Trex 500 Scale - No Tail Rotor Pitch Change

[prdp]

Hi Guys,
I have a situation here and would like your input. I have a Trex 500 (MD500 scale) which I have a problem with the tail rotor pitch control. I have installed an Align DS510 servo for the tail rotor which works fine while spooling up until I get to about 45-50% throttle and then the pitch control seems to freeze.

I currently have an Align GP750 gyro installed which I have confirmed is correctly setup, nevertheless even when I disconnect the gyro and re-connect the servo directly, the tail rotor still freezes during spool up.

It's as though the centrifugal load on the tail rotor is too much for the servo to handle. If I back off on the power, the pitch control comes back into action. I currently have the ball joint set in the 4 hole on the servo arm as per the specs. I realize that I could increase the leverage by positioning the heim joint closer to the axis point but whilst increasing the torque value I would lose blade pitch efficiency at the other.

Has anyone had or heard of this issue before and any recommendations considering that the Align DS510's are supposed to be up there with the best when it comes to servos.

Thanks in advance for your help.
Phil

[dani_r]

Well, the DS510 isn't strictly a tail servo. You need the DS520. It's the proper tail servo for the 500. Any of the Align range *10s (410, 510,610) are really cyclic servos, and the *20s (DS420,520,620 and the DS650) are tail servos.

Try getting a DS520 tail servo and having a go then!

Hope this helps

[matt t]

i have to agree put a 520 on it

[unwind-protect]

It's not the centrifugal load that's the problem* but rather the so-called tennis racket effect. If you're brave/ stupid/ reckless enough you can take off the main blades and disconnect the pushrod from the servo, spin up the heli and see how much force is generated for yourself. It's surprisingly high.

You can counter this force by using a longer blade bolt and spacing it with ball link balls. Overdo it though and you'll end up with an excessive force trying to push the blades away from centre. If you' taken off your main blades you'll be able to experiment to find the best arrangement.

Another problem I've found with the 500 is that the thin wire pushrod can bend if the guides are not fixed in place well or is out of line.


* well, I guess it could be overloading the bearings and binding, but we'll ignore that.

[prdp]

Thanks guys, Excellent advice. In regard to the DS 510 servo I had never realized that they were strictly cyclic servo's doh! Anyhow I had been to a few other forums in the meantime and decided to splash out on a decent tail rotor servo and so I'm now waiting delivery of a new Hitec HS5245 MG servo. Now that should cure any servo short comings.

@unwind-protect. Good explanation. You've got me really thinking on the wire push rod idea. I'll remove the mechs from the fuselage and give it test. Now that you've got me thinking about it, it just might be the issue. So obvious when someone points you in the direction.

Thanks again guys for your input, most appreciated.

Many happy landing to you all.

Cheers
Phil