Recipe for a stable stock T-Rex
Before you read this mod page, you should really read http://www.trextuning.com/setup1.htm if you havent done so before.
The T-Rex in clean stock edition is what some say twitchy, some suggest changing
ATV/DR/EXPO to get rid of it, but that’s only half the truth.
Many items on a stock T-Rex is sloppy, it doesn’t matter how much you tweak
your radio settings, but you cannot get good results with only radio settings.
So I decided to try to make it the best I could, CCPM is –the- way to get it
stable, but this was to make the best out of the stock version.
I’ve used only stock parts from a spare kit that I have, except for Zoom/Shogun
paddles that I had laying around.
Thanks to all at RunRyder.com that have helped by answering posts and submitted ideas.
1. Main rotor head.
Either use a 4mm piece of nitro fuel tube to replace the o-rings, or
as I have done it by putting in 2 o-rings on each side.
When tighten the screws back, make sure it doesn’t get too stiff, if you do
that, you will get a wobble when hovering.
Use lock tight to secure the screws in place when you have the perfect
configuration (not sloppy and not too stiff).
2. Linking system.
4 brass bushings that are too small compared to the hole where there sit, they
are in the SF mixing lever & Flybar control lever.
2 metal pin in each of the washout linkage.
For the bushings use CA to make the hole smaller so the bushings fit better,
or (as I did which works as well, use CA to nail them in there, just make sure
you don’t get the CA into the bearing)
For the two pins connecting the washout linkage use the spare screws from
your kit to replace them.
Note: Make sure holes are aligned when screwing them into place or you will easy break the arm.
3. Pitch, Elevator, Aileron servo arms.
I used the same rods as previous, cut them before the z-bends and glued on the
ball links.
Alternative you can use a tool to create new threads
4. Elevator control.
I’ve seen some posts where also the brass bushing it self had slop where
it was fitted into the frame, that was not the case for me.
I put on some JB weld(? I use swedish counterpart..) on the control arm and let
it hardened.
Then I grinded it down so I got a nice fit into the bushing.
5. Servo.
I did change my servos for pitch/elevator/aileron to HT-56HB, those servos has
a little bit more torque and a larger set of servo arms which was a little
firmer.
Don’t know if it was needed but I had them lying around.
6. Paddles.
I changed the stock-paddles to Zoom/Shogun/Zap paddles; you need the paddles
and the metal holders for them.
Drill out the hole with a 2mm drill, and put them on the existing fly bar,
tighten the grub screws and you are done.
I suspect however that if you do number 7 perfect,
then the need for weighted paddles isnt as great as if you have bindings in the
ballinks still, Havent tested that though.
7. Ball-links.
I went through all my ball-links and loosened them up (mine was _way_ tight
from the beginning).
I also checked that none of the balls was damaged. (Replaced those that I had
accidentally scratched.)
An easy way to see if you have bindings in the ball-links in the mixer:
If you place paddles along the tail boom and move pitch up and down the paddles
shouldn’t move, just the pitch of the blades.
Loose for free movement, but not sloppy, that is the trick. I guess there is
several ways to fix ball-links, soldering iron etc,
what I used was that I put a ball-link with a rugged surface in the drilling
machine, put on the ball ring and let it spin until it felt OK.
(Nicely balanced.)
(and with the slightest weight on it, it drops freely)
8. Throttle/Pitch, RPM settings.
Get a Tachometer, or borrow one or take your heli to someone that has one.
Especially if you are new to heli’s, I thought my T-Rex would come apart if I
got higher RPM, when I tached it, it was 1900…
You need head-speed (I run around 2200/2300) for a solid hover.
And make sure your pitch curve is linear between the points above and below the
hover point.
I made the mistake that I had too low on the point below the hover point
compared which resulted that when I lowered the pitch a just a little it gave
much altitude drop.
And keeping the correct headspeed will lower your motor's and ESC temp, as
little as 8% of the throttle (I tried 80% instead of 88%) can give quite large
temperature changes.
9. Center of gravity
If your heli is tail heavy or too much nose heavy it will perform
unstable in hover, either get it perfect CG under the main shaft or just a
little nose heavy.
10. Radio settings. (Futaba)
For indoor flying (like I am doing here in the middle of the winter).
I have set the aileron expo to -30%, DR to 100% and elevator Expo to -50%, D/R
to 50%. ( I like it because then the T-Rex follows the same sensibility as my
Zoom 400 )
Why I have 50% on the Elevator D/R is because of the size of the ball-links I
had to put it out on a further hole out on the servo arm then compared to my
aileron.
I’ve seen several debates if or if not to change ATV/D/R; it’s a personal
choice I guess…
11. Canopy
Just a sidenote, but when I changed to the Z-Bends to Balllinks I had
trouble fitting canopy again.
What I did on the back end of the canopy I fitted some 2 mm plastic parts on
the inside over the holes, and then I drilled the same size hole in as in
the canopy.
So the canopy was a total of 2 mm wider on each side, that was enough to get
free movement on all the servo's
Result:
After I did the parts above I can hold my Trex within one square foot
during hover.
If I fall outside that, its my fault, not the heli's
Still to do:
I still have some slop in the pitch control making the T-Rex drop or gain
altitude all of the sudden,
Will make further investigations how I can fix that or I will give up and go
ccpm ;-)
Picture of my HeliPark :)
Fredrik Melin
mel@dacsa.net
Stockholm, Sweden.