Most builders probably look at aircraft brakes as, literally, a drag. Most of the time, the brakes sit there, doing nothing but adding aerodynamic drag and weight-or so it seems until you land just a bit hot, perhaps a tad downwind for convenience, and discover that those devices are now your best friends. An exaggerated premise? Perhaps, but judging by the questions we understand are presented to the brake manufacturers, its clear that learning about aircraft brakes is far down the average builders to-do list, and that this essential system can be overlooked.
In todays Experimental world, two brake manufacturers dominate: Grove and Matco. Cleveland, the overdog in certified wheels and brakes, has largely priced itself out of the market-comparable wheel-and-brake sets from Grove and Matco run 25% to 40% of the cost of the Clevelands. This fact alone is why homebuilders have overwhelmingly moved away from the certified products. Plus there is variety in the offerings. Look at Grove’s or Matco’s catalog, and you’ll find brake and wheel sizes to fit an almost absurd range of aircraft weights and speeds, plus a score or more of master cylinder configurations and sizes. We talked to Grove’s Robbie Grove and Matco’s George Happ to get up to speed (or maybe that’s down from speed) on current braking issues.
Let’s go back to your first hydraulics course. (What, it was Friday afternoons, and you didn’t get to many classes?) Typical light-aircraft brakes operate with a closed hydraulic system; that is, the fluid is driven to the wheels under pressure and there is no recirculation method. Fluid is provided by a small reservoir to the master cylinder, typically one for each mainwhee—though there are examples of combined mainwheel brakes for aircraft with pedal-steerable nosewheels. In a sense, this makes the normal setup redundant, though if you lose one brake in a nosewheel-equipped aircraft, you’ll be making circles in one direction only.
The master cylinder translates the force applied by your feet (or hand, or whatever) into hydraulic pressure. This is important: Most master cylinders have a bore of ⅝ inch and a working stroke of around inch to 1 inches. Leverage comes into play here. The smaller the bore, the higher the line pressure will be for any given combination of pedal effort and pedal geometry-well get into those concepts in detail shortly. But the smaller bore can move only so much fluid for any given amount of stroke. Often, when a builder (or even kit manufacturer) chooses the incorrect combination of master cylinder and caliper, the brakes can feel powerful but mushy, or weak but with a rock-hard pedal.
In general terms, brake tech isn’t very different among the manufacturers, nor is it exactly leading edge by vehicle standards. But aircraft brakes are highly specialized and, in that sense, optimized for the application. Modern systems use a single-piece steel disc bolted to the wheel itself. How the disc mates to the wheel depends on the caliper orientation. Calipers that grip with the open end facing the axle line-so called external calipers-will generally have a hat-shaped disc that picks up the bolts used to hold the wheel halves together. Internal-caliper brakes typically have a single flat disc or rotor bolted to standoffs on the wheel rim; these calipers slip over the inside of the brake disc.
Wheels and brakes are, metaphorically at least, inseparable. Grove stocks a wide range of sizes and configurations.
Brakes are sized several ways, and there are important considerations to keep in mind. First is the kinetic energy the brakes are expected to dissipate-and this value is influenced by aircraft weight and landing speed. Obviously, the heavier or faster the aircraft, the more kinetic energy the brakes will have to absorb in a maximum-effort stop.
When you consider the dissipation of kinetic energy, you’re thinking primarily about the brake systems ability to shed heat. Remember, all you’re doing is turning the energy of the airplanes forward movement into heat through friction between the brake pad and the disc. Naturally, brakes with higher kinetic ratings tend to have larger and/or thicker brake discs; by giving more material to the disc, its heat-absorption capabilities increase. So, too, does its weight. It’s all a compromise.
How do you know how much kinetic energy you’re dealing with? Grove publishes a simple equation, kinetic energy = (0.044 x W x V2)/N, where W is the landing weight in pounds, V is the beginning-to-brake velocity in knots, and N is the number of wheels with brakes. So for an airplane weighing 2350 pounds that starts braking at 60 knots we get: K=(0.044 x 2350 x 3600)/2, or 186,120 foot-pounds. (If you don’t want to do the math, go to Grove’s site, where you’ll find an online calculator.) Notice that velocity is squared in this equation. The difference in kinetic energy between a 3000-pound airplane landing at 55 knots and the airplane landing at 85 knots is more than double.
In designing a brake system to handle a certain amount of kinetic energy, several approaches are taken, the main one being rotor thickness. For example, Grove forges its steel discs oversize so that it can set a final disc thickness by how much metal is machined off. Two sets of seemingly identical brakes can have vastly different heat-absorption capabilities based on disc thickness.
Torque About It
There’s also a torque rating. Think of the brake and caliper at the center of the wheel with a small 5.00×5 low-profile tire. Now imagine that same brake with a large-diameter tundra tire. The relationship of the tire contact patch to the brake pad has obviously changed, and the larger tire will allow the aircraft to exert more leverage on the brake system. In order to hold this big-tire airplane in place during a full-power runup, you’ll need a brake system with a higher static torque rating. The torque rating, for this discussion, is independent of the brakes ability to absorb heat, and is therefore a function of the hydraulic line pressure, number of active calipers, the size of the pad and, to a much smaller degree, the friction of the pads. In general, the more pad area you have touching the disc-whether thats by a single large set of pads or from two calipers-and the harder you can press that pad into the disc, the greater the torque rating.
You will also see specifications for something called a static load rating, which applies to the wheel assembly. Obviously, you want a wheel capable of handling at least half the landing weight of the aircraft; both Grove and Matco test their designs in-house to a generous safety margin. Nevertheless, its prudent to choose a wheel that can at least meet the expected load.
Choosing Your Own
A lot of builders don’t understand how all the pieces fit together, says Grove. “We make a big effort to create complete packages. Wheels, brakes, axles, spacers…everything needed for the installation. That’s important. In fact, we don’t permit customers to buy directly from our web site. We’ve found orders for wheels that use an inch-and-a-half axle, but [the customer] has ordered a inch-and-a-quarter axle. We’d much rather get this guy on the phone and walk him through the process.” It’s much the same at Matco. Says Happ, “You wouldn’t believe the questions we get, but were happy to answer them. Often, we can save a builder quite a bit of grief by helping him select the correct [combination of] components.” There is your official invitation to call and ask what you might think are dumb questions.
By far the most common problems found in the field relate to one-off installations. Leverage! “It’s about leverage,” says the normally soft-spoken Happ. “It got so bad, hearing complaints about poor braking action and finding out that there was a problem with the pedal, that we built our own rig to take to shows.” Indeed, this apparatus has two sets of brake pedals and pressure gauges to demonstrate how a simple change in pedal or master-cylinder geometry can have a big impact on line pressure and, as a result, performance.
Common master cylinders are not meant to be used without a force multiplier. Look carefully at the toe-brake pedals used by production aircraft; they will all have leverage ratios of 1.5:1 to 2:1 over the master cylinder. That is, the pedal is designed to travel twice as far as the master cylinder bore moves, resulting in twice the force applied, with a follow-on gain in line pressure. Matco says its brake systems are designed to use 450 psi, while Grove’s performance claims are based on line pressure of 600 psi. Normally, this is not a problem, but there may be installation limitations on pedal size, master-cylinder height and other factors that call for some clever workarounds-including using a different size of master cylinder bore and/or brake caliper arrangement.
Poor fluid-handling practices-kinked lines, incomplete bleeding, contamination-cause brake problems far more often than hardware issues. You may want to use nylon flexible lines, and many builders do with good results. But be sure you have the correct line: Nylo Seal is rated to 1500 psi, the Poly Flo to just 75 psi when hot. If you want to use aluminum hard line for much of the brake system, that’s fine, but be sure to leave a flex loop at the wheel to avoid cracking. The most successful installations use a combination of hard and flexible tubing.
So it really comes down to this: Read the information each of these two manufacturers provides—right there in the box, but online, too—and give your brakes more than a passing thought during the annual condition inspection.
For more information on Matco, visit www.matcomfg.com; for more on Grove visit www.groveaircraft.com.