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Hi folks,

Does anyone know the tensile strength of Hoop Pine?  I've looked everywhere, and can't find it.

 

Thanks,

Duncan

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The highest grade I found for seasoned hoop pine after a quick search was F17.

It's a softwood and AS1720 (Timber Structures: Part 1 - Design Methods) - gives a characteristic bending strength of 42 MPa and tensile strength parallel to the grain of 22 MPa for F17.  However, these basic values need to be modified by a whole lot of factors if you want to design something.

 

 

Edited by rgmwa
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Hi,

Yes, I came across that one, but no tensile strength listed.  I'm wanting to carve wing struts out of hoop pine, and need to know if the wings will fly off on their own at 1000 feet...

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Wing struts could also have a compression load in turbulence as well as static, so bear in mind column length, thickness ratio. Would you encase it in fibreglass?

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Just to add to above the fittings could be a weak point. 2 or 3 bolts in alignment at each end would seriously reduce the area taking the load.

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I agree with Yenn. You need to have a pretty good idea of the tension and compression forces in the strut and its fittings under all loading conditions, and also how to design in timber if you're not building from a plan. 

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Hi guys,

Thanks for your suggestions, but I've given this some more thought, and because I don't have any of the kit needed to hotwire foam, this is what I've decided to do.


Materials required:

 

  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 diameter)

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.

 

image.thumb.png.2f3688b74bec6ac5ab97edabaf1b5682.png

There are enough little riblets in a single sheet of XPS from from Bunnings to make several struts for several planes.  Just sit back and watch the router do its magic.  Of course, hotwiring things would be easier, but as I say, I don't have the kit.

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Duncan, I'm using chrome moly steel tube for the struts on my 701.   One idea I'm considering is simply to fold 0.016" aluminium into an airfoil shape and rivet to the tube.  Because the distances are so small and the curvature gives strength, I can probably get away with either no ribs at all or maybe one top and bottom.  Have you considered that as an option?

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Hi there,

All options are on the table at the moment.  I'll probably end up giving a few of them a go, and seeing which one works best.  But the folded AL sheet idea is a good contender.  Might be the subject of a nice photo-post for all to see.

 

Duncan

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11 hours ago, duncan_rtfm said:

Hi guys,

Thanks for your suggestions, but I've given this some more thought, and because I don't have any of the kit needed to hotwire foam, this is what I've decided to do.


Materials required:

 

  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 diameter)

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.

 

image.thumb.png.2f3688b74bec6ac5ab97edabaf1b5682.png

There are enough little riblets in a single sheet of XPS from from Bunnings to make several struts for several planes.  Just sit back and watch the router do its magic.  Of course, hotwiring things would be easier, but as I say, I don't have the kit.

Carbon fibre rods are overkill - dead weight and $$

 

Just the foam formers west epoxy to the tube + a fiberglass skin (definitely very light woven fabric and not CSM) will be entirely enough.

 

I've done similar on a set of lift struts for the sapphire (replacing a wrapped fiberglass hollow section streamline) and I did not have CNC so rough cut the sections, drilled the tube hole, assembled the stack with epoxy (glue to tube, flox filler between layers) and then used masonite templates on the ends and a sanding stick to bring to final shape before glassing and finishing. 

 

1.5m struts gained 400g which was acceptable to me as that was pretty much exactly the weight of the hollow skin forms I removed. 

Edited by kasper
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Hi,

Good to hear that our methods are identical, if not the means of getting the riblets cut.  I also intend to cut end plates out of ply, and a straight sanding block to tidy up any bits and pieces.  Mine are now cut and in a box.  The only trouble is I made the steel strut hole 20mm OD - I'm thinking I should have made it 25mm?  Thoughts?

 

The carbon rods are just for alignment - they get removed when the foam has been bonded.  a one metre length of 4mm CF rod is about $8 from hobby shops.  Similar aluminium/brass etc is a bit cheaper, but this is a tool, not part of the strut.  Buy once, use many times.  It just depends on the most convenient purchase.

 

I also checked on the price of streamlined 4130 steel tube - but at over $80/metre, that's not going to happen.

 

I need this for two front wing masts (650mm each)

Four rear wing struts (460mm and 520mm)

Six main gear legs (1000mm each)

 

Regards,

Duncan

 

 

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Posted (edited)
3 hours ago, rgmwa said:

How long is the strut?

Hi,

See post above/below.  Various lengths.  My only concern is whether I need to go with 25mm tube vs 20mm OD

 

Duncan

Edited by duncan_rtfm
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10 minutes ago, duncan_rtfm said:

Hi,

See post above/below.  Various lengths.  My only concern is whether I need to go with 25mm tube vs 20mm OD

 

Duncan

On the wing struts your OD of 20mm seems fine as they are primarily loaded in tension and it will all come down to wall thickness and how you do the ends

 

on the legs it’s a difficult question to answer as it depends on how you are springing the leg - rubber in compression - rubber in tension - shock springs in compression and then it’s also an issue of which leg component you add the spring to because at least one of the leg members will primarily be in compression and that element greatly benefits from greater OD to reduce the risk of compression failure of a hollow tube.

 

do some comparison design shopping. Read over the original hm290 or hm293 leg designs and consider how yours stacks up to them on design.  I can report that the original main hm290 leg and struts can take a whack sufficient to destroy the wings and collapse safely to the ground with the pilot stepping out of the aftermath unscathed. The tubes failed gently absorbing a lot before failure and that is what you want in a design. 

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4 minutes ago, duncan_rtfm said:

Hi,

See post above/below.  Various lengths.  My only concern is whether I need to go with 25mm tube vs 20mm OD

 

Duncan

The struts are not very long which is good, but I'd suggest "When in doubt, make it stout!"... especially when your life depends on it and you're not sure what the loads are.

There's a big difference in strength and stiffness between 20 and 25mm tube for the same wall thickness, and steel is a lot stronger than aluminium (obviously).

Also if you're putting bolts through it, the bigger tube will lose relatively less cross-sectional area.

 

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Hi again,

I am equipping the Fleabike with 18-inch tundra tyres, and no spring damping at all.  I am relying on the big soft tyres to do all the damping for me.  But yes, a study of the HM293 is certainly on the cards today.

 

Duncan

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3 minutes ago, rgmwa said:

There's a big difference in strength and stiffness between 20 and 25mm tube for the same wall thickness, and steel is a lot stronger than aluminium (obviously).

Also if you're putting bolts through it, the bigger tube will lose relatively less cross-sectional area.

 

I concur.  25mm it is.  I'll keep my box of 20mm foam riblets for another project perhaps.  Ha ha - I have so many let overs from this project "perhaps for a future project" that I need a separate shed to house them all!

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11 minutes ago, duncan_rtfm said:

I've just checked my HM293 plans, and it seems that they specify 25mm OD, 22mm ID.  Am I reading this correctly?

image.png.00a309b086c091679e748d894c541438.png

Yes, that is how he specs the tubes - OD and ID in your pic = 1.5mm wall 25OD mild steel and thats what works nicely in a gently failure mode when you whack the ground at speed ...

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My co-owner of a Pou many years ago whacked the flea into a raised bank next to the runway at near takeoff speed … the undercarriage took much of the loads as it came to a stop. The front wing twisted when it struck and the main spar snapped into match stocked at every pickup point. 
 

one new undercarriage.  One new centre section. One repaired engine cowl and it was up and flying again. 

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37 minutes ago, APenNameAndThatA said:

I’m confused about this. Aren’t you supposed to use maths when you design aircraft? 

If you want. But there are alternates. Take an existing know element from another aircraft and incorporate into yours is perfectly legitimate. As is design by eye and confirm by test. 

 

If you are designing a one off self build then you are free to do what you like. That's the point of experimental 😃

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