the ailerons are used primarily to directly roll the wings, and the rudder is used to "coordinate" (to keep the sideslip angle near-zero during the rolling motion). Sideslip otherwise builds up during an aileron-driven roll because of adverse yaw. Often, the transmitter is programmed to automatically apply rudder in proportion to aileron deflection to coordinate the roll. When an airplane is in a small to moderate bank (roll angle) a small amount of 'back pressure' is required to maintain height. This is required because the lift vector, which would be pointing vertically upwards in level flight, is now angled inwards so some of the lift is turning the aircraft. A higher overall amount of lift is required so that the vertical component remains sufficient for a level turn. Many radio-controlled aircraft, especially the toy class models, are designed to be flown with no movable control surfaces at all. Some model planes are designed this way because it is often cheaper and lighter to control the speed of a motor than it is to provide a moving control surface. Instead, "rudder" control (control over sideslip angle) is provided by differing thrust on two motors, one on each wing. Total power is controlled by increasing or decreasing the power on each motor equally. Usually, the planes only have only these two control channels (total throttle and differential throttle) with no elevator control. Turning a model with differential thrust is equivalent to and just as effective as turning a model with rudder. Lack of elevator control is sometimes problematic if the phugoid oscillation isn't well-damped leading to unmanageable "porpoising". See "Toy class RC" section. V-tail systems A V-Tail is a way of combining the control surfaces of the standard "+" configuration of rudder and elevator into a V shape. These ruddervators are controlled with two channels and mechanical or electronic mixing. An important part of the V-Tail configuration is the exact angle of the two surfaces relative to each other and the wing, otherwise the ratio of elevator and rudder outputs will be incorrect. The mixing works as follows: When receiving rudder input, the two servos work together, moving both control surfaces to the left or right, inducing yaw. On elevator input, the servos work opposite, one surface moves to the "left" and the other to the "right" which gives the effect of both moving up and down, causing pitch changes in the