The XF JR 421 EX Radio System

An Entry-Level Computer Radio For Everyone

 

 

Many new RC pilots start with the basic RC “radio” that was originally installed in their first Ready-To-Fly trainer. RTF trainers (except the Hangar 9 Extra Easy Two trainer which is equipped with this system) ordinarily use very basic, analog transmitters that feature servo reversing, but little in the way of additional features. Such “entry level” RC Systems are serviceable, and far superior to anything available just ten years ago, but are still somewhat limited.

 

Basic analog transmitters, for example, cannot store trim settings for more than one aircraft. Nor can these transmitters mix flight controls together to control V-tail or elevon equipped airplanes. Adjusting control surface movement to transmitter stick movement, called setting control “throws,” is strictly a mechanical operation done on the aircraft itself. Many basic RC systems have only 4 channels and cannot operate retractable landing gear or wing flaps. Neutralizing transmitter trim settings also requires mechanical airplane adjustments. 

 

Therefore, most new RC pilots who learned on a RTF trainer usually buy a more advanced radio system for their second airplane. Other new pilots, starting out with an Almost-Ready-to-Fly airplane, must purchase a radio set to control their new ARF.

 

Therefore, sooner or later, every RC pilot is going to purchase a new RC system. When they do, they will want their new system to feature a computer transmitter. Why a computer “radio”? Because the transmitter has the ability to do so much more. Computer-based transmitters can store trim and control settings for more than one aircraft, can ease the initial set-up by adjusting the amount of servo movement for any given transmitter control stick movement and can operate flaperons, V-tail and elevon equipped aircraft.

 

The only problem with computer transmitters is that some RC pilots are concerned about difficult operation. (Mostly those pilots over thirty whose VCRs’ still blink “12:00” like mine does.) Well, JR has answered this concern with the XF 421 EX RC radio system. The “421EX” features an easy to use, self-explanatory computer transmitter combined with a sport flight pack of exceptional quality.

Photo 1

The heart of this system is the transmitter itself. The large screen in the top center is easy to read, as the print is about one-quarter inch high. The screen is even easy to read when making field adjustments in daylight. When first turned on, the screen shows the transmitter battery voltage and the number or name of the model set as the active aircraft (photo 2). The 421EX transmitter will remember all settings for two different models.

Photo 2

The transmitter has two separate classes (menus) of operations that can be performed. The first menu is called the “System Mode.” The System menu contains the settings normally used only during model setup. Why didn’t JR just call it the Setup Mode? We don’t know, but this menu includes the setup functions of choosing the active model, setting the wing configuration (more on this later), resetting all data, entering each model’s name, and performing V-tail mixing.

 

The transmitter has just two simple input buttons, located in the transmitter’s top corners that perform all the data selections (photo 3). Accessing the System Mode is easy. Just hold the two data input buttons up while turning on the transmitter. Accessing the System Mode is difficult to do by accident; this protects the setup data from accidental changes. A good feature we think.

Photo 3

The black buttons on each side of the transmitter’s front panel are the input buttons. The left topside metal toggle switch works the flaps or retractable landing gear. The right toggle activates the buddy Box system.

The System Mode opens up to the data reset function for the active model. Push the left input button up once and the next menu item comes onto the screen, choosing the active model. This is model one (MDL 1) in photo 4 below. Push the right button once and the model two (MDL 2) becomes the active model (photo 5). This is pretty easy to do. The left button calls up the menu item while the right input button changes the item displayed. This button operation holds true for all programming.

 

Photo 4    Photo 5

Push up on the left button once more and the wing configuration menu pops onto the big screen. The 421EX can operate several types of wing control setups. When the screen reads “OF” (photo 6) it does not mean the wing is turned off. Instead, this setting is for normal aileron operation with a single aileron servo. This is the aileron setup used in almost all trainers today.

 

Photo 6   Photo 7

But if you use two aileron servos, one for each aileron, you can do some interesting things. Press the right button once and the transmitter goes into “flaperon” mode (photo 7). This changes your regular, everyday ailerons into flaperons. Think your model lands slowly now? Try a landing with the ailerons drooped as flaperons. Your plane might land so slowly that a little wind could blow it backwards. Flaperons also allow a steeper glide path without gaining airspeed. The student can keep the plane higher, (read “further from harm”) when landing with flaperons. The “retract” switch on the transmitter’s top left side controls the flaperons.

 

Pressing the right button once more brings up the delta wing (DLT) screen. Pulling down on the left input button when the screen is in the wing type mode brings up the V-tail mixer (VTL). Many gliders, pylon racers and electric models use V-tails for elevator/rudder control. Having this function in a low cost, yet very advanced, radio system is a real plus.

 

The transmitter’s other menu system is called the “Function Mode.” The Function Mode contains the model’s flight settings. Why JR didn’t just call it that is a question above my pay grade, but this menu choice contains the sub-trims, the servo travel adjustments and the servo reversing switches (you will love these).

 

Entering the Function Mode is simple; just push both input buttons upwards while the transmitter is turned on. The first screen up is servo reversing. Rather than a confusing set of toggle switches, where all channels are pictured simultaneously (sometimes identified by just a number), the 421EX calls up each channel individually with the channel listed by name. Photo 8 shows the reversing switch for the throttle while photo 9 shows aileron reversing.

 

Photo 8    Photo 9

What is servo reversing? All servos rotate in the same direction for a given transmitter stick input. When the servo rotates, the plastic control arm that connects to the control rod also rotates in the same direction. This moves the control rod, which in turns moves the control surface, such as the elevator. However, the control rod can be placed on the right or left side of the servo’s control arm and this might cause the elevator to move downward when “up” elevator is applied. Reversing the servo’s rotation direction solves this problem (photos 10 & 11).

 

Photo 10   Photo 11

 

If placed correctly, the elevator will move upwards as the transmitter’s elevator control stick is pulled toward the pilot. But what happens if the control rod is hooked to the wrong side of the output arm? The elevator moves downwards as the transmitter stick is pulled towards the pilot. This setup can really ruin your day.

Depending on a particular aircraft’s internal construction, it may not be possible to connect the control rod to the correct side of the servo output arm. This used to cause many problems; so many in fact, that RC manufacturers once made “reversing servos” that rotated in the opposite direction than standard. Today’s transmitters will reverse the servo’s direction of rotation internally. The 421EX’s reversing system is one of the easiest and least error prone available.

The 421EX transmitter also features “sub-trim”. Accessed in the Function Mode, sub-trim allows the pilot to fly and trim his model, using standard transmitter trim levers. Once the airplane is trimmed and landed, sub-trim is used to set the control surfaces in their same trimmed position while the transmitter’s trim control levers are returned to neutral.

This is important to do whenever a single transmitter is used to control more than one model. Having different trim lever positions over several models can become difficult to manage. Keeping all trim centered insures every model controlled by a single transmitter always has proper trim settings. The 421EX again uses an individual screen for each channel’s sub-trim settings (photo 12).

Photo 12

 

The final Function Mode menu adjusts the total amount each servo moves when the transmitter’s control stick is moved. This travel adjustment is a true gift when setting engine throttle movements. This feature also allows the pilot to adjust the airplane’s sensitivity to control inputs. The 421EX allows the pilot to adjust control surface movement in both directions for each channel. This means a pilot can set the elevator, for example, to provide a lot of downward movement, (good for outside loops), while leaving “up” elevator at normal sensitivity.

 

Photo 13    Photo 14

 

The 421EX transmitter has a few other good features. One not normally found on RC systems in this low price range is called Direct Servo Control (DSC). Using an inexpensive DSC cord (purchased separately) plugged into both the model and the transmitter, the pilot can operate all aircraft controls without transmitting a radio signal. This is great for making field and workshop adjustments.

 

Many new pilots are not aware that leaving their transmitter turned on, with the antenna collapsed, puts a severe strain on the transmitter’s Radio Frequency (RF) output electronics. Yet they often do this when setting up the model’s control surfaces. Too much of this can cause the transmitter’s RF output circuits to fail in-flight. Not a very happy way to end the flying day. Using the DSC for bench setup prevents this potential problem while also allowing field adjustments when someone else has the “pin.” In addition to the DSC cord, using this feature also requires the JR Deluxe Switch Harness, which must also be purchased separately. When using the DSC feature, the transmitter draws just 70 mAh of current instead of its normal 200 mAh.

Other transmitter features include control sticks that are adjustable for height and spring tension, an audible and visual low-battery warning indicator plus a “buddy box” trainer system. This last feature allows the student to operate a second transmitter while the instructor is flying the model using the 421EX transmitter. Pressing a switch located on the 421EX’s right top passes control of the model to the student. Releasing the switch returns control to the instructor.

 

Photo 15

In addition to the powerful, yet simple-to-use transmitter, the JR 421EX System has an advanced flight pack for such a low priced radio. The receiver is the JR R-700 7-channel, FM receiver. Even though the transmitter has five channels, the receiver can operate seven separate controls. Using flaperons requires one extra channel as does operating retractable landing gear. The receiver weighs 1 ounce and is just 1.9-in. long, 0.95-in. wide and only 0.6-in. thick. It fits most anywhere in the model’s fuselage.

JR receivers use a patented, multi-step process of signal elimination, they call it ABC&W, to insure that only the transmitter’s signal gets through to the receiver. Without getting too technical, the receiver has a series of electronic filters, each more discriminating than the one before it, that examine the incoming radio signal. Like progressively finer strainers, these filters strip the incoming radio signal of any interference until, at the last and finest strainer, only the transmitter’s signals actually pass through to the control circuits.

 

The servos are also high quality. The JR 537 servos supplied are ball bearing equipped; provide 43 inch ounces of rotating power (enough power for all models up to “90-size”) and rotate 60 degrees in just 0.25 seconds. This high-speed rotation means that most pilots cannot move the transmitter stick as fast as the servo can rotate. What the pilot “feels” is the model responding immediately, and exactly, to all control inputs. This is called a “tighter link” to the model.

 

The 1.5-in. long, .75-in. wide and 1.3-in. high servos weigh only 1.6 ounces each. All mounting hardware and grommets are included as are sixteen extra servo control arms of various sizes to ease setup chores.

 

Photo 16   Photo 17.

The best feature of the 537 servo is the ball bearing (photo 16). This bearing supports the shaft that moves the servo’s control arm. There are three types of shaft supports used in servos today (photo 17). One is a nylon disc while the other is the same disc made from metal (both called bushings). The shaft rotates inside these fixed bushings. The fit between the bushing and output shaft must be slightly loose to allow rotation without binding. This causes a very small output shaft flex during operation. Once the servo ages, the shaft and bushing wear because the bushing is not free to rotate with the shaft. As the wear progresses, the output shaft’s flex increases.

 

A ball bearing support is free to rotate along with the output shaft. There is no output shaft flex since the shaft fits tightly inside the inner bearing support (called a “race”). There is also no output shaft or bearing support wear during operation as both rotate together. All premium servos use ball bearing support and it is rewarding to see this feature in a radio system in this low price range.

 

This is only an opinion, and one not shared by every experienced RC modeler, but I think that life is too short to use servos without ball-bearing support. Within two hundred to three hundred flights, a bushing-supported output shaft flexes enough that control surfaces no longer return to their exact center positions. Trim settings change constantly during flight and this can become annoying. If you think that three hundred flights are a lot, remember that these servos will be used in many models, for many years to come in your modeling life. With that in mind, three hundred flights are definitely not a lot.

 

Photo 18

In case you are wondering just what the inside of a servo looks like, take a look at photo 18 above. Several nylon gears gradually reduce the motor’s high rotation speed until the 0.25 sec per 60 degree is reached. These nylon gears have very tiny gear teeth that sometimes break. Broken gears can be replaced for just a few dollars. Sport Aviator will shortly be featuring a Flight-Tech article on servo gear replacement. Unless you are an electronic master, there is never any need to go deeper into today’s servos than the above photo. For most modelers, there are no user serviceable parts inside.

A 4.8-volt, 4-cell Ni-Cad battery, powers the entire on-board system. This battery has a 600-mAh capacity. Combined with the transmitter’s 600-mAh battery, the 421EX provides about 2.5 hours of safe flight time before requiring recharging.

 

The 421EX does have a few limitations, as do all computer radios in this price range. There are no programmable channel mixes available. It is not possible therefore, to set the elevator to move just slightly when the rudder is deployed. This setting is used to prevent aerobatic models from moving sideways during knife-edge flight. Elevator trim cannot be automatically added when the flaps or flaperons are deployed. The pilot has to set this trim by hand. Such features are only found in RC systems about twice the 421EX’s price, make setup complicated and are not usually required in most first and second model airplanes anyway.

 

The JR 421EX is one of the best introductory computer RC systems available. Operation is simple, intuitive and easy to understand. The 5-channel transmitter allows either flap, flaperon or retractable landing gear operation. Like all computer systems, the 421EX makes model setup easy. But the 421EX does it in a more understandable manner than most other computer transmitters. The flight pack is very high quality for this price range. There is probably no better “first” computer RC system available.

For more information on the JR 421 radio, please go to: www.horizonhobby.com

 

Transmitter:         5-channel; computer controlled Battery:                9.6 V (8-cells), 600 mAh Receiver:             7-channel; weight = 1 oz. Servos:                43 in. oz. Torque; wt. = 1.6 oz.                             Ball bearing equipped On-Board Battery: 4.8 V (4cells); 600 mAh Total flight Pack wt.: 12.9 oz. Charger:              120 V, 50 mAh Rec. and Trans. Suggested retail: $269; Street Price- ~$175

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