For those in the scale helicopter world, the Bavarian Demon Axon gyro represents the latest and arguably best programming and operational technology out there.  My aim in writing this article is to clear out some of the cobwebs that exist in the minds of AXON owners when it comes to head gyro settings.  The instructions are of scant value. Most pilots will accept the default parameters and then find themselves sub-consciously compensating for settings that are sub-optimal for their particular helicopter.

This article will cover each of the 5 parameters in the head gyro menu. For this piece, I flew a Vario Bell 429 (48 lbs) running a Jakadofsky Pro 5000 at an altitude of roughly 6,000 feet above sea level during 83 degree temperatures (Chatfield State Park in Littleton, Colorado).  Each flight lasted approximately 8 minutes.  Here’s how I set up the flights:

• I used the yellow, red and blue banks in the software and set each bank identically to begin with.
• I started by changing the “Direct Input Proportion” variable differently for each of the three banks, while leaving the other parameters identical in each bank.  This allowed me to isolate the effect of changing a parameter during the same flight.
• For each parameter change, I landed the helicopter, kept the disk spun up and merely changed the 3-position switch assigned to the colored banks. This is NOT required. You can switch banks mid-flight if you want.
• For each parameter and each bank, I performed (a) a right-hand facing hover for at least 20 seconds, (b) slow forward flight both up and down wind, (c) steady, but fast, forward flight and (d) higher speed sweeping bank turns. 
• I'm flying a Jeti DC-24 with 20% exponential on the cyclic and -5% expo on the tail. 

FLIGHT #1 - DIRECT CONTROL PORTION PARAMETER

As with any FBL system, you do not have direct control over the swashplate with your radio.  Instead, you give commands to the gyro and the gyro filters those instructions before commanding the swash.  That's where the Direct Control Portion comes in. On a scale of 0-6, a zero gives you the least amount of direct control and a 6 gives you direct control.  When we talk about direct control, realize that it's not full, sweeping control.  There are other settings in the gyro that also have a job to do.  Those parameters will kick in based on how you set up your Direct Control.

Lower numbers on the scale translate into a more regulated reaction of the cyclic for any initial stick movement.  Obviously, this "softening" will be compounded by whatever exponential you have set for either elevator or aileron.  As you climb the scale, there is more of a 1:1 initial response from the helicopter for a given cyclic input. If you set your Direct Control to 3, the gyro will give you half as much direct control initially as it would if you were at 6, which would allow you to initiate movement before the gyro starts to regulate things based on agility parameters.  For 3D flying, you might want a setting around 5 or 6. Scale or smooth pod and boom flying would demand a "softer" 2-3 setting.  You can think of it as initial responsiveness. If you need immediate, precise responses, set the DC to 5 or 6. 

A Bit of Agility Training

Direct Control works in conjunction with agility to affect both initial response and maximum pitch and roll rotation rates. As we discuss below, agility controls the RATE at which the machine may rotate about an axis for a given stick movement. For example, let's say a setting of 10 for agility gives you 90 degrees of roll per second for a full stick deflection. If you move the stick halfway, you'll get 45 degrees per second.  You would get the same result by simply adjusting the agility setting to 5.  Importantly, the agility setting tries to maintain the rotation rate that you instructed it to maintain by moving your stick.

If you set your DC very low couple that with high agility, the results might be displeasing. Your initial stick inputs would be more slowly implemented while the attitude change rates would be aggressive resulting in a slow and sluggish initial response followed by Mr. Swash's Wild Ride as the gyro tries to maintain the roll rate you instructed it to maintain (you might keep feeding in roll because the initial response was low and then suddenly the gyro thinks you want to maintain a rapid roll rate).  You would feel like you were one step behind on the controls.  On the other hand, if you set the DC high but run agility low you can initiate attitude changes quickly but the regulated attitude change rate (say pitch) would be low and might provide insufficient swash movement to complete the maneuver.  If you were trying to loop, for example, the agility setting might prevent a pitch rate of change above the minimum needed to perform the loop.  

Back to the Test

For this test, we set the Direct Input Control parameters in a pretty tight band. The scale goes from 0-6, so a 2 is 33% of the full range. To keep things tame, we tried settings of 3, 4 & 5.  This test was difficult to measure just by flying. I had to review video to see how smooth things were in a hover, where I think this control has the most impact. My helicopter has a rescue guy on a winch so I have to hold the helicopter steady in a 90 degree (right) hover so I can see it work. In video, I could tell that I was holding the helicopter more steadily (fewer jerky adjustments) in a 10 mph cross wind at the 3 setting.  This setting is a great place to start anyway because it represents 50% of the scale.  The lower the number, the softer the response.  Again, combining this with 15-20% expo will give you a soft stick center but still leave you with enough authority to do emergency or aggressive maneuvers.   

FLIGHT #2 - AGILITY PARAMETER

As you learned above, the Agility parameter sets the maximum rotation rate about an axis for full stick deflection.  If you move the stick full right, the heli will roll at n degrees per second. As you decrease the agility, the gyro will slow that rate proportionately.  As you climb the agility scale (5-20) the heli becomes more sensitive for any given deflection. When you move the stick, you are commanding the gyro to rotate at a given amount of degrees per second.  The gyro will move the swash in order to honor the command.  Initially, as you give a rotation command, the swash tilts a lot until you hit the rate instructed by the stick location and the Agility setting.  Then, the gyro adjust swash movement based on what it needs to maintain that rate.  If you set at 10 and move full right aileron, it will roll the swash at, say 90 degrees per second, and then try to maintain it.  Then if you set Agility to 5 you will get 45 degrees per second for full deflection. If you set the parameter too high, you may run into mechanical limits at which point the helicopter will never be able to achieve your desired agility. 

The Test

On a scale of 5-20, we tested 6-10. For smooth scale flight, we elected not to test above 10 because at 10, the helicopter was reacting more  abruptly than I was comfortable with. The Bell 429 we tested weighs approximately 48 lbs so it's possible that a heavier model might handle an agility setting above 10 better than this one. For us, 7 was a nice compromise between smooth but not sluggish handling. At the 7 level, I flew the helicopter toward me at a medium rate of speed and then swung a 180-degree turn into a light tail stand to a landing. The responsiveness was terrific but not too violent. 

In our tests, we plugged the gyro into a computer to change settings.  But, there is an easier way. Set your travel adjustment to 100% (or close to that). If you then set Agility to 10 and your elevator and aileron dual rates (in the radio) to 50%, it's exactly the same as taking Agility from 10 to 5 because the radio is cutting the servo movement signal in half.  The beauty of this setup is your ability to simply bump your dual rates in the transmitter if you need more agility, rather than having to reprogram the gyro. 

FLIGHT #3 - DECAY PARAMETER

The "decay" parameter controls the speed with which the swash returns to neutral after the stick is centered. The higher the number (from 2-10), the faster the swash returns.  If the number is too low, you can get into situations in which the swash never returns to true neutral. That said, the higher the decay, the more likely it is that the helicopter will ascend in forward flight absent down pressure on the cyclic stick.  A low decay rate (sluggish swash return) combined with a high attitude hold setting (below) will make the helicopter feel robotic. The key is finding the balance.  For our test we tried decay settings of 5, 7 & 9.  For large scale machines, we recommend a starting value of 7. I have my Bell set to 8 because I like the return to neutral and am willing to babysit the ballooning issue.

FLIGHT #4 - ATTITUDE HOLD PARAMETER

The "attitude hold" parameter is designed to keep the helicopter in the orientation you had it in just before you re-centered the stick. If you pull up 45 degrees and then recenter, for example, the system will try to hold that.  For 3D helis, this has advantages. For example, if you pull the heli to vertical as part of a square loop and then center the stick, you are looking for the machine to stay on the vertical line.  If attitude hold too high, the gyro tries to push the nose back down (over-compensating). If the setting is too low, after re-centering the stick, the machine will coast over onto its back (under-compensating).  On a large scale ship, if your Attitude Hold is too high, the gyro can makes the heli look robotic and less smooth. If the number is too low, in a wind, the heli will not sit in a hover, but who cares? The pilot has to do SOMETHING! 

The Test

The scale runs from 2-10 and in our tests. At a 9 setting, I was questioning my ability to make a smooth turn because the heli held its banked posture! I came out of the tests most pleased with a 5-6 setting.

In the video below you will see a comparison between a low attitude hold setting of 5 (video says 6 because that's what we later determined was optimal for this model) and a high of 9. Although the video concludes that attitude hold was responsible for the gnarly turn in the second clip, it is likely that the decay rate contributed to the issue shown in the video. During the second clip, we had the decay rate at 5.  The optimal rate we found was 8 so it's likely that the slower return of the swash to neutral during the somewhat aggressive turn caused the helicopter to remain in a banked attitude (especially when coupled with a high attitude hold rate).  I was careful to simply return the stick to neutral after the turn vs. counter turning to level out the ship. 

FLIGHT #5 - ELEVATOR FILTER PARAMETER

The "elevator filter" parameter controls vertical bobbing of the tail. Using a scale of 0-10, try the lowest setting possible.  The higher the setting, the slower the helicopter's response rate (it's a filter, remember). Think of it as a dampening mechanism. On helicopters with large horizontal stabilizers, you can set the Axon to 3 or less based on our tests. We tested 3, 5 & 7.  Three was optimal with no discernable sluggishness in response time.

FINAL HEAD SETTINGS

We spent a good bit of time messing around with the settings.  It's fun, in my view, to be able to tweak your machine in such a way that these small adjustments combine to make for a more enjoyable model. At the extremes, you would not like what these settings do for a scale ship. Bavarian Demon knows this and their initial settings are pretty good.

There are two ways to go about tuning your parameters.  If you are flying a Jeti radio, you can connect telemetry functions to your transmitter and make quick head parameter changes right from the transmitter. This works provided you are using the SINGLE cable connection to the AXON.  If you are using the loom (like I do because I'm using the Central Box), you cannot make changes from the transmitter. Instead, you'll be schleping your laptop to the field and making those changes at a table (hopefully in the shade). But just imagine how impressive you'll look with all of these complex dials and your perfectly cogent explanations of each parameter to stunned onlookers who will now regard you as an RC genius. 

By the way, the Axon banks are not just for testing. You can essentially create flight modes by setting favorable parameters for different conditions, like windy vs calm or hover cs forward flight.

It took me a year of flying my Bell 429 before I ventured into these settings with any real understanding. Thanks to Danny Melnik of Aeropanda for great insight and just the right amount of sarcasm as I embarked on this painstaking process. I should also note that you may get different results from me and your flying preferences will surely differ from mine. Your comments below are welcome. In the end, this article and your comments can serve as an enduring resource for those who wander into the wonderful world of Axon gyros!