Understanding the JR Propo X-388S Radio – Part I

By Richard A. Eckel

©Copyright 1994 All Rights Reserved.

Ever since I started flying RC sailplanes about two years ago I knew that I would eventually move up to a computer radio. I’m an engineer with a strong interest in computers and technology. Obviously the high tech sailplanes and computer radios have a strong attraction for me.

I have always admired the JR radios and finally decided that I would purchase and learn how to program the JR388. Let me lay it on the table – the radio is not hard to program but I found the manual very difficult to decipher. Perhaps I find the manual difficult because I am a "top down" learner. That is, I want to understand the overview before I learn which buttons to push. If you are the same way or are just interested in a different view of programming the JR388 I think that this article will be of interest to you.

Intent of Article

The intent of this article is to supplement the manual in some respects and to present my understanding of the programming and organization of the JR388. Along the way I will point out some potential traps in the programming. After reviewing the organization of the radio and its functions I will recommended setup procedures and tell you about some programming tricks that might be of interest. I am hardly an expert on programmable radios. I do not know how other radios such as the Vision or Super 7 handle their progarmming and I will not be doing any comparisons. I have simply spent a good deal of time analyzing the JR388 and want to pass along what I have learned.

Receiver Channels

The receiver channel assignments are very important to the programming of the JR388. JR has preprogrammed a variety of useful mixes in the transmitter but they are dedicated to specific channels. If you want the radio to program easily and with maximum flexibility it is best connect your servos to the channels specified in the manual.

To keep you interested, the receiver channels are referenced in the manual by a variety of names. Table I lists the channels and the aliases I found in the manual. In this article I will refer to receiver channels by their sailplane assignment followed by the channel number – i.e. Rud(4). When I use the term Flap or Aileron without a channel designation it refers to the mixed channels together controlling both flying surfaces. Table I lists the receiver channel number followed by its receiver label alias followed by its sailplane setup alias followed by any other alias I came across. Keep Table I handy when you are reading the JR manual.


Receiver Channels

Receiver Channel Aliases
Channel Receiver Mark Sailplane Alias 3
1 Throttle Spoiler
2 Aileron L. Ail Aile 1
3 Elevator
4 Rudder
5 Gear R. Ail Aile 2
6 Aux1 L. Flap Flap
7 Aux2 R. Flap
8 Aux3

Transmitter Controls

The sailplane version of the JR388 is specifically arranged and labeled to be accomodating to the sailplane flier. However, not every sailplane pilot will like the layout or the labeling. (We’re an opinionated bunch!) But the biggest problem is that the manual references the controls with a variety of nomenclatures and the programming prompts do not corrospond to the labeling or, often, to the manual references. I had a lot of trouble identifying switch references in the manual. Again I have provided a table, Table II, of the switch names and the aliases I found in the manual and programming. Again, keep it handy when you are reading the JR manual.


Transmitter Switches

Switch Aliases
Transmitter Mark Program Alias Alias2 Alias3 Alias4
CROW/Aux3 BTF0,BTF1 BTFLY Landing Camber
FLAPS (3-positions) F-U,F-D,FU+D Preset SW Flap Switch Reflex Preset
Trainer TRN.

Before you set up any mixes the control sticks are the usual setup with the right stick controlling the elevator(3) and the ailerons(2). The left stick controls the Spoiler(1) and the rudder(4). Each of the pots, pot 5 thru pot 7 control the same receiver channel as the pot number. ie pot 5 controls channel 5 etc. Channel 8 is controlled by the CROW switch and any attached servo moves full throw when the switch position is changed. The P.MIX and FLAPS switches are unassigned and do nothing until mixing is set up. The TRAINER switch cannot be mixed but is used to switch in the trainer function or the timer function depending on the setup in the Function Setup Mode.


List of Functions





mix V-TL —> DUA.F




The mode that I refer to as the Model Setup Mode is accessed by holding down the UP and DWN keys on the transmitter while sliding the power switch from off to on. This mode has eight functions that are stepped through using the + or – keys. One additional function is available as a submenu to the V-TL function, the DUA.F function. The eight functions begin with MOL, model selection, allowing selection of model 1 thru 8. The NAME function allows you to name the model selection using 4 alphanumeric characters and the TYPE function lets you set the type of aircraft you want to set up in the programming. Setting TYPE to GLID, AIRP, or HELI changes the programming in the radio to specially support the type selected. I will only address the GLID mode.

The MOL RST function allows resetting all programming for the specific model selected. It resets all servo adjustment, all mixing, the model name, everything back to the factory defaults. If you hit this function your programming is tossed into the great bit bucket in the sky. The only way to get it back is to go back and start over again.

The V-TL function can be used if you have a v-tail plane and want to enable the radio’s preprogrammed v-tail mixing function. Preprogrammed v-tail mixing takes place between the receiver channels RUD(4) and ELEV(3) and reduces throws to 75% (I assume to avoid overtravel when full elevator and full rudder are pulled on the sticks). If you want to use other channels you will have plug the servos into them and mix them manually using the free mixes that will be covered later. You could also mix RUD(4) and ELEV(3) using the free mixes to accomplish v-tail mixing. I recommend using the preprogrammed v-tail mix, particularly if you have more than just v-tail controls on your plane.

The MODU setting sets the transmitter up to broadcast to an FM (PPM) type receiver or to either a SPCM or ZPCM type receiver. Just match the setting to the receiver type in your plane.

The FLAP setting has rather far reaching ramifications in the programming of the radio. Its default setting of POT.6 has Pot.6 on the transmitter directly controlling channel L.FLAP(6) on the receiver. The full throw of the servo plugged into that channel is available by twisting the pot. Setting FLAP to SW+T preprograms the control of the flaps to the FLAP switch with Pot. 6 becoming a trimming device. The really off the wall thing here is that with SW+T set the travel adjustment (T.ADJ in the Function Setup Mode) for the flaps changes from defining the endpoints of the flap servo throw to defining the offset of the flaps for the Flap switch up and down positions. Keep this in mind. It will come up again.

The COPY function allows you to copy all of your settings and programmings from the current model to one of the others. You can only copy from the current model to one of the others, not directly from another to another.

The sub-function DUA.F under the V-TL function is important for a couple of reasons. First and most simply is that it sets the preprogramming to synchronize (mix) the servos plugged into L.FLAP(6) and R.FLAP(7). It also is instrumental in full trailing edge control including mixing flaps and ailerons for full length ailerons. In the Function Mode settings it lets you setup and mix flaps as a single entity rather than having to work with each servo. If you have two flap servos and at least a 7 channel radio it makes your life easier to enable DUA.F. If you have only a 6 channel radio or want to use channel R.FLAP(7) for something else you can use a y-harness for the flap servos plugged into L.FLAP(6), mechanically install and adjust them to work together and disable DUA.F.

Some interesting things happen to the trim potentiometers on the transmitter when you select different combinations of DUA.F and FLAP. Rather than try to explain in paragraph form each of the possibilities and their effect on the trim pots I have provided the following outline. Each of the four DUA.F/FLAP possibilities is listed along with the transmitter potentiometer control use. This can be a handy quick reference when you are planning your setup for an airplane.

I have used the terms flap style and aileron style in the table. By flap style I mean that both flying surfaces trim up or down together. By aileron style I mean that the flying surfaces trim in opposite directions. Notice that the pots change not only in what they control but also whether they are full throw or trim controls.


DUAF=INH and FLAPS=POT.6 (Both are the defaults)

Pot.5 — Trim of R.Ail(5)

Pot.6 — Full throw of L.FLAP(6)

Pot.7 — Full throw of R.FLAP(7)

*Pot5 can be disabled using the POT function


Pot.5 — Trim of R.Ail(5)

Pot.6 — Full throw of Flaps – L.FLAP(6) and R.FLAP(7)

Pot.7 — Aileron style trim of Flaps – L.FLAP(6) and R.FLAP(7)

*Pot5 and Pot7 can be disabled using the POT function


Pot.5 — Flap style trim of Ailerons

Pot.6 — Trim of L.FLAP(6)

Pot.7 — Full throw of R.FLAP(7)

*Pot5 and Pot6 can be disabled using the POT function


Pot.5 — Flap style trim of Ailerons

Pot.6 — Flap style trim of Flaps

Pot.7 — Aileron style trim of Flaps

*Pot5, Pot6 and Pot7 can be disabled using the POT function


Essentially the Model Mode settings on the radio set up the radio programming for the type of aircraft, the type of receiver and special servo configurations to match the airplane you are setting up. Additional functions to select the model, name the program or aircraft, and reset or copy the program are handy convenience functions. An understanding of how the settings affect the programming can make life a lot simpler when you get into adjustments in the Function Setup Mode. Although the JR388S has a lot of flexibility I recommend that you try to use the functionality preprogrammed into the radio whenever possible. Stick with the receiver assignments that JR has defined and use the preprogrammed functions until you have a thorough understanding of the radio. Then go ahead and experiment with alternate setups.

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