Walt Thyng

My Feedback: (9)

I'm building a mid-80s Dan Santich plane called the "Cobra." It is a double gull wing sport racing biplane. The bottom wing spar also carries the landing gear loads. Projected weight is 20 lbs. The plan calls for four 1/4 inch seven ply plywood layers giving a total one inch thickness. I'm wondering if a sandwich of two 1/4 in. ply, one 1/2 in blasa and two carobn or kevlar layers (between the balsa and the ply) would be significantly lighter and sufficiently strong? Any input?
TIA
Walt

Ed Smith

I use a 1/8 x 1" x 24" lite ply spar in my Pylon Racer wings. On each side of the spar I lay on three strips of carbon Tow with epoxy resin. The completed spar weighs 12 grams. The strength and stiffness is phenomenal.

Your suggested spar construction with Carbon is very sound. Just forget the Kevlar. Kevlar will not give you what you want in this situation.

Ed S

Walt Thyng

My Feedback: (9)

Thanks Ed. Got any suggestions for a source? I know almost nothing about CF.
Walt

da Rock

Your idea sounds excellent.

Truth is, carbon fiber is great stuff. But it's great stuff that costs lots of money and is really overkill in most cases. At least, when you compare it's price to glass cloth. The structure you propose would be very much stronger than the original design with glass and it's going to be MASSIVELY stronger with CF, way stronger than it'll ever need to be. If I were you, I wouldn't make any effort to find CF for your lamination. I'd get some glass cloth for lots less money. And for way more than enough strength.

When you're laminating with heavy materials like plywood, it's also a good idea to look closely at where the structures will face the heaviest loads. Lots of our finished pieces could be easily lightened by drilling some lightening holes where the stresses are the lightest. All the ARFs I've run into so far that use heavy wood or ply wing joiners are completely solid and have way too much strength at the ends. I've cut their weights by as much as 20% just by drilling holes where little or no strength is needed. And when you're cutting weight from heavy items, youre CUTTING WEIGHT!

Walt Thyng

My Feedback: (9)

I'm a lot more familiar with glass than I am CF. Waht weight would you go with in my proposed application?

Walt

Ed Smith

Glass cloth will do exactly as suggested in the foregoing. However glass cloth will not add the STIFFNESS that carbon will. Removing some of the heavier ply from the structure will reduce the stiffness of the spar. Carbon will add stiffness to the spar for very little weight gain. Carbon Tow is not prohibitively expensive. I purchase my material locally so I do not have a lot of information on US suppliers.


This company has been around for a long time and does cater to modelers.

http://www.acp-composites.com

Ed S

Walt Thyng

My Feedback: (9)

I didn't realize it's been a year since I started this thread (truth be told, I had forgotten about it). I got side-tracked with both work and health issues, but I am now ready to actually start the project. In fact, I'm stopping at a LHS this afternoon to pick up some wood.

I crashed a TF Corsair this year and took a look at the spar in it which I had lightened significantly. I must have done something right as the wing was completely destroyed, but the spar was intact (-:

I'm also going to take a stab at building a cowl with the lost foam method. My stuff is all electric, so I don't have the vibration problems of I/C power.

Back to the spar: if I put cf tow in the sandwich would I also run a layer on the bottom edge?
Thanks for all your input so far,
Walt