actually Hamid they have 3 servos that have been available
2 analog and one digital
1 analog is for the tail and one analog was made for the cyclic, unfortunatly they use the same part number![] NO way to tell them apart except by operating them by hand. Which means its a coin toss which one you get when you bought them at the LHS.
If you have several you can tell the diff by carefully moving the arms by hand, the ones that are used for the cyclic move very slowely and feel geared down, the ones for the tail moves rather faster and needs much less force to move the arm.
Do not mix them! If you have a tail and cyclic as the tail one will move faster then the other cyclics and cause strange control problems, you can use the cyclic in the tail but not the tail in the cyclic. Also the tail will fail in short order if used in the cyclic even if all three are tail, it seems it can't handle the extra load.
I would suggest you check all your servos and put 3 that match together for your cyclic and the one that doesn't match for the tail (whether it is digital or not) just dont mix one digital into your cyclic or a tail into your cyclic and you will be ok. Actually you can buy much better (tighter) servos for about the same or less that will pretty much bolt right in. You will see a big diff right away in more precise handleing. Tower Pro 9 comes to mind as one. Stay away from Hitech 50 as it is too small (designed for 1/2A planes and small park flyers) and the HS 55 tend to strip gears and have centering problems with the stock tx.