Newbie with a question

[sersplot]

Hi Will,

I am in Rochdale and currently building myself so it would be good to put heads together, especially if you need help.

[bigsi]

How about pneumatics to cock the spring? Have plans for something similar...

Your pneumatics have to develop more force than the spring to be able to compress it - so there must be an argument for just using the pneumatics and not bothering with the spring!

Si

[Liftoff]

Your pneumatics have to develop more force than the spring to be able to compress it - so there must be an argument for just using the pneumatics and not bothering with the spring!

Si

Thats right, agreed. The main argument for the spring rather than pneumatics is the simpicity of the air system required to close the spring when compared to the air system required as a flipper. For an effective flipper using LP pneumatics you would require a buffer tank, large bore fittings and qev etc, basically you need a system which is optimised for flow and that takes up space.

On the other hand, if you use a spring, you get instantaneous power without the related problems of flow etc on pneumatic systems. The pneumatics to close the spring will not be effected by port sizes and flow etc because it does not matter if it takes a couple of seconds to cock the spring.....I think a set up with springs takes up less space and can possibly be more powerful...that said, it is not straight forward to build as it's not something I've seen built, so there's not much help around or build projects from where to take ideas.
I have a CAD design of one, which I'd like to build at some point...but I need to find time for it!

[bigsi]

Let me share an alternative with you. This arrangement is known by several names, including a 'cannon valve' as they are often used in Potato Cannons!

http://www.fightingrobots.co.uk/images/attach/jpg.gif
It's made from two concentric tubes. The outer forms the reservoir and the inner a barrel.

To charge, you push air through the hole shown. The pressure moves a sliding piston to the right. The piston has a thin diaphragm (thick rubber works well) bolted to the right hand face. This forms a seal with the end of the barrel but can flex under pressure allowing air into the reservoir.
http://www.fightingrobots.co.uk/images/attach/jpg.gif

Once it's charged, you release the pressure in the space to the left of the sliding piston (3 port pneumatic valve with exhaust). The pressure on the reservoir side of the piston is higher than on the charge side - so the piston is shot to the left.
http://www.fightingrobots.co.uk/images/attach/jpg.gif

Once the piston reaches the end of it's travel, the diaphragm seals - and the only place the air can go is into the barrel pushing the plunger (or potato!) out.
http://www.fightingrobots.co.uk/images/attach/jpg.gif

This gives you a valve with the same port size as the bore of the barrel which opens incredibly fast.

I've used this to fire a potato over 500m at 8 Bar pressure - but I've also used it in the configuration above to move a load attached to the piston rod very fast (known as a 'Jerk Rig').

They are very easy to make, even out of drain pipe (so long as the pressure is not too high). You need to calculate the volume of the reservoir to be about 25% greater than the volume of the barrel for the best power / size ratio.

Also, the sliding piston only needs to move 15mm or so - I drew it bigger to make the diagrams clearer.

Si

[bigsi]

Let me share an alternative with you. This arrangement is known by several names, including a 'cannon valve' as they are often used in Potato Cannons!


It's made from two concentric tubes. The outer forms the reservoir and the inner a barrel.

To charge, you push air through the hole shown. The pressure moves a sliding piston to the right. The piston has a thin diaphragm (thick rubber works well) bolted to the right hand face. This forms a seal with the end of the barrel but can flex under pressure allowing air into the reservoir.


Once it's charged, you release the pressure in the space to the left of the sliding piston (3 port pneumatic valve with exhaust). The pressure on the reservoir side of the piston is higher than on the charge side - so the piston is shot to the left.


Once the piston reaches the end of it's travel, the diaphragm seals - and the only place the air can go is into the barrel pushing the plunger (or potato!) out.


This gives you a valve with the same port size as the bore of the barrel which opens incredibly fast.

I've used this to fire a potato over 500m at 8 Bar pressure - but I've also used it in the configuration above to move a load attached to the piston rod very fast (known as a 'Jerk Rig').

They are very easy to make, even out of drain pipe (so long as the pressure is not too high). You need to calculate the volume of the reservoir to be about 25% greater than the volume of the barrel for the best power / size ratio.

Also, the sliding piston only needs to move 15mm or so - I drew it bigger to make the diagrams clearer.

At school (40 years ago!) I made one out of a Bic Biro as the barrel and a piece of plastic tube as the reservoir. I used a syringe and two non return valves to pump it up. It fired bits of tissue paper with enough speed to really hurt over the width of a classroom! It nearly ended my time at school prematurely!!! - don't try this at home (school) kids!

Si

[Liftoff]

That's a buffer ram, spent some time toying with that idea, and whilst I agree with you it that you avoid all valvetry and flow problems, to design and machine that ram into a size which will fit inside a featherweight will be difficult. For a heavyweight it would probably be a good option but it's been mentioned that the velocity of the piston will be so high that there is a high possibility of blowing the cylinder top cap off...and that sort of put me off.
But I like the concept.

[maddox10]

I made one of those rams, feather size.

After a few dry fires the endcap was getting new and interesting shapes. But non that inspired confidence in the survivability of the setup in RW.

Also, the weight was higher for the same amount of power as the classic Cuttlet setup.

[Will]

The latent heat of vaporisation of CO2 is 574kJ/kg the energy density of a typical LiPo is about 500kJ/kg - so not much to call it.
Unfortunately you need a container for the CO2, which for the relatively small amounts involved drives the density right down.

Not impossible - but probably not an advantage.

Si

I was thinking more along the lines of maximum power output, electric motors tend to be pretty poor for mass vs peak power. it would be relatively simple to spin something up using a jet of co2 onto an fan/turbine.

Hi Will,

I am in Rochdale and currently building myself so it would be good to put heads together, especially if you need help.

Sounds good 😀
As I said I've got a lot of other things still to finish off before i start in earnest but I'm happy to help out if I'm free.

[Jakebowman420]

Hi guys really sorry to bother you with this! I'm a new member as well and it wont let me post a new thread? My account has been activated.. Any help much appreciated. Will delete post when I work this out cheers

[Will]

My other projects are nearly completed now. Missed the boat for applying to RW this year but I see they have introduced a featherweight category now. So that will be my target to build something in that size range. I'll aim for sub 10kg which gives plenty of space for finding it doesn't have enough armour/battery/rigidity.

Im happy to gang up with someone else who has better ideas for me or with some experience of build these. I've got good electrical knowledge and general engineering skills (just rebuilt an MG with an alternative engine and got it MOT'd) but almost zero experience in the radio controlled world.