Eric Clapp has retired as Jet Central’s managing director after working for the company for more than 10 years. Although he no longer works for Jet Central, Eric has not walked away from the hobby. He will continue to grow his own business, Jet Central USA. In addition to selling Jet Central turbines, he will offer FeiBao Jets.
Jose Melendez is Eric’s replacement at Jet Central, so change your contact information to what is listed in the “Sources” when looking for Jet Central service and support.
I have a maintenance program for my jet models, and I conduct a thorough inspection roughly once a year. My most recent inspection was on my Xtreme A.R.F. SH-Discus 2 A. This large, 6-meter fiberglass composite sailplane is powered with a Wren 100 turbine on a power-pod setup. I put this model together last year and rushed to have it ready to fly before winter set in. Now, with approximately 25 flights on it, I have a list of improvements and minor repairs to make.
First, the pesky landing gear doors broke off, snapping the glue joints because of excessive mechanical force during operation. The Discus has a single-wheel retract with two gear doors that are pushed open by the tire, then the landing gear pushes the doors the rest of the way open. The gear doors are pulled closed by a spring on each door, again rubbing on the landing gear, and then the tire, as the landing gear closes. After determining why the doors broke off, I found a tight spot when opening them.
The wheel no longer contacts the doors and the doors continue to open while being pushed by the extending landing gear structure. The geometry of the system puts an unusual angle between the landing gear and the doors, aggravated by strong gear-door springs. The solution is softer springs and a slight geometry cleanup with a bumper to aid in the operation. Now, I look forward to many flights with full gear-door operation. One maintenance project down!
The next task I wanted to complete was simplifying the model’s refueling, which is a clumsy process on this glider. The hatch is hinged in the front, tilts forward for scale canopy operation, and cannot be removed. Removing the cockpit/seat assembly includes turning four twist locks while holding the canopy open. With the seat removed, I can finally access the fuel system. It’s a hassle because the hatch has to be held open the entire time.
I chose a simple fix for this: a fuel-filler dot. The device plugs the fuel-fill line. When the fuel tubing is pushed back inside the airplane, it is secured by an aluminum receiver. The unit I used is slightly larger and fancier than a typical fuel dot, but it also adds a bit of bling to the model. Now fueling will be a simple matter of pulling out the fuel dot, removing the plug, and filling the tanks.
When fueling is complete, I can reinsert the plug and push the tubing back into the fuselage. I now also have an external fuel overflow extending outside of the fuselage, allowing a ground tank to be connected during long waits for takeoff clearance.
These types of issues are usually resolved during the initial build, but because it is a sailplane, I concentrated on aerodynamic cleanliness and kept external systems to a minimum. Never again! When it comes to being able to enjoy a day of flying, convenience is more important than a clean appearance.
On the subject of fuel systems, watch your fuel fitting and hose sizes. For the 100 newton, 22-pound-thrust turbine I am using on the Discus, I added large Tygon tubing to assure plenty of kerosene would be supplied to the turbine fuel pump. Large-diameter Tygon tubing has a 1/8-inch inside diameter (ID) (approximately 3mm ID), and the fuel is sucked through all of those tank fittings and the tubing on its way to the fuel pump. I have used this setup for years with no problems. My philosophy on fuel tubing, clunks, and fittings is to switch to larger sizes as the turbine power goes up.
On my Jet Central Cheetah, a 147 Newton turbine (or 30- to 32-pound-thrust-class turbine), I prefer to use extra-large Tygon fuel tubing, which has a 5/32-inch ID (approximately 4mm ID). It has proven adequate, but has roughly 50% more cross-sectional area for fuel flow. I use it on a turbine with roughly 50% more thrust. You could use the larger Tygon tubing, but you would be pushing the limits.
My Cheetah turbine is temporarily installed in my Der Jet Vampire and replaces a 100 newton-thrust turbine. The 140 newton Cheetah runs well on large tubing. This airplane is an exception, though, because the fuel tank setup allows short fuel tubing runs combined with an ultimate air trap (UAT) consisting of a simple 1/8-inch ID tube centered in the UAT tank. No filter or bag is used with a low-restriction setup.
The larger turbines continue to climb in power and fuel flow, so as you build your fuel system, appropriately size your tubing, fittings, and clunk. Failing to prevent restricting fuel flow to the fuel pump can lead to power issues. Throughout the years, with the move to turbines in sizes 160 and larger, experience has proven that undersized fuel tubing and fittings have caused poor power performance because the fuel pump could not overcome the kerosene flow restrictions.
I prefer to use 3/16-inch Tygon tubing for turbines with more than 40 pounds of thrust. This is more than double the cross-sectional area of large tubing. I am referring to fuel flow rates, and the fact that increased suction required to pull the kerosene could result in additional air leaks.
Follow your turbine manufacturer’s recommendations. Remember, this is not only the Tygon tubing—all fittings, brass tubing, and the fuel clunk need to increase in size and ID to prevent restricting fuel flow to the fuel pump.