Need help with Mini-Titan tail

[DWSchmidt]

A friend of mine is getting into helis and asked me to help him with his V1 Mini-Titan, of which he is the second or third owner.

After going over it, I think I have everything working properly, except the tail. I am not sure if the problem is mechanical or electronic, but I am using equipment that I am not used to since it is his heli.

Here are the specifics:

Futaba 401 gyro
Hitec HS-65HB servo
stock tail, with standard white plastic tail blades (installed right from package)
Spektrum DX-7 radio

Current radio/gyro settings:

Gyro sens - Rudd D/R (0: 85%, 1: 15%) (plan to fly in HH (AVCS) exclusively)
GY 401 - DS: off, limit all the way CCW (minimum) - this gets full mechanical throw without binding
servo ball at 10 mm, everything centered and/or at 90 degrees on initial setup)

The problem:

When I spool it up, the nose will easily turn CCW (looking from the tail), and I have to hold full right rudder just to keep the nose from spinning. By the time I get enough power to put it into a hover, the nose is starting to drift left again. I have triple checked the direction of the servo and the gyro and both are correcting the proper direction. Basically, it's like the gyro is trying to counteract the torque, but just doesn't have enough power to counteract the main rotor. The only way I could even get it into the air was to turn the throttle curve down to the point that it was hovering at about 50% throttle. Any more and it would spin before I could even get it off the ground. I have tried tweeking that gains for the gyro, but even at max there isn't enough.

Since I am not a Spektrum guy, I am wondering if there is something I am missing in the radio that it causing the problem.

Thanks for any help you can give!

D.W.

[th0rg0d]

Sounds like you need to reverse the gyro direction.


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[DWSchmidt]

That was my first thought as well. Unfortunately, as expected, the servo and gyro direction are correct. I even tried reversing them from what I thought was correct and it just spun faster.

D.W.

[th0rg0d]

Is it possible the tail is spinning the wrong direction? Are the grips set to leading edge control? Is there excessive binding under load?


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[DWSchmidt]

Turning the correct direction, leading edge control and everything seems very smooth. Very little slop either.

Also the belt tension is correct and nothing feels notchy (at least to my experience).

D.W.

[B335]

Everything about those symptoms suggests to me that the tail servo position needs adjusting, because you currently have not enough right trim to counteract the natural tendency to yaw nose left.

I would suggest moving the tail servo forward 1mm at a time and re-testing. You should eventually find a place whereby the heli no longer tries to turn left.

[DWSchmidt]

OK, maybe I don't have enough throw. Instead of moving the servo, I should be able to just increase the travel throws on the the gyro then, correct? Maybe it just isn't getting enough throw in AVCS even though it is almost binding in Normal?

Thanks!

D.W.

[B335]

No. The servo has to be in the correct, ie central position, so that its position gives the correct amount of right trim to counteract the rotor torque.

Adjusting the throw won't solve the issue you've described.
Moving the servo to the correct position along the boom, will.

[Scott Bowersox]

you want the limit pot on the gyro set close to 100. That's where the 401 likes to be. All the way CCW will limit the travel thus not giving enough authority in either direction. Moving the servo arm to increase CW pitch will only move the small amount of travel being used to a different position and hold it in hover, still not give it any more authority left or right. It's only using a small part of it's movement.

Might have to play with servo ball length to achieve this. With that servo being a bit slow for the tail, a little longer length to the ball might help speed up response.

[antij0sh]

Is the light on your GY401 solid or blinking ? It should be Solid red in HH mode, if its blinking rebind your reciever, the gyro can initialize before the radio and cause the gyro to not go into AVCS mode correctly, and for some reason rebinding seems to fix it.

[DWSchmidt]

Gents,

Appreciate all the feedback!

I'll try increasing the limit adjust toward 100.

Should the tail rotor bellcrank at the back still be at 90 degrees to the boom or offset a little? I know the directions and a couple of the other books I have talk about having it setup at exactly 90 degrees, but I am starting to think this is just a starting position and it may need to actually be a little more or less?

The light is on solid, but I will try the rebinding and see if that helps.

D.W.

[Scott Bowersox]

If you can't get 100 on the travel limit pot, you want at least 80 and no binding or servo whine.
100 is better, but here it is a trade off for a little mechanical speed by having the ball out farther than ideal for limit range

For the bellcrank, if I remember, the MT might be designed with a little CW (right rudder) at 90 degrees to the boom. If not you can add a few degrees at servo center like B335 said. This simply makes servo travel more linear in both directions. However, the 401 only has a total limit pot and not separate left and right travel so you have to find the sweet spot that give full travel with no binding both directions. It takes a bit of messing around with.

I'll try to help more when I get home and have the ship in front of me. You may want to suggest to your friend that most people use mini sized servos on the tail instead of the micro class like the H-65. Something like a JR 3500g or Futaba 9257 or anything else similar to these. They play really well with a 450 machine and 401 gyro.

[B335]

The bellcrank gives you 90deg. What you are supposed to do is make sure your tail servo limits are set without binding, and then move the servo to a position along the boom until you find the spot that just gives a degree or two of right trim and in doing so holds the heli steady without yawing left or right.

Got that?

1. Set servo limits
2. Move servo to point on boom that doesn't yaw either direction (usu a deg or two of right offset).
3. Repeat (2) until you find the right position.

[DWSchmidt]

Sounds good. I'll give those a try this weekend. Unfortunately, 18 hour work days are getting to be a drag right now!

D.W.

[scottward]

Are you picking up the heli and moving it to a different location after it has started up in AVCS? Just wondering if you might need to flick beteen rate and AVCS rapidly, 3 times I think, to reset centre?

[DWSchmidt]

Scott,

I'm sure. I am very careful to let the heli sit and everything "calibrate" before I move it.

D.W.

[scottward]

Yes - but are you moving it after it has calibrated? If you swing it around 180 degrees the HH mode will try to swing it back once you spool up.

Try starting up in AVCS, then flick to rate, then move the heli, then back to AVCS, then try spooling up.......

[Scott Bowersox]

you never have to go into rate mode. If you want to center the rudder, just quickly go left/right three times on the rudder stick and it will center itself if you want before take off.

[DWSchmidt]

It flies!!!

Thanks again for all the suggestions everyone! in the end, I turned the limits up on the gyro and it works. Since I (and my friend) will always fly in HH (AVCS) mode, I figured it will be OK if there is binding in rate mode. There is still some hunting going on when I stop the tail, so I obviously need to tweek the gains. I also need to do some tuning on the cyclic/collective, but that shouldn't be too hard.

My own heli is a Raptor E550, so I like TT products. The min-Titan definitely shows its 450 roots relative to its bigger brother, but it still seems to fly well. Looking forward to getting it tuned and teaching my friend to fly!

Also need to get the new Trac-X head on my E550...

D.W.