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karmarepair

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On a completely separate note:

We plan to use 3/32” stainless steel cables for the control system for our ultralight, and we’re not sure whether to use tin-plated copper or stainless steel nicopress sleeves to secure our cables.
Unless something has radically changed in the 40 years since I made a living as a sailboat rigger, but Back In My Day, there was no such thing as Stainless Steel Nicopress sleeves.

Stainless end fittings of all sorts I attached to many things, using a scary punch press with the relevant dies that was older THEN than I am now, and had NO guards of any kind, or various roll swagers (Kearney being a common brand name) or rotary swaging machines the size of a Honda Fit, but 3 times as heavy. IF you have accurate lengths, you can get your local sailboat rigging shop or sailmaker to make up cables to length with stainless fittings for Small Money using the aforementioned devices, but if you want stainless end fittings, there is NO WAY you can Do It Yourself.
 

M Clarke

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Copper sleeves will be plenty strong. If you put enough force on your airplane to break the cables you’ve probably got bigger problems… There would likely be a structural failure in the control surfaces/airframe before the cables broke.
 

maya.ayoub.32

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Unless something has radically changed in the 40 years since I made a living as a sailboat rigger, but Back In My Day, there was no such thing as Stainless Steel Nicopress sleeves.
Copper sleeves will be plenty strong. If you put enough force on your airplane to break the cables you’ve probably got bigger problems… There would likely be a structural failure in the control surfaces/airframe before the cables broke.
Awesome! Copper Nicopress it is - thank you!
 
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Hello Homebuilt Airplanes!

We’re nearly finished with our fuselage design, and are hoping to weld it in the coming weeks. Previous simulations have shown us that the fuselage might break at its joints, but we’re not sure if we can trust these simulations, so we would like some opinions. Our current design uses ⅝” OD 0.035” wall 4130 chromoly steel pipes. The triangles at the front of the cabin (highlighted in green) are doors; they aren’t structural. Is there anything we should be worried about? Is this likely to break in flight?

Fuselage with Wing Diagonal.pngFuselage with Wing.pngFuselage Diagonal view.pngFuselage Green Door.png
 

TFF

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Looks like a flyer.

You say it can break it at certain joints. All joints can break. Which ones and what load to they break at first? That’s the key, if that point can’t be reached, it can’t break.
 

mcrae0104

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Would you not want the struts in plane with the spars? Also, is the aft spar up to the task? It looks kinda skimpy.

Consider how the load is getting from the wing into the fuselage. It looks like you have a good attach point at the rear spar, but not at the front (the front spar attach wants to happen at a joint in the fuselage truss, not somewhere along the length of the tube).

Your door opening can be larger (and more structurally sound) if you consider something like this:

1624344209725.png

Keep in mind, a truss isn't truss if it has "T" intersections. You can do Ts, but it puts the members in bending and presto, you have a "frame" and not a "truss" (muy mucho extra weight).

Read up a little bit about what makes a truss a truss (axial loading only on the members - aka two-force members; don't worry for now about the fact that the weld clusters aren't true pin joints; loads applied only at joints). Then you will be in a better position to design the geometry before running a simulation. Otherwise you're stabbing in the dark. I think Peery might be a good reference for the team. Don't be intimidated by it; just start by absorbing some of the simpler examples in chapters 1 & 2. Another very good source would be Jeff Hanson on statics. Here is his truss intro.

And keep up the great work. It's really fun to see the project evolving.

 

GeeZee

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Looking good! One thing to consider if it’s not too late…. Since you have a “cabin” style fuselage you could have a center section that the wings attached to. This would allow a Kitfox style wing fold if you relocated the struts to align with the rear spar.
 
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Would you not want the struts in plane with the spars?
The strut and spar are in plane, but you're right, we need to revise the area around the front strut. The reason why we had the spars attach aft of the joint was because the beams were going straight through our ribs... not the greatest idea on our part. The new cabin layout looks great though! We'll probably be using that. And thanks for the resources!
Screen Shot 2021-06-22 at 8.34.33 AM.png
 
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Since you have a “cabin” style fuselage you could have a center section that the wings attached to. This would allow a Kitfox style wing fold if you relocated the struts to align with the rear spar.
I don't think that's something we can do this time around, but we'll definitely look into it in the future. It would make storage and transportation so much easier. Thank you!
 

Dana

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Back to the nicopress question, stainless sleeves do exist, but you don't want to use them, they're for specialized applications. The stainless terminals that require a hydraulic or roller swager are something completely different.

But, you might consider galvanized steel cable instead of stainless, it's cheaper AND stronger. In that case you would use unplated copper sleeves.

Another thing... although 3/32 is probably fine for your ultralight, 1/8 is generally considered the minimum for primary flight controls. This is not for flight loads, but based on the force the pilot can exert. Also you should have stops on the control stick to limit its travel before the control surface hits its own stop.
 
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On an unrelated note, we're planning to use tangs to attach our cables to our bellcranks and control stick, similar to how to the Legal Eagle attaches their cables. Are there any specific scenarios where we should be using shackles, or is using tangs everywhere fine?

Screen Shot 2021-06-22 at 1.42.10 PM.png
 

maya.ayoub.32

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but you don't want to use them, they're for specialized applications
Okay gotcha. We reached out to our local EAA chapter and were able to borrow the official tool to press the copper too.

For your recommendation on Galvanized cables, we checked it out and it actually saves us a lot of money. All our thimbles are stainless steel, though, so will that be a problem?

We were basing the sizing off the legal eagle and affordable, but better safe than sorry and we'll switch our cables to 1/8 as well. Thanks!
 

karmarepair

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For your recommendation on Galvanized cables, we checked it out and it actually saves us a lot of money. All our thimbles are stainless steel, though, so will that be a problem?

We were basing the sizing off the legal eagle and affordable, but better safe than sorry and we'll switch our cables to 1/8 as well. Thanks!
We used stainless thimbles with galvanized wire all the time in sailboat work, as by that time (the late 1970's to the 1980's) it was getting hard to get decent galvanized thimbles. To a certain extent, the zinc on the wire will be anodic to the stainless thimble, but land airplanes typically don't see immersion in salt water.

I would NOT up-size the wire - you need every ounce to meet Part 103. Particularly since each size wire has it's own size corresponding thimble. If you already have the smaller thimbles, go that way, IMHO.
 

proppastie

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The original plans for the Carbon Dragon used 1/16 cable (rudders, elevator only) flaperons were torque tube with the attach at the root of the wing. ,,,, 1100 lb for 3/32 is a lot of force to be pushing on a flimsy UL
 

maya.ayoub.32

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We used stainless thimbles with galvanized wire all the time in sailboat work, as by that time (the late 1970's to the 1980's) it was getting hard to get decent galvanized thimbles. To a certain extent, the zinc on the wire will be anodic to the stainless thimble, but land airplanes typically don't see immersion in salt water.

I would NOT up-size the wire - you need every ounce to meet Part 103. Particularly since each size wire has it's own size corresponding thimble. If you already have the smaller thimbles, go that way, IMHO.
Okay gotcha - would you recommend taking any corrosion resistive measures for the galvanized to stainless surfaces or is it negligible? We just changed the design back to 3/32 cable and ordered off West Tech Rigging, which is almost 75% cheaper than Aircraft Spruce if this will help anyone else!
 
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We've been looking at ways to redesign the upper nose and cabin area, and we haven't been able to think of anything that would better support the front spar without severely restricting nose and cabin area (our pilot will be pretty far forward, so we need space in the nose). Is there an option we're missing or should we stick with the old design?

Our main problem throughout the fuselage design has been that our wing chord is massive (6ft) and we've had to make the cabin larger to accommodate that. Naturally the pilot would go in the center of the cabin, but our center of gravity would be too far aft if we did that, so our pilot is going to be somewhere near the nose, which means our nose has to be large. Any recommendations for mitigating this?

Screen Shot 2021-06-23 at 4.58.41 PM.pngScreen Shot 2021-06-23 at 5.01.56 PM.pngScreen Shot 2021-06-23 at 5.13.28 PM.png
 

mcrae0104

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The computer models are neat, but may I suggest a different approach that will give your team a more intuitive feel for space frames? Buy a bag of gumballs (weld clusters) and bamboo kebab skewers (steel tube). Mock them up and see which designs work when you bend and twist them in your hands. (Hint: #3 might be more flexible than you want in fwd upper part of the cabin.) There's also a lot to learn from the Legal Eagle fuselage as a study in minimal elegance.
 
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