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mcrae0104

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Plywood’s not the best choice for a slender column. Only roughly half the grain is running in a useful direction. Solid wood is a much better choice (but will need to be designed for buckling and may be fatter than you’d like) and steel or AL is a better choice yet. Wood is possible but it will be larger, heavier, and will have more drag.
 

poormansairforce

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Ok, first, I am way too lazy to break down an airfoil shape to get spot on numbers so I rednecked it and used 70% as the average thickness(.82") and adjusted the width(3.5")to get the area for tensile numbers and just plugged it into the spreadsheet. I attached both an airfoil and a square PDF for the spruce numbers. I also plugged in common size 4130, 2024, and 6061 tubing numbers just for fun. The first four sections deal with compression numbers and the last four deal with tension numbers. Pages 3-4 belong to the right of 1-2 if you print them out. Due to rounding of decimals the numbers for the comparison of the spruce shapes vary a little. I just used the strut dimensions/area you posted, no weight or G factors used. Some observations:

1. Airfoils are horrible way to get to buckling strength with regards to weight unless the airfoil has been adjusted for wall thickness,etc in the right areas. Round is right.

2. Plywood could possibly work but the compression/buckling values will be significantly lower than solid wood due to fiber orientation and the tensile values would be half of solid wood. With that in mind, make the airfoil as thick as absolutely possibly. Some channels to accept pultruded CF would help immensely. And then you get to deal with the hardware and the weight of it.

3. If used correctly in a structure then wood is good.

4. However, a piece of 4130 tubing that has some straps welded on looks really good right now.
 

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rtfm

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All I seem to have are questions at the moment. Here's another one...
Nearly all Fleas have:
  1. Two wing masts (triangulated, so as to ensure no fore/aft movement.)
  2. Two single cables attached further outboard to ensure sideways stability.
Something like this:
1627190289599.png
However, what about this arrangement?
  1. A pair of triangulated spars to the mid-span position
And that's it.
1627190522318.png

Being triangulated, there can be no fore/aft movement of the wing, and the spars on the other side of the wing prevent any side-to-side movement.

Thoughts?

Duncan
 

rotax618

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When considering slender struts of non-circular cross-section, only the minor axis need be considered. Unless the connectors are poorly designed tension will not be the mode of failure, struts will invariably fail by buckling in compression. Have a look at Euler’s column formulae.
A very narrow fuselage presents special problems with load paths, I would fabricate a triangular steel frame to connect the wing to the top of the fuselage main bulkhead, and connect the struts/wires from the outer wing hinge point to the bottom of the fuselage main bulkhead
 
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n45bm

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See - this is what I mean. Your sketch is better by far than my plan. The finger joints, the flange at the top... I like your approach much better. Back to the drawing board...
It DOEs look like a coffin...
 

Beragoobruce

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Re your strut design, I used aluminium tube on my MiniMax, and faired it with balsa wood (from Bunnings). The balsa shapes easily, & weighs next to nothing. I bonded it to the strut with filled WEST. I was going to coat it with GRP tissue and epoxy, but ended up just coating with unreinforced epoxy. Lasted very well in service.
 

challenger_II

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Suggestion:

Round tube strut. Make 3mm ply airfoiled ribs, and build the strut as a mini wing, fabric covered. Search for "Bellanca Skyrocket" for a reference.

Light, relatively simple, and adds a bit of lift.
 

rtfm

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Re your strut design, I used aluminium tube on my MiniMax, and faired it with balsa wood (from Bunnings). The balsa shapes easily, & weighs next to nothing. I bonded it to the strut with filled WEST. I was going to coat it with GRP tissue and epoxy, but ended up just coating with unreinforced epoxy. Lasted very well in service.
Hi,
Now this is the best idea I've heard for a long time. I'm going to give this a go and see how I get on...

What aluminium tubing did you use?
Thanks.
 

rtfm

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Wait - I've had a better idea.
For this I'll need:
  1. 2x 25mm steel or aluminium tubes
  2. 1x sheet XPS foam (local hardware store, $12.50)
  3. 2x steel/aluminium/brass/carbon fibre rods (4mm)
Method:
  1. Chop the foam sheet (1200mm x 600mm x 30mm) into a zillion 100mm chord "riblets"
  2. Grease up the tube and the big hole in the riblets with West epoxy
  3. Thread required number of the riblets onto the steel pole, bonding each new one to the preceding with some West
  4. Thread the two brass/aluminium/ carbon fibre rods through the 4mm diam alignment holes
  5. Wait for it all to set
  6. Rinse and repeat
All that's left to do is to affix the attachment hardware to the ends of the steel/aluminium tubes, and to coat the foam with a layer or two of epoxy and possibly a layer of glass fibre.

1627280006814.png
 
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rotax618

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The strut diameter should be larger than 25mm at least 32mm the foam only needs to be at the trailing edge of the tube and would be better hot wire cut sections say 600mm long, glued to the tube with gorilla glue and covered with glasscloth/epoxy.
 

Beragoobruce

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What aluminium tubing did you use?
The rear strut is 6061-T6 ally tube, 1.25" dia, 0.58"wall. The front strut is same dimensions, but 4130 steel.

The chord/thickness aspect ratio of your fairings should be >3:1 to be of real benefit. I went for 4:1.

Your foam idea sounds good. I just prefer working with wood given the choice.
 

rotax618

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Duncan, if you get in touch with Nick at Lightwing at Ballina, they have an alloy streamlined strut extrusion that doesn’t cost an arm and a leg, is about the same size as your foam shapes.
 

challenger_II

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How about getting making a pattern from poster board, make a streamline fairing, a tad over-size in the fat section, and using Adel clamps on the tubing to attach the aluminum fairing? No holes drilled in the strut, this way.
 

poormansairforce

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Get a strip of aluminum sheet, bend it into a deep V shape, and rivet it onto the tube so the cross section is like this. <o
That's kind of what I did on my Mini Max except I glued it to the strut and glued the rear edge together, no rivets. Made very little difference in speed which was disappointing.
 
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