ORIGINAL: EHFAI

Here're some data that may help you decide. I consider fast / constant servo speed critical for precise flight performance and choose the fastest servo with sufficient torque. I wanted more information as to what ''sufficient torque actually was, so I designed an experiment in an effort to find out.

Peak current draw was measured in flight on each servo during high load maneuvers, snaps, knife edge loops, high speed rolls, etc. Servos were of both brushless & digital variety. Servo voltage was regulated 6v from a LiPo source and the aircraft was a typical 2m E powered pattern model. These data were then used to measure servo speeds under load at the same current draw on the bench using a rig built for this purpose.

A significant observation is that the servo speed is relatively constant until current draw reaches 60% of stall current. Probably a good target is 40% of stall current - here're data observed in this testing.

On elevators, 2 X 60 oz in servos exceeded 60% of stall current (SC) and slowed measurably - changing to 75 oz in servos dropped the current draw to around 50% SC and speed is maintained.

On ailerons, 2 x 128 oz in servos drew 30% of SC.

On rudder, 1 x 300 oz in servo drew 20% of SC.

Basis these results, it appears that minimum digital servo torque for this ariplane might be 150+ oz in for elevator, 75+ for each aileron, and 100+ for rudder to prevent servo slow down during stressful maneuvers. Be aware that linkage ratios will factor into this and that for this test ariplane the elevator linkage amplifies surface movement to increase travel speed (which will also increase servo load.

Generally it's believed that stronger servos have more ''holding power'' and I didn't address that issue in this experiment. My ''feel'' over the years is that the digital servos provide more solid flight which may be related to better holding power. It was noted that servo current draw is extremely low in level flight in calm and very rough air.