"Those pesky servos—why can’t I ever find one that’s properly centered? Every time I attach an arm, it seems as though the servo center shifts! What’s going on here?"

Sound familiar? Well, if you’re using an Airtronics, Futaba or JR radio, it sure seems as if that’s the case. If you’re using a HiTEC radio, the problem doesn’t seem so bad. What causes this, and what can you do about it?

All (standard-sized) servos today have splined shafts on which those servo arms are bolted. The problem arises because of the number of splines (teeth) on those shafts—Airtronics and JR use 23 splines, HiTEC uses 24, and Futaba uses 25. (Your radio may be different—grab a servo and count the splines on the shaft to find out. Use a magnifying glass!) This is a really neat feature, and you should take advantage of it when you set up your airplane!

Put a servo arm on a servo. Now, every time you lift and rotate the arm by one spline, you change its position by a fixed number of degrees: for Airtronics or JR, this is 15.65º, for Futaba it’s 14.4º, and for HiTEC it’s an even 15º. The formula is simple: 360º divided by the number of splines.

Now consider that your servo arms have an even number of fingers—2, 4, even 6. You can see by experimenting that rotating the servo arm and putting each finger as near as possible to where its predecessor was (about 90º, or 180º or 60º) will result in a shift in position of 3.91º, 3.6º or 3.75º for Airtronics/JR, Futaba and HiTEC respectively. The formula is equally simple: 360º divided by (the product of the number of splines times the number of fingers). So, for Futaba, finger 1 is assumed at 0º, finger 2 (rotating clockwise) is placed at 3.6ºoffset, finger 3 at 7.2º, and finger 4 at 10.8º. (For Airtronics/JR, use multiples of 3.91º, and for HiTEC use 3.75º.)

"Whoa, that’s too complicated for me!", I hear you exclaiming. Well, don’t worry about it—just keep rotating and pressing on the servo arm until you get a finger as close as possible to that magic 90º position. ONE of those fingers will be right. (Actually, Futaba makes it simple—the fingers are numbered! Choose number 1 and you’re there. JR has a raised dot in the lower right of its number 1 finger. It doesn’t matter as much with HiTEC, as there are an even number of splines, and two of the fingers (out of four) will be right at any time.

Incidentally, the number of splines being different is the reason why servo arms are NOT interchangeable between servos of different brands—don’t try to use Futaba arms on JR servos, etc.

This also clears up the apparent servo-centering shift. Most of the servos today have electronics that is so good that mechanical centering is a thing of the past and isn’t necessary. If you in fact have a servo that won’t center properly, or consistently, it’s probably BAD! Send it back for repairs!