Adverse Yaw and Aircraft Control
Coordinated controls and control surface size counter adverse yaw
With most of the smaller indoor electric RC model airplanes, three channels are used for aircraft control. Rudder for turning the model, elevator for climbs and descents and a throttle input allow for good control and safe maneuvering. Ailerons are not compulsory for small sport models, the only penalty being certain maneuvers that require a roll command cannot be performed.
Coordinated rudder and aileron are required to turn the Horizon Hobby 4-Site
The introduction of remarkable micro RC electronics such as the radio system used on the E-flite Ultra Micro 4-Site or one of the new micro RC helicopter designs allows practical four channel control for all types of indoor RC model airplanes. With the more widespread use of ailerons, indoor flyers will face the challenges RC modelers previously faced with understanding aircraft design parameters and countering the effects of adverse yaw during turns.
Adverse yaw describes the situation whereby a pilot moves the aileron control to the right, for example, to produce a bank to the right. The RC pilot anticipates the model entering a turn to the right. Oddly enough, even though the aircraft is banked slightly to the right, the plane’s nose can start to swing in the opposite direction to the left. Aircraft nose movement opposite the desired turn direction is called adverse yaw. In some aircraft, you can actually fly a complete turn to the left with right aileron control inputs and vice versa. Certain airplanes can have a very pronounced case of adverse yaw, especially with antique model aircraft plans. There is no sure way to find out other than a test flight. Note that there are no worries regarding adverse yaw with airplanes that do not employ ailerons, such as the UFO flying saucer.
With right aileron input, adverse yaw can turn nose to left
Adverse yaw is easily explained. In the above example, the RC pilot moves the aileron control to the right. The left wing’s aileron goes down, and the right wing’s aileron goes up. In an aircraft that does not suffer from adverse yaw the model will bank to the right and the nose will start to move to the right as the airplane enters a right hand turn. For most sport RC models the ability to turn with the ailerons alone is so straightforward that many pilots do not even use the rudder to coordinate the turn.
To continue with the above example, let’s suppose that this particular model airplane is susceptible to adverse yaw. Adverse yaw is most prevalent in slow flying aircraft that use large control deflections to account for the low velocity of air flowing over the control surface. One should be especially alert for adverse yaw tendencies when flying biplanes such as World War I fighters and sport aircraft of the post war era.
With the left aileron going down and the right aileron going up to commence a right bank, the left aileron produces more induced drag due to an increase in that wing’s lift (by the lowered aileron increasing the outer portion of the left wing’s camber) than the right wing with its aileron now above the wing (decreased camber). This increase in aerodynamic drag on the left wing can cause an airplane susceptible to adverse yaw to actually turn its nose opposite to the intended direction of turn commanded by the pilot.
Early aircraft are especially susceptible to adverse yaw
The fix for the pilot faced with adverse yaw is simple and involves proper pilot technique. The pilot needs to add rudder in the same turn direction as aileron is applied. In the above example of a turn started to the right, the right rudder control is inputted as the right aileron is applied. The rudder will prevent the airplane’s nose from wandering to the left, opposite the turn, and the use of rudder and aileron together is called a coordinated turn.
The coordinated use of rudder and aileron for all turns is good basic pilot technique, and should be used at all times. Once you get in the habit of automatically using some amount of rudder at the start of all turns, it will soon become second nature. This procedure is of special importance with slow flight, as the rudder is usually the last control to lose effectiveness prior to a stall. Someday, if you get slow and close to a stall on landing approach, the instinctive use of rudder to control turns may save your aircraft.
Trainers from the 1940s benefit from coordinated controls for turns
As you gain experience with flying scale RC model airplanes, you will be able to anticipate the possibility of adverse yaw in a model airplane design. As discussed previously, World War I biplanes can expect to encounter adverse yaw in their turns. As a model aircraft designer you can take additional steps to counter adverse yaw by increasing the area of the vertical tail surfaces (rudder and fin).
A final design technique to counter adverse yaw that is common on full scale and model aircraft is differential aileron control. With differential ailerons, an aileron goes down half the distance as the opposite aileron going up. The aileron going down (to raise the wing for the turn) is the cause of adverse yaw. By limiting the distance an aileron can go down, differential aileron control is a very effective design technique to improve an airplane's turning characteristics.