Definition
The time, expressed in seconds, required for an electric motor to accelerate from rest to approximately 63.2% of its final steady-state speed when full operating voltage is applied. It is a measure of how quickly a motor responds to a change in input.
Plain English
A number that tells you how fast a motor speeds up after you switch it on. A short time constant means the motor reaches running speed quickly; a long one means it takes longer to get up to speed.
Context Anchor
Seen in aircraft electrical system and maintenance discussions, especially when comparing how fast electric motors respond after power is applied or load changes.
Derivation
From physics and electrical engineering, where a 'time constant' is the standard way of describing how fast a system responds to a change. The 63.2% figure comes from the math of exponential rise: after one time constant, a system has covered roughly 63% of the gap between its starting and final values.
Why Pilots Care
Motors that drive trim, flaps, gyros, and autopilot servos must respond predictably. The time constant tells engineers and technicians how quickly the motor will react to a command, which affects how smoothly and accurately a system performs.
Analogy
Think of turning on a fan. It does not jump instantly to full speed; it rises toward that speed. The time constant is a standard marker for how fast that rise is happening.
Grounding Statement
It tells how responsive the motor will feel when commanded to move a flight control surface or other load.
Intuition Check
Do not read time constant as the total time for the motor to reach full speed. It is the time to make about 63.2 percent of the change toward the final speed.
Example Sentence 1
The trim motor's short time constant allows it to respond almost instantly when the pilot makes small pitch corrections.
Example Sentence 2
A longer time constant on the flap actuator motor means the flaps take more time to reach full extension after the switch is moved.