jester_s1

I doubt this is new but I got creative this week and decided to see how much strength to weight I could pull out of an all wood spar. Being an experiment, I'm all ears about how to improve it or about any concerns about its effectiveness.

The idea is to skelotonize everything that can be taken away and add weight where it will help. I know about I-beam construction but this approach is even lighter. The basic idea is to glue up a double spar with two 10" doublers in the middle, and a 2" vertical grained doubler every 8 inches. Then mill out all the wood in between the doublers. That gives a structure with 4 rods to support the load and doublers to prevent splits and stiffen it from twisting. I also drilled lightening holes toward the middle but left plenty of meat there to keep the strength where the spar needs it most. Here are the pics.









I tested the spar by placing the point of the dihedral on a bathroom scale and reached out with both hands near the ends to press down. I didn't hold it straight to keep it from twisting. I only used straight down force with open palms. The spar held nearly 40 pounds without flexing significantly. It broke at 50 when the thinnest part of the wood buckled under the compression.



If this had been glued to a piece of coroplast, it wouldn't have bucked like that. None of the rest of the spar had even started to give way, so I think just the spar design itself has the potential to hold upwards of 70 pounds or so. Add to that the strength of the coroplast and it doesn't sound crazy to me for a wing built with this spar to take over 100 pounds of force. If it were reinforced with fiberglass or carbon fiber, who knows?

For comparison purposes, I made a standard spar like you see in the Debonair plans. I didn't trim the wood from the doubler because I was making it just to break anyway.



This one could only take 15 pounds before it twisted and warped. Even with me holding it vertical, I could get more than 20 pounds of pressure on it before it twisted sideways. I wasn't able to break it because of the flexibility.

Now this took about 4 hours to do the first time. My next one will go quicker. The standard spar weighs 6 ounces. I used lattice wood from Home Depot and only picked pieces of hearwood with straight grains. The skeletonized spar weighs 9.5 ounces, made also with lattice wood and poplar vertical doublers. So for 3 ounces, you get more than double the strength and probably 4-5 times the twist resistance. The cost is an extra couple of hours building time.

What remains to be learned is how well this spar handles sudden loads like rough landings or high-G turns. I would think pretty well, since the vertical doubler won't let it split unless the glue joint itself breaks.

Any thoughts from guys far more experienced than me?

DLSmith2

Jester,

Have you tried this in a wing yet? How did it go?

Is the weight/time worth it?

Just curious...

Blue skies,

DL

batchelc

My Feedback: (7)

Jester,

I think you need to put some math behind your experiences. The first thing you need to do is define your loads and then build your spar to withstand those loads. Basic stress analysis is not hard. The thing you want to do is define a weight (~5 lbs model etc) and then multipy by the number of g's that your plane will pull. 10 G's is a good place to start for a trainer, 23-30 is better for sports, acro or racing.

Once you have your loads then you can use some math to define the stresses in the spar.

If you post the wing span and weight that you want your spar to be I can work you through some of the math. We will also need to know the thickness of the airfoil.

jester_s1

I never did build up a wing with this spar in it. The strength was pretty good, but the design is fragile. As I researched spar design some more, I found that the I beam construction reigns supreme in strength to weight ratio. Some of the more experienced guys recommended a foam core with a strip of cedar on the top and bottom. Either than or a foam core with a carbon rod on top and bottom.

kbear

This is my lightest yet. CF arrow shafts put together tip to tip with 8-32 all thread and ca. The wood is .25" Poplar. So far pretty strong.

ChrisSpad

it'll be really strong when you glue it to the coro - that'll mean it'll be mostly loaded in tension and compression - which c/f tube is extremely strong in.
I've done something very similar - except i use sheet foam, and poly glue the c/f rods into grooves in the edge, and polyglue that to the coro. Super strong spars.

somegeek

My Feedback: (2)

Begs the question... how much were the shafts and what is the weight compared to a wood spar of similar length?

somegeek

ChrisSpad

they're not cheap, and really, on a 40 size plane, the yardstick is tough to beat - light, and cheap. However, if your building a 3" thick wing, foam and c/f makes a much lighter spar than you can rip out of a 2x4.

batchelc

My Feedback: (7)

I know the combat guys are using 0.125 solid fibergalss rod in foam core wings. at 2.50 for 6' its pretty cheap.

huck1199

Instead of CF arrows which are expensive, could you use FG driveway markers? I bought some for $1.75 at HD. They are about 4' long.
Huck

ChrisSpad

arrow shafts are ok - but, really, they're overkill. I use c/f tubes - normally 5/32" od. Most kite shops have them. Here's where i get mine - http://www.cstsales.com/Hollow_Carbon_Rods.html

kbear

30"shafts around here are 4 or so bucks. Not the cheapest option but the spare is working great. I think I'll try the 5/32 CF rod next time, as it would be about $4 or so cheaper. I was mainly doing it to save oz. Put enough of those oz. together and it makes a difference in certain applications. I had been using .25" thick Poplar. Yard stick are hard to come by around here, and I haven't got around to ordering some from the yard stick lady. Anyway, always trying new things. That's the fun of SPAD.

batchelc

My Feedback: (7)

kbear,

As you mentioned earier the I beam is very strong. You can use the CF for the upper and lower part of the I beam and you will need a shear web in the middle (The tall skinny part of the I beam). The next isse that you will have is that an I beam (espically one with two little circles at the top and bottom (in cross section) is that it will twist long before it breaks. This is where the wing skin (coro, monokote, etc) comes into play. It keeps that spar from twisting.

The loads in the spar will be along the arrow shafts in tension and compression (this is a good use of the arrow shafts). The stresses in the shear web will be at 45 degree angles to the verticle. The shear web is as important as the arrow shafts.

Since the arrow shafts are composite you could make a composite shear web by gluing the whole thing together. I would lay the shafts on a flat board on some waxed paper (after roughing them with some 100-200 grit sandpaper) and then cleaning with aceatone. Then I would lay 3-4 layers of glass cloth between them as a shear web.

I hope this helps.

kbear

I already had this spar in a plane before I posted. It is working wonderfully. It's very strong, very rigid, and very light. I use ca to glue the shafts to the skin, and then fold the small flaps down and glue to the wood blocks.

batchelc

My Feedback: (7)

kbear - what is the rest of the wing made from? What did you cover it with? Looks good from here!