RC Jets

Landing Gear

By Jim Hiller | [email protected]

As seen in the January 2023 issue of Model Aviation.

While at the Bill Beach Jet Classic event, held by the Rosewood RC Flyers in Rosewood IN, in October 2022, Ben Burden (center) discussed with Gerry Bowling (L) and Dave Brawley the proper gloss clearcoat for a nicely weathered F4 Phantom. Ben and Gerry have both enjoyed flying this scale jet.

ELECTRIC RETRACT LANDING GEAR are all in vogue right now and are proving to be dependable when properly installed, although they have their own unique issues. With that stated, the next assembly in the queue for me is a T1 Eurofighter with pneumatic retracts and brakes. I’m going back to my roots.

Pneumatic retracts have a long history of reliability when properly installed and well maintained. Both my Cougar and my Prowler have air systems. The only thing the air systems have suffered from is air-line issues. I used the supplied air lines and both aircraft have had blue air-line degradation throughout the years, with soft spots and a complete degradation to the point of falling apart near the nipples. The red, yellow, and white air lines have held up well with no issues—so, no more blue air lines.

The plan for my Eurofighter is new air lines from Bob Violett Models (BVM Jets). I love the BVM air lines because they have proven themselves throughout the test of time. I have found Robart Mfg. air lines tend to get soft in high temperatures. They are okay away from the turbine bay, but in the Eurofighter, most of the air lines pass through the turbine bay, so I will stay with the tried-and-true BVM air lines.

Air systems can be reliable, but care must be taken to detect all air leaks. They are always lurking and ready to make a bad day. The best way to avoid this problem is to check every component for air leaks before installing them in the airplane. I prefer to check my devices under water when possible; electrical air solenoids are the exception and should be kept dry.

Air cylinders tend to be the worst culprit, with a leak both at the rod end and internally at the piston seal. Check your air cylinders with only the pressure line attached. Push the air cylinder rod out with full anticipated air pressure and place the cylinder under water to look for air bubbles where the rod exits the cylinder.

The next test is to move the air line over to the opposite end, and again under water, to check for air leaks out the opposite air cylinder’s nipple. A leak out the open nipple suggests a bad O-ring seal. I prefer to disassemble the air cylinder and replace the O-ring at that time. They never heal themselves, even with lubrication. A good source for rubber O-rings is McMaster-Carr, which is my favorite find-all supplier.

If you are going to use standard slide air valves, lubricate the O-rings and test them too. The O-rings tend to dry out with age and don’t always seal as well. My preference is electronic air valves such as those from Xicoy. They look slightly expensive until you compare them to the cost of an air valve and servo for actuation, then the premium cost is not so bad.

The Xicoy two-stage brake valve is really a nice piece for proportional braking and uses little air per actuation. The weakness is that it must be properly set up, which is a bit of a fiddly process, so don’t be surprised if you have to go through it once or twice to figure out how to properly calibrate the valve to your radio.

Gyro Off Switch

We have been told to always have a switch to turn the gyro off should it have a problem. Well, it finally happened to me. Although troubleshooting is still underway, I did have to turn it off in flight.

The ailerons started a gyro rock while cruising at a moderate speed. I simply flipped the gyro switch to zero gain and it stopped immediately. A quick check on the return pass confirmed that it was a recurring problem, so the remainder of the flight was with zero gain and the gyro effectively off. This was the first time that I had experienced a problem such as this. It sure was nice to have a switch to shut down the gyro.

Here is more about gyros and the importance of properly setting the gain without damaging your aircraft. A high-speed model can be damaged by an overly aggressive gyro gain. The big question is how to set up the gain for your model. The answer is proper flight testing.

The problem with the gain is that the faster you fly, the less gain you will require. I write about this often, but I still see many airplanes with improperly set up gyros, either not obtaining the true advantage of a stabilization gyro or having too much gain and damaging the airplane from gyro flutter. We don’t normally call excess gain gyro flutter, but too high of a gain setting can set about flutter dynamics caused by the gyro’s actions. It’s the same basic dynamics as aero/mass flutter.

Let’s review the two accepted methods of setting your gyro gain. First, the simplest and most common method is putting your gyro gain on a rotary knob and, at high speed, turning it up until the gyro causes an oscillation in an axis, whether it is yaw, pitch, or roll, then turning it down slightly and calling it the right setting.

Typically, at this point after landing, that setting is recorded and the gain is moved over to a switch with the maximum travel set to that value. The weaknesses of this method are that you must first subject your airplane to gyro oscillation (e.g., flutter), and secondly, this must be done at high speed to truly find the limits of the gain prior to the onset of gyro oscillation. A higher gain can be used as the air speed slows down and controls become less effective.

Now let’s review a second method and one many of us use. This technique is based on the results we want to achieve from the gyro—that of improving the stability of each axis. Again, the gyro gain is on a rotary knob to allow adjustments. You slowly turn up the gyro gain as you conduct this test.

First, set the gyro gain and again, at high speed, bump each axis one at a time and observe the airplane’s response. You are looking for a positive stop to each bump, with little or no overshoot of the gyro correction stopping the aircraft’s axis motion. When the aircraft responds with an overcorrection (example: in a roll, a slight overcorrection results in bouncing back in the opposite direction while the gyro stops the roll), you know you have gone too far.

Ron Ballard came to the Bill Beach Jet Classic to feed the pilots lunch and dinner. They were great barbecue meals made by a master himself. Thanks for the good food.

This method sneaks up on the proper gyro gain setting without going so far past it that you experience gyro oscillation, or flutter, and the resulting stress on the airframe. This method also allows you, as the pilot, to experience the true advantages that a gyro has to offer on your particular model, such as crisp control response when releasing the control sticks and, in many of the 1950s scale jets, an improved yaw stability.

You now have two methods in your toolbox to adjust the gain on your gyro. Use them as appropriate and enjoy the pluses that a gyro has to offer in flying your model.

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