BVM Smooth stop.
 

I am currently carrying out deep winter maintenence on the fleet ready for 2014 flying season . I currently have 6 Smooth stops, all some years old. 3 work great, (Phantom, Sabre, BobCat) always have. 1 has been intermittent, (original BobCat) fully serviceable now after O ring change and lube.

2 others (spares) have me beaten. Despite O ring changes and lube and set up exactly as per instructions, they STILL leak badly. I have even tried changing the piston assembly to the latest, long, type but still they leak. These things are hardly complex devices so can anyone shed any light on the problem ?

I am not the only one in the UK with this problem, several flying colleagues have ditched them, being too unreliable.

Yes, I know there are two obvious answers (bin them, ship to BVM) but I would like to try and fix the problem first, and perhaps help my colleagues who have given up with them !

Anyone know the secrets ?

David Gladwin.

Hello Dave,
Just send them to company maintenance for a periodic inspection! Like the "good old days"!!

Skeet

Purely as a guess, it could be a minor and nearly invisible imperfection in the cylinder. Look for a brake cylinder hone that's small enough. If you find one its an easy procedure to hook that up to a drill an make a few slow and well-oiled passes back and forth in the cylimpnder to clear any burrs.

From the BVM website ........ Notice about Smooth Stop Leaks

Hi David

I had one do the same thing after it was lying up for a bit changed the O rings and lubed them still leaking
Next on to plan B took it all apart and honed out the cylinder very lightly reassembled it all and all is working well no leaks

Andy

Hi David,
I guess I know the Smooth Stop secrets better than anyone since I designed the thing.

Some time ago the supplier of the brass tube that forms the cylinder changed the spec on the tubing without telling their customers. It would appear that the effect of the change left the tube with a larger ID, so the O-rings don’t seal as they should. It may be that you have a couple of these. BVM updated the design when they found out about this. BVM has always offered a free repair for this, which I know is a bigger pain for your location.

These can be fixed, but it is a pain, I did one for a friend once. The only way I could figure out to fix it was to wrap some fine copper wire in the bottom of the glands and add little silver solder. Then, using a lathe, re-cut the glands so that the O-rings are compressed very slightly when inserted back into the brass tube cylinder. Unfortunately the fix requires changing things by a few thousands of an inch, so just changing something out probably will not help. I am sure that there are other ways to approach this. Of course this might not be the cause of your problem.
Steven

The best fix for a BVM smooth stop?

Get rid of it.

David

I can't help with repair as I have never tried. But does anyone really use proportional brakes or do we just switch them on or push on a slider until they are are full on? I don't think I know anyone who uses them as proper proportional brakes . I always use either a jetronic single valve or as in the Bobcat a Clippard button valve on the retract servo with a mix to take it over centre.

Jason

David, you can do it yourself but need the proper size brass tube. Be prepared to pitch it if it doesnt work. If you pull it apart you will see the brass tube pressed fit into an aluminum plug. Heat to pull that brass tube out and save for the hole location you will have to drill later. clean, and look for debris. look to see if that brass tube was deformed. There is an oring in there also that if you are lucky can reuse if you didnt use too much heat. press fit back together with new tube, reassemble, silicon grease and see if it works. I also channelized the action on mine since it was a very old version.

I use proportional brakes in 2 ways:

1) in short fields, apply 50% of brakes before landing
2) I put my brakes on a momentary switch and use servo slow. So on the ground, when you hit the swith, the brake comes in gradually, looking a lot more scale, and you have the chance to let go of the switch before the brake fully locks

On my mig, with the trailink link type of gear, if you hit a normal non proportional brake, the gear compresses and looks really bad (it works, but looks toyish)

so yes, I do see a use to proportional brakes... but it is not about being able to brake a little, is about the way the brakes engage... makes sense?

+1

David,
Have you ever tried the Festo pressure regulator. JP introduced me to this great little item. Its a minature demand valve and therefore you can have a tank with air at 100 psi but the actual air pressure to the brake is constant adjusted by trial and error but very accurately. The pressure does not change even though the air tank is reducing in volume ( until it reaches the brake pressure of course). Simply run the plane out and try the brake. Adjust as necessary and its always as you set it thereafter.

Part no. pressure regulator is 267-3379 price £13.91 plus VAT Festo / RS components stock it

Regards
David

David,
here is what it looks like.

Same for me Jack...works great..!

The only other thing I do is add some brake mixing with down elevator. As I taxi in, just some down elevator and mild break is applied.

Thanks for the feedback guys. What I found curious is that 4 of my brake units work fine (They are still holding air today after filling yesterday and don't seem to leak with brake applications) but I just can't fix two of them.
Steve's hint that larger id brass tubing is the culprit is the most plausible as the leaky units show very little resistance when the piston is slowly pulled out. Both units hold air when freshly lubed but soon start leaking after a few cycles.

They will go back to BVM who, I am sure, will sort them.

Jason, I do like proportional braking, saves tyre wear BUT I do like the chance to stands on the brakes if need be ! The Smooth stop when working normally does a very good job of this.

David, never used this valve on brakes , but Wren use it as gas regulator, nice device as used on my SuperSport and XL 200.

Other brake units I use are PST Proportional, Jet Tronics, and Ultra Precision all work fine and have proven 100% reliable.

Thanks again.

David