OK guys help me out. I need a sanity check. We lost the twin to my Macchi today. I'm pretty sure I know the cause but would like some feedback from you guys who have been racing warbirds for a while.

So here is the question. How much torque would you be running on the elevator of a 5 lb .40 size airplane that does just a little better than silver speed? Would you use a stock plastic arm? Those who saw my Macchi fly at the Last SAMs race tell me how you would equip,

A stock arm on a good quality 45 oz/in or higher servo should have been fine.
My Silver class Kyosho KI-61 is bone stock except for some airframe beef up for a YS 1.10.
I have used the Hitec 425 BB in it for three or four years now with the stock hitec plastic arms with no issues at all.

Tom. I;m glad that you responded. In fact it was the Hitec 425BB that was used. I'm thinking the difference was the linkage setup. When I built the Macchis I wanted internal linkages for the elevator and rudder for the obvious reason. I was able to do this on elevator but to do so ment the control horn is only 3/4" from hinge to clevis. Not really a problem as long as the servo horn is 3/4" or shorter.

The 425BB is rated at 57 oz on 6V. The setup on the second Macchi was a 3/4" control horn with a servo horn of 1" with a ball link and stand-off. The result IMO was this reduced the servo power getting to the elevator by 25% thus the servo output was more like 43 oz.. My setup is a Hitec 5645 which is rated at 168 oz and I am using an aluminum arm and steel clevis at 1/2".

What I think actually failed was the arm. My thought is there was enough flex in the arm due to arm length and linkage offset below the arm that the arm itself was twisting. I think that was the cause of the flutter and the flutter caused the arm to break right where the screw for the ball link passed through the arm. When we examined the wreckage the arm was broken but that end of the pushrod was not bent. This tends to make me beleive the arm did not break upon impact. All theory of course.</p>

WOW, just WAAAYYYYY over kill....

I used to use a JR 2721 metal gear and about 90 oz I think on 4.8 v
3/4" nylon arm and horn with metal clevises. You dont need much throw at those speeds only about 1/4" up or down and use high rates when you land. Also another tip be careful not to "Bank and Yank" I hace seen so many plane tip stall and eat the dirt.

Tim

Very true about the tip stalling, I tend to setup my planes to only have enough pull (yank) to not tip stall but give me a good turn. I then vary based upon wind conditions. I find it easier to fly the course like this.

Bulldog