My Zenith CH 701 (N701TX) brakes had always been marginal during runup, and stopping on an incline required throwing a wheel chock out the door to block the nearest wheel. But I found a solution for these issues with a couple of upgrades, which we’ll take a look at in more detail.
After landing and rolling down the runway, we use our aircraft’s brakes to slow down. What gives us the power to slow our mighty beast? It is mechanical advantage (MA). In physics and engineering, MA is the factor by which a mechanism multiplies the applied force. Of course, there is no free lunch. The input force must travel a longer distance (D1) than the output force (D2) to gain any advantage. At balance, if the input and output forces are multiplied by their associated moment arms, we have equal moments (much like computing the center of gravity of your airplane). Figure 1 shows a simple lever, and from this illustration we see that a beneficial MA is achieved when the input travel is much greater than the output travel. (Note: The direction of travel is not a factor, only magnitude of the distances traveled.) Mechanical advantage can also be determined from the length of the arms or the forces involved.
Figure 2 shows the geometry of the N701TX toe brake pedal and the master cylinder. Without going into a mathematical proof, I will just say that the more vertical the toe brake and closeness of the master cylinder, the better. In this configuration, the master cylinder downward motion is small compared to the toe-brake motion. But this difference, and the resulting MA, decreases as the pedal moves away from vertical. However, this is not critical, as the output motion of the hydraulic disk brake system is small, requiring only minimal input travel.
The next MA source is based on the implementation of the master and slave cylinders. Mechanical advantage for hydraulics is the ratio of the slave (aka wheel) cylinder piston area to the master cylinder piston area. Here is some good news: My brakes have dual slave cylinders on both wheels. Thus, I have twice the slave piston area providing twice the braking force. With diameters of 1.25 inch for the slave cylinders and 0.625 inch for the master, I have a hydraulic MA of 8. This relates to 0.016 inch of slave cylinder motion and 1⁄8 inch of master cylinder motion.
Now for the bad news. Figure 3 shows the moment arm of the N701TX tire tread (L2) to be much longer than moment arm of the brake pucks (L1) on the disk. Thus, the MA of this component is less than one. (Airplanes with small diameter tires can require less robust brakes.) During runup for magneto checks, I had to really stand on the brakes. Those big tires had way too much leverage to even think about going to full throttle when stopped.
However, there’s also more good news. Matco (the manufacturer of my brakes) has a master cylinder intensifier kit. By reducing the diameter of the master cylinder piston, the same hydraulic output force requires less pedal input force. Now we are getting somewhere. Figure 4 is a photo of the kit (courtesy of Matco). A master cylinder insert, smaller master piston, some seals and keepers make up the kit. I removed the master cylinder from the rudder pedal and secured it upright outside of the airplane. I removed the snap rings, cover plate and piston. Using a syringe, I extracted most of the hydraulic fluid. Per the accompanying instructions, I trimmed the cylinder insert to the proper length and slid it inside the master cylinder. After replacing the old piston with the new smaller one, installing the new seals, and returning the spring and cover, I was home free. I topped off the hydraulic fluid in the master cylinder, and, after pumping the pedal a bit, the brakes were solid; no bleeding was needed. I now have an increase in braking power of about 60%, with the master cylinders moving about 0.2 inches. All in all, N701TX is much more friendly during runup.
Matco sells single and dual parking brake valves. In the park position, the valves will allow the brakes to be applied and will then hold the pressure to the slave cylinders. By turning the valves to the open position the brakes are released. With dual toe brakes, I would need two valves to lock both wheels. As my brake hydraulic lines do not run together, the dual parking brake valve was not an option. However, with a non-castered nosewheel, I figured locking one wheel should be sufficient for most needs.
First, I determined an out-of-the-way, but easily reached, location for the single valve. Two straight fittings (1⁄8 inch NPT to inch compression) were installed at the valve ends. A mounting bracket of 1⁄16 inch thick by -inch aluminum angle was fabricated and drilled for the valve mounting tabs and fuselage frame attachment. A 1⁄16 by 3⁄4 inch template was drilled to match the fuselage attachment holes. This template is used when drilling the holes in the fuselage frame. See Figure 6.
Figure 7 shows the drilling of the fuselage frame. If you want the holes in the right place, hold the drilling template in place with a clamp and then Cleco the first hole before drilling the second hole.
With the valve temporarily installed, the brake line is marked at the valve ends (flow arrow toward wheel). See Figure 8. Do not cut the line here, but allow inch on each end so the lines can be inserted into the inch compression fittings.
Remove the master cylinder from the brake and rudder pedals, being careful not to spill any fluid. Secure it to the outside of the airplane and upright, as shown in Figures 9 and 10. To avoid much fluid spillage when cutting the hydraulic line, the master cylinder should be slightly lower than where the line will be cut.
Using a razor blade, cut the line going to the wheel, insert it into the compression fitting and tighten (with open end of valve facing up), and cut and plug the line going to the master cylinder. Using a syringe, fill the valve body with fluid. Insert the line from the master cylinder into the valve compression fitting and tighten. I hung the master cylinder from the wing to allow air bubbles to move up the line as shown in Figure 11. With a screwdriver, carefully move the brake pucks away from the disk. This forces fluid up the line and any remaining air out of the open valve. Slowly, the bubbles should move toward the master cylinder and again alleviate the need to bleed the system. If necessary, cycling the master cylinder will seat the brake pucks, which can then be used to force any remaining air from the parking brake valve.
Finish up by reinstalling the master cylinder and attaching the valve bracket to the fuselage frame as shown in Figure 12. The system now works great, and there’s no more unwanted rolling when I depart the airplane.