Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/307497
32 Vol.3 No.5 / May 2014 UNDER THE COWL be applied over a larger brake pad, in turn allowing a larger pad and lighter backing plate. A multiple-piston design also allows a smaller caliper that more closely follows the arc of the disc profile. The downside is that multiple-piston calipers are more complicated (with, say, two moving parts instead of one) and are more expensive. Sometimes, when loads increase and space for available brake components does not, multiple-caliper designs are necessary, as in high-energy or high-leverage (e.g., tundra tire) applications. Here two calipers are mounted opposite each other, and brake application then energizes multiples of pistons. Note that master cylinders need to be matched to the system; you often can't change or add calipers and expect the original master cylinders to handle the job. Master cylinders: Master cylinders contribute to brake efficiency at least as much as the brake itself. They are often neglected; some wear, then leak or get blocked. Remi Bering- er said, "Many 'brake problems' come from master cylinders that are worn or mismatched, resulting in a spongy or 'too hard' feel, even if your caliper is the best in the world." The job of the master cylinder is to move brake fl uid to the pistons in the calipers. The amount of fl uid that must be moved and the pressure at which it needs to be moved are critical for proper brake operation. For any given amount of toe movement from the pilot, a small-diameter master cylin- der will exert more pressure on the fl uid, but less fl uid will move. If the calipers have large or multiple pistons, there may not be enough travel in the master cylinder actuation to move enough fl uid. If the diameter of the master cylinder increases, it can move more fl uid, but it requires more pedal pressure for the same result. The entire system—from pedal and actuation geometry, to the stroke of the master cylinder and its diame- ter, to the size and number of pistons in the calipers—must be balanced. Master cylinder diameter and stroke must comple- ment brake caliper requirements; a mismatch can result in oversensitive brakes or require excessive pressure from the pilot's toes. Either scenario detracts from control. Brake lines: Small-diameter tubing translates the pilot's signals from feet and master cylinders to the caliper's pis- tons, containing the high-pressure brake fluid. Non-flexible metal tubing is inexpensive, excellent for transferring pres- sure without loss, light…and non-flexible. On lightly loaded systems, flexible "clear" polymer line is used throughout; stronger and more reliable steel-braided Teflon hoses are both tough and flexible. Many systems alternate lines ac- cording to where the flex is needed. This may or may not save weight, depending on how many fittings are incorporat- ed; and more fittings mean more opportunities for leakage or line failure. Where long lines are necessary, consider using flexible lines only where flexibility is needed, and plan well to minimize the number of fittings. Fluids: Typical in aviation is the familiar red mil-spec fluid. It has the advantage of not destroying paint when it's spilled or when it leaks. If leakage isn't a problem (and it Master cylinders come in a myriad of sizes and confi gurations. Match them to the rest of the system. No matter what brakes you have, bleed them frequently and properly, as this RANS employee demonstrated at Sebring. Photography courtesy of Tim Kern E A A E X P _ M a y 1 4 . i n d d 3 2 EAAEXP_May14.indd 32 5 / 5 / 1 4 3 : 1 9 P M 5/5/14 3:19 PM