Hang bolt safety cable...do you have control?

[John.com]

FYI . . . here is a picture of a typical hang block (Aquilla), showing TWO rotations of the Safety Cable - with ONE rotation on FRONT of the Hang Block.

Also, safety cable bolt points are aligned with hang bolt, so there would be no rotation of the trike airframe in relation to the wing on failure of the hang bolt.

Hang Block, showing TWO rotations of the Safety Cable

Hang Bolt & Cable.jpg (28.85 KiB) Viewed 1214 times

Just to clarify, the hang-bolt is the TOP bolt in this picture.

Although it "should never fail", I replaced mine 25 hours back at 600 hours. I would suggest ordering one from the trike manufacturer, not the hardware store!!

Safe Skies!

John

[Bundy]

Thanks for all the input guys, some interesting thoughts and theories coming out in relation to what the trike will do in this situation....

The looping of the cable is critical, to prevent the cable sliding forward and aft in an emergency, thanks for the pic John.

Glad you replaced the bolts on your trike....just not worth taking a risk on. A new Hangpoint assembly from Solowings is now R520.00 if anyone wants to know.....

Ordered one last week for DBP. But....what is also of critical importance is not just these bolts....but the amount of wear and tear on the end of the pylon. The bottom bolt goes through the pylon as well as a nylon bush. When this nylon bush starts to visibly form a gap between the aluminium edge of the the end of the pylon and itself, it indicates that the hole through which the bolt goes through the pylon is becomming "ovalled" or "elongated"....there is therefore less aluminium material left to "hang on"

After the crash, and stripdown, I noticed a fair amount of wear on our pylon (385hrs at the time, bolt hole was elongated by 1mm)....something I will keep a good eye on in future.

We should have no problems in the short term though as we are replacing both the pylon and the hangpoint assembly on DBP.

One thing I question is the "limits" of this vital joint at the top of the pylon. The limits on almost all trikes are positive 6 and negative 2 if I recall correctly. With an all up weight of 450kg, I doubt whether this little bit of aluminium will handle that kind of force exerted on it? 450KG at 6 G is 2.7 tons! (unless i've screwed up the calculation??)

.....Not that you will ever get close to that figure in normal flight of course...

[John.com]

But....what is also of critical importance is not just these bolts....but the amount of wear and tear on the end of the pylon. The bottom bolt goes through the pylon as well as a nylon bush. When this nylon bush starts to visibly form a gap between the aluminium edge of the the end of the pylon and itself, it indicates that the hole through which the bolt goes through the pylon is becomming "ovalled" or "elongated"....there is therefore less aluminium material left to "hang on"

Hi Alan, thanks for this. Do you maybe have a picture of the wear so that I can better visualise it? Tx

[John.com]

One thing I question is the "limits" of this vital joint at the top of the pylon. The limits on almost all trikes are positive 6 and negative 2 if I recall correctly. With an all up weight of 450kg, I doubt whether this little bit of aluminium will handle that kind of force exerted on it? 450KG at 6 G is 2.7 tons! (unless i've screwed up the calculation??)

Your calculation of 2.7 tons (6 x 450 kg) is spot on! The one thing we laypeople tend to forget is that in normal design there is ALWAYS a large factor of safety applied when the load is "dynamic", which it would be in this case. However, let's not venture there as it gets a little too deep into areas I am unfamiliar with, i.e. aircraft design.

Firstly, we must remember that this 12mm bolted connection is in "double shear", meaning that the load of 2,700kg is shared between two failure points on the bolt (one each side of the pylon), so only 1,350 kg is the failure load on the bolt at 6Gs!!

I stand to be corrected by if my memory serves me correctly the yield (failure) strength of high tensile steel (which is what these bolts are - which is why you should order them from the manufacturer and NOT the local hardware store!!) is 700 N/mm2 (Newtons per millimeter squared).

Therefore the shear capacity of a 12mm diameter high tensile steel bolt is = 700 N/mm2 x Cross-Sectional Area of the Bolt = 700 N/mm2 x π x r2 = 700 x 22/7 x 6 x 6 = 79,200 N!!

OK, so back our load of 1,350 kg . . . . how does this translate into "force"??

Well, as you will remember from Matric maths, F = m x g (force = mass x gravity) = 1,350 kg x 9,81 m/s2 = 13,243 N!!

So load capacity of 79,200 N plays a static load of 13,243 N!! A factor of safety of almost 6!!! (back to my comment about dynamic loading)

A factor of safety of this level would be taking into account other factors such as metal fatigue, etc.

Out of interest: Assuming a STATIC load of 1,350 kg (i.e. double shear), a 5mm diameter high tensile steel bolt could handle this!! We have a 12mm diameter hang-bolt!!

Anyway, I hope you found this useful.

Anyone mathematically/scientifically-minded, please feel free to challenge the calcs!!!

Safe Skies!

John

[Bundy]

[/quote]

Hi Alan, thanks for this. Do you maybe have a picture of the wear so that I can better visualise it? Tx[/quote]

Thanks for the maths lesson John... (prob not a good time to tell you I failed Mathematics? )

As far as I can tell from the above, your calcs are based on the shear strength of the bolt. But I am not really worried about that...rather the shear strength of the Aluminium through which it passes.... have a look at the photo's below...That bolt will prob never break (even at 12g??)... but your whole life rests on the integrity of the 30mm (approx) of aluminium tubing above it. The Alunimium will shear long before the bolt does....thus my post regarding the wear and tear on the pylon bolt hole.

Here.... have a careful look at the nylon bush starting to stick its head out of the pylon... should be flush! Hole elongated by approx 1mm, thus allowing the nylon bush to protrude. This was not accident damage (even though the RHS appears to protrude more)...when we did our ATF in Nov, Hans called me over and showed me the gap. At the time, he said we would prob have to replace the pylon soon....I didnt think he meant the whole trike!!!

Here are just a few more of how stuffed the pylon is....

[John.com]

As far as I can tell from the above, your calcs are based on the shear strength of the bolt. But I am not really worried about that...rather the shear strength of the Aluminium through which it passes.... have a look at the photo's below...That bolt will prob never break (even at 12g??)... but your whole life rests on the integrity of the 30mm (approx) of aluminium tubing above it. The Alunimium will shear long before the bolt does....thus my post regarding the wear and tear on the pylon bolt hole.

Hi Alan,

Firstly, we need to be sure we are talking about the right BOLT . . . . the Hang-Bolt, is the TOP bolt, not the one you are referring to with the "elongated hole".

With that said, it brings into question the concept of "joint efficiency". A perfectly designed joint, in this case the pylon-to-keel joint, will be designed in such a way using materials and thicknesses of materials so that ALL components fail at precisely the same time, under the design load (load x load factor)!!

Of course this is never the case due to using components that are largely "off the shelf". So back to YOUR question around "will the aluminium tubing in the pylon fail first?"

I can almost guarantee that this would be the case!! Under what load, I wouldn't hazard a guess! Lab testing would need to be done to determine this.

The subtle, yet critical difference, is HOW the components fail. What I mean is this: A hang bolt, if it failed, would probably be the result of metal fatigue (repetitive loading over time) and would be catastrophic - BANG, and you find yourself hanging on your safety cable. Whereas the failure of the aluminium tubing would probably be more a gradual deformation before complete failure, something you could pick up through inspection, as you correctly highlight.

So, the take-aways are:
1. Replace your hang-bolt at regular (500 hour) intervals (remember: only x-ray will pick up metal fatique cracking!!)
2. Inspect your COMPLETE Hang-Block Assembly including ALL surrounding fittings, aluminium tubing, bolts, wing-nuts, safety-clips, and cables for damage or wear, especially deformation.

Safe Skies!

John

[Bundy]

Firstly, we need to be sure we are talking about the right BOLT . . . . the Hang-Bolt, is the TOP bolt, not the one you are referring to with the "elongated hole".
John

Gotcha, thanks J