Servo Arms for 100cc (2.6m) IMAC Airframe

[scruffmeister]

I'm building my first 100cc airframe to use exclusively for IMAC. There will be absolutely no 3D on this airframe - I have other airframes for that so minimum required travel/maximum resolution is what I'm after!

Unfortunately I don't really know where to start with servo arm length t get what I want. What length servo arms should I use on 100cc (2.6m wingspan) for ailerons, elevator, rudder* to get throws necessary for IMAC but no bigger?

(Note that the rudder is direct drive, not closed-loop)

Thanks in advance!

 

[bobzilla]

1-1/4" would be min. Some day you may want to sell the plane. Buyers won't be too impressed with anything less.
Just because you have longer arms you can alway use the inner holes. Also you should be using Dual Rates to control your throws.

here is the arm I use on all my 30-36% planes:
(They have arms available at USA warehouse to fit Hitec, JR and Futaba.)
This is the Hitec arm.

http://www.hobbyking.com/hobbyking/store/__23605__CNC_Alloy_Servo_Arm_Long_Hitec_USA_Warehouse_.html

 

[scruffmeister]

Thanks @bobzilla, but I'm not building this plane for future buyers, I'm building it for me! With that in mind, I'd still like to find the shortest arms people are using for IMAC on this size of plane.

Using dual-rates to control throws is fine, but I don't want to do it at the expense of servo resolution - i.e the maximum mechanical throw should be the maximum I actually need for allowed IMAC manoeuvres. In this setup the "dual" rate is likely to be a flight condition switch of between normal, spin, snap and rollers - all subtle variations of similar amount of throw.

 

[ChickenBalls]

Try this fella @Pistolera for info , there are other IMAC gents here also that may help.

 

[Capt.Roll]

Scruff........

What level of competition will you be flying?

I just finished up my first season in Basic and used 1" arms on the elevators and ailerons. On my high rates I did not have any more than 20 degrees of throw and rarely flew in high rates. Most of the time I flew with my low rates which were set around 12 degrees of throw.

The rudder had a 1.5" servo arm and was setup for full throw on all of my rates (35 - 40 degrees) to make sure I had enough movement to get the tail kicked around on hammer turns, especially when the airspeed drops as you approach the start of the turn. The full throw also seemed to help when windy which is most of the time in Texas.

The maneuvers in Basic class are pretty tame and everything is a pull to the canopy. I'm moving up to Sportsman in 2015 but will be setting up my new plane the same as the ARF I flew in Basic. At this point I'm not sure if the addition of more rolls, 2 point rolls, a positive snap, and some inverted lines will require more aggressive throws on the controls surfaces. I've flown the Sportman sequence a few times and right now I think the 1" and 1.5" servo arms will be good; I may need to fly more in my intermediate or high rates.

I've used the HK servos arms (for Futaba spline) for three years without issues. I've also used the SWB arms without problems.

 

[scruffmeister]

@Capt.Roll, thanks for the info. I currently fly Sportsman using a 50cc airframe with 1.5" arms all around - tamed with dual-rates. The info you gave is very valuable as the deflections you have in degrees for those arms are around what I'm looking for - obviously it's airframe specific, but these seem like a good starting point for me.

 

[tl3]

There is more to consider than simply the length of the servo arm. You also need to look at the control horn set up on the flight surfaces. Are you using Rocket City type bolt / clevis horn, or is it a double truss style horn that's supplied with many arfs? What is the distance from the the pivot point of the horn to the hinge line of the flight surface? Is the setup such that the pivot pt is directly over the hinge line? Generally speaking, you want the relationship between the servo arm and pivot point / hinge line to be about 1:1. If you can go less, and still get the throw you need (shorter servo arm as compared to pivot pt - HL) you'll increase the mechanical advantage on the servo side and improve the resolution of your setup. If this is to be strictly an scale aerobatics plane I would use the shortest arms that you can use while still getting the throw you need, plus a little for good measure, but make sure your ratio is as close to 1:1 as you can get, or in the servo's favor.
t

 

[scruffmeister]

@tl3 thanks

 

[tl3]

FWIW, there is one caveat I should add re: shortest possible arms. While shorter is generally good, if you get too far below the 1:1 ratio (.5:1 for example) the mechanical advantage on the servo side is great, the drawback is a slower control surface. Not an issue on the ail, but when using a .5:1 ratio on the elev I noticed a somewhat "numbed" response on pitch initiation in snaps.

 

[exeter-acres]

you have pitch in your snaps?