering+diagonal ribs vs stiff covering without diagonal ribs.

Taping the tip panels on: perfectly fine. I've seen people forgetting taping and the tips staying on just by joiner friction (not recommended, though).

If you have to make a joint on the D-box, you will need to reinforce it to carry all the forces the D-box itself is capable of handling. Details depend of the material and construction of the D-box. On a balsa sheeting, a 2" wide strip of glass over the seam would be enough.

Forces on the tailplane will be low, you might be able to get away without a spar of any kind, relying just on the frame for stiffness. Depends on the details of the design.

Glass alone will be perfectly fine to handle motor forces. Carbon doesn't interfere with electronics, but it will block the radio signal. Make sure that there will never be any carbon between (at least one) receiver antenna and the transmitter, in any attitude or aircraft orientation.
Jun 08, 2021, 12:06 AM
B for Bruce
BMatthews's Avatar
If you're using 1/16 balsa for the D sheeting then I'd say that it can in fact just butt up against the spar caps. the edge of the 1/16 will give you enough glue area to do a pretty good job for any loads outside of a major flutter event in the air or a crash.

In my experience a D tube setup and with a good piece of C grain balsa on the trailing edge you don't need diagonal ribs. The D tube itself is already very torsionally stiff. But if you want to use "WWW" style diagonal ribs for the open area it can add a slight bit more stiffness plus it looks pretty dope.

If you look at Mark Drela's Allegro Lite and the balsa version of the Bubble Dancer plans I don't think you'll find full span kevlar wrap. But at the wing joiner where there's more bending load focus? Then sure.

If weight is a concern then also keep in mind that less is more. And with balsa that you want to pay attention to the wood density and use lighter grades in the extremities. It's the small things that add up in the end.
Jun 08, 2021, 01:20 AM
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Thread OP
Completely forgot to say, the D-box would be made of bidirectional 45 degree bias carbon fiber, probably purchased from CLMPro.
Last edited by undrosaram; Jun 08, 2021 at 01:42 PM.
Jun 08, 2021, 10:18 AM
B for Bruce
BMatthews's Avatar
In that case most certainly lap it over the spars. That's the only way to ensure a good bond to the spars and webbing. But as jkettu mentioned there's no need to carbon the back of the spar and webbing filler. A lot of work and a little more weight which would do next to nothing over and above the spar caps and webbing.

You mentioned that this will use the AG35 airfoil. And that means that the top of your D box should extend back past the spars anyway to give you the covering support intended for that airfoil. I'd probably put a support stringer for the edge of the D molding at that rear edge as well.

Also now that we know that you're using a composite layup to mold the D box I think you'll want to do a bit of testing. If the layup you are going to use is less rigid in bending than 1/16 balsa your idea of using half ribs in the LE boxed area starts to sound like a good idea. Such layups can be great for torsional stiffness but still be more flexible than ideal in bending where tension from shrinking the covering can cause the layup to flex inwards. And the whole point of sheeting the D tube on the AG3x series is to use sheeting of some manner to maintain the airfoil shape over the upper side back to the end of the support at around the 40% point.

You've clearly looked at all this and I'm sure it's old news to you. But you didn't show it that way in the sketch.

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