Uprated spar for Dodgson Lovesong? Ollie?

[kingbee]

I've been watching the Houston Hawk project with great interest as composite spar construction is an area where I have no experience. One of my winter projects is a Dodgson Lovesong, and it has occurred to me that it might be a prime candidate for an uprated spar design.

However, I lack the know-how to design such a spar, so I wonder if any of you engineering types could spec a design for me.

Specs of plane: 134" span, 1075 sq. in. area, advertised flying wt 73 oz. Basic wing is white foam core, with 1/16" balsa sheeting. Eppler 214 section.

The existing spar is as follows: the first 32" from the root has a full depth spar consisting of 3/8 x 3/16 spruce caps, full width balsa shear webbing (grain horizontal), and additional 1/32" ply shear webs at the root, 12" long on front of spar, 17" long rear of spar. The joiner tubes are 3/8" O.D. brass tubes 4-1/2" long, potted in epoxy. The joiner tube area is the wrapped with thread. Depth of spar at root is 29/32".

At 32" from the root, the spar changes to 3/8" x 3/32" spruce caps, with no webbing.

The cores are already cut full depth for the main spar. The cut is 7/16" wide at root, tapering to 3/8" wide at 32" from root. The slots for the outboard spars are not cut, needing to be sanded into the cores.

How strong do I want it? Certainly strong enough for an aggressive winch launch. I don't own a winch currently, but that could change, or I could find myself flying at a field that does have a winch.

Winch PROOF would be nice, if such a thing is possible. :D

Thanks in advance.

Cheers,
Dave

[Ollie]

This spar design is based on a minimum modification of the existing wing cores. I think that the cores are white beaded foam. Beaded foam is not strong or stiff enough for the shear web. If I am correct, then balsa or spyder foam shear webs will have to be added and the cores cut for the spar assembly at the tip ends of the cores.

Aerospace Composite Products makes precured carbon laminates that are long enough for your wing panels. Get four packages of 0.007 X 3/8 X 72 inch strips, two strips to a package. Each package will make one spar cap. The cut the strips to the length of the foam core. One strip remains the full length of the foam core. The other strip is cut in half. One of the half length strips will be laminated to the root end. The other half length strip will be cut in half. One of the quarter length strips will be laminated to the root end of the spar cap and the remaining quarter length strip will be cut in half again. this process is continued until the last lamination is about 4 inches long and a 4 inch piece is left over. That will give you five laminations to 1/16 of the core length, four laminations to 1/8 the core length, three laminations to 1/4 the core length, etc. The 0.007 inch thick strips are laminated with laminating epoxy under pressure. The result is one spar cap that is thickest at the root end and tapers exponentially to the tip.When planning the height of the spar assembly, allow for the thickness of the kevlar tow wrap to be added later. Make three more spar caps like the first and assemble the spar caps to 3/8 vertical grain contest balsa shear webs to about 2/3 the way between the root an tip using cabosil thickened epoxy. Use spyder foam shear webs with thickened epoxy the rest of the way. Wrap the joiner box with two turns of 1.5 ounce fiberglass cut on the bias before adding the kevlar wrap. Wrap the spar assembly with kevlar tow in epoxy. Close space the wraps in the joiner box area and gradually increase the spacing to about four turns per inch at the tip. Attach the cores to the bottom sheeting. set the spar into its slot with microballons in epoxy and fair the top into the contour of the airfoil. Add the top sheeting.

This spar will take around 180 pounds pull on the tow line before it breaks. That is much more than enough to put a permanent bend in the stock wing joiner.

[jgleigh]

Once I had an Anthem, and the spar was bombproof. After I sold it to a friend, he managed to put in an extra 20 degrees of dihedral by bending the wing rod from an over-enthusiastic winch launch. And Dodgson never economized on the wing rods. I'm trying to remember the spar construction on that; I seem to recall some carbon and kevlar thread wrapping near the root. If someone with a better memory than mine, or better information, can verify the construction, I'd suggest using that as a model.

[Ollie]

In a private E-mail Badger has alerted me to the need to consider the stiffness of the spar because at high loads, the balsa skin will buckle long before the spar breaks. Taking that into consideration the wing spar should be derated to roughly 100 pounds pull on the line. This is still more than enough to put a set into the stock 11/16 inch joiner. It should still allow quite a vigorous zoom launch if not a full pedal to the metal zoom. If you want a wing that will still take the 200 pound line pull without excessive flexing then you could double carbon raising the cost from about $30 to $60 just for the carbon. This would result in ten laminations at the root with intermediate length strips interleaved with the strips from the first design. There is not much point in a stiffer spar unless the steel joiner is increasd to 1/2 or even 5/8 inch.

Also, such high speed launches might blow the stab right off. When you start beefing things up one thing at a time they just tend to reveal the next weakest element in the structure. This observation is a reason for choosing a design that is properly integrated from the beginning like Dr. Drela's are.

[kingbee]

Ollie,
Many thanks for the info. It sounds as if it will be plenty strong for my needs. You are correct that the cores are white beaded foam.

2 questions: ACP sells tapered carbon strips, they have one that tapers from .042" to .007"; would it work? I assume its taper is linear rather than exponential, so that's why I ask.


Second: Should the "steps" in the spar cap assemblies go towards the shear webbing?

jgleigh: Reading about the Anthem in some old "Second Wind" issues is what got me thinking about the upgrade. Dodgson incorporated the Anthem spar system into the Windsong/Lovesong wing, and sold them pre-built as the "Windsong Classic". I think that version also had obechi sheeting rather than balsa.

Thanks again.

Cheers,
Dave

[kingbee]

Ooops. one more question: does the kevlar tow wrap go all the way to the tip?

Cheers,
Dave

[rdeis]

Originally posted by kingbee
full width balsa shear webbing (grain horizontal), and additional

Isn't this incorrect from the beginning? Seems like turning the balsa grain vertical would add an awful lot of strength all by itself?

[Ollie]

Dave,

The 0.042 tapered carbon spar caps will work just fine, be slightly stronger and save some labor. The down side is that they will be slightly heavier and cost twice as much.

Along the tip end of the spar where you have to cut a new spar slot in the core, you can taper the width of the spar from 3/8 inch to zero at the tip and cut a tapered spar slot in the core to match. This is more work but it will save weight near the wing tip where it will do the most good. The tapering is best done after the caps are assembled to the web but before wrapping. don't inhale carbon sanding dust. It might be carcinogenic.

If you use enough fumed silica (cabosil) thickened epoxy between the vertical grain shear webs and the carbon spar cap, the 0.007 steps won't matter and either side down will work.

Yes, the kevlar tow wrap goes all the way to the tip. It starts close spaced or even overlapped a bit (12 turns per inch) at the root and gradually opens up to about four turns per inch at the tip.

I forgot to mention that the corners of the assembled spar should be sanded round a little so that the carbon doesn't cut into the kevlar tow wrap.

[kingbee]

Ollie,
Thanks for all the information thus far. I'm looking forward to building the spars, it's always fun to try something new.

If I do taper the outboard half of the spars (which I probably will do), is spyder foam still adequate for the webs, or should I use balsa? I have a bunch of 3/8" contest balsa, which I wouldn't mind using up if it would be okay to use at the tips.

Just about ready to place my order with ACP....

Cheers,
Dave