Stringers

Luke68

Luke68

Junior Member
What is everyone doing for stringers?
I would prefer to use the heavy extruded aluminum ones but my kit came with the aircraft Spruce stringers and they are really flimsy, Mark G told me to get in touch with Superior Mason Corp but they have since discontinued making the 5/16 x 1 extruded
 
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If they are the same stringers that came with my Patrol kit from AviPro, once they are in place they are amazingly strong and rigid. I'm happy with mine.
 
I bought Aircraft Spruce aluminum stringers and didn't like them. I'm sure they would be ok but I wanted something stronger. I looked up wood stringers and how the grain is supposed to run and cut my own. They are much stronger than the Aircraft Spruce stringers and I am very happy with them.
 
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I have also started making my own, they would work fine but on a champ we covered a few years ago the wood stringers tend to move a little with temp and moisture changes.
 
I opted for the ACS stringers (5/8 x 5/16 x .035) for weight, price, and build time. There is a huge weight difference between these and the ones that used to come with the kits. I'm also scratch building so I was able to custom-size the standoffs and add an extra couple of standoffs. I haven't covered the fuselage yet, but the stringers have been fitted and I haven't looked back.
 
Sitka Spruce has about the highest strength to weight ratio of any wood. I calculated that Stringers made of Spruce would weight 2.7 pounds. The weight of Aluminum Stringer from Mark I calculated would weigh 10.3 pounds.

I used these numbers in my calculations.
  • Avipro - 3/8 x 1 x .050 Al - .163 lb/linear ft
  • spruce wood strips 1/4-5/16 x 1 - .043 lb/linear ft
  • Plans called for 63.1 feet of stringer material.
Brooks Cone
 
I think your calculation of 63 ft might be short. I send seven 16 ft stringers with the kits, and there is some left over but I do not think half. Mark
 
I went with the ACS aluminum extruded stringers. They work fine and are lightweight. When you shrink the fabric they may pull to the side you are shrinking. Just be sure to shrink the fabric evenly. You can scooch them to be straight when you are done.
 
I recalculated the weight of the stringer material that came with the kit. The kit came with 112' of 1" x 5/16" aluminum stringer material. Actual weight of the stringer material, using an uncertified scale, is 15 pounds. I believe this to accurate within a 1/2 pound. That gives us a weight of .134 pound per foot. The weight of the same volume of Sitka Spruce is 6.5 pounds, and Douglas Fir, also having a high strength to weight ratio, is 7.7 pounds.

Three coats of 2 part epoxy varnish from Poly Fiber will add weight to the lumber.
 
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I was just about to start installing the stringers after moving my shop location. I had to move the bundle of stringer material as well. I haven't picked it up in 3 months. A quick search got me to this thread. It sure seems like spruce stringers are a cheap and cheerful weight reduction. Maybe the cheapest per pound by quite a bit.

For those of you who used wood stringers, any regrets? I would do 1 coat of epoxy as a sealant, with ALL excess wiped away.
 
I have restored several Aeronca Champs and machined my own mushroom hat stringers from Spruce or Doug Fir. I apply two coats of Randolph epoxy and will probably do the same on my Patrol. Making the stringers is a bit tricky but the jigs are easy to make for use on a router table.
 
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These stringers are indeed a little heavy, and it is out of the normal for Bob to want to use something heavier than another option. But Bob had a plane damaged by someone leaning on the side of his plane at an airshow. He likes the stronger aluminum stringers for that reason. Mark
 
A broken stringer would be a bummer. I will have to think about it. It sure would be nice to take 5-8 lbs out of the back the fuselage.
 
Thinking about it more, Cessna never offered a "steel panel" option for their fuselages. I will get a little of the quarter sawn spruce and see what I think. Weight out of the tail is pretty valuable to me.
 
Univair stringers, strong and light, I don't remember the cost, wasn't outrageous. Shipping would be the issue, I was able to pick them up at their facility in Denver.
 

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rodsmith said:
Univair stringers, strong and light, I don't remember the cost, wasn't outrageous. Shipping would be the issue, I was able to pick them up at their facility in Denver.
Click to expand...

Used these on my Bearhawk and Pacer... can’t post a picture from Tapatalk?
f59f667b21bdfa59507736b540a1836d.jpg
 
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Looking at that Univair stringer cross-section, there wouldn't be much difference in the total volume of aluminium compared to the box section stringer which the kit includes. Is the alloy thinner or lighter perhaps, to save the weight?

The rounded head would save a lot of taping and filing, which would be nice.

That 'mushroom' cross section is a more flexible (weaker?) cross-sectional shape than the box section, in terms of both twisting and folding sideways. Bending strength would be about the same as a box section. Although it may not matter at all, if the strength is high enough overall.

I have had several cases of 'hangar rash' involving stringers, despite it being a private hangar with two family aircraft. I wouldn't count on plane being handled carefully.They can take some knocks. In the same breath, weight in the tail is worth avoiding!
 
I didn't measure the wall thickness of the kit stringers. I will check tomorrow. If they are the same as the Univairs .032, I don't see much difference in volume of material used. Still leaning towards spruce.

I was just about to start installing the aluminum ones, and I picked up the whole bundle. Got me thinking.........
 
svyolo said:
.048 wall for the kit stringers. Univair website calls theirs .032.
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That is good information, that would explain the weight saving.

Mark Goldberg said:
FYI - these stringers are extrusions made of 6061. Mark
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That is worth considering too, as the 5052 which Bearhawk Aircraft supplies does not need painting and will not corrode. The 6061 can definitely corrode, particularly if it gets wet - think bottom stringers. I believe 5052 is also a softer metal, which means fatigue resistance in the most general terms. They probably use a thinner walled 0.032 because they are using a stiffer metal.
 
Univair got back to me about their stringers, but I only asked about the "small" ones that are 1/4 X 1/2. They said .66 lbs for a 13 foot section, which is .05 lbs/ft. I didn't ask the alloy.
 
A follow up to the stringer length and weight discussion for those of you searching the forums in the future:

Stringer lengths:
2@ 165"
1@ 136"
2@ 120"
1@ 91"
1@ 83"
for a total of 73'

I used Douglas Fir cut at 5/16" x 1"

Total weight is 6.33lb with two coats of a heavy duty spar varnish.

I plan to wrap a few layers of fiberglass around the side stringers in front of the lifting handles just in case someone pushes on the side of the plane or slips and falls into while trying to move it.
 
If using the supplied kitset stringers, is it the norm to spray them with a corrosion inhibitor before mounting them ?
 
As with everything, it's a personal choice. However, in cold, damp NZ I would ...... Simple 1k etch primer would do.
 
I ended up using spruce stringers, with a thin coat of epoxy. I can't say I every thought of priming the aluminum stringers. I would have let aluminum oxide do its job. I have had my kit stringers sitting in my shop for 2 years, in an environment identical to the south island, including being 1 mile from the ocean. They are, as new.
 
Nev said:
If using the supplied kitset stringers, is it the norm to spray them with a corrosion inhibitor before mounting them ?
Click to expand...

No need, they are 5052 and will never corrode. They will passivate as svyolo suggests.

I would recommend taking the outside edges off with a file though, to smooth the edges for covering and prevent cracking the paint in the fullness of time.
 
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I did't remember that they were 5052. That is marine grade. Good to go. I did look to see if they were anodized, they don't appear to be.
 
Thanks - well I’ve taken the burrs off (and applied corrosion proofing before I checked here !)
 
The stringers are actually 6061. I had thought they were 5052 for many years, but when I ordered a mill run of them a couple years back learned that they are 6061 and not 5052. I have not primered mine on any of my planes. Mark
 
This is prolly dumb... But I appreciated the other data folks provided which made me consider other stringer materials. But as a first time builder I'm trying not to get side tracked and just build a "stock" model 5.

I took an 1/8" roundover bit to the standard kitset stringers. Hitting one side (front and back) I lost 0.10 oz/ft... Only a half pound over the ~73 feet of stringer materials.. when rounding over all 4 sides I'd lose a little over a pound out of the tail. I tried crushing and bending the stringer and it didn't seem to lose strength so I upped the ante and decided to shave the bottom edge of the stringer off. Which would result in losing 2.5 pounds. I checked the 1 foot piece and it still had a decent bit of resistence to bending but when it let go it did so spectacularly lol. I turned it around in the vice and hit it with a hammer ~5 inches away and it didn't budge. All anecdotal things but maybe interesting to someone lol.

I think I will use the 1/8 roundover bit, not for weight savings but just to not have a hard 90° edge pushing into the fabric unless someone here wants to talk me out of doing that.
 
2.23 oz/ft as it came from the factory
2.13 oz/ft after 1/8" roundover one side
1.99 oz/ft after roundover both sides (not sure why I got an extra 0.04 oz on the second pass)

1.7 oz/ft with the bottom cut off and top rounded over
 

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Has anyone thought of adding lightening holes to the factory provided stringers? Perhaps as Mark mentioned, for the underbelly only?

If introduced on the longer side, depending on hole size and spacing I'm curious if they would remain ridged enough.
 
Daniel said:
Has anyone thought of adding lightening holes to the factory provided stringers? Perhaps as Mark mentioned, for the underbelly only?

If introduced on the longer side, depending on hole size and spacing I'm curious if they would remain ridged enough.
Click to expand...

I very much plan to, but it will be awhile before I get there. Planning to do lightening holes after shaping, using a Unibit carefully to reduce deburring effort.
 
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