RADIO CONTROL JETS
Jim Hiller 6090 Downs Rd., Champion OH 44481
I am writing this month's column as I prepare to leave for the Superman Jet Rally. The routine inspection of systems on my Razor is complete, and it's ready for another major weekend of flying.
Dubb Jett and Fan-Jett engines
I had a nice conversation with Dubb Jett concerning his ducted-fan engine line, his philosophy on engines, and what the future may bring for ducted-fan modelers.
Dubb has two engines on the ducted-fan market: the Fan-Jett 50 and the Fan-Jett 95. These engines are reasonably priced. Dubb's primary design goals when building engines are performance with reliability, in terms of durability and ease of operation. Art Arro has been flight-testing these engines, and he has always had good things to say about them.
Local flier Joe Lewis has been running a Fan-Jett 50 in his Jet Hangar Hobbies F-86 Sabre for approximately two months. Joe saw the success Art was having at the Nationals, so he decided it was time to upgrade his engine to get the same performance. Joe is turning a Turbax fan with a MACS quiet pipe, left over from his K&B 7.5 days. The Fan-Jett's performance is significantly better than the old K&B 7.5 engine it replaces.
Joe has roughly 20 flights on the engine, and it is coming up on pipe nicely. He is on only his second glow plug, and it has more than a dozen flights on it with no sign of problems. The Fan-Jett has turned out to be a friendly engine to tune and fly.
Check out the Jett Engineering engines on Dubb's web site at jettengineering.com; e-mail him at jett@pdq.net; or call him at (713) 680-8113.
The Internet and staying current
The Internet age is upon us, so I hope you don't mind my listing web sites; they are an effective way to stay in touch with all the new products coming at us in the ducted-fan and turbine community. I am not very computer savvy, but even I can find my way around the web.
A hard landing reminder
I had to relearn an old trick last weekend. I have been flying my new jet for a couple months, and I have been happy with its flying characteristics—but the "dragon" is always waiting around the corner.
I got low and slow on final, using way too much up-elevator on the final approach. That resulted in a hard descent to landing, with no elevator left for the landing flare. No big deal, just a bump—or it should have been. I forgot to set up the nose-gear strut with a weak spring. The nose-gear strut spring collapsed and launched the nose into the air with the power of a regular-strength main-gear spring still in the nose-gear strut, leaving me with no airspeed and the nose pointing toward the sky.
The model had a scraped nose, wingtips, and stabilizers. Remember to put light springs in the nose gear for the day the blown landing comes. Springs of various strengths that fit our landing-gear struts can usually be found at the local hardware store. Take the time to change that nose-gear strut spring before your first bad landing turns into a truly humbling experience.
Summer flying and the need for maintenance
This past summer was great for flying jets. I haven't flown anything but ducted fans and turbines all summer, and I don't miss my propeller twisters one bit. Sport-flying and jet meets kept me busy. All this flying does mean one thing—the need for a good maintenance schedule to keep the reliability we demand from our high-performance models.
Since I started flying ducted-fan models more than 10 years ago, I have kept fairly accurate maintenance logs on them. My original need was to assure myself they would be ready for the few opportunities when I found a jet meet or an adequate field from which to fly. The first few years were tough. I never flew my models within 100 miles of home because adequate model sites did not exist. I continue to keep these logs even though I have access to a couple of local flying sites now, because the complexity of the models has increased.
Living with turbine-powered models
I'll look at turbines in general, but remember—not all turbines are created equal, so some issues vary.
The first thing these turbines bring with them is Jet-A fuel, and the stink of kerosene. This makes transporting the model, and the fuel, a memorable experience. Many modelers have trailers for this reason; I am also on the lookout for a trailer for my models and that Jet-A fuel.
Purchasing Jet-A fuel has not been a problem, except at one location: the major airport closest to my house. This really doesn't matter, though; I get my fuel at another local airport that welcomes our model-airplane traffic. We operate with well-defined rules for models and how we mix with full-scale traffic.
The proper mixture of kerosene or Jet-A and turbine oil for most turbines is 5%. This is an easy mix—five gallons of fuel to one quart of turbine oil. Find a container with a large-diameter mouth to make it easier to fit the commonly found nozzles on the fuel trucks. The usual small-mouthed bottles at most department stores are difficult to fill; the mouth is smaller than a common Jet-A pump nozzle.
An alternative way to fill your container is from the bottom tank drain of a fuel truck, with a small funnel. However, this is where all the dirt and water collects in the tank, so I recommend against filling your container in this manner. The normal cost of this mix is less than $20, and it provides roughly 10 flights.
Common flightline equipment and starting systems
The normal turbine requires:
- A starter box (can often be obtained from the turbine manufacturer)
- A compressed-air source (usually a scuba tank; small tanks are available and full-size tanks are not required for field operations)
- Starter gas (commonly a simple propane canister from a camping store)
- A fire extinguisher
The methods for carrying these supplies vary from simple shoulder packs to beautifully constructed carts. Move over NASCAR pit crews; the jet modelers are on the flightline.
Turbines with ground-support requirements are normally started with the sequence manually controlled or controlled by an auto-start box. The turbine's ECU (electronic control unit) requires specific operations after starting to warm up the turbine and calibrate engine parameters.
If I fly a Golden West Hammer P-80 with the onboard auto-start feature (unique in field operations at this time), the aircraft is fueled with approximately 80 ounces of Jet-A with a butane starting gas. The turbine's fuel pump and starting glow plug get their power from a six-cell, 1,200 mAh battery pack on-board the model.
I am spoiled; all I bring to the flightline is the airplane, the transmitter, and my fire extinguisher. The starting sequence is a simple matter of turning the radio on, then initiating the startup with the auxiliary channel and throttle channel. The channels are used in a specific pattern to avoid accidental starts. The turbine spins up to 54,000 rpm after startup to bring the engine up to temperature, then it returns to idle before returning normal operation to the throttle stick. This is far below the full throttle setting of 118,000 rpm, hence the turbine is reasonably quiet during the starting sequence. The disadvantage is the onboard starting system's additional weight, but everything has its tradeoffs.
Fuel effects, wiring, and maintenance vigilance
No matter which system you use, the kerosene fuel will play heck with the model. The fuel lines cannot be silicone, and even the best products seem to have aging effects during the course of a season. The most critical thing to watch is the effect the kerosene has on insulation found on servo wires. Exposure to kerosene, or sometimes even the fumes, makes the insulation hard and brittle. Combine this with servos mounted aft of the turbine receiving tailpipe heat, and you have the makings for disaster.
This is where good maintenance comes in—look for a crisis before it happens.
Normal vibration has not been an issue on the turbines I have flown. These engines run so smoothly vibration is minimal. I find myself violating many established rules for installing receivers, battery packs, and ECUs with no negative effects. However, I still respect vibration because it is always present with rotating devices.
Watch out for loose items—especially those located ahead of the turbine—that may get sucked into the turbine or make contact with the hot tailpipe.
The general maintenance cycle of a turbine model beyond these issues is similar to any ducted-fan model. We fly rather heavy, high-performance models that do stress the airframe and landing gears, so watch critical areas for wear and tear.
- Inspect wing spars and wing-mounting systems carefully for stress cracks as they age.
- Check landing-gear plates and mounting pads; they are heavily loaded and can fail if neglected.
- Inspect plastic parts on retract units; they often crack and wait for the next blown landing.
Closing
It's time to sign off and hit the road. The weather forecasts are good, and you have to take advantage of good flying weather when you live in the North.
See you at the field. MA
Transcribed from original scans by AI. Minor OCR errors may remain.
