wing weight

[gnirwin]

Are balsa sheeted wings , with the spars farther apart, heavier than plastic sheeted wings with the spars closer together? Also how much stronger are the balsa covered wings? I know these are rather vague questions but I am talking in general, apple to apple terms.

[Ollie]

You just can't get a valid answer to your question until you put dimensions on everything or cite two specific examples to be compared. Also, while most balsa is around 8 to 12 pounds per cubic foot the extremes can vary from 4 to 20 pounds per cubic foot. Two wings built to the same plan could have weights where one was four or five times heavier than the other. Plastic film covering materials can vary in weight by a factor of two or three.

Until you get completely specific the answer could go either way.

Do you mean ribs or spars?

What about strength? Do you expect the strength to be the same?

What about stiffness? Unsheated wings are not very torsionally stiff.

[gnirwin]

I did mean ribs.What I am wanting to do is build my first scratch built (from my own plans I am drawing up) plane. I have always built balsa "kits" with specific wing builds. This time I have a choice and I want it to be light but strong. What I am building is an Islander 36" wingspan. wings have to be fairly stiff.

[Ollie]

You can use medium density balsa for the LE, TE and ribs with open rib bay (unsheeted) wings if you cover it with Monocoat and stick the covering to all the ribs and leading edge. This covering will provide adequate torsional rigidity. Space the ribs about 2 inches apart. Put a hard balsa spar cap (1/8 X1/4 ) top and bottom at the thickest part of the airfoil. Connect the spar caps with vertical grain 1/16 balsa shear webs between the ribs.

[HELModels]

It sounds like gnirwin is struggling with the same things I am when it comes to building wings. I still havent left the ground as I am trying to build a 32-34" span 4.25" chord scratch wing which is light and torsionally strong. I plan to use your suggestion of the monocote for added rigidity, but I have only half a wing completed and already am not satisfied with the direction it is going. I placed my ribs about 1-1/2 inches apart and used 1/16 balsa sheet to cut them out. I put a very thin layer of 0.5 oz/sq yd fiberglass cloth on the balsa sheet and then 3m77'd photocopies of the airfoil onto sheet and cut ribs. This really did not add much weight as resin to cloth ratio was good(wipe off excess resin with paper towel). The fiberglass added some strength to these ribs and actually helped accurately cut intricate airfoil shape - sometimes real thin balsa tears instead of cuts.

One problem I am having is shaping leading edge and trailing edge. I compensated for sheeting thickness when I drew up the foil, but it was not perfect and so shape has changed some. Probably not a serious factor on a 4" chord, but aim for the stars anyway.

I used 1/16 sheeting from leading edge to 20% chord and from 70% chord to trailing edge. The trailing edge is tough to get accurate too - a knife edge it is not due to sheet thickness and sanding will affect shape again - If not careful I could actually put reflex into the trailing edge and that would not be fun to trim. I'm not building a Strovarious is what I will keep telling myself.

Wing tip shape is another problem as is dihedral bracing, but lots of people build their own wings from scratch.

Good Luck to gnirwin

[Ollie]

On small models, every gram counts. with a 4.5 inch chord and 1/16 balsa ribs spaced 1-1/2 inches apart, no glass reinforcement is needed!

For accurate construction you can find many tips at:
http://www.charlesriverrc.org/articles_construction.htm

[gnirwin]

electrostorch, that was what I was trying to avoid , to a certain extent, building more than one wing style and not being satisfied with either.

[HELModels]

1-1/2 inches apart, no glass reinforcement is needed!

I remeasured and the ribs are about 1 3/4 apart. The fiberglass reinforcement was primarily to make cutting the ribs easier, without tearing the trailing edge or the spar notches or anywhere. It works! Each rib is very true.

I saved several grams in the fuselage by again using 0.5 oz. fiberglass and 2 part foam - then sanding and cutting away excess foam. This is not exactly lost foam technique because I kept some foam for strength and did not use any solvents to remove any foam.


That link is a good one.