Light wing strut material

[Bill Mixon]

Got a project in mind that would require structural wing struts. They would not need to take much negative load, just the load of the wing itself. Wing will be built up (no "D" tube) so the struts also need to provide some amount of torsional support.
Total length of the "V" strut would be somewhere between 50-60".

I've used the K+S streamlined aluminum tubing in the past on smaller models, but for struts this long it seems too flexible. I know I could use larger maybe 1/2" round tubing or other materials but the weight starts getting up there. I'm looking for light weight here.

Never built anything with structural struts before, so I'm not real sure about this. Advise greatly appreciated.

Thanks,
Bill

[Ollie]

When a wing supported by a V-strut is subject to a torsional load, one leg of the V- strut is under tension and the other leg is under compression. A long thin column under compression will buckle under a relativel small load compared to its compression strength. To prevent buckling the strut under compression has to be supported from the side. That is why you often see two small vertical sub struts between the wing and the middle of the V-strut and a horizontal sub strut at the same location between the legs of the V-strut. Those three sub struts are providing side support to resist buckling. The leg of the V-strut under compression will still fail in buckling but at a much higher compression load.

The torsion generated by the wing is proportional to the pitching moment of the airfoil, the wing area, the wing chord and the square of the airspeed. The strongest measures to limit the torsion are to limit the airspeed and to use an airfoil with a small or zero pitching moment like the NACA five digit series or a symmetrical airfoil.

A hard bouncy landing can put quite some negative load on the wings. That's why biplanes have landing wires in addition to flying wires.

I guessing that you are planning a scale model with a span in the range of 12 to 15 feet. This is approaching a size where professional structural engineering is the prudent approach from a safety point of view, IMO.

[Bill Mixon]

Thanks for the tips, Ollie

12-15' is in the range. Model is my own design though not scale and will be a low performance design. Right now it's in cad and nothing is final. Planning on using Clark Y 26" chord with elliptical tips.

The torsional forces applied to the struts in flight was my main concern. As it is right now there are too many unknowns to base any calculations on. I'm mainly trying to find some materials to start with to do the strength calculations. The wing construction method might change depending on what I can find for strut material. In other words I might build a stronger more torsionaly rigid wing that wouldn't have to rely so heavily on the struts.

I was planning on using sub struts also and maybe two sets per side vs. one.
I should also mention I'm shooting for a weight goal around 15 lbs flying weight. Thats the major hurdle..

Bill

[Ollie]

You could save weight by putting a guy wire pylon above the center of the wing and guying the wing from above and below. This gets rid of all the strut compression and buckling by replacing the compression forces with tension forces. Kevlar or Spectra guy wires will have much, much higher strength and stiffness to weight ratios than struts.

In such an arrangenent the wing spar carries a lot lower bending load but the spar becomes mainly a compression element. A thin walled carbon fiber tube will be a very light weight, high strength structure for this type of spar. Pultruded or prepreg unidirectional carbon has about 49 times the strength of spruce and about 20 times its strength to weight ratio. By making two tubes of generous diameter for the spars, buckling will be avoided.

Another approach would be to use an NACA 23012 airfoil instead of the Clark Y. This will reduce the pitching moment coefficient by a factor of about 10 without reducing the maximum lift coefficient much. With the NACA 5-digit airfoil the torsional loads on the wing will be far smaller. A single thin wall carbon tube spar of sufficient diameter, with the some of the fibers at + or - 45 degrees to the length and some along the length will carry both the bending and torsional loads of the NACA 5-digit wing without the struts and guys.

[Ollie]

I just thought of another very light weight method of adding torsional rigidity to an open bay wing structure. Use kevlar tow or unwaxed dental floss to spiral wrap the main spar and leading edge diagonally across the rib bays from spar to leading edge in the first rib bay and from leading edge to spar in the second rib bay, etc until the spiral is complete from root to tip. Then add another spiral starting at the leading edge so that the second spiral wrap crosses the first in the middle of each rib bay. Then add two more spiral wraps so that each rib bay is criss crossed both top and bottom by the spiral wraps. CA the spiral wraps to the structure. If the top and bottom spars are flush with the rib edges and the spars are set back from the leading edge a distance equal to the rib spacing, the spirals will be at about 45 degrees for maximum effectiveness of the spiral wraps. This type of structure is called a tension truss. At each rib and spar, there are two choices of routing the spiral. You can route the spiral vertically between the top and bottom spars or you can route the spiral diagonally between the top and bottom spars across the rib bay. If you route the spirals diagonally between the top and bottom spars across rib bays, the spiral wraps will also carry the shear forces between the top and bottom spars to add bending stiffness. Six spirals will be required in this case. The best rib spacing will be the same as the airfoil thickness at the spar location. in this case no external struts will be required and all bracing will be internal. A small balsa compression post should be added between the top and bottom spars at each rib.

If the structure involves a secondary spar, additional spiral wraps can be added between the two spars for even more torsional rigidity and spar shear strength.

For maximum strength to weight ratio the spars can be made from hollow carbon pultrusions such as those available from Model Research Labs. See:
http://www.modelresearchlabs.com/current_product_listing_and_pric.htm
In the past I have been able to negotiate supply of longer lengths from this supplier.

For a description of the merits of this material, see:
http://www.continuo.com/marske/carbon/carbon.htm

[Bill Mixon]

Thanks again,

This gives me a few things to think about.
I had considered building it with diagonal interlocking geodesic ribs. Not sure I want to go down that road though.
I have built a couple of planes with the guy wires (Kevlar) to a pylon. I really like the strength and light weight of this construction, but for this project I don't want that look.

Off to do some figuring...

Bill

[Jeff Meyers]

4000 some sq. inches of wing area....

wonder what'cha have in mind??:D