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Frame materials


ganers
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Hi all.

I'm just throwing this out there but why are cages and frames made from aluminium?

The first thing that springs to mind is weight. But given how easy it is to ding the frame is the weight saving that great?

Also alli is so hard to weld and specialist kit is needed where as if it was steel a cheap tig and your off and running.

Just wondering really mainly because I've tripped over and bent the hell out of mine and an getting it repaired. Whereas if it wasn't made of Ali I could have sorted it myself, and the question Is bugging me.

I'm sure there are some really good reasons gained from tons of research so if anyone knows and has a few minutes can you let me know.

Cheers Lee.

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Hi badger.

I was thinking about knocking up a cage frame in stainless to go onto the chassis and looking at the tube thickness needed with Ali against stainless I wouldn't have thought there was that much in it. How much heavier is your pxp?

Cheers Lee

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I'll have what you say about titanium but for the sake of a couple of kilos the steel benefits look rather attractive.

On another note I've had the chance to crawl round my dads new bulldog and the chassis and frame on that make my ec extreme look rather wimpish!

It's quite an impressive piece of kit.

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Choice of material for a space frame is more complex than simply weight and strength. Stiffness is a significant factor.

Consider two tubes of the same weight, one made from steel and the other from aluminium:

Aluminium is around 1/3 the density of steel, so we can use 3 times as much aluminium for the same weight.

Aluminium is around 1/3 the stiffness of steel, so a tube of the same outside diameter and the same weight (with three times the wall thickness) will have about the same stiffness in either material.

Aluminium tube (6000 series) is about half the strength of mild steel tube, so the tube described above will be stronger in aluminium than in steel. Stronger steels (4130 etc.) are available, but require special care when welding and need to be heat treated afterwards.

In practise, if one were replacing a steel tube with an aluminium tube, the aluminium tube could be larger diameter. Stiffness of a tube is proportional to the square of the diameter, so large diameter thin wall tube is stiffer than smaller thick wall tube of the same weight in the same material.

Unfortunately, large diameter tubes with thin walls are easy to dent. Filling the tube with expanding rigid foam helps.

Titanium has the strength of 4130 steel, density and stiffness about halfway between steel and aluminium.

Stainless steel is not a good choice. In the grades available in tube (316 is the usual high strength one) it's weaker and less stiff than mild steel, but with the same density. It's also prone to fatigue, especially when welded. Our machines make enough vibration for this to be a concern.

The only thing stainless has going for it is appearance. It's nice and shiny and won't go rusty.

The choice of material is an integral part of the design. If you've got the space to use large tubes, aluminium can give a good balance of strength, stiffness and weight. If you need to use small tubes, steel is good and titanium is better if you can afford it.

Edit: Mild steel is available in a wide range of grades. I am using figures for commonly available tube grade A106.

As a slight aside, I don't think a space frame is the best possible solution to paramotor design requirements.

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Excellent reply Pete.

For me aluminium wins in its workability, strength, lightness and easy of repair.

I looked at titanium but it doesn't really excel in its main application. As Pete describes, sectional size is where strength comes from, hence most of the Bulldog is 25mm tubing.

In tests that I done I found titanium bent in a crash but resisted straightening afterwards.

A good trade off would be a mix of different materials (aluminium, titanium, steel, fibreglass (or as some call it Carbon-fibre)) in the right places.

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"titanium bent in a crash but resisted straightening afterwards."

This is called work hardening. The areas of the amterial which have been deformed become harder and attempts to straighten the tube will result in a wave, with a new bend to each side of the original bend. This can be avoided by annealling before straightening, then re-treating afterwards. This is often inpractical.

Sectional size gives stiffness more than strength. An aluminium drink can can support my weight without buckling (high stiffness) but I can crush it with my hand (low strength).

If I took the same mass of aluminium and made it into a solid bar of the same length as the can, it wouldn't support my weight without buckling but it would be harder to bend by hand.

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Thanks gents.

I guess this is where all the r&d has pointed towards Ali. Also I guess the shape has a lot to do with it. My ec has lots of lovely curves where a simple straight piece would have done the trick. Where as the bull dog is very simplistic in design almost basic.

But now it's bent the ec curves are proving a pain in the arse.

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I have been looking inti getting one of the renegade "plastic frames" in from the states, with all the shipping costs and import duty I think I could get them to sell for about £1,500 +vat, this is for the swan neck low hang point frame and harness only.

The set up would best suit a moster engine with a 125 prop, the harness is the new acpo light weight one.

It looks like a great design, all the frame spars are interchangeable so if you manage to break one just replace it with the spare and off you go!

If anyone else is seriously interested I would be happy to get in a couple and possibly stock them and some spares, but would like to gauge interest first, so please get in touch if you are

Sorry Simon if this is the wrong place to "advertise" this.

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Aluminium will fatigue, but it is detectable by visual inspection. The surface finish (usually anodising) will show a series of fine cracks where the material has been yielded or repeatedly stressed anywhere near to it's elastic limit.

The problem with glass reinforced nylon is that the matrix (nylon) is very ductile compared to the glass strands. It is possible to break the strands while the surface appears unchanged.

Certainly, if in doubt, replace the part. My point is that short of Xray inspection, there will always be doubt with composite materials.

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  • 2 weeks later...

I have repaired the bottom left of my Bailey frame and bottom left of the cage twice now.

I bought a new AC/DC tig welder (£350) aluminium tubing, argon bottle and BOC license. All that cost about £800

I'm not a welder as is obvious by the welds I do but Aluminium can easily be filed to look better.

I can repair my Bailey aluminium parts in a couple of hours now.

Quarter frames are £160 and main frame, I think over £600 so the kit has paid for itself.

Pitty props aren't repairable :lol:

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Never had to replace a prop, bend or weld my frame, etc, and I have had some very bad launches, landings, and even turtle'd a few times. The last fly-in I went to I had to avoid a rather large object (no comment), and landed on my ass at full power. Most were surprised I didn't bust my prop, as many did that day.

Clearance and a good frame design probably helped a lot.

Touch wood... a lot of it...

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