When casual modelers see a turbine-powered jet, they see a hodgepodge of wires and fuel lines running all over and think “Wow! I could never figure all that out.” The truth about it is that it isn’t all that complicated once you understand what everything does and how it fits into the big picture and contributes to the result, your jet in the air. This article will help demystify the unique steps to configure your radio system to operate a gas turbine engine.
Programming for a turbine jet isn’t vastly different than any other model. You need to apply your rates and exponential throw so that you’re comfortable flying your model, whether it be a turbine trainer like a Turbinator or a big scale F-18. Where programming a turbine jet differs is mainly in setting up the throttle because you no longer have direct throttle control like you do with a combustion engine or electric motor.
A BIT OF BACKGROUND
Turbine engines are controlled by an Electronic Control Unit, or ECU. The ECU is the brains of the outfit and is usually a separate box about the size of a receiver. Plugged into the ECU is the ECU battery that is either a 3-cell LiFe or LiPo that provides power for the fuel pump and starter motor. Also connected to the ECU is a control output to the fuel pump, an output to the turbine itself, and a servo connection to the throttle channel port of the receiver. There is also a connection provided for the Ground Support Unit (GSU), a small screen that allows you to view and change turbine parameters, and optionally a telemetry output to provide data to the radio system such as turbine status, rpm, voltage, fuel flow etc.
Controlling the fuel pump allows the ECU to add fuel to the motor as the throttle stick is advanced and reduce it as the throttle is retarded. This is carefully controlled by the ECU via parameters like acceleration delay and deceleration delay. If you were to jam the throttle stick fully forward and the pump voltage exactly followed the throttle stick, it’s likely that you would dump so much raw fuel into the engine that it would put out the flame and drown the motor. Similarly, if you reduced it too quickly and the pump responded instantly it would starve the motor of fuel and shut the motor off. Given that dead stick landings with a turbine jet are an adventure many jet pilots wish to avoid, we let the ECU manage the fuel flow to the engine.
All turbine engines that I have experience with, which includes Jet Cat, Kingtech, Swiwin, and Jet Central, require you to do a process known by various names but can be universally referred to as “Learn RC.” There are three points that the ECU needs to learn with respect to the radio system: Full Throttle, Idle, and turbine Shutoff. Pilots of electric-ducted-fan models will be familiar with calibrating a speed control to the throttle stick on the radio, this is a similar situation with an additional parameter.
Those three positions are controlled by a combination of the throttle stick and throttle trim. The shut off position is low throttle stick and full down trim. Idle is full down stick and throttle trim advanced to maximum. Full throttle of course is full up throttle stick. With most radio systems the default for the throttle trim is that it only affects the lower half of the throttle stick throw and has no effect over half throttle. Some radios this require this limitation to be defined in the trim setup screen, check your documentation and servo monitor.
Activating the Learn RC function on the GSU will cause the system to ask for the three respective points and at each point it will measure the pulse width of the throttle signal and when the user indicates they have the desired stick/trim configuration they press a button to save it and move on.
Sound simple enough? It really is but there’s some advanced features of our radios we can use to make our life easier. More on that later.
Now that our ECU knows the throttle configuration, when we are ready to go fly, we go through the following steps. First, with the throttle and trim both down the GSU or telemetry unit should indicate that the turbine is in Shutoff status. Advance the trim to full up trim and the status should change to READY. That means everything is clear to start the engine. Advancing the throttle stick to full for a second or two and bringing it back to idle tells the ECU to initiate the start sequence.
From that point on, the GSU is in control of spinning up the motor, lighting the glow plug or ignitor, advancing the fuel pump voltage and verifying that certain temperature and rpm parameters are met as the start proceeds through each stage. I should note at this point that the user can abort the start sequence at any time and shut down the fuel by lowering the trim to full down. If the ECU detects any issues like failure to light off, low RPM, too high an exhaust gas temperature, air bubbles in the fuel supply or anything that doesn’t match exactly the parameters for a clean, safe start, the ECU will immediately shut things down and throw and error code telling you the reason for the abort. (Tech note: while we covered the common startup sequence some turbines have their own procedure, always read and follow the manufacturer’s instructions.)
If everything goes as it should the status will change to Running and the user will assume control of the throttle again. At this point the motor is running and your throttle stick is active, but it’s important to remember that you’re requesting a certain power setting of the ECU with the throttle stick, the ECU determines the fuel flow required to meet that request and adjusts the pump voltage accordingly while monitoring the rpm and temperature and making minute adjustments as required.
Now that we’ve got the basics out of the way it’s time to acknowledge that jet pilots are often advanced radio users, and we want to get the most out of our investment. The first advanced setting we need to look at is throttle trim step. Most computer radios allow us to adjust how course or fine and adjustment the trims affect the various surfaces. My Spektrum NX10 at default trim step of 5 (0 through 10) takes about 50 clicks of trim to go from Shutoff to Ready (you can hold the trim button, but it still takes a couple of seconds to go from low to high trim). Advancing the trim step to 10 and it takes about 20 clicks. Better, but you can select Trim Type and change that to a two- or three-position switch. Now to arm the turbine I only have to give one click (two-position) or, and this is my preference, two clicks (three-position) to go from Shutoff to Ready.
Using the two click method has several advantages. You can arm the motor almost instantly and more importantly you can shut it down immediately. Using the three-position gives you nearly instant control yet allows you one click of buffer, so you don’t activate one or the other direction by mistake and is the best of both worlds and what I teach when working with new turbine pilots. An alternative to this is that some radios allow you to move the throttle trim off the trim tab completely and assign it to a toggle switch. Assigning trim to a toggle is a common practice of turbine helicopter pilots.
The key is to find what works best for you because the ability to shut down the turbine quickly in case of a fire or imminent crash, without the need to fumble around on your radio, can save you serious damage and money.
Most jets have some things in common and one of those is the presence of a nose wheel is both steerable and retractable. I’m going to teach you a trick that gives you isolated control of your nosewheel steering independent of the rudder and allows you trim it without the need to enter a menu. Various radios accomplish this using slightly different terminology but allow you a similar functionality. I’ll describe the process using the extremely popular Spektrum radios, but the process can be adapted to radios from most manufacturers.
First, create a mix from the rudder channel to the nose wheel steering servo. There are linear, or 1-to-1 mixes and curve mixes. I use a curve mix and use the points on the curve to adjust the expo on the nose steering because nose wheel steering can be highly effective. Set the mix to be activated by a switch and make that switch the gear switch such that the steering is active when the gear is down. This centers the steering and shuts off the servo while the gear is retracted saving both the receiver battery moving a servo that’s contributing nothing during flight and keeps the nose wheel from banging back and forth in the gear well, possibly preventing mechanical damage.
The last step is to assign the input for the steering servo to LTRM or RTRM, the small trimmers on the face of most transmitters that often go unused. This step gives you the ability to precisely trim the nose wheel steering during taxi tests without the need to enter the transmitter programming menu to adjust the sub-trim. I’ve seen the steering assigned to a rotary knob and while that works the knob can be disturbed when changing models or even handling the transmitter, so it’s not the ideal solution.
Text & photos by Andrew Griffith