The Stearman uses a Bendix, hydraulic, self-energizing brake. It is quite easy to maintain and adjust if the mechanic knows a few simple facts about it.
It has two shoes. The secondary shoe is the upper and larger of the two. This shoe is anchored on the upper forward end on a fitting which allows it to hinge up and down, and is supported at it’s center by an eccentric bolt. During adjustment, this bolt is turned, causing the secondary shoe to move up or down, thereby setting the proper clearance between the shoe and the brake drum.
Note that both shoes form almost a complete circle and the eccentric pulls this circle up or down to it’s proper position (within the circle formed by the brake drum) and is locked into place there.
The star wheel between the shoes is nothing more than a turnbuckle which makes the circle formed by the brake shoes either larger or smaller, thereby fitting it to the brake drum.
The front end of the primary shoe is attached to the hydraulic piston, which actuates the brake. The piston pushes the primary shoe down against the moving drum. The friction set up between the shoe and the drum, pulls the primary shoe around with the drum causing the secondary shoe to be engaged. Thus the brake tends to tighten itself when the wheel is moving forward, or in other words, is self- energizing.
Any looseness in the adjustment causes the primary shoe to throw the secondary shoe violently into contact with the drum, thereby causing grabbing or chattering brakes.
Another “theory” which might explain the grabbing tendency of the two-shoe brake is described as follows:
The rear shoe is long enough to extend toward the front of the drum past the axis of rotation. When the front part of the rear
shoe contacts the drum, and there is any “stickiness” between the two, it forces the rest of the shoe against the drum, and the rear shoe becomes “self energizing”. Since the top or other end of the rear shoe pivots against a hard point, there is no way to release the contact pressure on that shoe and it “grabs”. If this theory is correct, the grabbing problem could be significantly reduced by minor shortening of the front, lower part of the lining on the rear shoe.
The experimental modification that is being evaluated is as follows:
On the longer rear shoe, remove 2 3/8 inches of the lining on the bottom end of the shoe. Cut on a downward angle just below the second row of rivets. Stop just before going clear through the lining so that you don’t nick the metal shoe. Remove the two lower rivets and the short piece of lining. Sand off the edge of the remaining lining to create a smooth bevel. This can be done on the plane with a finger holder for a hack saw blade.
Make sure the lining is free of oil or grease. You should install two new brake springs on each wheel. Also make sure the wheel cylinder piston is clean and works freely. It might take flushing out.
Set the clearance on the rear shoe to the drum with the eccentric bolt to .010 inches. Then set the clearance, on the front shoe the same with the starwheel. Recheck the rear clearance, then the front again.
A few owners have tried this on their Stearman with very good results. Since it is experimental, you and your A&P are on your own in trying it, but if you do we would like to get a report on the results.
Feedback on Brake modification results
author John Lohmar SRA 2188
As you know there are generally two problems with Stearman brakes. They stick and they grab. Mine were doing both, to the point that it was getting dangerous. Normally when they start acting like this the problem can be taken care of by just cleaning and adjusting the brakes. I did my usual cleaning, scuffing and adjusting, but still had problems with both sticking and grabbing. I decided to do the mod that we had talked about. The result was almost no “grabbing”, but the brakes were still very sticky. I tried several attempts at cleaning and adjusting the brakes, using various clearances on the brakes, with largely the same results. We also installed new brake return springs – no help. One of my mechanics suggested taking the brake cylinders apart. Sure enough, the old DOT 3 fluid in the system (automotive master cylinders on my airplane) had absorbed moisture over the years and had formed a gelatinous residue inside the brake cylinders. We thoroughly cleaned and flushed the entire system and refilled it with DOT 5 fluid and installed new brake cylinder seals. My brakes are now very smooth and do not stick at all. Prior to the mod, if my airplane had not been flown for 2 or 4 weeks the brakes would be very “grabby”, until they had warmed up. I always attributed this to moisture collecting in the brake material, then “burning off” as the brakes warmed up. In any case, they do not behave like that anymore. I have let my airplane sit for over three weeks, then found that the brakes behave very well. I attribute most of this improvement to the mod. I think that the mod is a good idea, and I have told several of my customers about it. Until we get a field approval I do not want to actually modify any of my customers aircraft. I may use my airplane as a test bed for an FAA field approval, then start doing it for customer airplanes. I think that it is still very important to keep Stearman brakes clean and properly adjusted. I would also suggest using DOT 5 fluid for those Stearman owners with automotive fluid in their brake systems.
Editor’s Note: The above information, including the feedback from members, is offered for reference and information only, and is not to be solely relied upon.
Cut the lining on the bottom of the rear shoe just below the second row of rivets.
Stop at about 1/16th inch from the shoe so that you don’t nick it.
Remove the short piece of lining, then bevel and smooth the edge .