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  #1  
Old 08-11-2019, 08:57 PM
WingnutWick's Avatar
WingnutWick WingnutWick is offline
 
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Location: Fresno, CA (KFCH)
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Default RV-6A Brake Fire - What is salvageable?

Well,

The girlfriend’s RV-6A continues to give me headaches as I slowly learn more about this plane. I recently just finished doing her annual following a new paint job. New brake rotors, pads, and O-rings. I also had to remove the piston from the pedal do get the valve open to allow the fluid to come up through the bottom. I finally finished this (along with a lot of other things) so her plane would be all good to go before I left for a couple months. Did a few flights uneventfully. Then on the last flight before I left, naturally, as we were pulling off the runway she said the right brake felt weird. Sure enough it seemed locked up. I tried pumping the pedal to see if I could free it, which it did but then subsequently seemed to start dragging again. Shortly after that smoke was coming up and we jumped out.

Sure enough the right brake had caught fire. Fire was going inside the wheel pant for about a minute before we could get an extinguisher on it. Newly painted pant - toast. Smoke had traveled up the leg fairing which blackened the leg and wing root with soot but I believe (hope) that it’s limited to just that vice any real heat damage to the leg itself or wing root.

Brake line at the brake assembly burned through, and it looks like some burning of the fiberglass/carbon fiber around the wheel leg. See pics.

I am looking to get some guidance and opinions on what is salvageable from this fire and what to look for to make that determination. Also any ideas what caused the locking? When I put the pistons back on, I tightened them....can this cause binding?

What kind of temps can these wheel and brake parts withstand? Also, I am unfamiliar with the construction of the leg. Is this carbon fiber surrounding the metal leg? What is that for? Is the burn damage at the base ok? Also, unrelated, it looks like there is a crack at the top (red arrow in pic) that we noticed in the carbon fiber material. Is this an issue?

Finally, wheel pants, what’s the process to make new ones after you order them from Vans? A royal pain or pretty easy?

Her original post was here: http://www.vansairforce.com/communit...d.php?t=173877

Any and all help is greatly appreciated!
Thank you, cheers!

Charles

















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Last edited by WingnutWick : 08-11-2019 at 09:00 PM.
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  #2  
Old 08-11-2019, 10:25 PM
SPX SPX is offline
 
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Someone will be along shortly to answer your questions, no doubt.

In the meantime, good to see that RV with the wood prop removed. It?s nice to see that the plane went to a home that can give it the TLC that it deserves.
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  #3  
Old 08-12-2019, 12:02 AM
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Is that a crack in the gear leg, or just the powdercoating? I would definitely disassemble the gear leg and remove/inspect it. New powdercoating should be applied to alleviate any issues with future rusting of the gear leg.

The brake line looks to be coming into the bottom of the caliper. I think that it should be coming in to the top of the caliper, and the bottom should have the bleeder valve. At least that is what the RV-9A plans show.

You'll probably need a new tire, wheel pant, gear leg fairings and brake line at a minimum. The wheel pant might be salvageable. Hard to tell from the photos. You might be able to salvage some of the hardware involved, like nutplates and hinges. Definitely will need to rebuild the calipers/pads, and please check on doing the brake pedal springs and long bolt modification for the pedals. It makes the brake pedals work much easier.

Making a new wheel pant should be an easy job for an experienced builder. The fiberglass work can be a pain, just because it seems like an endless process of waiting for epoxy to harden, then sanding/filling repeatedly. Actual time is not that long.

Once you get the gear leg/brakes back in airworthy shape, you can just take the other wheel pant off and fly pants-less while you get the new one fabricated and ready for installation.
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  #4  
Old 08-12-2019, 12:46 AM
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Tdeman Tdeman is offline
 
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Default A couple things...

The cloth you see wrapped around the bottom of the axle is likely (burnt and blackened) fiberglass. Many early RV builders built the "gear leg stiffeners" from wood and fiberglass to try to alleviate shimmy. Later on, the practice has been to carefully balance tires and let a few PSI out rather than glass on the stiffeners.

With that in mind, I'd suspect the large crack you see right at the gear leg socket, is just a crack in the epoxy/fiberglass of the top end of the stiffeners. You could try picking at it a bit with something like a flat head screwdriver or a pick/scribe. If it comes off in flakes/chips, it's most likely just epoxy thats cracked.

Considering the mechanics of that joint, any attempt to re-epoxy from the gear leg to the socket, will almost certainly crack again (flexible gear leg, rigid socket, and a brittle material adhered to each). If the other side was glassed identically, I'm sure it will have identical cracks. Probably not reason for concern, but have a closer look to be sure. I'd clean up the area, smooth out the cracking with some sandpaper, and apply paint to any raw steel you can see.

Regarding heat damage to the wing root: Is that burnt paint, or just soot from the burning fairing bits? (Can it be wiped off to reveal paint underneath it?) I suspect and hope it's just soot. If there was real heat in that area, those fuel lines and wing tank could have made a bad day, a disastrous one. Check for any heat damage to the inboard wing tank rib and the proseal that seals it.
If the soot cleans off, leaving behind undamaged paint, you probably don't have any re-heat-treated aluminum to worry about there.

You might have some salvagable hardware from the wheel area... But I'd spend the extra 10 bucks for new nutplates, fasteners, and hinges.

Good luck with the repairs! Fly the thing naked for a while if you have to (the airplane that is), plenty of RV's running around without their pants.
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  #5  
Old 08-12-2019, 06:56 AM
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Steve Melton Steve Melton is online now
 
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Quote:
Originally Posted by bruceh View Post
Is that a crack in the gear leg, or just the powdercoating? I would definitely disassemble the gear leg and remove/inspect it. New powdercoating should be applied to alleviate any issues with future rusting of the gear leg.

The brake line looks to be coming into the bottom of the caliper. I think that it should be coming in to the top of the caliper, and the bottom should have the bleeder valve. At least that is what the RV-9A plans show.
difficult to flush those lines with the bleeder on top
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  #6  
Old 08-12-2019, 12:45 PM
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WingnutWick WingnutWick is offline
 
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Quote:
Originally Posted by Steve Melton View Post
difficult to flush those lines with the bleeder on top
Yeah, I noticed that when we first got the plane. Is it simply a matter of swapping the two brakes? Or even easier, swapping the position on the valve on the assembly?

What?s the recommended way to fix the brake line? Splice it high up with new line? What?s the process and hardware needed for this?

Thank you!
Charles
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  #7  
Old 08-12-2019, 07:12 PM
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bruceh bruceh is offline
 
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Aluminum tubing is cheap, so no need to splice it. I would buy a new coil of tubing from Van's or your local industrial hardware store.
You can simply unscrew the bleeder valve and the 90 degree elbow from the caliper body and swap them. You'll need to have access to a tubing cutter and an aviation flaring tool to make the ends of the tubing fit onto the AN fittings.
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  #8  
Old 08-12-2019, 10:24 PM
F1R F1R is offline
 
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Consider using braided SS over teflon all the way from the calipers to the master cylinders with the external return springs as posted a few weeks past. You can make your own or get TS flightlines to supply professional made lines.

Viton Orings in your caliper pistons and Royco 782 or Aeroshell fluid 31 will raise operating temp limits to over 400F.

Somebody recently posted about installing thermocouples on their brake calipers. Several EFIS and engine monitors have extra channels available that would allow you to monitor brake temps.

Thicker discs have more heat capacity and will not heat as rapidly. I am not sure what make your calipers and brakes are but Bruce hill posted his retro upgrade to thicker discs with his Matco brakes.
http://www.vansairforce.com/communit...d.php?t=172644
Thicker discs provide more braking ability with less fade as they do not heat up as rapidly.

Installing external return springs on your master cylinders at the foot pedals should prevent almost all dragging brake issues.
From Tom in Australia:

Quote:
Originally Posted by tgmillso View Post
To answer the OP's question in post #10, I'm not sure how similar the RV-r brake pedals are to the RV-7, but it is likely you need to do the "long bolt" mod that puts the brake pedals on a common axis, then also the "spring mod" which is described below to stop binding if you have any. Unfortunately the pictures have evaporated because the hosting site I was using has disappeared, but I'll dig some more up and repost them. I've run this mod now for about 200 landings and they work great and feel they way brakes should. I am with the other guys regarding run-in of new pads being unnecessary. So long as your disks are clean and you've made sure they adequately stop the aircraft, it is really a non issue.:


"After a recent post in another thread from someone asking about brake return spring mods, I thought I should finally update this thread with my brake modifications. I assembled my Matco master pistons with the springs recommended in this thread and there was only a barely perceptible increase in return force. I subsequently took some measurements of the spring force in the matco's, ran a few calcs, and came up with an alternative spring solution that will double the current spring force. The original spring upgrade was a maximum of 6lb in compression. This one is 30lb. I've tested them out over the past week during my taxi tests and they feel perfect, with no risk of dragging brake, even if your toes are naturally leaning on the pedals, but not too much force that they become an incomberance when you want the brakes. I also modified the collar sleeve arrangement so that even at full brake compression, there is no way that the spring can rub on the ram. The sliding collar is also a self lubricating polymer, which should help with longevity. Just make sure when you assemble things that the collar is low enough to just clear the top mounting tab, but not so low that you restrict compression of the ram. My aircraft is an RV-7, and I would absolutely recommend the long-bolt (single axis) mod to the brake pedal to eliminate binding, in addition to the spring upgrade. You'll also see in the photos some another mod I did to make some standoffs. These are just aluminum split collars and a piece of tubing cut into thirds, then riveted in place with some long AN470AD4 rivets (the collars are countersunk on the inside and I back riveted it against an appropriate diameter bolt shank). I've also stacked two more short pieces of the cut tubing in between to give myself a little further distance.
Anyway the McMaster Carr parts list is as follows.
Spring 9657K435
Lower nylon sleeve bearing: 6294K441
washers (for seating the lower nylon sleeve bearing) 90295A470
Top nylon sleeve bearing: 6389K625
Set screw shaft collar: 9946K13

For the standoffs:
Two piece collar: 6436K136
Aluminum tubing: 9056K79"

Hope this helps.
Tom.
RV-7

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Last edited by F1R : 08-13-2019 at 10:39 AM.
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  #9  
Old 08-13-2019, 09:52 AM
rapid_ascent rapid_ascent is offline
 
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I was curious what type of brake fluid you were using?
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  #10  
Old 08-14-2019, 11:15 AM
F1R F1R is offline
 
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Default Perhaps worth a closer study

By your own description, " I also had to remove the piston from the pedal do get the valve open to allow the fluid to come up through the bottom." (when bleeding the brakes)

Normally, when a person takes their foot of the brake, the fluid should be free to flow back into the master cylinder and reservoir. There should only be pressure and checked flow when you press on the brakes. With feet off the pedals, there should be no pressure possible, other than the 0.4psi per foot of static head elevation from the reservoirs to the brake cylinders at the wheels.

Likewise when bleeding from the brake / wheel cylinder/ bleed valve back up through the system, logically the fluid should be free to flow to the master cylinder reservoirs with no need to take anything apart.

There have been various installations of master cylinders (upright and inverted) and different pedal arrangements, so there might be exceptions.

Perhaps check with the manufacturer of the master cylinders and their instructions ( And / or send them a photo ) to verify you have a normal installation, operation and bleeding procedures.- in their opinion

If the pedal pivots are binding or if the piston shafts are not fully extending it might prevent the master cylinders check valves from opening and preventing normal reverse flow. If that were the case it might explain the dragging brake and excessively hot disc.

Last edited by F1R : 08-14-2019 at 11:35 AM.
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