Paramotor weight

[powerlord]

ah, yes nothing wrong with a shear load. Though it's a pin in the parajet - not a standard bolt.

I dunno what it's rated at though.

As I say, I'm not a mechie engineer - but all I'm saying is bolts (like screws) tend to be hardened for  tensile loads don't they ? there are usually no published specs for the shear loading of bolts. though I believe it's typically 60% of tensile - but that's across unthreaded part too - which there is none if I remember correctly?

Having a specific pin, parajet at least had the opportunity to specify specify shear load requirements for the bespoke part. though frankly I've never like that either in the parajet.

But as I say - frankly I have no real idea what I'm talking about - all I can do is compare against others where there's a bit more than just a basic bolt through a hole. But you are probably right -safe as houses.

But a safety straps wouldn't add much weight at all - and that helps not just if it does shear, but if by user error the bolt works itself loose (either lose enough to take too much shear force, or all the way out), you've got a safety strap there that might avoid and extremely dynamic event ? basically - what Jeff says.

I remember flying my PAP once, and later, noticed that bolt through the swing arm was loose, and hanging out about 1cm... totally user error. should have noticed in pre flight check, yada yada.. but I didn't. it wiz fine. but if it had vibrated itself all the way out, having a safety strap might have saved my life.

Again, some other machine don't have em either.. everything is a compromise, etc, etc - but that doesn't mean we should just ignore it or sweep it under the carpet imho - no harm talking about the cons as well as the pros of a machine - there has yet to be a perfect paramotor created afaik....and until one comes out that weighs 1kg made of pure Adamantium, produces 200kg of thrust and runs for 4 hours on the built in lithium-unobtainion battery, that's unlikely to change any time soon

 

 

stu

[AndyB]

lol, standanddeliverium maybe? Showing age!

[admin (Simon W)]

22 hours ago, cas_whitmore said:

polini's reputation is the only reason apart from moneys I haven't owned a parajet '

Cas . 

Take a look at the Maverick with the EOS booster engine in it mate 19KG ready to fly oodles of thrust. 

SW

[AndyB]

At Parafest I looked at Maverick with Moster engine. It was same weight as AC Nitro, but had no reserve or prop fitted. There are no weights listed on the Parajet site any more?

[Madasapig]

24kg it was i think

[Steve]

The Mav Pro is 21.5kg from memory. About 24kg for the other two, give or take 0.5 kg.

Andy, you saying the Nitro had a reserve and prop fitted and was the same weight as the Maverick without both of these things? 

[AndyB]

Steve, I didn't weigh them. I only picked them up with one arm and did a bicep curl!  They felt about equal. 

[Steve]

6 minutes ago, AndyB said:

Steve, I didn't weigh them. I only picked them up with one arm and did a bicep curl!  They felt about equal. 

Did the Nitro have a reserve and prop fitted but the Mav didn't? 

[AndyB]

yep, NItro had flown. Fuel dregs as well. Mav was brand new.

 

[custom-vince]

Yep, but scruffy but nice for pilots to see how it's aging with use. My AC nitro was fitted with 125cm e-prop, prop covers and the reserve chute mounted in the side. 

In standard form it is 19.4kg, plus my reserve and bridling etc that's about 1.5kg

with soft fuel tank and split leg harness the nitro 200 can be reduced to 18.8kg

 

I like your ideas on the swing arm design. What I'm often told though is no recorded incident ever of an AC bolt breaking. The idea of a graded bolt is that it does have known breaking & working load limits. They often exceed carabiners and the tubular metal frame it's mounted to. 

[powerlord]

the grade on graded bolts are for tensile strength Vince. Not for shear load. afaik no bolts have a shear load grade. There are 'finger in the air' estimates for different material for what fraction of the tensile grade would be there in shear - but no agreed standard tests, no standard grades.

Happy to be proved wrong, but that's my current understanding of it (from the interweb - im an electronic engineer, not a mechanical one so don't claim any expertise in the area that's for sure).

And unlike tubular steel and carabiners, bolts don't tend to bend, twist or deform when they reach that limit - they snap.

Plus, as I say, a safety strap protects not just against catastrophic failure modes like a bolt snapping, but also protects a bit from the most likely problem source - the pilot not checking the bolt was tight, etc.

That's the far far more likely situation where a safety strap could save a life imho.

With quick release arms of course, that's an even more likely occurrence - all it takes is a pilot to forget to put one nut on in the excitement of setup (and who of use hasn't forgot something one time or other), and it's likely to lead to a very very dynamic event which might be fatal. A safety may just help that situation be a little bit less dynamic, and be survivable.

stu

[AndyB]

The design should have a spigot so the swing arm flat plate locates into a hole in the frame flat plate, with the bolt then holding it together.

I don't know what size the AC bolts are, but just as an example, here are the figures for just ONE M16 high tensile bolt (link to table below).

In tension the bolt will hold 30.6 KN, or over 3 tonnes.

The shear load of the same bolt is 25 KN, or 2.5 tonnes.

To put 2.5 tonnes shear load on one bolt would require the following.

Pilot + machine weight 130kg. Initial impact time 1 second. Impact speed 188 m/s, all vertical so bolts hit in shear.

This gives impact force of 48.9 KN, or 2.5 tonnes per bolt.

However, 188 m/s is 420 mph.  If the bolts are smaller, even half the x-sectional area, then 210 mph would not be any better!!!!!!!!!!!!!

Spigots would be a better engineered solution.....but they aint going to break anyway!

http://www.engineeringspreadsheets.co.uk/pdf/bolts/Bolt Load Tables to BS5950 01.01.03.PDF

 

[custom-vince]

I agree there can be a better more engineered design solution, but that is not viewed as being required as the proof is in the pudding. Simplicity has always been key.

 

Some data:

 

Class 8.8 & 10.9 bolts have been used in size M8 (8mm)

 

 

https://www.boltdepot.com/fastener-information/materials-and-grades/bolt-grade-chart.aspx

 

 

[custom-vince]

Powerlord, forgetting to do up your weight shift in the excitement can be likened to forgetting to secure your carabiner. I hope the shear load table above helps you.

AndyB, thanks for calculations but the bolt breaking in the event of an impact, I would think is not a great concern when reading the forces involved, by the time there is an impact of that scale, bolts are not the issue, bones are more important. The forces involved to get a bolt to break in flight surpass the structure of the aircraft as a whole.

Arm failure scenario has been tested and it is land-able but not ideal, Secondary safety backup straps are inlace, it can be improved again with a 3rd safety strap if one feels they need it. Email me for that. I do not see the requirement given the history and design of the current setup.

Arm failures in the past on other weight shift designs have been down to: thin wall stainless weight shift. 6mm bolts. non graded bolts. poor design offering greater leverage. All these have been addressed by design, 8mm bolt and the simple 2 disk system dispersing the levering forces, double thread design, high grade, greater density titanium in the arms all combine to improve the situation.

It has been suggested in the past to add a spring to the inside of the bolt. We see this as incorrect as then you eliminate the effectiveness of the double disk system and then when under load and spring is compressed you then rely on bolt strength alone.

I hope this goes some way to help people understand the key differences by design that is not obvious to the eye. 

[AndyB]

Cheers Vince. I was showing that to break a bolt takes a very terminal event. Nothing we could ever do in flight would load to a sufficient level. The only other worry would be fatigue, but again, the loads are very small and not an issue.