Atmospheric Flight

5-8 Grade Reading

Trim, Stability and Control

The key concepts for aircraft control are trim, stability and controllability.

Trim is equilibrium of moments. Moments can be viewed as a rotational version of forces. Forces cause a body to translate, or move position, while moments cause a body to rotate, or change angle. Moments can arise when forces do not all act through the center of gravity of a body.

Consider a board balanced on top of a triangle shaped support, a seesaw if you like. If the board has a regular shape and uniform density, and you put the triangle support underneath the board halfway, it will balance horizontally. If the support is not directly under a mass, it will not balance. A moment is created by the offset between the support and the center of mass.

The weight force acts at center of mass (the black and white dot) which is in line with the supporting force. The see-saw is in trim.

a picture of a seesaw
with the pivot point moved toward one end and the other end of the board
touching the ground

The weight force acts at center of mass (the white and black dot) which is NOT in line with the supporting force, producing a moment. The see-saw is NOT in trim.

When calculating moments for the seesaw, we used the distance between the force and the pivot point as the moment arm. A free body (one that is not attached to anything) pivots around its center of mass, so we calculate moments by using the distance between the force and the center of mass.

Stability is the tendency of a system to return to an equilibrium state after it is perturbed. It is commonly described by talking about a ball in a valley, on a plain or on a hill. If a ball is nudged in valley, it returns to the valley floor: the system is stable. On a plain, there is no tendency to accelerate or return to the initial position: the system has neutral stability. If a ball is nudged from its position on top of a hill, it tends to accelerate down the hill: the system is unstable.

a line drawing depicting
a
ball at the bottom of a valley labeled stable, a ball on level ground
labled neutral stability and a ball at the top of a rounded mound labeled
unstable

Adding heavy blocks to the top of the see-saw shifts the center of mass above the pivot. When the left end of the see-saw is shifted down, the center of mass moves to the left, and a moment is produced that rotates the left end further down. The system is UNSTABLE.

an unstable seesaw,
with blocks placed on top of the ends of the board

Adding heavy blocks to the bottom of the see-saw shifts the center of mass below the pivot. When the left end of the see-saw is shifted down, the center of mass moves to the right, and a moment is produced that rotates the left end back up. The system is STABLE.

a stable seesaw with additional weight added underneath the board on
the bottom of the seesaw

Stability is the ability of an airplane to return, of its own accord, to its original attitude in flight after it has been disturbed by some outside force, like wind gusts. It also refers to an airplane's response to the pilot's use of the controls.

An important component of stability for an airplane is the center of gravity (CG). The CG is an imaginary point about which the weight of an airplane balances. If you put a ruler across your finger and place it so it balances, your finger is at the CG of the ruler. The CG of an airplane does not stay at the same place at all times. The loading of heavy cargo onto an airplane will shift the CG as will the drainage of fuel from the tanks during flight. Pilots have to recognize shifts in the CG and respond accordingly. Sudden shifts in the CG can be catastrophic. For example, if an airplane experiences turbulence during flight and a large cargo load shifts, the pilot may have trouble reacting quickly enough to the change in the airplane's center of gravity to maintain stability of the airplane.

Imagine putting a weight on the back of an arrow. Because the CG has shifted, it will be less stable and tend to wobble. Now consider the arrow flying with feathers in front. If the tip is nudged up, the angle of attack for the feathers (wings) is increased. The lift force points up, and causes the arrow to rotate so that the angle of attack increases further. The moment continues to rotate the arrow until feathers are at the back. Now flying in its proper orientation when the tip is nudged up, the angle of attack for the feathers causes a lift force and a moment that rotates the tip back to its equilibrium position.

a drawing of an arrow with the
center of gravity closer to the front of the arrow and lift at the end
of the arrow by the feathers

and arrow with a steeper angle toward the ground

an arrow flipped and
facing into the airflow

The empennage, or tail, plays an important role in the stability of an airplane-much like the tail feathers of an arrow are critical to the stability of the arrow's flight. If an arrow is shot without its tail feathers, it will wobble. The tail feathers keep the arrow stable, and help it stay on course. The empennage works the same way. The vertical fin helps to maintain stability in the direction of yaw. The horizontal stabilizer helps to maintain stability in the direction of pitch. The empennage structures do produce drag, however. Researchers have developed ways to fly tail-less airplanes, but the airplanes must use computer-based control systems to maintain stability. While the empennage structures of different airplanes can be very dissimilar, most modern airplanes still do have some form of fin and horizontal stabilizer. The arrow is stable when the lift force on the feathers from a disturbance acts behind the CG. In the same way, an airplane is stable when the combined center of lift of the wing and the tail acts behind the center of gravity.

an airplane in a
climbing position with arrows illustrating wing lift up and
tail lift down

Controllability means that different trim conditions can be commanded. A system might have a natural trim condition, and be stable about the condition, but be uncontrollable, because the operator is unable to command a different condition. A controllable system has actuators to introduce moments that can drive the system to a new position. Imagine that there is a mechanism that can move the support under the seesaw.

a diagram of a
seesaw
with
equal weights on each end and the center of gravity marked in the middle
of the
seesaw and a diagram of a seesaw with unequal
weights on each
end and the center of gravity marked toward the heavy weight on the
seesaw

If the mass distribution changes (perhaps if we use up fuel from one end of the seesaw) we need to adjust the pivot location to restore trim. The system is CONTROLLABLE.

Most airplanes have horizontal and vertical tails to make the airplane stable. These control structures behave like the feathers of an arrow. These surfaces can also be used to control the orientation of the airplane. The horizontal tail has an elevator that controls pitch moments. The vertical tail has a rudder that controls the yaw moments. The airplane also has controls near the tips of the wings called ailerons. These move in opposition to each other, so that one side of the wings produces a little more lift while the other produces a little less lift. These controls cause a moment that makes the airplane roll.

An airplane has three control surfaces: ailerons, elevators, and a rudder. The control stick controls the ailerons and elevators. The rudder pedals control the rudders.

a picture of a plane showing
its
rudder, elevators, and ailerons

A more detailed discussion of how control surfaces work

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