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Tight brakes

Old 07-24-2018, 06:09 PM
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roydefiant
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Default Tight brakes

I have an FB jet with pneumatic brakes. Getting ready to maiden and applied brakes. They seemed to only partially release. Checked pressure at wheel and with brake off is 0 psi but wheel friction still high. I assume I need to lube up the brake.

What is the best lubricant to use for this purpose?
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Old 07-24-2018, 07:45 PM
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Never had that problem but if I had it I would think they will get loose after the first landing/use, the O rings may be a bit tight as you mention, I wouldn't use lube at this time.
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Old 07-25-2018, 05:05 AM
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Steve Collins
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I have been having this exact same problem with my new Fei Bao Dolphin and it has been exasperating trying to find out why it is happening and trying to remedy the problem!

When I received the jet last fall I noticed that the wheels could barely be turned because the brakes were so tight. This was before air pressure was ever introduced into the equation. I removed the wheels and used a sanding stick to sand a "flat" on the outer diameter of the "O" rings. The wheels went back on nicely and were loose enough that they would nearly freewheel when spun by hand.

Fast forward to Memorial Day 2018. I take the plane to the field for its maiden flight only to find that the brakes are once again stiffened from what they were when I had worked on them in the fall. Decided to go ahead and try a takeoff which, while it used a lot more runway than normal, was successful. A second flight was also successful on the takeoff. So, I am good to go for the first jet fly 10 days later, right? WRONG!!! I get all the way to Nebraska only to discover that the brakes are now tighter than ever!!! Silly me, I thought that since I now have a 5,000 foot runway I can still fly. NOT! Attempted a takeoff but the plane could not reach liftoff speed.

By now I am ready to take a hammer to it!

I took the wings over to a friends house when I got home to do some diagnostic to try to figure out what is going on. Just for grins I take one of the wheels off and seeing nothing obvious reassemble the wheel and brake hub. Now the darn thing is freewheeling like nothing was ever wrong!! Did the same to the other wheel with the same result.

A couple of days later I take the plane out and assemble it to see if the brakes are now OK as the last result would seem to indicate. As soon as brakes were applied to stop the plane, the stiffness had returned. Where's that durned hammer?!?!?!

A couple of days later we are again trying to assess what the problem might be and ended up ordering new "O" rings of the same size. Installed the new "O" rings but got the same result of stiffness. So, at this point we are wondering if the original brake "O" ring is the wrong size since it doesn't seem to return to the channel on the brake hub when the air pressure is released. The original "O" ring was found to have a 45mm inside diameter while the diameter of the channel in the brake hub was much less. An "O" ring with a 40mm inside diameter was tried next with the reasoning that there would be more elasticity pulling the "O" ring back into the channel when brakes were released. Brakes were still a bit stiff after air pressure was applied but seemed to be somewhat better.

So, fast-forward to last weekend and another opportunity to fly from a fullscale runway. The damn brakes have stiffened even more. Disassembled the wheels and used a thin lube of silicone oil which seemed to free up the brakes enough to attempt a takeoff which was ultimately successful but with the smaller "O" ring, there is less braking action to the point where the wheels will not lock up even with full braking pressure.

Why am I having such a difficult time trying to get this to work properly? Just returned from Kentucky Jets a couple of weeks ago where there were many Dolphins flying without any brake issues at all!

What the hell is going on with my brakes? I can only conclude that the original brake "O" ring supplied with the plane is the wrong size or that the machining on the brake hub is off as in too narrow by a small amount which is not allowing the "O" ring to seat properly when the brake air pressure is released.

Has anyone else been faced with this problem and been able to solve it?
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Old 07-25-2018, 07:21 AM
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I have a similar problem with my SM F-16.
While the wheels don't lock up, when you release the brakes the wheels aren't free but drag.
SM doesn't use O rings but discs and a rotor, somewhat similar to the full size setup.I tried everything I could think of including lube, oiling the wheel bearing, constant cleaning, etc but could never solve the problem, just learned to live with it.
At least I can takeoff but after a long run.

Last edited by F106A; 07-25-2018 at 07:30 AM.
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Old 07-25-2018, 08:04 AM
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Disc and rotor you need to set the axle position with the brakes pressurised, the disc moves out a fraction when pressurised.
The others I guess there could be a tolerance issue on the wheels, or if more than one FB model the brake o ring has been produced with a fractionally thicker wall. Skimming the wheels on a lathe would be the only solution...BUT never try and run in tight brakes, heat just makes it worse and one wheel jamming you risk a turn off the runway you can’t control, or worse into the pilot box.
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Old 07-25-2018, 11:24 AM
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Lube, lube, lube!

BVM's O-ring lube would be a good first try.
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Old 07-25-2018, 11:50 AM
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Steve Collins
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BVMs "O" ring lube is bad advice since that is meant to restore the seal on rings in air cylinders by causing them to swell which is a problem not needed.
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Old 07-25-2018, 12:05 PM
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This is what you want to use:

Super-O-Lube (for brakes) [BVM5779] - $9.50 : ShopBVMJets.com, your one stop jet shop!
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Old 07-25-2018, 12:10 PM
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Originally Posted by Steve Collins View Post
BVMs "O" ring lube is bad advice since that is meant to restore the seal on rings in air cylinders by causing them to swell which is a problem not needed.
Bob's o-ring lube, helps my brakes be "looser", so, in practice, your theory doesn't quite mirror what I, and many others, have experienced.

Last edited by ravill; 07-25-2018 at 12:21 PM.
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Old 07-25-2018, 02:03 PM
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Steve Collins
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Trust me when I say this:

Lubing was the first thing tried to remedy this situation. I have used many different lubes including the ones that have been mentioned already and also very thin silicone oil etc., etc. Lubing is not the answer as it has been tried yet the problem persists.

Surely Roy and I can't be the only ones with Fei Bao jets that have an issue with the brakes. Would love to hear from someone who solved their issue with tight brakes on Fei Bao equipment.
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Old 07-25-2018, 02:26 PM
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A friend of mine has a their Panther he Removed one of the two O-rings and sealed the air port with epoxy. He also switched to viton O-rings. I can ask him for the part number
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Old 07-25-2018, 02:31 PM
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Steve:
i had a Fei Boa Hawker Hunter a couple years back. Great flying airplane but had problems like yours with the brakes. First the rep sent me some smaller o rings which kinda helped. But they would not release each time the same. What I did was to sand the oring so the cross section was “D” shaped and the flat side was riding on the wheel hub. It worked well but if I took the oring off it would never go on the same way and twist so that the flat was not riding on the inside of the wheel.
Someone told me you could by “D” rings. Never looked into it but not sure if you could get the ring in the slot without twisting it.

my 2 cents
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Old 07-25-2018, 03:38 PM
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On the SM brakes, the order of discs is important. Something to check(see s/m 1/6 f16 in my hot little hands!). And of course most brakes need silicone grease for lube.
That's the case for most except the Dreamworks/Intairco ones for the Flash which need little if any silicone grease.

Last edited by dbsonic; 07-25-2018 at 03:41 PM.
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