I am finishing up a CA Models 27% Extra (no more than 15lbs. AUW) I plan to use strictly for learning IMAC and not 3D. I don't want to put a ton of money into servos because I view this plane as a learning tool, so what's the minimum servo torque I can get away with?

Ailerons: one servo per wing
Elevator: two servos per side in tail
Rudder: pull pull, one servo

Thanks.
Bob

Whatever the manufacture recommends

CA Models' instructions recommends 4-6kg (141 to 211 oz), however I believe that would be for all out 3D. I've heard that 70-80oz. servos would handle the plane nicely not flown 3D, but since I'm really green at this and simply do not know, I thought I'd get more opinions from the more experienced.

Anyone else willing to help out a really green beginner here? Thanks.

When you start getting into bigger planes you don't want to chance pushing the limits of servos. Plus if you are planning to fly IMAC digital servos are more precise and the extra torque helps in many maneuvers. You can use whatever you want, but someday you will be pulling out of a loop and an aileron will go into a flutter and rip out of the wing because it didn’t have enough torque, and you’ll ask yourself “why didn’t I listen�. Trust me it WILL be a learning tool if you skimp on servos, just not a good one.[:'(]

flymeaway,
a little correction to your torque numbers: 4-6 kg equals 56-84 oz, not 141-211.

Hmm..... Two different conversion programs I use say different.

I would say you could use the Jr 4721 servo, it strong and I used this on an aerotech 84 inch extra that I did all out 3d with. I know you said you are strictly imac only but if the servos are good enough for all out 3d on my plane, they should work great for oyu, plus provide you with a little insurance against flutter.

Mike

Your conversions were correct the first time. However, you are missing some units. You are talking about torque, which is force x distance. Servos are rated in oz-in, not oz. To know what the 4-6kg rating converts to in Imperial units, you must also know the moment arm measurement being used. If it is also 1 inch, then your original numbers are correct. If it is different, then you need to convert for the arm length as well.


Mark

I'll assume you'll be practicing Basic routines and hence probably might not even notice the extra precision of digitals. If you set up the linkages for strictly Imac you will have significant mechanical advantage over the same throws on a 3D set-up. In other words 1/2" elevator deflection at full servo throw needs a lot less torque than the same 1/2" using only a 1/4 of the servo movement. If you want to go cheap use 645's on 6 volts or something similar (stay away from plastic gears) but, the money spent on digitals now will pay off down the road when you move them into a bigger plane.

I'm pretty late coming into this discussion, but I would like to throw in my 2c. When I started IMAC flying Basic, I used some relatively inexpensive servos in what I thought was a pretty nice plane. That was probably the biggest mistake I could have made and it was extremely noticeable (but only after I went to good, precision servos the next year). Things I noticed with better servos include:

Centering: servos returning to center precisely. This helps SO MUCH when trying to fly straight lines because you won't always be fighting a plane that is just slightly out of trim (climbing, rolling, falling, etc). The plane should be rock solid in horizontal trim so that your workload is decreased. Ideally on a calm day, you should be able to establish a horizontal line and let go of the transmitter - the plane should fly from one end of the field to the other without you having to touch a thing. Believe me, this makes precision flying SO MUCH easier.

Precision: this is harder to see, but basically you will notice (particularly with 1 elevator servo per stab) very precise control - if both servos track the same, then the looping figures will be much easier to control. Also extremely important in snaps (not a big deal in Basic) because the servos will always move to the same position and therefore the snap will be more predictable and controllable. This will also be extremely noticeable on throttle control at idle. A good, precise servo will allow you to have a very predictable idle that won't die at really bad times (like during a spin).

Power: this is probably the most important factor. Read through Peter Goldsmith's article on the IMAC website about trimming a plane for precision. ANY control surface blowback will make trimming a pain because the servo won't be at its intended position as airspeed changes.

I think the best advice I ever got was to spend my money on good servos. You will probably never know just how much you apprecaite them until you've tried the alternative, inexpensive route. Save your money on graphics and spend it on servos if you want to get serious about precision flying!!

Ken

AMEN, very well said. That should close this thread.

to clear things up a bit; I was referring to oz-in vs kg-cm. 1 kg-cm torque equals approximately 14 oz-in

Thanks everyone. After I read this I was thinking it'd be a rather bad idea to add any element of failure just to save a few bucks in a plane that is already expensive in the first place. Good advice.