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Omega

Omega Slope Model Build

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Omega

Yeah that's greatly reduced. The material I have chosen is 290Mpa which is around 43kgs / sq.in. 

I have powered the calculator back up lol

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mikef

I think 290 Mpa = 290x145 lbf/sq.in.= 42050 lbf/sq.in. = 19074 kgf/sq.in.

 

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Omega

So thinking about this a wee bit more. The majority of the beam is supported in the wing joiner tube. 

The length of the joiner is 350mm

fuse is 40mm across leaving 310mm in the joiner tube / 2. Or 155mm

so with 40kgs of force applied via the winch line at breaking strain (worst case)

each end of the joiner is supporting 20kgs. 

But this is distributed along the length of the joiner, not just at the end ?

so does that mean I have to calculate from the root of the wing to the hinge point ?

or just assume it's end loaded ?

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Omega
16 minutes ago, mikef said:

I think 290 Mpa = 290x145 lbf/sq.in.= 42050 lbf/sq.in. = 19074 kgf/sq.in.

 

So I reckon I'll be ok then. Lol

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Omega

Area calc went squiffy. 

My 3/8" rod is actually 0.346 sq.in

so 19074 x 0.346 = 6599 kgf/sq.in ?

Due to size of my rod (oh err)

plenty of head room then ?

thanks mike :) that's brilliant. 

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Omega

Area calc wrong again. Must be my age. Lol

pi x (r)2 or 3.14 x (0.375)2 = 0.442

whatever imgoing for a lay down. Lol

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mikef

The X-section area of a 0.375 dia rod is (0.375/2)x(0.375/2)x'pi' = 0.110 sq.in.

Make yourself a cuppa too.

 

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Omega

Time to order material. 

Suggested suppliers ?

metals4u any good ?

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mikef

For a rough calc, assume each wing is loaded half way down the semi-span by half the line tension of 40 kgf.  So 20 kgf each side acting at (wing span)/4.  That will give a bending moment at the centre-line of 20x 2950/4 = 14750 kgf.mm.

For purely elastic bending (no yield at all), the maximum stress in your 0.375 in. Dia rod is given by:-

stress (N/sq.mm.) = [Bending moment (N.mm.) X radius (mm)] / [second moment of area of section (mm to 4th power)]

= [14750 x 4.7625] / [’pi' x (4.7625 to power 4.)/4]  = 174 kgf/sq.mm. = 1700 Mpa.

The yield stress is exceeded.

Can someone check this for method and maths please?

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Omega

Thanks mike perfect. 

After my lay down lol

so it would bend a wee bit. 

Somif I went to 7/16" same material ?

the wing is so thin that I would need two joiners if that's not enough ?

 

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Ribs
4 hours ago, mikef said:

For a rough calc, assume each wing is loaded half way down the semi-span by half the line tension of 40 kgf.  So 20 kgf each side acting at (wing span)/4.  That will give a bending moment at the centre-line of 20x 2950/4 = 14750 kgf.mm.

For purely elastic bending (no yield at all), the maximum stress in your 0.375 in. Dia rod is given by:-

stress (N/sq.mm.) = [Bending moment (N.mm.) X radius (mm)] / [second moment of area of section (mm to 4th power)]

= [14750 x 4.7625] / [’pi' x (4.7625 to power 4.)/4]  = 174 kgf/sq.mm. = 1700 Mpa.

The yield stress is exceeded.

Can someone check this for method and maths please?

Seems pretty good to me Mike.

Omega - see attached quick analysis plot based on Mike's approach of putting the vertical load at half way out. A more accurate model can be done with an elliptical lift distribution, which might relieve your stress a little.

Note that even two or three rods of EN1A are going to be pretty prone to bending under your proposed 40kgf load. I would seriously consider other materials - en1a or 070M20 as it is also known to close friends, is pretty basic mild steel. It would normally be used for lightly stressed parts or parts that need to be bashed out cheaply and welded or bolted together.

Consider tougher stuff like silver steel or EN16 (605M36 in new speak). Lots of people sell small quantities of better steels - I just did a quick gander on EBay for EN16 - loads of options there for you. If you don't find the size listed, just give them a call.

One of the other good things about higher grade steel is that you might be able to get ground bar - this will have a much higher tolerance on diameter and straightness. This in turn means you can make a better joiner tube that fits more tightly and helps to transmit the bending moment from the wing to the joiner. Doing this might lessen the chance of nasty stress raisers that may give problems - more likely in the airframe than in the joiner.

One more thing - I have an Erwin XL from PCM. They use what looks like EN16 or similar for their wing joiner and this can be swapped for a carbon rod, depending on ballast options. The standard fit for the slope layup is two such bars and they are about 16mm in diameter each! I think their slope and Medium layup models use the same diameter bar (you could Email Markus at PCM - I'm sure he'd give you some advice). The XL has a 3m span and is designed for fast slope soaring (not even racing really), so that perhaps makes your 3/8 option seem a bit on the small side, without even looking at stresses and material choice. Saying that, it might depend on your planned AUW and what you want to do with the model.

Best of luck with the project - following it with keen interest.

cheers,

Chris

joinerAnalysis.pdf

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mikef

Thanks Chris,

I thought there'd be someone out there who knew what they were doing.  The family PC was busy last night so I could only use the back of an envelope.

My direct experience with sizing joiners comes from F1A (free-flight glider). I was bending 1/4 in. piano wire and changed to a decent steel to solve the problem.  That's a 410 gram model on a hand tow (pictures below).  We estimated that we were pulling about 15-20 kgf in those days (1990s).  The model weight was negligible on tow of course.  A fast glider in a tight turn is going to generate another load case worth considering.  Slope racers must pull tens of 'g's in those turns.

It's a long time since I've done any aircraft detail stressing but it used to be my job.

image.jpeg

image.jpeg

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Ribs

No worries Mike.

Love the design above. Something about a plan that is so pleasing on the eye. Have you seen some of Mark Drelas? They are incredibly detailed and thought through - your's remind me of his.

Scratch designing a model is on my to-do list, but I somehow never quite get there. I made a good stab a few years back, but I shelved it - I think it is great to see projects like this and the homemade F3j build being done and presented here.

With regard stress analysis - it's such a tricky thing to do - how do you really know your load cases? I would imagine that doing it for the aerospace industry is very time-consuming and tricky - no doubt there was stress all around!

 

 

 

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Witch_1

Nice to see a good design-and-build going on.

Just chipping in with my experience of a steel joiner.  My Race M came to me with a made-up 14mm high x8mm thick steel joiner (with plywood both sides to fit the joiner box).  It had 6.deg written on it.  After a couple of rounds F3F with moderate wind (16mph) and moderate ballast (900g) the dihedral now measures 10 deg and it has not been used again!  Unless using good quality steel like Multiplex blade type joiners do, I would steer away.

I made up a carbon joiner by laying up tows between two aluminium angles that were glued together at the right width, and pressing it by clamping a strip of wood on the top - no problems since.  The size is 20mm x 14mm.  For a new build it should be easier, using a steel or aluminium (or even hardwood) bar as a blank to make the joiner boxes and as a form for a simple mould for a carbon joiner.

Good luck with the project!

P1030457b.jpg

WP_20170505_20_59_17_Prob.jpg

P1030453b.jpg

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martynk

How is the build going? Looks very interesting

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