"Conventional" use of rudder

"Conventional" use of the rudder

Steve Seibel
www.aeroexperiments.org

This page is still under construction!
This page was last modified on August 24, 2006

 

In "The rudder as a roll control: aircraft with dihedral" and "The rudder as a roll control: aircraft with anhedral", we described how the rudder could be used to create a roll torque.

More "conventionally", the rudder is used for a simpler purpose: to keep the nose of the aircraft exactly aligned with the direction of travel through the airmass at any given instant, i.e. to keep the nose of the aircraft pointing directly into the "relative wind". When the rudder is used in this manner to augment the aircraft's natural yaw stability or "weathervane effect" and overcome "adverse yaw" and other related effects, so that the nose of the aircraft is kept pointing directly into the relative wind, drag is minimized.

Sailplane pilots always use a "yaw string" (photo to be added) as a guide to rudder usage: if the yaw string blows to one side, the nose of the aircraft is not pointing directly into the relative wind. Applying rudder on the side opposite the deflected yaw string will swing the nose back into alignment with the flight path and relative wind.

Airplane pilots typically use a slip-skid ball (or bubble) (photo to be added) as a guide to rudder usage: for reasons that are explored in detail elsewhere on the Aeroexperiments website, if the nose of the aircraft is not pointing directly into the airflow, the slip-skid ball (or bubble) drifts to one side. Applying rudder on the same side as the deflected ball (or on the side opposite the deflected bubble) will swing the nose back into alignment with the flight path and relative wind.

As we noted in the related article on this website entitled "What is a turn", as a pilot begins to roll an aircraft to the left (for example) to enter a left turn, "adverse yaw" can cause the aircraft's nose to swing to the right, both in an absolute sense, and in relation to the actual direction of the flight path and relative wind at any given moment. In a "conventional" aircraft, the pilot can prevent this "adverse yaw" motion by applying left rudder along with left aileron as he rolls the aircraft into the turn. The "proper" amount of left rudder will keep the nose of the aircraft exactly aligned with the actual direction of travel through the airmass at any given moment. In other words, the "proper" amount of left rudder will keep the nose of the aircraft pointing directly into the relative wind as the aircraft begins to roll into the left bank and the flight path beings to curve to the left. In most "conventional" aircraft, overcoming "adverse yaw" in this manner is one of the main functions of the rudder.

As a natural extension of the ideas explored on "The rudder as a roll control: aircraft with dihedral", if the nose of an aircraft with dihedral is allowed to adverse-yaw to one side with respect to the flight path and airflow, the aircraft will tend to roll in the same direction. This means that if a pilot applies a left roll input and the aircraft adverse-yaws to the right, an adverse roll torque to the right will arise that will oppose the pilot's left roll input and slow the aircraft's roll rate. If the pilot applies an adequate amount of left rudder along with his left roll input, so that the aircraft is not allowed to adverse-yaw to the right, this will keep the nose of the aircraft directly aligned with the flight path (i.e. pointing directly into the relative wind). This will eliminate the unfavorable right roll torque that would otherwise be caused by adverse yaw. Because of these relationships, the rudder ends up being a very important flight control on any 3-axis aircraft that has dihedral (or has a high wing placement, which creates a dihedral-like effect) and also has "simple" ailerons that lack differential movement, asymmetrical hinging, or other features that serve to minimize adverse yaw. This is particularly true if the aircraft has a long wingspan, which exacerbates adverse yaw. Regardless of the degree of adverse yaw that an aircraft suffers from, using ailerons as the main roll control and applying exactly as much rudder is needed to counteract adverse yaw and keep the nose of the aircraft exactly aligned with the flight path (i.e. pointing directly into the relative wind) is generally a very efficient way to turn an aircraft.

Many RC model aircraft have no rudders, and use ailerons for roll control. Due to the relationships described above, such aircraft typically have very little dihedral, or no dihedral. The lower aircraft in this photo is an example of such an aircraft: it has some sweep, but no dihedral.

As a natural extension of the ideas explored on "The rudder as a roll control: aircraft with anhedral", if the nose of an aircraft with anhedral is allowed to adverse-yaw to one side with respect to the flight path and airflow, the aircraft will tend to roll in the opposite direction. This means that if a pilot applies a left roll input and the aircraft adverse-yaws to the right, a favorable left roll torque will arise that will augment the pilot's left roll input and boost the aircraft's roll rate. If the pilot applies an adequate amount of left rudder along with his left roll input, so that the aircraft is not allowed to adverse-yaw to the right, this will keep the nose of the aircraft directly aligned with the flight path (i.e. pointing directly into the relative wind). This will eliminate the favorable left roll torque that would otherwise be caused by adverse yaw. However drag will also be reduced, because the nose of the aircraft will be kept pointing directly into the relative wind. In nearly all cases, if an aircraft has both a rudder and ailerons, roll control is accomplished with the ailerons and the rudder is applied as needed to keep the nose of the aircraft exactly aligned with the airflow (i.e. pointing directly into the relative wind), even in the rare cases where the aircraft has anhedral and the rudder inputs actually slow the roll rate. This situation is very rare in actual practice--few "conventional" 3-axis aircraft have enough anhedral, in relation to other competing factors like sweep, to actually create a "wrong-way" roll torque in response to the rudder. However, less "conventional" aircraft that lack rudders and use ailerons for roll control are sometimes designed with anhedral so that adverse yaw can be "harnessed" to create a helpful roll torque. Flex-wing hang gliders, which lack rudders and use weight-shift for roll control, nearly always have anhedral so that the sideways airflow arising from adverse yaw can be "harnessed" to create a helpful roll torque.

 

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