JR 770T Gyro
Simon Lockington

CLICK ON PIC FOR FULL SIZE IMAGE

The 770 mounted on the Stratus.

For many years now I've never thought much of JR gyros. Their transmitters and servos are all first rate, but for some reason, their gyros never really held a candle to the Futaba GY range of gyros. Futaba gyros just seem to work, take next to no setup and perform extremely well. While I have seem some JR gyros go well, it's often because the user spent a hell of a lot of time setting them up, invoking all sorts of black magic and just happened to get one that worked, because there were plenty that didn't.

Could it be now that things have changed? In 2007 JR released the new JR 770T gyro which when you look at it, reminds me of a GY401, it's just a little box with no head unit, a couple of dip switches and a dial to set end points. This in itself is good because modern helicopters seem to have less and less space for all of the electronic wizardry we're inclined to use these days!

The 770 also comes with a new high speed 8900 servo with a heat sink case built into it like the Futaba GY6XX series servos. The 8900 has impressive statistics, 0.5 second transit time and 1.36kg of torque.

SETUP
I went through the instructions for setting the 770 up on my 14MZ in a MA Stratus. Set the ATV's at 150 it said. Hmmm, if you do that with a GY series gyro, the piro rate is going to be blinding! Moving on, I then set the servo limits using the same method as the GY401.

One thing that wasn't clear in the instructions was what to do if the servo wheel is slightly off center, are you able to use sub-trim? Luckily I did not need to, but still, would be interesting to know.

I then checked to see if the rudder needed to be reversed, which it did. I then swung the tail to see if the gyro needed to be reversed and found the gyro wasn't responding at all! Hmmm. Another look at the instructions yielded no help, but I suspected that the gyro gain lead may have been plugged in wrong. It's not clear which way the plug goes into the reciever as it only has one wire attached to it and it's not clear which way up that goes into the reciever. Sure enough, reversing the gyro gain plug saw the gyro come back to life. The gyro didn't need to be reversed for the Stratus.

Now onto the gain setting which had me going for a while due to my own fault. I hadn't taken the model program's gyro setting off AVCS and so moving the gain under 50% which should have yielded normal mode operation (above 50% is heading hold) was yielding nothing! Once I switched it off AVCS, everything came right.

The instructions talk about using Dual Rate and Expo to adjust the feel of the gyro during flight. I'm not sure why you wouldn't just adjust ATV to change the piro rate? Surprisingly, JR recommend setting the expo of the rudder to 50% (for Futaba that's -50%)! I very much look forward to finding out how that's going to feel!

After a few flights I decided to reduce the ATV's back to 100% and adjust the Expo setting to my taste just like you do with a Futaba. I really don't understand why JR advocate setting the limits to 150%.

FIRST FLIGHTS
The helicopter lifted off with no troubles and in hover seemed ok. The initial tail response around center was good, but I didn't give it any full rudder commands just yet.

In forward flight the tail started wagging, so I landed and wound the gain down, much better this time, however in forward flight the response around center felt a bit strange, like there was not much going on, then all of a sudden a whole lot, just like you expect with a lot of expo...

Once back in the air I decided to test the full piroette rate out and not surprisingly, the helicopter turned into a blur with a piroette rate so fast it was unusable. I landed and reduced the ATV from 150 to 100% to try and get a more controllable piro rate.

I began some piroetting flips to test the response when the tail on the Stratus failed due to the boom slipping back and losing tail drive. I hit hold just before it went in, unforutnately when I flicked out of hold after it hit, I was still in Idle Up so a fair bit of damage was caused.

SUBSEQUENT EXPERIENCES
I haven't flown the Stratus since it went in, but I have had a number of experiences with other JR 770's since then. One on a T-Rex 500, where the 770's very small footprint is ideal and another on a Synergy N9. I met the Synergy when it's owner wanted a hand to get the engine running right and get it to fly nice. However the machine's tail performance was terrible. The tail was wagging a lot and letting go very easily. Changing gains didn't seem to yield much change at all, it was really annoying the owner who was ready to throw the 770 out for a Futaba GY611.

Given that I have seen and experienced 770's that go well, I figured we should be able to get it to work. One thing that did stand out on this machine was that the gyro was tightly strapped down on it's mount. I see this a lot, and I'm not a fan. I think strapping the gyro is OK if you do it LIGHTLY. If you strap it down tight, you'll just be transferring vibrations from the airframe directly into the gyro sensor.

The first thing we did was remove the gyro strap which yielded an immediate improvement. The next item we tried was swapping the thin and hard gyro tape for some of the thicker green Zeal tape. Once this was installed suddenly the tail came good. No more wagging.

Then I spent some time on the engine and got that running smooth so that the gyro wouldn't be affected. After that we just gradually wound up the gain until the tail was holding as required. Once that was done, the tail performance was very good indeed.

The tail never stepped out during rolling tail slides or fast backward aerobatics and I didn't detect any changes in piroette rate during piroetting maneuvers such as piro flips.

WOULD I BUY ONE?
I must admit to being vary wary of trying this gyro. I've never really seen a JR gyro go well, and deep down, didn't expect this one to be any different. However I was pleasantly surprised at how it just worked and showed a lot of potential right from the start. The 770 seems to be a lot more sensitive to problems than say a GY series gyro which are very forgiving of less than ideal setups, but once all the components such as mounting, engine vibration etc are satisfied to it's requirements, then it performs very well indeed.

Are you missing any functionality from a 770 that you get in a gyro with a head unit such as the GY611? I don't think so, especially for 90% of pilots who wouldn't touch the advanced settings in the GY611 menus anyway.

I would definately be happy to buy one of these for a small heli like the T-Rex 500 where the small footprint is just awesome, and I'd certainly buy one of these over a GY401, however I'm not yet convinced the gyro is better than a GY611 on a 90 size, simply because I haven't had enough time with it yet. However given that the 770T is US$120 cheaper than the GY611 (based on Ron Lund's prices in April '08), you have to give the 770T some serious thought.

Overally, JR have finally produced a decent gyro whose performance is easily replicated on many different machines that is finally able to take the fight to Futaba's range of gyros.