Backward, inverted flight with tail blow outs?

[Gladius]

Everyone,
I have been practicing inverted flight for several months now on the sim, and have only recently been comfortable enough to really try it on my real helis. One of the things I have noticed is that when flying backwards while inverted, I can get the tail to blow out about 180 degrees if I flip the tail up to go back upright. I've been searching the forum for backward, inverted, tail blowout issues to see what could be causing it, and haven't found anything real definite.

I'm flying a Trex 500 ESP that has been converted to FBL, using a ZYX controller. It has an 1800KV Turnigy motor with a 13T pinion, and a Hobbywing 80A Platinum Pro ESC governed at 2650 rpm. I am running Sky Lipo 3000mah/40C batteries, and also Turnigy Nanotech 3000mah/25C batteries. I have had the blowouts happen on each kind of battery. I'm running a CC 10A BEC. The tail servo is a Protek 230S full size servo.

The tail is set up in rate mode (although I've found a Finless Bob comment regarding the tail on the "certain Align models" that indicates this may not be helping matters). The tail is rock solid in upright and inverted punch outs, and all upright orientations. I can fly the heli backwards while upright with no issues. I need to test it during pitch pumps (just don't do those enough to have noticed any issues yet.) There is no binding, that I can tell (yes, I have checked). I've read that it could be caused by my own lack of skills, which is a definite possibility as my precision is not anywhere near where I want it to be yet.

Does anybody have any pointers for likely causes of this? If it's just my own lack of collective management, that's fine, and I will enjoy working on that. But I don't want to ignore a mechanical or electronic problem because I chalked it up to my own sub-par skills.

[blade450]

I would say something is off on your machine.
The only thing I can think of is the tail gain. Keep increasing it until the tail starts to wag and drop five points.

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[FailureToFly]

There are a lot of forces on a heli during these moves and if you're going fast and are hard on the collective, it'll more likely blow out.

Anyone can blow out a tail at high speed if they're ham fisted but tail blade size is also a factor.

Have you tried just flipping the tail down rather than up?

[Gladius]

My tail gain is about as high as I can get it. I actually lowered the gain by a point on my Tx the other day, because the tail was wagging slightly.

I was doing the maneuver kind of fast, so I guess I'll try it again slower and see what happens. I have not tried dropping the tail. Once I realized what had happened, and tried to do it again to see if it was a fluke or not, I wanted to keep it moving away from the ground! When it stops raining, I'll give it another shot. Thanks guys!

[suthae]

If it's not the tail gain... they most probably there's an issue with your tail mechanics...

Make sure it's buttery smooth.... the slider and bearings.... re-build your tail if you have to...

Also, make sure you've set your tail travel limits correctly... they can blow out if you have not given enough travel for your tail...

[sl4ppy]

Check that you have the blade grips facing the right way. Most modern heli's are leading edge control on the tail blade grips and it's easy to put them on the other way on accident. The heli will fly fine this way, it just has a LOT less authority.

[Gladius]

The tail blades are leading edge control.

Right now, my tail is set up per a rate mode setup. I found an old thread where Finless makes a pretty good argument against doing this. He says that positioning the slider away from the center of the tail shaft (0* pitch) allows the tail servo less resolution to provide fine control of the tail. I think I may try to redo this setup to see if it is any better.

[Russell Harding]

Try carbon tail blades.

The maneuver you describe (especially at high speed) is one of the hardest on the tail. You need to achieve and maintain maximum tail blade pitch to avoid blowout.

Double check your endpoints, and eliminated flex in the pitch system. Pushrod, pushrod guides, pitch lever, and tail blades. One of the first things I would look at is tail blades if you have the stock blades on.

KBDD have more surface and are stiffer. Anything carbon will likely go a ways towards solving this, too.

[Gladius]

I'm running CF tail blades already. It came with KBDD tail blades, but I switched them out for CF blades pretty early on, due to the stiffness - made a huge improvement too. My tail pushrod is also CF, and I have an upgraded dual pivot pitch slider. The endpoints of the pitch slider are just about maxed out (maybe a millimeter of travel before they bind).

Pretty much every shaft and bearing on my tail has been replaced since my last crash. I suppose I can always check them again!

When I do the flip more slowly, I can pull it off without blowout. This may just be a case of my ham fisted control exceeding the capabilities of the helicopter. Inverted flight is still really new for me, so I have very little finesse while doing it, and so I'm not that surprised.

[Himilou]

Does it always loose authority in the same direction? If so have you added a tail preload to the blades? About 8 degrees of pitch CCW when the servo is centered.

If this doesn't help there is a setting in the zyx setup ( pre comp I think ) which helps tail input to high pitch maneuvers. Though I don't know if it has any affect in rate mode.

Do you never use HH mode? I don't know of anyone that flies rate mode outside of setup purposes.
for clarification HH mode = heli auto adjusts to keep same compass position with no stick input
Rate mode = total manual control, wind blows heli and heli heading changes

[Gladius]

I only fly in HH mode, just use a rate mode setup. So yes, the tail blades have some pitch with rudder at midstick. I am also using pre-comp to keep the tail rock steady during max-pitch pumps. I appreciate the advice! I think I was just flying the heli beyond its capabilities though. If I give less cyclic, the tail doesn't blow out. I've also manage to get the tail to blow out in upright, backward flight by flipping it too quickly. I just need to learn how to finesse the sticks a little more.