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Using a 3-D Aircraft Stabilizer



What products are out there?

Two of the powerful and relatively cheap solutions on the market now are by Feiyu Tech. Eagle Tree is also about to come out with a unit which at least on paper looks superior.

FY-20A was Feiyu Tech’s first dedicated stabilizer. Now available for about $75. It’s the only one I have used, so far.

FY-30A is their new unit. Available for about $125. It’s supposed to have much better performance, it can be used to stabilize a camera, and it can be re-flashed when software upgrades come out. It also outputs pitch and roll angle data via a data port, which is useful for FPV flight and for autopilots.

Eagle Tree is coming out with another one, the Guardian. It’s expected to sell for about $75, and be more powerful (and considerably smaller) than the FY-30A. It’s due to reach stores “late Spring” which I hope means by June 30. As soon as it becomes available, I expect it to push Feiyu Tech out of the market or to much lower prices. Keep that in mind when you make your buying decisions.

Some other manufacturers also have these devices, but AFAIK they are more expensive and not used as much. Stabilizers are also built-in or options for most autopilots. For example, Eagle Tree has a “Guardian Expander” for its popular data logging system ($55). Feiyu Tech has several autopilot systems that incorporate stabilization.

30A Gyro

Eagle Tree


Do they work?

The short answer is “Yes.” The rest of this note is my “long answer.” Remember I’ve only used the FY-20A so far.  Jim Bonnardel reports that the FY-30A is indeed superior, but I also talked with another club member who crashed with an FY-30A on its maiden flight. So some of my cautions about setup at the end of this article still apply.

3 modes of operation. This is the key to these devices, so I’ll try to explain clearly. Control the FY unit with a 3-position switch, which needs its own channel. The first position is “off,” the second is “3-D mode,” and the third goes by various names including 2-D, but amounts to “straight and level flight (S&L).” The 3D and S&L modes are completely different behaviors – it’s like having two different personalities.

The first and most important mode is “off.” Be very sure this works when you start out, because if anything goes wrong, you can go back to normal flying. There’s a “quirk” in the FY units that means the “off” mode does not always work – I will explain later.

The mode that I mostly use is the 3D mode. This uses the gyros to sense, and compensate for, any non-commanded changes in pitch, roll, or yaw. You still fly the aircraft normally, and it responds normally to your control sticks. But acrobatic maneuvers are much smoother, landings track very well, and so forth. I am now at the point where I customarily fly in this mode for about 80% of my flight time.

Here’s an example of what 3D mode does. Most aircraft have some adverse yaw when banking, which we either control by hand with the rudder, or use mixing to add rudder automatically. With a stabilizer, adverse yaw is automatically compensated for. When you roll right, any attempt by the aircraft to yaw left is countered by automatic rudder movement.

The S&L mode is intended for emergency recovery, and hands-off flying. Let go of the sticks, and the aircraft will recover from whatever it was doing and fly straight and level. You can also work the sticks to turn or climb, but only very slowly. For example to turn, you have to hold the stick hard over, and keep it there.


Bugs and problems

The FY-20A has some problems. Most serious, it has twice behaved in ways I cannot explain. (One of them was this week.) The temporary solution is to turn it Off and land asap. Size of the unit is a bit large for small aircraft. (Jim just cut a hole in his foamy!) Physically installing and removing is awkward, so you need another $100 unit in each plane.

Gain adjustment and control oscillation: If the gain on any channel is too high, the corresponding stabilizer will “hunt” back and forth around the proper setting. This works the heck out of the servo, and puts stress on the airframe. The gain is adjustable on the ground, but the best gain setting depends on the airspeed. (The new Guardian unit will have gain partially adjustable in the air.) So I have my gains set to be OK normally, but at top speed and especially in a power dive, I get aileron oscillation (looks like flutter) when in 3D mode. So I’m trying to train myself to turn the unit Off before going to high speed. Note that it’s the airspeed that matters, not the power level.

Servo wear and power consumption. When the stabilizer is on, it is constantly making tiny adjustments to the servos. You can hear this on the ground – if there is any wind blowing at all, you’ll hear servo movement even if it’s too small to see. This supposedly increases power consumption on the BEC, and may cause servo wear. I don’t know much about servos, and don’t know if this is real. When I switched from Spectrum to Hitec with telemetry, I decided I needed an external BEC on all my models anyway, so power consumption has not been an issue for me. (Current draw of the FY-20A itself is spec’d at 52ma at 5 v.)

S&L mode is somewhat sensitive to airspeed. As speed increases, you will start to climb. The technical reason is that the accelerometers can measure attitude, but not changes in altitude. At a fixed attitude, more airspeed = more lift = climb. This is inherent in the physics of flight. Only a true autopilot can deal with it (one which keeps track of the barometric altitude). I explain some of this at

Bad default on startup: This caused two early crashes, I suspect. When the FY-20A powers up, it starts in one of the stabilized modes! If anything is wrong with the unit or the setup, this means your plane will behave in unexpected/ incomprehensible ways. Checking that the transmitter switch is “off” won’t help, because the unit is  “on” anyway.


Setup and Use

Mounting: Put it in your plane, as near the 3-axis CG as convenient, with the arrow pointing either straight ahead, or straight back.  It should be as close to level as practical, although you will be able to “trim out” small deviations from level once you get airborne.

All MEMS accelerometers are sensitive to vibration. I’ve never found vibration to be a problem (with electric motors); just 2 strips of Velcro for mounting seems to provide enough vibration isolation. But each FY unit comes with an anti-vibration “trapeze.” It needs a lot of space to mount, and I have not found it necessary – so far.

Setup: Program an additional channel on your TX, tied to a 3 position switch. The channel should be set up similar to a “flaps” channel. Position 1 is no signal, position 3 is full motion, and 2 is a halfway signal. You might need to play with End Point Adjustment on your TX, but I never had a problem.

Connect the Aileron, Elevator, and Rudder channels on your RX to the appropriate stabilizer inputs. Then connect the actual servos to the corresponding stabilizer outputs. Therefore, it’s not convenient to switch these devices from one plane to another. Attach the new “control” channel from your RX to the appropriate input on the stabilizer. Caution: the pins on the FY-20A were not well designed (the FY-30A is better), and I always worry about bending a pin.

Now you have to adjust the gains on all 3 channels, using a Phillips screwdriver. Low gain on a channel means the plane responds more gradually to deviations in that axis. I won’t go into how to set them, except to say it’s best to start with low to very low gain settings.

Flight Conditions in your TX for turning stabilization on and off. This gets tricky, it’s not well documented, and it took me a while to figure out. They are the easiest way to take care of changing mixing, expo, end points, and other settings automatically when you change stabilizer mode. My TX (Hitec Aurora 9) makes them relatively easy. I have my old hand-flying setup as “Normal Condition,” and a “Stabilizer Condition” which shuts off  mixing, re-trims, and makes some other changes. The Flight Condition is selected by exactly the same switch that changes stabilizer modes. One big virtue of Flight Conditions is that trimming on the A9 is separate for each Condition. Since the stabilizer is hard to install exactly level in the aircraft, when I turn it on I would otherwise need to re-trim. See for an explanation of flight conditions.

Calibration is almost never needed. I calibrate when I first install. Thereafter, the manual recommends calibrating only when there is a large temperature change (20 degrees – I assume that is 20 Celsius). Around here, that is rare. I think I’ve recalibrated once since I got mine installed.



Test, test, test. I found out the hard way. On my first flight with the stabilizer, the aircraft acted erratically the moment it took off. I couldn’t control it, and ended up flying my Parkzone T-28 into the ground, breaking the fuselage. I rebuilt it, tried again, and flew a few circuits. But again something went wrong, and although I frantically flipped back and forth between Off and S&L modes, I crashed again. (This time the wing separated.) I was a novice flyer at the time; when something similar happened to me this week I was able to recover and after some trial and error got it working again.

The key test is simply moving the aircraft in three dimensions by hand, and watching what the servos and control surfaces do. Change the stabilizer mode, and do it again. Repeat. Run the motor up to full to see whether there is a lot of vibration. (If so, the red light on the FY unit will go on.)  Then try taxiing in different modes, although the control movements are too small to see at any distance.



Always know which mode you are in, especially S&L mode. Other than that, it’s ridiculously easy to use – try different maneuvers on the different settings to see what happens.


Preflight checklist:

The stabilizer is always connected between the radio and the servos, so there are no additional connections or adjustments before a flight.

•    Power up the transmitter and aircraft in the normal way. Check the lights on the stabilizer.
•    Test all 3 axes of flight control with the stabilizer in S&L mode.As you tip the aircraft, the corresponding control surface should try to compensate.
•    Then test all 3 axes again with the stabilizer OFF. It is very important to switch the FY units “On” and then “Off” again before takeoff. The reason is bad design – when it first powers up, the FY-20A defaults to S&L mode. Bad, bad, bad.
•    Check that the vibration light does not come when you run up the engine.
•    Taxi out normally.  Flip the stabilizer control to either 3D or Off before takeoff. Do not try to takeoff in S&L mode, because if anything goes wrong, you won’t be able to react quickly.
•    Take off, and go up to safe altitude before you change the stabilizer mode – just in case.

Landing: I am still experimenting. 3D mode seems to work very well for lining up and landing. S&L mode won’t let you line up (turning radius is too large), but once you are  in the groove you can switch S&L on and drop the power.

I will try to write a follow-up to this article in a few months, when I have gotten my hands on the Eagle Tree unit.  Until then, if you see me at the field I will be happy to let you fly my Trojan for a few minutes to check out the feel of the FY-20A.


Links for more information for running discussion of FY-30A. for announcement of the new, standalone, Guardian stabilizer from Eagle Tree. The other version, which only works with the data logger but actually exists and has been used a lot, is discussed at