Definition
A condition during the takeoff roll in which the rudder is not yet fully effective for directional control because airflow over its surface is still too low. The pilot must supplement rudder input with steering from the nosewheel or tailwheel until the airplane gains enough speed for the rudder alone to keep the airplane straight.
Plain English
Early in the takeoff roll, the rudder pedals don't yet have enough air moving over them to steer the airplane on their own, so the pilot has to combine pedal input with ground steering until the airplane is moving fast enough for the rudder to do the job by itself.
Context Anchor
Encountered during instrument takeoff procedures, especially while tracking the runway centerline and heading during the first part of the takeoff roll.
Derivation
Partial' here means 'not yet complete.' The rudder is doing part of the work but cannot do all of it until airspeed builds. Naming this phase reminds the pilot that rudder authority grows with speed -- it isn't fully present the moment the throttle goes in.
Why Pilots Care
Awareness prevents runway excursions by prompting use of nosewheel steering or differential braking until sufficient speed restores full rudder effectiveness.
Grounding Statement
At low speed, there is only light airflow over the rudder, so it can help, but it cannot yet respond as strongly as it will later in the takeoff roll.
Intuition Check
Partial rudder control does not mean the rudder is broken or only partly connected. It means the rudder is working normally but has only limited effect until the airplane gains more speed.
Example Sentence 1
During the early part of the takeoff roll, the pilot used nosewheel steering to compensate for partial rudder control until the airplane gained enough speed for the rudder to take over.
Example Sentence 2
As airspeed increased, partial rudder control gave way to full effectiveness and the pilot transitioned to primary rudder inputs.