Feet control the rudder. Useful for keeping straight on the runway on take off especially with prop engined aircraft which will tend to try and throw you off one side of the runway due to various effects, or landing in a xwind. Otherwise just a footrest for most heavy metal and indeed fast jet mil pilots although light aircraft pilots will use it to balance a turn. Edited to add-useful for keeping the thing straight as well if you loose a donk on one side as the assymetric thrust will yaw you in the direction of the dead engine.
On aircraft with long wing spans/low airspeeds such as gliders rudder is used to counter adverse yaw in a turn. If you put on say hard right stick, the down going aileron on the left wing drops into a higher pressure area than the upgoing aileron on the right wing, causing the a/c to yaw to the left, so you need rudder, indeed in a glider substantial amounts of rudder to counter this adverse yaw.
Similarily in some high performance mil jets the down going aileron can stall the wing on that side at high alpha causing the a/c to roll in the opposite direction to what is demanded. In the F4 and a/c of that ilk the primary roll control at high alpha was the rudder, you didn't touch the ailerons, and at landing speeds with the flaps down a device called ARI (aileron/rudder interconnect) used to kick in which automatically put rudder in with stick movement to counteract the tendency for adverse roll. Some big jets have a similar device which will put a bootful of rudder in if you loose a donk on take off.
Of course a lot of aircraft use the rudder pedals for steering on the ground, forgot about that bit!
Last edited by thing; 25th Sep 2015 at 07:12.