dodging a bullet! (T-rex 500 Pro DFC - in-flight failure)

[srhammett]

This T-Rex 500 Pro DFC is my baby. I did all the mods when I built it, but it's about two years old now, and I haven't touched anything on it in almost that long. I have a couple of fixed-wing FPV projects going, but I always come back to the 500 for fun LOS flying. I have (had?) lots of confidence in it. Still, I know from hard experience that you can't cut any corners with a heli on maintenance and inspections.

I took it out early today to beat the heat and enjoy some calm air. The first flight was great. I'm still rolling out some new stunts (basic stuff!) that I wired on the sim over the winter; it was going well, and I was getting some rewarding commentary from my neighbor, who often tags along.

Three minutes into the second flight, I had it doing a backwards pass at speed as I considered my next moves. Then, in a flash, the heli does this perfect, lightning-quick half piro, for no apparent reason, and continues along in forward flight. Like it didn't think I would notice, right? I started to bring it in, but it seemed fine. I thought maybe it was just a one-time hiccup, so I continued with some conservative cruising, watching for further signs of trouble.

To state the obvious, that was a dumb move. Always check things out while you still can! About a minute later, the tail kicked and it started spinning wildly in the yaw axis. It was far enough out that it became difficult to stay oriented and keep it level. I was losing it, but at the last second the spinning stopped. I was able to regain control and start bringing it home. I nearly had it over the field when the spinning resumed.

The cycle of crazy spinning and loss of control followed by a few seconds of normal flight repeated itself maybe seven times. A couple of times I barely kept it from the clutches of trees on the steep hillside opposite me; once I brought it back and was about ready to land when it glitched again, with the heli spinning away just inches above some nearby tombstones (I fly in a cemetery). I nearly gave up before I finally got it back over open grass one last time. I immediately hit throttle hold - which I should have done earlier, duh. With torque eliminated, it stabilized nicely.

Outside of the sim, I'd never had the nerve to try an auto from more than about 10-15 feet above the deck. This one was a good 40 feet, and it wasn't pretty, but thankfully it did hit the turf upright and stay that way. A cracked landing skid was the only damage.

There are no answers yet; I've only just gotten home and let my pulse rate return to normal. At a glance, nothing appears to be amiss, except that the rudder servo (Savox SH-1290mg upgrade) is partially jammed. Obviously, it's hard to be sure exactly when or how that happened, or whether it was a cause or effect of the problem. Aside from that, all of the tail rotor linkages look fine.

The forensics can wait for later, though. Right now I'm just ecstatic that I still have my bird, with hardly a scratch, and I'm not up there sweating on the mountainside with the rattlesnakes, trying to collect a bag full of heli fragments.

Plus I pulled off a sweet emergency auto. Yay!

[rstekeur]

Sounds like a pucker moment no doubt

[Gladius]

The backwards flight with a quick spin into forward flight is a classic case of tail blowout. I'm going to guess that you were going pretty fast when it happened? Take heart! It's a sign that your skills have progressed to the limits of your heli (as currently set up). Check everything for binding. If everything is buttery smooth, then chances are your tail servo is giving up the ghost. That's probably part of the reason for the blowout. Some Chinese weights on the tail will also help your servo to work easier, and hold the tail better.

[srhammett]

Originally Posted by Gladius

The backwards flight with a quick spin into forward flight is a classic case of tail blowout. I'm going to guess that you were going pretty fast when it happened? Take heart! It's a sign that your skills have progressed to the limits of your heli (as currently set up). Check everything for binding. If everything is buttery smooth, then chances are your tail servo is giving up the ghost. That's probably part of the reason for the blowout. Some Chinese weights on the tail will also help your servo to work easier, and hold the tail better.

Thanks for the feedback. It was moving backward into a moderate headwind, so high airspeed, that makes sense about the blowout. I just hadn't seen that happen before. Or, I should say, since my cheap fixed-pitch trainer days.

So a blowout would/could happen under the right conditions even with a completely healthy system, is that right? In this case, though, it must have been a precursor to what happened next, i.e., the servo was dying?

Kind of disappointing, since I thought the Savox was a pretty robust upgrade for this ship, plus I spent a ridiculous amount of time tuning the tail mechanics. To be fair, it has a good number of hours on it by now, but I'm also a fairly gentle pilot.

Anyway, I'll be looking it over this evening and I'll report back. Thanks again!

[Russell Harding]

Bad servo or tail linkage... Or more likely: A stripped tail drive gear somewhere.

[srhammett]

Originally Posted by Code3Medic

Bad servo or tail linkage... Or more likely: A stripped tail drive gear somewhere.

I just recently did a inspection on this tail setup. It still looks as good as ever, and it's been a good flying machine. I haven't had a chance to check the servo and power it back up yet. I suspect it will all become clear.

[Epoweredrc]

The drive gear for the tail drive could be worn my father had that happen on his trex 450 same thing just like ur story is what happened to him

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