Hi, well im up to the stage on my 2m craft where I have to work out the installation of my elevator servos. Im just wondering which is the better method, using the MK ballraced bell-crank using 1 servo or going for the 2 servo in the stabs, what are the pros/cns of each method. Thanks

MY vote.... pull/pull one servo.

With one servo and a bellcrank your control response will be quicker than with two servos in the tail and long extensions. An other thing to consider is getting the elevators matched up with two servos operating at the same speed and throws ect.. Tom

I think that with one servo on pull/pull you have many more adjustment options. You have adjustment up and down on the control horn top and bottom for more precise adjustments.

If done right the MK bellcrank is a far better system than pull pull. You have virtually infinate range of adjustment of both elevator halves and there is not worry when taking the stab off that the elevators will be out of alignment (with pull pull that will happen from time to time). The trick with the bell crank is on the pushrods that run from the external arms of the bellcrank to the elevator, the thread pitch on either end must be different ie: 6-32 one end and 4-40 other end. Doing this that pushrod can become any length that you want, if they are the same 4-40,4-40 at each end then you limit yourself to a half turn minimum adjustment.

I havent tried the bellcrank method yet. I'm sure it has its advantages especially with a removable stab. However, wouldnt a pull/pull setup be lighter then a pushrod and bellcrank with rods and clevises?

Yes, but you will be saving very little, there are places that a lot more weight can be saved without the hassles of pull-pull. My current FG/foam/balsa 2M pattern ships are running at 9.6 lbs, no Duralites.

Thanks guys, fast and great help . Ive decided to go for the MK bell-crank which is quite a bit cheaper than the two servo option., how far back in the fuse is the elevator servo mounted Chad as the Matrix (www.pbgcomposites.com) has a fiberglass fuse, theres nothing shown on the plans ,thanks again.

I put mine in as far back as I can get access to easily. I mount the bellcrank somewhat forward of the LE of the root of the stab. I think that my main shaft (3/16" carbon rod from Central) is about 25"-30" long and the ones out to the elevator are (5/32") and are about 10" long.

The other trick is to make a pull-pull arm from printed circuit board and bolt/glue it to the bellcrank, and run pull pull from the servo to the bellcrank. I think that I am going to try that on my new planes this year.

Lot's of great discussion on pull-pull but what about twin mini's in stabs? I also have to make a choice for my Demonstrator, which is designed for twin minis inside removable stabs.

I will probably go for MK bellcrank - twin servo speed/matchup has always been a problem for me. But I would still like to hear from others who have done it successfully. It's looks cool with the minis in the stab! What servos are you using, and how do you align the throws?

I use many Hitec 85MGs in my gliders, will these work for the elevators?

Yes there is some headache in programming and matching up a pair of seperate servos. Speedwise, it is strictly servo dependent, extention length is a non-factor compared to the speed of the elctrical signal travelling it.

BUT...... You get one thing with the dual servos that I have come to really like, failsafe redundancy. If one servo or linkage fails, you can in all likelyhood limp the plane in. I have had an aileron servo fail on the left side of my stinger 120 but easily flew the plane in with just the right aileron working. I know size and weight are concerns, but if it fits, dual servos are cheap insurance.

I agree with the extra insurance of dual servos. But aligning them really gives me a headache. With super fast servos it's not a major problem, but with minis they are not very fast and it's difficult to get them matched up exactly.

I'm used HS-85MGs in my G-Trick. I can never get the servos to move exactly. Either throw is little off or speed is a little off when I adjust with ATV. Just a little bit off. But it bothers me a lot because I start imagining that my loops will not be strait etc...

Maybe faster servos? Any recommendation for minis?

Huang, I have dual servos in the tail of my Nolimit 3D. They are standard servos, but I had no troulbe with matching throws or speed. With the dual servos you can do some neat things for 3D. My only complaint is I could not split the elevators one up one down, and still get extreme throw. Tom