Nuts & Bolts

Craft & Technique

Show Stoppers

FOR ALL THE EFFORT HOMEbuilders put into their airplanes to get an extra nickel of speed, only one thing can make them stop on a dime: the brakes. Airplane brakes aren’t much different than automobile brakes. Step on the brake pedal, and you increase the pressure on an incompressible fluid in a closed system. This “liquid push rod” moves the only movable thing in the system, a piston, which pushes against a set of pressure plates in the brake. This, in turn, causes a set of pads to squeeze against a disc that is attached to the wheel, like a hand catching a Frisbee. The only real difference is that an airplane requires each main wheel to incorporate its own system—at least it does if you want the ability to apply differential braking while turning. Installing brakes involves three steps and work in three different places—in your cockpit, on your landing gear, and everywhere in between. First you’ll install your master cylinders, brake pedals, and plumbing, then you’ll install your wheels and brakes, then you’ll connect it all and make it work.

On the Inside Each brake pedal requires its own master cylinder to make those brakes function. There are two styles of master cylinders. One incorporates a brake-fluid reservoir, and one doesn’t. Obviously, you need a separate reservoir for the style that doesn’t have one. A reservoir can be a simple container with a fitting attached to the bottom that gravity feeds fluid into the system. Reservoir master cylinders are 104

MAY 2003

Installing hydraulic disc brakes GREG LASLO

easier to install because they need fewer connections, but they’re a little more difficult to fill with brake fluid. Most airplanes can use the reservoir-style, but some aerobatic aircraft require a separate reservoir to keep from leaking during inverted flight. The reservoir, whether incorporated into the master cylinders or not, should always be the high point of the brake system. In aircraft with dual controls, builders can use a set of each style

Each brake pedal requires its own master cylinder to make those brakes function. master cylinder, with the reservoir style providing the fluid for the nonreservoir side. The left or PIC seat can use either a reservoir-style master cylinder or nonreservoir style with a separate reservoir. The right/passenger seat needs only a nonreservoir style cylinder. Plumb the reservoir “upstream”—that is, first in the system. The rule of thumb for installing brake pedals and master cylinders is to have a two-to-one ratio of pedal travel to cylinder travel. In other

words, when you press the brake pedal in an inch, the cylinder arm should move down half an inch. That requires the base of the master cylinder to be mounted as close as possible to the base of the rudder pedals. Roger Dickson of Cleveland Wheels & Brakes recommends that reservoirs be connected to master cylinders, and master cylinders to brakes, with “standard installation”—1/4-inch aircraft quality aluminum tubing with AN T-connectors and fittings. Even so, some homebuilders use Nylaflow tubing with standard brass fittings. Aluminum lines are more durable, but they’re also more expensive and harder to work with—remember, they’re rigid. However, since normal brake operating pressures range between 300 and 600 psi, with occasional spikes to 1,000 psi when a pilot really jumps on the brakes, durability is a genuine concern that some kinds of nylon tubing can’t provide. You’ll need a tool to make a flare on the ends of the aluminum tubing so you can use aviation connectors. If you use aluminum plumbing, you’ll also need MIL Spec flexible woven medium-pressure hose to connect the master cylinders to the rigid line. If you choose to use the nylon tubing, add a 5/8-inch reinforcing sleeve of 1/8-inch diameter brass tubing into the ends for some extra strength. Affix whichever tubing you choose to the firewall with brackets to keep it organized and away from your feet. If you’re installing more than one set of brake pedals, connect them in parallel—that is, the right

pilot master cylinder should connect to the right passenger master cylinder—and connect the passenger master cylinders to the lines to the wheels. This is a failsafe system. If either the pilot or passenger brakes fail, the other set of brakes will continue working.

Main wheels are sized to match the aircraft’s weight and landing speed. The most common are 5.00 by 5 and 6.00 by 6. The first number is the footprint of a mounted tire where it contacts the ramp, while the second number indicates the wheel’s diameter in inches. Five-inch wheels have a grossweight limit of 1,285 pounds with a six-ply tire, less with a four-ply tire. Sixinch wheels are limited to 1,750 pounds for six-ply tires. Within those weight classes, you can use a heavier-duty wheel to compensate for higher-than-average landing speeds. More likely than not, your kit manufacturer or airplane designer has a wheel size in mind, but you can choose a larger size if you want extra rubber on the road (so to speak). Light aircraft wheels are made of magnesium, which corrodes easily. Before priming, clean them with a prep treatment for magnesium, such as Magna-Dyne. Roger recommends a gray primer by Columbia Paint Corp., but most non-etching epoxy primers should work. Once primed, any paint is suitable. Aircraft wheels should be painted everywhere, but Roger recommends applying fewer coats to mating surfaces to ensure torqued bolts hold properly. “If you don’t paint it, you’re going to buy a new one,” he says.

FROM SPORTPLANE CONSTRUCTION TECHNIQUES, BY TONY BINGELIS

Installing Wheels

the connecting bolts through the holes in the disc and wheel, and fasten on the outside of the wheel. Insert inboard and outboard bearings packed in AeroShell 22, Mobil 28, or other grease that meets MIL-G-81322, and cover with grease seals. Dampen your wheel bearing seals with 10weight engine oil, and make sure the inboard bearings are seated before tightening the axle nut. Torque the nut to 150 to 200 foot-pounds to fully seat the bearings, and back the nut off to zero torque. Retighten the nut to 30 or 40 inch-pounds while turning the wheel, then rotate the axle nut, align it to nearest slot and cotter pin hole, and insert the pin. The wheel should rotate freely without any play. Inflate your tire according to its manufacturer’s specs. To complete the brake installation, attach the brake cylinder to the torque plate so it cups the disc. Spray the anchor bolts with silicon spray to ensure friction-free movement, and attach to the brake back plate. Screw one end of a flexible hose onto each cylinder inlet fitting. The other end will soon connect to the rigid brake line descending from the brake pedals.

Once the brakes and wheels are painted and dry, you can install them. Attach the brake torque plate assembly to the axle of the wheel, and torque the connecting bolts according to the manufacturer’s recommendations. Unlike cars, most light aircraft use tube tires, and the tube and tire have to be mounted before you can assemble the wheels. Before installation, rub the tube and the tire bead with denatured alcohol, wash them with soap and water, and dust them with talc when dry. Inflate the tube just enough to round it out. Put it inside the tire, and align the “heavy spot” marking on the tube (or if there is no marking, the valve) with the balance dot on the tire. Install the tube and tire on the outer wheel half, and put the valve stem through the valve hole in the rim. Insert the inner half of the wheel through the tube and tire. Place the brake disc inside the inner half of the wheel, and stick

Finishing Touches With the plumbing attached, fill and bleed the system. Aircraft brake systems require MIL Spec red brake fluid. Do not use automotive brake fluid—it will damage the seals of aircraft master cylinders. If you’re inclined to go shopping, there are devices to help bleed brakes, but they aren’t necessary. Most airplane brakes can be bled from the bottom. Open the master cylinder filler plug, and connect a hose with 1/8-inch threads. Stick Sport Aviation

105

Craft & Technique the other end of the hose into a jar to catch the overflow. Loosely connect a designated oilcan to the brake caliper bleeder screw with a 1/8-inch clear hose. Pump the oilcan to fill the hose with fluid and to remove air bubbles. Tighten the hose to the bleeder valve, and open the valve a quarter turn. Pump fluid into the system until it fills the master cylinder and dribbles through

the hose into the overflow jar. Tighten the bleeder valve bolt, remove the hose, and reseal the reservoir. Check to make sure you have a hard pedal. If the pedal is soft, pump the brakes several times. If it doesn’t fix the problem, drain the fluid and repeat the process. Torque the brake caliper bolts to the recommended force and then safety wire to finish the brake installation.

Put some POWER in your Panel Our NAV 122D is a self-contained 200 Channel NAV Receiver, 40 Channel Glideslope Receiver, plus VOR, Localizer and Glideslope Indicator/Converters. The NAV122D/GPS interfaces with your GPS and serves as the required external CDI for IFR-Approved GPS Installations. (Also replaces Narco NAV12 or NAV122)

Our new AT 155!

FAA TSO’d and FCC Approved

New surface mount design. High power 100% solid state transmitter. EXACT replacement for all Narco AT 50, AT 50A, and AT 150 transponders.

Call us or visit our web site, and ask about The New NARCO FACTORY EXCHANGE PROGRAM

Avionics that make sense

(800) 223-3636

www.narcoavionics.com For more information, visit SPORT AVIATION on the Web at www.eaa.org

106

MAY 2003

Before flying you must break in the brake linings. This smoothes the microscopic high spots on the linings and generates enough heat to apply a thin glaze to their surface. By properly breaking-in your brakes, you’ll get longer pad life and better braking action. The break-in process depends on the material used in the pads’ manufacture. Organic linings, identified by their solid pads and most common in light aircraft, require a three-step break-in process, says Cleveland’s Sandi Schickel. First, taxi the aircraft about 1,500 feet at 1,700 rpm, and apply enough braking action to keep your speed between 5 and 10 mph. Second, let the brakes cool for 10 or 15 minutes. Third, apply brakes, and rev the engine to full throttle. If the brakes hold, the break-in process is complete. If they don’t, let the pads cool again, and repeat the process. For metallic linings, which have easily identifiable deep grooves in them and which are used more often in heavier, faster aircraft, the break-in is significantly different. Accelerate to 30 or 35 knots, and then brake to a complete stop. Do it again without letting the brakes cool. Cool the brakes, and then test them. If they hold, you’re done. If not, try again. Normal braking generates enough heat to maintain the glaze on the linings, which is an important part of brake “maintenance,” Sandi says. Some pilots think staying off the brakes on landings ensures longer brake life. Actually, the opposite is true. Brakes need to be used regularly to operate correctly. Whether it’s installing master cylinders, painting brake cylinders, or bleeding the system, hydraulic disc brake installation is a straightforward project for any homebuilder. With the right tools and a plan of attack, you’ll come to a complete stop in no time. Just the way you like it.