Many Eflite foamy planes sold by Horizon Hobby come with SAFE® technology based on AS3X® receivers from Spektrum. While Spektrum’s AS3X receiver can be purchased separately and installed on any planes to give them 3-axis gyro stabilization, Horizon Hobby has coupled the AS3X receiver with most of their Eflite foamy planes, with additional programming, to make their planes more accessible to beginner flyers. The receiver/plane package is known as SAFE®. And the main features of SAFE® include the famed “Panic” button, as well as beginner-friendly receiver-side flight modes.
These SAFE® features annoy veteran flyers to no end, and render most of these fine aircraft useless to experienced flyers, unless the $80 AS3X receiver is replaced with a normal receiver. Take the NIGHT VisionAire that I can’t stop flying, for instance. This is an exceptional 3D plane that delivers great value for the price. But its built-in receiver forces the pilot to choose either the Precision flight mode with intermediate gyro gains and ridiculously low throw rates, or the 3D flight mode with very high gyro gains, and 100% control surface throws. The gyro stabilization can never be turned off.
Perhaps Horizon Hobby can’t fathom why anyone would want to fly their VisionAire with low/zero gyro gains and very high throw rates, or high gyro gains and low throw rates. The end result is that one can never fly the VisionAire at very high speed without the plane swimming in the air like a dolphin. And one can’t have a relaxed cruising experience under strong winds.
Sarcasm aside, I do understand why gyro gains are coupled with expo and throw rates in these receiver-side flight modes. Often beginner pilots have nothing more than the free DX5e that comes with their plane. So Horizon Hobby must make it such that pilots with a dumb radio can still fly these sophisticated planes. Receiver-side flight mode is one way to achieve this goal.
The Seldom-mentioned Solution
But it turns out that most of these planes actually come with an extremely capable $80 Spektrum AR636A Sport receiver, and it can be reprogrammed using just a smartphone and a special programming cable. You can change gyro rate gains, as well as heading gains. You can completely turn off AS3X stabilization in any of the three receiver-side flight modes, if you want to fly your plane without gyro stabilization.
The fact is, Horizon Hobby does not go out of its way to advertise that these supposedly locked-down “AR636A Sport” receivers can be reprogrammed without losing SAFE® features. There isn’t a lot of information online talking about repurposing the AR636A receiver, except on rcgroups.com and some YouTube videos, and only if one knows what exactly to look for.
Updated March 2019: I’ve finally spent time understanding the AS3X firmware, SAFE and SRM (Spektrum Receiver Model). I have now written a much longer and more comprehensive article which you may want to visit, in addition to this one. Go to: What on earth is SAFE® or: How I hacked AR636 from V900 to fly the Freewing Mirage 2000, with SAFE and AS3X
Many experienced pilots chuck the AR636A and install a new receiver in order to make the plane fly the way they want. What most do not know is that AR636A is simply a normal AR636 Sport receiver with preconfigured parameters and some built-in customization tailored to particular Eflite planes. For instance, here is a version of AR636A customized for the Sukhoi. In replacing the AR636A, not only does the pilot waste an $80 receiver, they also lose Panic mode and other SAFE® features that may come in handy when lending a plane to a newbie.
The solution in most cases is a $12 AS3X programming cable, plus a free app called Spektrum AS3X Programmer for one’s smart phone. These are designed to work on normal AR636, AR6335, AR7350 and AR9350 receivers. But they work just fine for the AR636A, too. In fact, the app knows about these custom AR636A receivers – when connected to one, it will properly set the receiver type to “AR636 SAFE” which is not a choice one can explicitly set in the app under normal circumstances. (Update 2018-12-15: this 3.5 mm audio cable is discontinued. It is replaced by a Bluetooth version: SPMBT1000)
Identify Your Receiver
Not all Eflite SAFE® planes come with AR636A. The VisionAire and Carbon-Z Yak 54, for instance, in 2014 shipped with the AR635 receiver. While this was a fine gyro-stabilized receiver, it could only be reprogrammed using transmitter sticks. Another example is the Apprentice S 15e – it comes with a one-of-a-kind receiver.
If you already have an Eflite plane, make sure the receiver looks like this picture. Note the extra “A” at the end of the model number, the “Sport” designation, and most importantly the “PRG” label by the bind port which serves as the programming port.
But even then there is no guarantee that your receiver is “easily” reprogrammable. Page 428 and Page 429 of the AS3X receivers thread on 2016-07-28 clarified the issue. As of this date, only a few AR636A receivers can be directly reprogrammed using the app. Most previous AR636A receivers, even if they look identical to the above, must be connected via an USB cable to a PC, and have its receiver firmware flashed, in order to unlock the receiver for reprogramming. If you want to program the receiver from your Windows PC, and not from your phone, buy the USB Programming Cable (SPMA3065) instead. Then download the PC software.
At this time, it seems like one reliable way to find out if the receiver from your plane can be reprogrammed without requiring a firmware flash is to look at the Programmer app. In the New Model page, planes with reprogrammable receivers are listed explicitly.
If you search in rcgroups.com, you’ll find enthusiasts discussing AR636A reprogramming, such this discussion in the Night VisionAire thread around 2015-10-29. Note that, like this article of mine, no online information can be trusted alone. And not all information from the past you dig up will still be current and applicable by the time you read it. That discussion from 2015, for instance, makes one believe that wiping out the receiver firmware via a PC and losing the panic mode in the process is the only way to reprogram the AR636A from the Night VisionAire. That is not true, or at least no longer true as of the writing of this article (2016-07). Use the app itself as the definite source of truth.
If you are considering buying an Eflite plane, and want to make sure you can reprogram its receiver, with or without the need to first flash the firmware, start with the bindnfly.com page which lists all planes. For some planes it lists their exact receiver model number. These are planes listed as having the AR636A receiver:
- NIGHT VisionAire
- Pulse™ 15e
- P-47D Thunderbolt
- Rare Bear
- FJ-2 Fury
The page alone is not to be trusted, however. It lists the ParkZone® Sukhoi SU-29MM as having a normal AR636 receiver. I have the Sukhoi, and I know it comes with a locked-down AR636A Sport receiver. And AndyKunz from Horizon Hobby confirmed on rcgroups.com that no Sukhoi receiver can be reprogrammed without firmware flash. Now you know what keywords to search for online, when researching a plane. Do your homework first.
Learning to Reprogram the Receiver
So you’ve got the AS3X Programming Cable. And you’ve downloaded the Spektrum AS3X Programmer app. Furthermore, your receiver does not require firmware flash. Where do you begin now?
If you are relatively new to RC flying, you should start with the AS3X page at Spektrum. Just remember that almost everything they write about the AR636 applies to the AR636A. Spektrum has a 16-video tutorial series on AS3X on Youtube, designed for users who buy an AS3X receiver to install on their own planes. It is useful to go through the entire set if you are new to the scene, starting with Spektrum AS3X Step 1: Transmitter Setup.
But for the purpose of reprogramming AR636A, what you need are Step 13, 14 and 15.
Spektrum AS3X Step 13: Exponential and Dual Rate explains how to set expo and dual rate on the receiver, in one of the three receiver-side flight modes. Honestly, I recommend that you change expo to 0% and D/R to 100% in all receiver-side flight modes. I set up expo and D/R on my radio instead. This allows me to decouple expo and throw from gyro gains. Recall that the bundled expo, throw and gains were what caused pilots grief in the first place with these locked-down receivers.
Spektrum AS3X Step 14: Gain Adjustment explains gyro rate gain and heading gain very well. You must watch this video. You should note that there is a small difference between AR636 and AR636A. SAFE® planes hijack the heading hold feature of AR636A to implement the Panic Mode. When you hold down the panic button on the radio, it forces the receiver into heading hold mode, to move the plane back to a stable, upright orientation. So you will want to leave the heading gain settings in N/A, if you want to keep the panic mode. You can, however, freely change the rate gains.
Spektrum AS3X Step 15: Relative Gain Settings is the key to understanding how to find the right gains for ailerons, rudder and elevator for different flying scenarios. This only applies, however, if you have a modern Spektrum radio such as the black DX6, DX9, DX18, etc. that support telemetry, and is updated with modern firmware. Your AR636A receiver is able to send back its expo, D/R and gain settings as telemetry data, to the transmitter. The transmitter then allows the pilot to dial up and down the relative gain, on the radio, while flying the plane, until the right gain is achieved. Once the right gain is found, you read the “actual” gain shown on the radio, and reprogram the absolute gain on the receiver to match what you found.
Actually Reprogram the Receiver
So you patiently learned to reprogram the receiver. And now you are actually going to do it. Just know that you are doing this at your own risk. No one can save you if you screw up.
First remove the propeller from your plane. You never want to program your receiver with your propeller on the plane. Most tutorials advice you to remove your receiver, and power it with a battery pack for programming. But no one does this in practice. Everyone just plugs a flight battery pack into their plane, and power up the receiver that way. So you want to remove the prop. See how John Adams does it in this picture? Connect the programming cable to the bind port of the receiver as shown.
Power up your receiver. And launch the programmer app on your phone.
Don’t do anything in this app yet. Do not follow the initial setup instruction from the Spektrum video. Instead, just connect the programming cable to your phone. The Spektrum app will detect your receiver, and prompt you to create a new model FROM THE RECEIVER. This is the key to your success. You must let the app download current factory settings from the receiver to your phone. The app apparently tracks the model of the receiver it connects to, and it will not allow you to synchronize model changes you make in the app to the receiver, if it did not create this model from the receiver in the first place. For the Night VisionAire you will see default factory settings of AR636A reflected in the app. Flight mode 1 and 2 are identical.
You should take this time to name the auto-created model. And from the Settings pane, you can email this model to yourself. Save it as a backup, so you can go back to factory settings later, if you wish.
In the Settings page you’ll find that the app sets the “receiver type” for this model to “AR636 (SAFE)”. This is not a receiver type you can normally choose in the app, if you were to create a model file anew, without synching it from the receiver. The app apparently knows that it is dealing with a special, customized AR636. Note how it sets heading gains to N/A? The app knows that SAFE® uses heading hold to implement panic mode.
You can now tweak rate gain, D/R and expo on each axis. And you can try it out in real-time, too. After you tweak a value, the app will automatically synchronize it to the receiver. You can move the sticks on your radio to see these changes right away. Just remember two things. First, for the AS3X to engage in the receiver, you must first throttle up past a certain percentage – it’s a safety guard. So if the plane doesn’t seem to respond to your gain changes, make sure you have activated the AS3X once by throttling up, and then back to 0% throttle. Secondly, you have already removed the prop, right?
All I wanted was a cruise mode with less pitch rate gain to avoid the porpoise-like motion. I also did not want the receiver to impose any expo and throw constraints on any axis. So I edited flight mode 2 to make it so.
I also wanted a mode where stabilization is completely turned off. So I made the same changes as above to flight mode 1, and in addition zeroed out all rate gains.
Now I have three flight modes: 1. no stabilization, 2. low gyro stabilization, and 3. high gain for 3D flights.
That’s it for today. Fly responsibly. And try not to make garbage.
ps: 2016-07-26. Eflite is about to make available a second generation Sukhoi SU-29MM. This is the first time Eflite publicly states that a plane with the AR636A receiver can be tuned using the programmer app on a phone. But there is scantly any additional information on this detail. Instead, the gen-2 page and video go to great lengths to explain how pilots can now easily disable the Panic mode at binding time. I think Eflite is solving a non-issue – no one cares about disabling the panic mode. People want to learn to free their planes from the tyrannical regime of bundled receiver parameters. Instead of releasing a gen-2 Sukhoi with a disabled panic mode, Eflite and Horizon Hobby can better engender enthusiasm by simply providing proper documentation on reprogramming of AR636A receivers in existing planes. That said, this gen-2 plane probably comes with a receiver that can be programmed without firmware flash. I expect this plane to show up in the known plane list in the Spektrum Programmer app soon.
ps: 2016-07-28. There is apparently a new set of planes with the so-called “SAFE Select Receiver” (Spitfire and Timber). These are advertised as easily reprogrammable. I take it to mean that you don’t need to flash the firmware and lose SAFE features. Perhaps the Sukhoi Gen 2 comes with this receiver as well.