Hello everybody,

I am passing the following information along as an FYI about gyros. I found this especially interesting since there have been so many mysterious anomalies reported with the BobCat rudder servo/gyro setups.

I only have a few flights on my BobCat, however, the thing has been a hand full with the gear down and the gyro gain turned on. It has snapped on me twice during slow flight and flying it slow was akin to balancing a ping pong ball atop a pin head. Then, on Sunday I put in three more flights at Warm Springs and noticed that, after landing, the rudders were deflected significantly and that the deflection was proportional to the gyro gain. On each flight I ensured that the rudders were lined up perfectly prior to take off, however, upon landing they were deflected between 1/4" and 3/4". ugh...

So, I placed a couple of phone calls to Horizon yesterday and then did some testing last night. My BobCat's JR 450 gyro is basically defective with a phenomena called "excessive thermal drift". Basically, after you turn the gyro on, it heats up internally which can cause the gyro's neutral point to drift. The factory tests each gyro for thermal drift and Horizon says that JR has strict tolerances on what is allowed. Supposedly, they discard all gyros with any measurable thermal drift as part of their QA testing. On the other hand, I think that this could be a fairly common failure mode. FWIW, I will hence forth be checking all of my gyros out for this phenomena.

During my tests I powered the BobCat's receiver (and gyro) up and verified the rudders were at neutral trim. Then, I left the BobCat's radio electronics powered up for 10 minutes to simulate the time elapsed during a flight. During that time, the rudders gradually deflected approximately 3/8" out of trim. The factory tests them by also applying a hair dryer to the gyro for a few seconds to simulate worse-case in-flight heat buildup so Horizon recommended that I do that as well. My BobCat's gyro then displaced the rudders over 3/4" past neutral. Also, the displacement is proportional to the gain setting. In other words, at zero gain, the rudders line up perfectly and then at 100% gain the rudders are skewed to their maximum out-of trim deflection.

In addition, after heating the gyro with the hair dryer, the gyro started making the rudder/NG servos oscillate wildly; Sorta vibrate real fast; Shake, rattle and roll if you know what I mean. Unnerving...

I then tested a spare JR 450 gyro that I had. My spare gyro didn't deflect at all away from neutral trim after 30 minutes in operation. The hair dryer caused 1/32" of deflection at most and the spare didn't produce any oscillations or vibrations. So, I am installing this spare gyro into my BobCat. Horizon has offered to replace my defective gyro.

I am fairly certain that this is why my BobCat had snapped earlier. Just imagine flying an airplane around slow with the rudders out of trim between 3/8" and 3/4" while possibly in some sort of wild oscillation as well. I am probably lucky that I still have the airplane.

Gordon