Kevlar Thread for Joiner Boxes Specifications?

[gdjsky01]

What or whom is everyone using as a source for Kevlar thread for wrapping joiner boxes?

There a guy (or gal) on EBay selling various sizes that seem a decent price, but there is my confusion, what specification of Kevlar thread do most use?

Best wishes
Jeff

[Ollie]

Any kevlar tow or thread will do. The less the breaking strength the more turns of wrapping are needed at each end of the joiner box. Take the maximum bending moment at the center of the wing and divide it by the length of engagment of the joiner to the box. That is equal to the bursting force that the kevllar wrap must resist at each end.

For example: If the maximum bending moment at the center of a 3 meter span wing is 3000 inch pounds and the joiner engages the box for 3 inches then the wraps must carry a tension of 1000 pounds. If the breaking strength of the thread is 27 pounds then each turn can resist 54 pounds of bursting force. Each end of the box would require 19 turns plus a few more for safety. I would use about 24 turns of kevlar thread at each end of the box for this case.

[gdjsky01]

Honest Ollie you ought teach this stuff! I've read your messages all over the Thermal group and it's always sage and sane. Good stuff!

;)

Thanks.
Jeff

[GNuse]

Pro Bass Shop in the fly fishing department. I think 50yds is less than $2 I am sure other fly fishing shops would cary the kevlar thread also.

[Rob Nelson]

I puchased a couple of 25' spools of kelvar thread to wrap the spars of my BOT. With 25' I made it about half way down the main panel. It worked well for the most part but I found that the thread strands separated and became a little more trickey.
I decided to try dental floss for the rest of the spar out to about 1/3 way out on the outer panel. Tried a "pull" test and the dental floss actually seems stronger than the kelvar thread that I had.
I used non waxed dental floss...Anybody else try this. Looking for ideas that my be less expensive, but work as well, or close, to kevlar.

[JFarlette]

Originally posted by Ollie
For example: If the maximum bending moment at the center of a 3 meter span wing is 3000 inch pounds and the joiner engages the box for 3 inches then the wraps must carry a tension of 1000 pounds.

Ollie - Can you tell us how you calculate the load at the center of a wing and also at some point along it's span? For example, say I wanted to build a 4 meter 3 piece wing that is going to be winch launched. The center is 2 meters and each tip is a meter. What is the load at the center and at each joiner box?

Thanks,

Jason

[Ollie]

The ideal lift distribution is elliptical but for a simplified calculation that is conservative, assume that the lift is uniform. Using Jason's example of a 160 inch wing span and assuming the wing should be strong enough to break a 240 pound test winch line, each side of the wing would produce 240/2= 120 pounds of lift. With the average distance of that lift force from the center of the wing is 120 X 40 = 4800 pound-inches. The actual bending moment would be about 16% less but this method provides some safety factor. More to follow.

[Kestrel]

Rob

I built a Laser Arts Majestic. They included a small spool of dacron thread to wrap the spar. It worked just fine with the exception that the thread is a little bulky and required a reasonable amount of sanding to smooth out the balsa sheeting that went over the top of the spar. I read a thread a while ago that was also discussing spar wrapping and I think it was Ollie that suggested several ideas, one being unwaxed dental floss. I have used this ever since. Cheap, easy to get and comes in the dispensing container with the cut off device! It also lays nice and flat when wrapped. I did crunch a wing on the Majestic that was wraped with the dacron. The joint broke right thru the ply joiner but the wraped area on each side stayed intact.

[Rob Nelson]

Thanks for the input Kestal. Just finished wrapping the BOT spars. Had a heck of a time finding the kelvar thread (195 aramid yarn), and was not thrilled with the way went on. Maybe there is a better product out there,but, like you say, the unwaxed dental floss went on very nicely, seems very strong, and certianly economical.
Cheers,
Rob

[Ollie]

Knowing the bending moment at the center of the wing and that the bending moment at the tip is zero, we can calculate the bending moment distribution along the semi span. the bending moment increases as the square of the distance from the wing tip. At 0.1 of the semispan to the wing tip, the bending moment is 0.1 X 0.1 X 4800 = 48 pound-inches. At 0.2 of the way from the tip to the center of the wing the bending moment is 0.2 X 0.2 X 4800 = 192 pound-inches, etc.

To find the compression load on the top spar cap, divide the local bending moment by the distance between the centers of the top and bottom spar caps. If the spar height center to center of the wing's center was one inch then the compression force in the top spar for the maximum positive G load would be 4800/1 = 4800 pounds. To find the required crossectional area of the top spar cap divide the load by the strength of the spar cap material. For a defect free spruce spar cap the compression strength is 5610 pounds per square inch. Therefore, the spar cap crossection required at the center of the wing is 4800/5610 = 0.86 square inches. this is hardly a practical size spar cap to be able to break that 240 pound test winch line. So, let's see what a precured carbon fiber spar cap needs in the way of crossection. The compression strength of unidirectional carbon made from pre preg is 275,000 pounds per square inch. the required crossection at the center of the wing is 4800/275,000 = 0.0175 square inches. Stock thicknesses of this material are 0.06 and 0.084 inches. A 3/8 inch wide spar cap of the 0.06 thick material would do the job or a 1/4 inch wide piece of the 0.084 inch thick stock would also serve. Does anyone want to know how to determine the shear web required?

[JFarlette]

Originally posted by Ollie
Does anyone want to know how to determine the shear web required?
Of course! :)

Thanks for all the info - this will come in very handy!

Jason

[Ollie]

Let's try vertical grain balsa for a shear web. The shear strength with the grain is 300 PSI for 11 pound per cubic foot (medium) density balsa.

For a uniform spanwise load the shear is zero at the tip, increasing linearly to a maximum at the center of the wing. If the maximum lift of half the wing, in this case, is 120 pounds then the shear at the center of the wing is 120 pounds. The cross sectional area of the shear web is aproximately 3/8 x 1 = 0.375 square inches. However, the shear at the mid point of the shear web crossection is zero, cutting the effectiveness of the area by half. So the shear load capability is 300 x 0.375/2 = 56.25 pounds. This is way too low. One possible solution that doesn't weigh much more is to reorient the fibers in the shear web at 45 degrees to the length of the spar to better carry the load and to use denser, stronger balsa. Balsa plywood consisting of a 3/16 inch thick core and two 3/32 inch thick faces with the grain at right angles to the core will take advantage of medium balsa's 1700 PSI compression strength. It will double the shear load capacity to 112.5 pounds which is almost enough. By increasing the density to hard balsa of 14 pound per cubic foot density the strength of the shear web would be increase about 17% which would make it strong enough.

Another design solution would be to use 0.042 thick carbon spar caps 1/2 inch wide with 45 degree balsa ply shear webs consisting of four layers of 1/8 inch thick medium density balsa.

Because the shear load decreases linearly from the center to the wing tip, the shear webs can also be tapered in thickness linearly to the tip to save weight. Also, the commercially available tapered spar caps taper linearly in thickness but not in width while the compression load decreases exponentially. To save some more weight the spar caps can also be tapered linearly in width to match the shearwebs. When the spar caps taper linearly in both thickness and width, their strength will very closely match the load that they must carry. It just takes extra work of tapering to make a spar of extremely high strength to weight ratio. Compared to a spar of untapered shearwebs and untapered spar caps in thickness and width, the fully tapered spar caps will weigh 1/3 as much and the shear webs will weigh 1/2 as much, all without sacrificing any bending strength. Tapering gets rid of material that is just along for the ride and which is never stressed to the fullest. The best part is that mass is reduced at the wing tips which reduces the yaw and roll moments of inertia. This reduction makes the plane a better indicator of lift and reduces the control surface forces to maneuver. When the control surfaces can be deflected less for the same control effectiveness, the drag associated with maneuvering is reduced too. Drag reduction results in more range and duration.

There is the trade off between performance improvement and construction effort. In my opinion performance suffered too often because of designers' desire to save labor. I came by this mind set from studying the outstanding designs of Dr. Mark Drela.

It is a truism that in aircraft design everything affects everything else.

[markdrela]

Originally posted by Ollie
One possible solution that doesn't weigh much more is to reorient the fibers in the shear web at 45 degrees to the length of the spar to better carry the load and to use denser, stronger balsa.

Careful. This doesn't work if the web has ribs going through it. This ribs will be a lot weaker than the web, and will fail in shear first.

For a given density, +/-45 balsa "plywood" should be about 2.5x stronger in shear than 0 or 90 degree wood. So if the rib wood is 2.5x denser than the web wood, THEN the +/-45 web looks very attractive.

[Ollie]

Mark,
Thanks for saving my bacon yet again.

There is an ingenious method of assembling a wing from one piece ribs and a one piece spar assembly that allows the ribs to be cut apart for the spar insertion without disturbing the relationship between the cut-apart ribs. See:
http://www.mvsaclub.com/articles/dark_star2.htm

As Mark has pointed out in the Allegro Lite forum, a carbon spar, of the proportions in my previous postings, will bend a great deal before it breaks. This bending can be enough to buckle the associated balsa D-tube sheeting. Such spars should be designed to a stiffness criteria rather than a strength criteria. As a rule of thumb, double the spar cap crossections to get sufficient stiffness in the spar to prevent damaging the balsa sheeting.

[gdjsky01]

Ollie and Dr Drela,

I am WAY out of my league here. But I have studied Tony's Dark Star building techniques many many times with the intent of someday building one. Until then, his method of building the spar outside of the wing seems excellent even in conventional designs that have upper and lower spars directly under each other. All one has to do is notch the ribs for an addition of some jigging spars to keep everything straight until the main spar is inserted.

I am giving that method a try in a 2 meter kit. Seems like a wonderful way to get a strong and straight wing built.

Best wishes
Jeff