iron eagel

I am in the process of building a very light weight built up wing. It has a combination of balsa and carbon fiber wing spars, the balsa spars also have sheer webs. The carbon fiber spars only extend 60% of the span, while the sheer web balsa spars are full span. The fuesalage when full loaded with .40os engine, radio, servos, and sheathing, glassed it weighs 52 0z I want the wing it self to finish out at about 10 oz or so.
One thing I am concerned with is, twist or flutter of the wing tips. I was thinking of using truss bracing using 1/8 inch square stock, and alternating it, high low for the span of the wing. I really would like to keep the weight of each wing under 4 oz I am at three now each half of the wing is 32 inches from root to tip and will have a 4 inch middle section over the fuselage. Overall the wing span will be 68 from tip to tip. The wing will be fully sheathed and glassed. As it is now there is a small amount of flex about 1/4 inch when 7 lbs of pull is applied at the tip with the root secured.
But the wing will twist when only 2 lbs of force applied to the LE.
Each wing has 24 X 2.5 inch ailerons ( which extend from the tip in) it has a 10.5 inch cord and the ribs are 12%.
Am I being overly concerned about the twist?

BMatthews

By "sheathed and glassed" it sounds like you're planning on sheeting it with balsa and then light glass cloth and finishing resin? If so then the sheeting and glass will provide all the torsional strength you need and then some. So don't bother with the diagonals. I'll also suggest that adding that sheeting and glass finish will result in wings that are far over 10 oz total... especially after painting.

But the good news is that you've got lots of wing area so the model should still fly just fine.

PS: wrong forum. This is about structure and not aerodynamics. Moved....

iron eagel

Thanks Bruce,
After I posted it I said dummy wrong forum......

I am using 1/32 balsa for sheathing where I have close rib spacing. Yes I know 10 oz might be a bit light but you know shoot for mars settle for the moon...

BMatthews

That'll help but most of your weight will come from the glass and resin over the balsa. Most of the torsional strength will also come from that too.

You may want to consider using nice light weight chosen 1/16 sheet for the sheeting. It would not weigh that much more and it'll be MUCH more resistant to finger pokes. Even with light glass cloth and resin on the outside the 1/32 sheet would still be prey to being ruptured inwards by errant finger pressure.

Use a scale and balsa density chart to chose 6 lb/cu ft or less for the 1/16 sheet and the weight of the extra thickness will be negligable.

It's quite a large model you're building and fairly heavy in the fuselage (not overly but still more or less average for a basic design) and so the overall weight of the model will be high enough that a reasonably firm grip on the wing will be required when moving it around. It would be a shame to rupture the sheeting for the sake of saving at most about 2 oz of balsa.

iron eagel

Thanks,
I thought that I may be cutting it a bit close by using 1/32.....
I have the ribs 1.25" OC and though, gee that may work, but I would hate to poke a hole in a wing just moving it. Because I wanted to cut 2 oz in the weight.

One of the reasons for the weight in the fuse. is that, I incorporated ducted cooling system for the engine. The engine is fully enclosed for reduced drag, not to mention it looks cool...
The cowling is a cross between a Saber Jet, and a P-40, looks sharp. If I can ever figure out what I did to my firewall settings, or filters that is blocking my uploads I will post some pictures. I have been working on this one over two years.