File: 09hiller.ht1.doc
[Headline: Inspect landing gear and brake systems before taking off]
I am writing this month’s column following a successful day of flying jets with some friends. I plan to help some new turbine jet modelers next weekend, and it got me thinking about how good our equipment and skills have become, and how much easier their path to jet flying would be if they knew what we had learned. The existing quality of equipment is great, but what is important and how is it used?
Let’s start with the pneumatic basics that most of us use for powering our landing gear retracts, gear doors, and brake systems. Air systems worked great for years, but reliability has decreased and many modelers are replacing them with electric systems. (Pneumatics originally were introduced to replace unreliable electric systems.)
So what is the problem? The answer is low-quality air cylinders and a lack of understanding.
This year, as I pulled my aircraft out of winter storage and flew them, I found a multitude of air system problems that were similar to what my FeiBao Hawk had last season.
My first issue was with intermittent leaks in the landing gear system. Some flights went well with no leaks, while others had a significant loss of air pressure. I began by inspecting the air fill valve and worked through the system.
Most fill valves can be disassembled for cleaning. In this case, the aircraft had an O-ring on the backside that allowed the floating piston and seal to be removed. I cleaned all of the components and lubricated the O-ring seal with Parker Super O-Lube, a silicone-based lubricant. Most jet suppliers carry this product. The seal on fill valves can accumulate dirt, which is why I start at the fill valve.
I pulled apart the retract air valve, cleaned it, and added lubricant to the O-rings. I still could not find the problem. I pressurized the air system and it held air with no leaks. When I began moving things around near the main gear doors, there were occasional air leaks.
I found that any side load on the inner landing gear door air cylinders caused leaks around the rod seal on the air cylinder. It was time to pull out the air cylinders and see what could be done.
I pulled out the air cylinders and could easily duplicate the air leak problem by adding side load on the rod. The rod end seal was not doing its job. I didn’t have replacement air cylinders and could not figure out how to repair the seals, so I removed the air lines until I could obtain replacement air cylinders.
Years ago, I had the same problem with my Der Jet Vampire’s new air cylinders. I later determined that they were poorly assembled and the repairs were easy. On that set of air cylinders, the rod end seal was an O-ring compressed under a brass sleeve that was not visible until disassembled. The brass sleeve was not pushed down far enough to hold the O-ring in place, allowing the O-ring to move up and down with the rod.
Tapping the sleeve farther into the cylinder rod end cap, compressing the O-ring, solved the problem. On cylinders with this design, if you push the brass sleeve in too far the O-ring will be overcompressed and put a heavy drag on the piston rod. This simple fix has stayed in place for a couple of seasons with no problems. I did lubricate the O-rings with Parker O-Lube by applying a thin coat on the piston rod.
I removed the landing gears and disassembled, cleaned, and lubricated the air cylinders on them. I now have a proper pneumatic system with no air leaks, and I’m having a great flying season with no landing gear problems.
I next inspected the brake system. By the end of the previous flying season, the FeiBao Hawk’s brakes were gripping hard, so I decided to clean and lubricate them. The Hawk’s brakes are expanding O-ring-type brakes (rubber O-rings rubbing against aluminum).
I use two methods for cleaning and lubricating these brakes. If they tend to lock at low air pressure, I use the Parker O-Lube. It’s a slick lubricant, and after it’s applied, only high brake pressure can make the brakes lock.
For brakes that only lock at high pressure, I use Vaseline as a lubricant. It’s not as slick as Parker O-Lube, allowing the brakes to be more effective. Vaseline is not as durable as the Parker O-Lube and must be reapplied a couple of times per season.
These methods for O-ring brakes have served me well for years.
It was time to review the disk brakes found on some heavier models such as my Der Jet Vampire. This model has three disk pads pressing against a stainless steel disk in the center of the assembly, with one pad pressing against the aluminum wheel, creating an effective brake system.
This system worked well until last year when my landing gear strut’s shock absorber developed a leak and flooded the associated wheel and brake assembly with silicone-based shock oil. What a mess! The fluid lubricated the brake pads and caused the model to pull to the left when braking.
I disassembled the right brake assembly and cleaned all brake pads and disks with isopropyl alcohol. The work paid off and the Vampire now brakes straight to a stop.
How much brake force is enough but not too much? The rule of thumb that I follow is to apply the brakes, then push the model on the ground. If the tires turn with resistance, you’re all set. If the tires slide along the ground, they are locked, causing trouble when braking on landing.
Locked tires can cause two problems. First, the tires can develop a flat spot and will need to be replaced. You also won’t have much control, causing the model to spin out without much directional control.[dingbat]
SOURCES:
Jet Pilots’ Organization (JPO)
www.jetpilots.org
Parker Hannifin Corporation
(800) 272-7537
www.parker.com
