More on -A brake failures

I was reading the 'Not necessarily life threatening but could ruin your day' thread and its a good one. GDU000 relates what happens when a brake fails on a Cirrus. On the -A RVs, it can be catastrophic, as a friend of mine learned the hard way. Ironflight wondered what's different about the RV brake system so I thought I would relate what we found out after the right brake failed suddenly during taxi. He pressed both brakes to slow down. One worked, the other did not with the subsequent loss of control. While there was damage to the plane, he did a very good job of realizing the problem quickly, securing the engine and minimizing it. Good show there, but what happened?

There have been several instances of brake failures in the RVs reported on the boards, most without damage. My friend was not so lucky but that's beside the point. Picking up the pieces, we investigated the causes and here is what we found.

Looking at the parts:

The brake:
There was no real obvious reason that the brake failed, but all of the fluid ran past the single 'O' ring on the caliper piston, causing failure. The 'O' ring looked OK and was still pliable (and the puck was in the right way). There was evidence of the brake assembly getting hot (wrinkly label) but not too much different than the solid left one, which also had a hot looking label.

The pedals:
Seemed to be OK with no obvious friction in the linkage.

Braking action:
Seemed to be OK too but as we will see later, may not have been so good.

While waiting for new parts, we decided to look into why the caliper seal failed. Like many avaiation accidents, it summed up to be a combination of things.

First stop: Kinetic energy requirements for aircraft brakes and found this:

http://www.infinityaerospace.com/Brake_Kinetic_Energy_Req.htm

It discusses the FAR Part 23 requirements for how much kinetic energy (essentially how hot they get) that brakes should be able to absorb. If you crunch the numbers ( http://www.groveaircraft.com/braketechinfo.html for a calculator) you might be suprised at the required Ke that your brakes have to absorb to get you stopped.

The ratings on the standard -7A brakes are in the 110K to 120K ft-lb range. I looked it up but don't have it handy..

Result for MY plane: the standard brakes are about 1/2 of this figure. Take an aborted takeoff in Amarillo in the summer (>9K density altitude) and we are way short..

Now, I know that pilot technique enters into this. Minimum speed landings and all that. But, if my engine chatters on takeoff from Amarillo on a summer's day, I want to be able to hit the brakes and get results.

Second stop:
Looking further into the brake system, we were told by some brake guys, some selling us stuff but others, too, that the standard Matco master cylinders do not have enough spring pressure to overcome friction in the pedals et al. I was also told that the Matco piston has to be fully extended to relieve the pressure on the calipers and that, combined with the low spring pressure can cause the brakes to drag. The RVator had an article on this too I think, with slots being milled into the shafts to get some slop. My friend thought this might have been happening on the right side but it failed before he could look into it. (He thought all it would cost him would be an early pad replacement..). MORAL: As in all things aviation, fix it NOW.

For us: We decided on the Grove 56-1A wheel/brake. Bolt right up. Minor changes to pants. Look at numbers and decide for yourself. Mine look good in the box. Buddy is flying with his. Very smooth. Easy to modulate. Lock tires at will. I blew off Van's AirHawk tires after seeing buddy's. Worn. AvConsumer rated Goodyear Flight Custom II tops. Bought em. Look good on wheel. 1yr before flight to tell. Robbie Grove knows brakes.

Regards,
RJO

Matco master cylinder assemblies

Your note that the Matco's have a problem returning to their extended position rings pretty personal for me. About the only problem I've had with my 7A was a problem with intermittent draggy brakes. Since there is so little movement of the brake pedals when you use them, I didn't realize that's what was happening.
On takeoff a couple of weeks ago, I didn't realize I had a slightly draggy left brake. When I applied takeoff power and the airplane began the takeoff roll (straight down the runway, with no turning tendency), I had applied back pressure to the stick to take the weight off the nosewheel. At the same moment the nosegear lifted, the plane took a hard left turn toward the edge of the runway and became airborne. With the windsock coming straight at me, I banked (at a low angle!!) to avoid it and flew on (with my heart beating about 250). Not knowing if my brake was still draggy or locked, I flew around for awhile and looked the brake pedals over. They all looked to be normal (pistons fully extended on the Matco's). I then landed at MINIMUM airspeed and didn't touch the brakes at all. The plane rolled to a nice straight ahead stop. As soon as I began to taxi off the runway, the brakes started hanging up again.
Further investigation (checked pedal friction at the mounting bolts, checked pads, etc, etc, etc) proved that the Matco's didn't have enough SPRING to return themselves. This was impossible to verify on visual inspection, it took an assistant to push/pull the plane while I messed with the pedals. I installed a 3/8" ID compression spring and 2 washers on each of the Matco's. Since I did that, I've had zero symptoms of brake drag or brake lockup.

Booster Springs on Matcos

Interesting you said that, Bob. Part of the lost post described exactly that except we used a 3/8" shaft collar with setscrew on top so that nothing contacted the tab on the rudder bar.

Sounds like yours worked. Its working fine on my buddy's plane too. Cheap insurance.

Glad you recovered from the unexpected swerve. I wonder how many other similar incidents in the accident records were caused by brake issues but chalked up to pilot error or poor technique.

Other things lost in the post:

Parker Cleveland sells a high temperature version of the 'O' rings in the calipers. I don't know the P/N but it only costs a few cents more than the standard one.

I broke down and bought Grove's master cyls to go with the wheels and brakes. They don't suffer from the return problem and have screw shafts so that you can set your pedal rake as desired without re-drilling the pedals. Very nice stuff.

It was pointed out that the big Sensenich FP props generate considerable thrust, even at idle, compared to a CS causing more use of brakes on the ground to keep taxi speed down. Another buddy says his -8 taildragger with a CS prop will be stationary on the runway with no brakes applied at all. On the -7A with the fixed pitch, the airplane will accellerate at idle to 30-40mph, necessitating brakes to hold it down.

Taildraggers, especially those with CS props, are much easier on brakes than the -As since you steer with the tail wheel.

lucky333 said:

...and have screw shafts so that you can set your pedal rake as desired without re-drilling the pedals.

Click to expand...

Ooooh. That sounds nice. Can you provide the part numbers for master cylinders, brakes, and wheels you used? The Grove brakes truly used the same bolt holes on the leg bracket as the Cleavelands?

I've noticed the return spring problem and I'm not even flying yet. When I was tightening down the brake pedal screws/nuts, I was having a heck of a time finding a good balance between the brakes not feeling loose and the master cylinder returning to fully extended.

question

Bob Brown said:

I installed a 3/8" ID compression spring and 2 washers on each of the Matco's. Since I did that, I've had zero symptoms of brake drag or brake lockup.

Click to expand...

Bob:
Could you describe this fix in a little more detail? I am having a problem visualizing what you did. Your fix is within the master cylinders? Pictures would be great, if by chance you have some. thanks, Jack

Dave,

The master cyls are these. They made a modification to the bottom for me at no extra charge to match the Matcos. You'll have to order direct. I sent them some pix of the Matcos with the dimensions of the 1/2 round and hole location. They fit just fine.
http://www.groveaircraft.com/670-15.html

The wheels/brakes are 56-1A. Got them from Spruce. They go right on the axle and bolt up to the original bracket. My buddy had to make a little bulge on the inside of his wheel pant to clear the bleeder fitting.
http://www.groveaircraft.com/56-1a.html

As for the booster springs, I don't have a pic but it goes like this:

Take one of the Matcos to a hardware store.
Find a flat washer for a 3/8" bolt and slide it down the shaft to bottom out on the top edge of the cylinder.
Find a compression spring (one that you push on for spring action rather than pull) that will fit over the shaft.
Cap it off with another washer like Bob did or use a 3/8" shaft collar, basically a steel doughnut with a 3/8" hole through it to slide along a shaft and a set screw to lock it into place. Either the washer or collar will work fine but I like the latter since the assembly stays together by itself rather than having to hold the spring down while assembling the cyl. to the rudder bar. You also can get away with a shorter spring which may be easier to find. With the washer on top, the spring must be longer than the shaft length so it will be compressed when you assemble it.


Hope this helps.
John

Pictures!

Here is how we did the Matco booster spring.



The spring is a Hillman #181 (3 1/16" x 9/16" x .054) but others will work I'd imagine. At the bottom is a nylon 3/8" shaft bushing. The set screw shaft COLLAR holds it all together. Several RVers on the field are using this setup.

I went with the Grove master cylinder since I hadn't used my Matcos but the springs are definitely cheaper.



John

More on Brake failures

Take a look at the two small brake pedal pivot bolts, when force is applied to the brake pedal the two bolts have a tendency to cant and cause additional friction. The brake petal is not pivoting on one common axis.
This may prevent the cylinder from returning to the fully extended position and cause dragging brakes.
A solution to this is to replace the two pivot bolts with one long bolt insuring a common pivot axis and reducing the friction.
See the following for photo.
http://websites.expercraft.com/n51pw/index.php?q=log_entry&log_id=4964

DOH!

You know I actually ran a long drill through the pedal axis to line up a slightly off weldment tab. The pedal spun nicely on the drill. Then I put in the same old bolts. Good call!

dpansier said:

Take a look at the two small brake pedal pivot bolts, when force is applied to the brake pedal the two bolts have a tendency to cant and cause additional friction. The brake petal is not pivoting on one common axis.
This may prevent the cylinder from returning to the fully extended position and cause dragging brakes.
A solution to this is to replace the two pivot bolts with one long bolt insuring a common pivot axis and reducing the friction.
See the following for photo.
http://websites.expercraft.com/n51pw/index.php?q=log_entry&log_id=4964

Click to expand...

I am getting close to the point where I will be working on this section of the build. I like your solution with the one long bolt. I think I may see about doing that mod when I get to it.

Single Long Bolt

dpansier:

Love it. Copying it. Thanks.

Very nice. And much cheaper than $150 a pop cylinders. Thanks for that info John, but I have passenger side brakes. I just realized that replacing all the cylinders would be way too much. I'm going to try the long bolt, and then add the spring if needed.

dpansier,
Did you put a bushing in the middle, or is the brake pedal just riding free on the bolt? From the picture it looks like it's riding free. That really would enable it to spring back fully.

Bob Brown said:

Your note that the Matco's have a problem returning to their extended position rings pretty personal for me. About the only problem I've had with my 7A was a problem with intermittent draggy brakes. Since there is so little movement of the brake pedals when you use them, I didn't realize that's what was happening.
On takeoff a couple of weeks ago, I didn't realize I had a slightly draggy left brake. When I applied takeoff power and the airplane began the takeoff roll (straight down the runway, with no turning tendency), I had applied back pressure to the stick to take the weight off the nosewheel. At the same moment the nosegear lifted, the plane took a hard left turn toward the edge of the runway and became airborne. With the windsock coming straight at me, I banked (at a low angle!!) to avoid it and flew on (with my heart beating about 250). Not knowing if my brake was still draggy or locked, I flew around for awhile and looked the brake pedals over. They all looked to be normal (pistons fully extended on the Matco's). I then landed at MINIMUM airspeed and didn't touch the brakes at all. The plane rolled to a nice straight ahead stop. As soon as I began to taxi off the runway, the brakes started hanging up again.
Further investigation (checked pedal friction at the mounting bolts, checked pads, etc, etc, etc) proved that the Matco's didn't have enough SPRING to return themselves. This was impossible to verify on visual inspection, it took an assistant to push/pull the plane while I messed with the pedals. I installed a 3/8" ID compression spring and 2 washers on each of the Matco's. Since I did that, I've had zero symptoms of brake drag or brake lockup.

Click to expand...

Same thing happend to me right at take off. I felt like I hit something with the left wheel(Big thump ). Stayed in the pattern and did a full stall landing hopping the left brake wasn't locked up.
I just did my first annual and had to replace both brake shoes at only 60 hrs.
Has anyone contacted Van's about this yet?

Chuck Stuhrenberg
N180WC
RV-7A

(dpansier,
Did you put a bushing in the middle, or is the brake pedal just riding free on the bolt? From the picture it looks like it's riding free. That really would enable it to spring back fully.)

Davepar
The brake pedal rides free on the shaft without a center bushing; I used thin AN washers between the alum angle and the steel welded assembly to provide the maximum clearance. The Matco cylinder snaps back to its fully extended position when released if the pedal is pivoting on one common axis. (And of course if the common pivot bolt is not over tightened)
The additional spring on the cylinder will work fine however I believe it is masking the real problem of the bolts canting and causing two different pivot axes.

Regards,
Don Pansier

Might want to do both..

I was able to dispose of my unused Matcos to offset the cost of the Grove units so it wasn't that big a hit for me but if I had not been able to, I doubt if I would have gone that route. Everybody else I know just added the external springs and they are working fine.

Do check some of the other posts about the Matcos not fully releasing on their own, regardless of other friction in the system. I've heard this from several sources both from RV pilots I know, an RV builder A/P and posts on various forums.

That said, I'm also going to add the long bolt.

Great info, all. Thanks!

Any updates from the guys adding the long bolts to the brake peddles?

Gone quiet

Yeh, the long bolt guys have gone quiet......maybe it's not working.

Any update on the long bolt theory...from practical experience.

Long Bolt Guy

I have the brake hanging up problem, 7A, and it was always on the right side. I tried adding a spring over the shaft at the brake pedal and that helped but did not completely solve the problem. After reading the thread about the the long bolts I tried that, only had time to install it on the right, but it seems to be working fine. I have probably flown 30 hrs. since and have not had any trouble.

If you are having trouble with brakes be carefull not to overheat the calipers, the o-ring will flatten out and you will lose your brake fluid.

Not flying but big improvement.

I switched from the standard 2 bolts to the long bolt and it was a huge improvement. I didn't realize how much friction/binding there was with the 2 bolt arrangement until I switched to the single long bolt - I would definitely recommend it. BTW, the -60 bolt was a bit too long, I needed the equivalent of 3 washers on one side & 2 washers on the other side.

The solution with one long screw is good, but before you install it, you should drill with a long drill throu both holes at the same time. Why this you will ask, because the design of the pedal is not parallel. Both sideangles are monted parallel to the breakpedal outline, but the outtline has a taper towards the tip. This is the mainreason why things are not moving free. Also the pedals are to wide, with the original setup there was no possibillity to put a standart or l-washer between the pedalangle and the steel-latch!

The second is, that when you tighten the two bolts to less, they start to get out of line and start to jam. If the two bolts are tighten to much, then they jam anyway.

I research the sticking pedal issue of a friends installation, we align the welded steel-latch as parallel as possible, grind off the highspots from the latch-welding (this should vans do prior powdercoating), reamed all the boltholes (mostly only the paint was clamping the bolt), add 0.5mm washers between the pedalangle and the steel-latch on both side. On the matco zylinder we add washers to bring the centerline of the zylinder really 90? to the screws on top and bottom and GREASE all the bolts and holes.

This worked well enough so that the original Vans-design not has to be changed yet, flying will tell.

Parallel Breakpads, enough sideplay to add washers, well aligned steel-latch, one long bolt instead of two, this should Vans really improve!


Regards,

Dominik