Large Model Association Forum

After being into modelling all these year--you think you know it all and it wont happen to you !! well yesterday it did happen to me.
My retract man Alec who has ben building my Vulcan retracts,was due to come today to do the final fit of the noseleg---i had concerns about the size of air tank i was using,so i went out to the supermarket and bought a 3ltr pop bottle ---need i say more.Many years ago at Cosford,many of us will remember the bottle in the model parked in the sun [yes we did get warm sunny days back then]on the flightline exploding and demolishing the structure around it?---anyway i set up a test bench with valve and pipework on and connected to one of the retraccts to test.I started pumping up the system and disaster struck,i can remember standing there watching the gauge slowly rise and can remember seeing 8o lbs showing--after that i can remember a large explosion--and me on the floor with hand in blood and pain----the bottle had demolished,and the amazing force had blown the top fitting into the back of my hand and i must have fell onto the floor----after several hours once my hand had been patched up,i went back to look at the damage,its frightening to see the bottle and fittings,and the thought of this happening inside a model
Over the years i have had many models using pop bottles,and indeed my 18 year old Vulcan has two smaller ones installed and never been a problem---i can only presume the modern bottles are made much thinner---so please be warned---DONT use bottles for your retracts.Once i am" allowed" back into my workshop,i will look for more metal tanks--i had fitted several smaller ones in tandem,and will go back to this method with maybe an extra one etc,o no workshop for a few days,its put feet up and sit and sort out LMA paperwork for the forthcoming show season

Know what its like Dave,
I remember a concreted hole 5meters square and 3 meters deep that was the nitrogen test pit for a 3"-30 thousand psi valve when it burst we never found the concrete only a very big hole.
Hope you are back Vulcanising!!! soon.
Hugh.

Thats the nature of accidents- they can happen to any of us, any time. I have always worried about using pop bottles, after doing some work in a pop factory, and hearing bottles explode a dozen times a day. It sounds like a cross between a gunshot and an explosion. I bet your ears are still recovering, Dave!

Bob---you are dead right--the noise when unexpected was horrendous--DAVE

should have bought a 3ltr cider bottle, once you had drank the contents, you would not have felt a thing lucky booger boss, after our phone conversation last night, hope you didnt spill any of the body claret on the wifes carpet!!! you sounded--a little shocked, to say the least,

Don't watch anything exciting,,,,, and clap your hands

Have a good recovery.

Well let it be a lesson to all of us. Hope you will be OK pretty soon. I dont think the plastic bottles of today are as strong as they once were. I know if they are used for various things in the green house etc they are liable to crack they seem brittle.

Can you fly single stick?

All the best. Will send you a few sprouts

My sympathies to Dave and I commend his making this accident known for us to learn from. As an expert in hindsight wisdom and shutting stable doors may I offer the following comments.

There seems to be a move to bio-degradable plastics as I notice that things like the margerine containers I save for storing small items now become brittle and fall apart in quite a short time. Even so I would have thought that pressurised drinks bottles would have a decent shelf life for safety.

Coming from model engineering including some boilermaking, I recommend an hydraulic test of any pressure vessel, be it plastic or steel. It involves; filling the vessel with water, fitting a pressure gauge, and pumping up the pressure (with a WATER pump) to 1 1/2 time the working pressure and then holding it for 10 minutes. The point is that water is incompressable and if there is a fault then the pressure will simply disappear and there will be a puddle! Pump and gauge are available from model engineering sources such a Reeves but the local model engineering society will have a boiler tester who will have them and be happy to assist. I also recommend a pressure relief valve in the system to cope with over-enthusiastic pumping.

Since Zenoah's have that lovely power take off shaft in the tubular mounting how about a pumped system with just one air bottle sufficient for one gear-down operation if the engine stops. . . .

Chris

Here's a video (without the Hand!) 120psi this one blows at, but is only 2 litre. 3 litre will likely be weaker and blow at lower pressure.

http://www.youtube.com/watch?v=AERonjBB0Oc

But it could have been so much worse.
Many years ago and before i retired from the Worlds Favourite,i remember a terrifying episode when the undercarriage shop were stripping a Trident main gear assembly.
All that was left was the undoing and removal of the large gland nut that held the inner fescalised piston into the outer leg.There is a small pillar bolt with a central witness hole that had always to be undone to insure that the suspension medium ( Nitrogen at 1000psi ish) had been discharged to atmospere.In this instance that same witness hole had been blocked by baked on paint.
You can probably guess the rest.The piston flew out of the outer leg like a 9" shell from a gun and sadly hit a chap passing by the section taking his arm off at the elbow.
At the time i was running a RR Spey at T/0 power on test several hundred yds away and we heard the explosion from there.
A salutory lesson indeed.
Hope you make a quick and full recovery,
Best regards,
Alan

Did anyone see Mythbusters on Quest last night ?

That should have been enough to put anyone off using plastic bottles for air tanks. All bottles tested burst at below 120 psi. That is way below a safe level for a container for an air retract system.

Example
Working pressure say 100 psi. Safety factor 50% = 150 psi.....and then leave it out on a flightline in the sun for a few hours, pressure will increase further due to heating effect and you have a small bomb sitting there.

Food for thought ?

John

I seem to remember a discussion about this problem some years ago and one of our Dutch members said that plastic pop bottles manufactured in Holland had thicker walls than other manufactures so would withstand higher pressures. Maybe that was then because it was quite a while ago but could be worth checking. Just as a matter of interest I have tyre pressure monitors on my car, the cold pressure is set at 32psi but on on a warm summer day the heat from the sun can show an increase in pressure to 38psi. I'm sure the inside of a model can get much hotter than what my tyres are subjected to.

John Greenfield wrote:Did anyone see Mythbusters on Quest last night ?

That should have been enough to put anyone off using plastic bottles for air tanks. All bottles tested burst at below 120 psi. That is way below a safe level for a container for an air retract system.

Example
Working pressure say 100 psi. Safety factor 50% = 150 psi.....and then leave it out on a flightline in the sun for a few hours, pressure will increase further due to heating effect and you have a small **** sitting there.

Food for thought ?

John

I saw that too John. The bigger the bottle the lower the bursting pressure too. That WaterCooler bottle was impressive at 80psi

Unitract bottles were tested to something like 200 psi and just went white. I've used nothing else in twenty years. They are small bottles though, I use two linked and have never felt the need to add more than 70 psi. Never any issues in all that time.

The Comet has a 'European' coke bottle.....very thick walled and has been tested to 200 and odd psi when the compressor gave up! Still, it is nearly 10 years old now.............

When I stripped out my old 1/3 scale Bucker Jungmeister to refurbish it, it had a home made smoke system in it, the air tank consisted of old large car spray can that had a tyre type valve brazed into the top of of it, now that's what I call good old fashioned modeling.

As Chris stated, pressure vessels should always be proof and ultimate tested hydraulically as the structure will rupture and leak where as pnuematically tested items will fail explosively.
As Steve has mentioned his Comet it is perhaps relevant to mention the fullsize aircraft?

Following the initial problems with the Comet 1, the fuselage was pressurized as part of the fatigue test cycle, which also applied simulated flight loads. Pressurisation was done hydraulically for this very reason and, in this case, the working pressure was only 7 or 8 psi (ie ~ 1/2 atmospheric pressure). Because the volume of the fuselage was so great (and water weighs 1kg / litre) the aircraft had to be immersed in a tank in order to prevent it being damaged by the weight of water held within. Therefore two large (aeroplane size!) tanks were built, one at Farnborough and one at Hatfield for the purpose of conducting these tests.

http://www.flightglobal.com/pdfarchive/ ... 01835.html

Many's the night I've spent running the fatigue test, albeit on a different aeroplane. Watching the sunrise over a misty Hatfield airfield from the top of the test rig (which was considerably higher than the tank itself) was somewhat surreal! Such a shame that Hatfield airfield, along with countless others, are but a memory.

By the way, those of you who have not discoverd the Flight Global archive should be warned - you can lose many hours wandering around in there!

Alan

Alan, that is really interesting. Must be the biggest purpose built pressure test tank ever! I used to work in Petrochems and we had to do pressure tests on the large vessels on site after we did repairs on them. Sometimes we also had to add extra piled foundations because the weight of water in big vessels was huge.....up to 3000tes in one we did!!!