ailerons

A force may be thought of as a push or pull in a specific direction. When a force is applied to an object, the resulting motion of the object depends on where the force is applied and how the object is confined. If the object is unconfined and the force is applied through the center of gravity, the object moves in pure translation, as described by Newton's laws of motion. If the object is confined (or pinned) at some location called a pivot, the object rotates about the pivot, but does not translate. The force is transmitted through the pivot and the details of the rotation depend on the distance from the applied force to the pivot. If the object is unconfined and the force is applied at some distance from the center of gravity, the object both translates and rotates about the center of gravity. The details of the rotation depend on the distance from the applied force to the center of gravity. The motion of flying objects is described by this third type of motion; a combination of translation and rotation.

A force F is a vector quantity, which means that it has both a magnitude and a direction associated with it. The direction of the force is important because the resulting motion of the object is in the same direction as the force. The product of the force and the perpendicular distance to the center of gravity for an unconfined object, or to the pivot for a confined object, is^M called the torque or the moment. A torque is also a vector quantity and produces a rotation in the same way that a force produces a translation. Namely, an object at rest, or rotating at a constant angular velocity, will continue to do so until it is subject to an external torque. A torque produces a change in angular velocity which is called an angular acceleration.

The distance L used to determine the torque T is the distance from the pivot p to the force, but measured perpendicular to the direction of the force. On the figure, we show four examples of torques to illustrate the basic principles governing torques. In each example a blue weight W is acting on a red bar, which is called an arm.

In Example 1, the force (weight) is applied perpendicular to the arm. In this case, the perpendicular distance is the length of the bar and the torque is equal to the product of the length and the force.

T = F * L

In Example 2, the same force is applied to the arm, but the force now acts right through the pivot. In this case, the distance from the pivot perpendicular to the force is zero. So, in this case, the torque is also zero. Think of a hinged door. If you push on the edge of the door, towards the hinge, the door doesn't move because the torque is zero.

Example 3 is the general case in which the force is applied at some angle a to the arm. The perpendicular distance is given by trigonometry as the length of the arm (L) times the cosine (cos) of the angle. The torque is then given by:

T = F * L * cos(a)

Examples 1 and 2 can be derived from this general formula, since the cosine of 0 degrees is 1.0 (Example 1), and the cosine of 90 degrees is 0.0 (Example 2).

In Example 4, the pivot has been moved from the end of the bar to a location near the middle of the bar. Weights are added to both sides of the pivot. To the right a single weight W produces a force F1 acting at a distance L1 from the pivot. This creates a torque T1 equal to the product of the force and the distance.

T1 = F1 * L1

To the left of the pivot two weights W produce a force F2 at a distance L2. This produces a torque T2 in a direction opposite from T1 because the distance is in the opposite direction.

T2 = F2 * L2

If the system were in equilibrium, or balanced, the torques would be equal and no net torque would act on the system.

T1 = T2 or T1 - T2 = 0

F1 * L1 = F2 * L2

If the system is not in equilibrium, or unbalanced, the bar rotates about the pivot in the direction of the higher torque. If F2 = 2 * F1, what is the relation between L1 and L2 to balance the system? If F2 = 2 * F1, and L1 = L2, in which direction would the system rotate

n flight, any aircraft will rotate about its center of gravity, a point which is the average location of the mass of the aircraft. We can define a three dimensional coordinate system through the center of gravity with each axis of this coordinate system perpendicular to the other two axes. We can then define the orientation of the aircraft by the amount of rotation of the parts of the aircraft along these principal axes. The roll axis lies along the aircraft centerline. A roll motion is an up and down movement of the wings of the aircraft as shown in the animation.

The rolling motion is being caused by the deflection of the ailerons of this aircraft. The aileron is a hinged section at the rear of each wing. The ailerons work in opposition; when the right aileron goes up, the left aileron goes down.


As described on the shape effects slide, changing the angle of deflection at the rear of an airfoil will change the amount of lift generated by the foil. With greater downward deflection, the lift will increase in the upward direction; with greater upward deflection, the lift will decrease in the upward direction. Since the ailerons work in pairs, the lift on one wing increases as the lift on the opposite wing decreases. Because the forces are not equal, there is a net twist, or torque about the center of gravity and the aircraft rotates about the roll axis. The pilot can use this ability to bank the aircraft which causes the airplane to turn.


On this page we have demonstrated an aircraft roll induced by movement of the ailerons, but there are other ways to produce a rolling motion on an aircraft. The Wright brothers used a method called wing warping. Their wings were wired together in such a way that the outer panels of each wing could be twisted relative to the inner panel. The twisting changed the local angle of attack of sections of the wing which changed the lift being generated by that section. Unequal forces on the wings caused the aircraft to roll. Many modern airliners use a spoiler to roll the aircraft. A spoiler is a plate that is raised between the leading and trailing edges of the wing. The spoiler effectively changes the shape of the airfoil, disrupts the flow over the wing, and causes a section of the wing to decrease its lift. This produces an unbalanced force with the other wing, which causes the roll. Airliners use spoilers because spoilers can react more quickly than ailerons and require less force to activate, but they always decrease the total amount of lift for the aircraft. It's an interesting trade! You can tell whether an airliner is using spoilers or ailerons by noticing where the moving part is located. At the trailing edge, it's an aileron; between the leading and trailing edges, it's a spoiler. (Now you can dazzle the person sitting next to you on the plane!)


You can view a short movie of "Orville and Wilbur Wright" explaining how wing warping was used to roll their aircraft. The movie file can be saved to your computer and viewed as a Podcast on your podcast player

A fundamental aircraft motion is a banking turn. This maneuver is used to change the aircraft heading. The turn is initiated by using the ailerons or spoilers to roll, or bank, the aircraft to one side. On the figure, the airliner is banked to the right by lowering the left aileron and raising the right aileron. The lift of the wings of the aircraft is a vector quantity which is always directed perpendicular to the flight path and perpendicular to the wings generating the lift. As the aircraft is rolled, the lift vector is tilted in the direction of the roll. We can break the lift vector into two components. One component is vertical and opposed to the weight which is always directed towards the center of the earth. The other component is an unopposed side force which is in the direction of the roll, and perpendicular to the flight path.

As long as the aircraft is banked, the side force is a constant, unopposed force on the aircraft. The resulting motion of the center of gravity of the aircraft is a circular arc. When the wings are brought level by an opposing motion of the ailerons, the side force is eliminated and the aircraft continues to fly in a straight line along a new heading. Notice that the rudder is not used to turn the aircraft. The aircraft is turned through the action of the side component of the lift force. The rudder is used during the turn to coordinate the turn, i.e. to keep the nose of the aircraft pointed along the flight path. If the rudder is not used, one can encounter an adverse yaw in which the drag on the outer wing pulls the aircraft nose away from the flight path