Commercial Ground School AVF 221
Stability and control and spins
Stability is the inherent quality of an aircraft to correct for conditions that may disturb its equilibrium
It is the ability of the aircraft to maintain uniform flight and to recover from the effects of disturbing influence
These disturbing influences may include such things as gusts, cg range, and pilot input
It must have enough stability to minimize pilot workload but enough controllability to allow utility
Thus aircraft designers have to strike a balance between stability and controllability
Stability in general
Stability is generally discussed with reference to the 3 axis
Longitudinal stability which is pitch stability
Lateral stability which is roll stability
Vertical stability which is yaw stability
Stability is further categorized
Positively stable – resists any displacement
Negatively stable – favors displacement
Neutrally stable – neither resistant or favoring displacement
Aircraft Design Characteristics
Engineers design in specific control characteristics based on the job the aircraft needs to do
Training aircraft generally are quick to respond to inputs
Transport category aircraft are usually slower to respond and are heavier on the controls
Stability affects 2 areas significantly:
Maneuverability & Controllability
The capability of the aircraft to respond to the pilot’s inputs
Especially with regard to flightpath and attitude
The quality of an aircraft that permits it to be maneuvered easily
Also the ability to withstand the stresses imposed by those maneuvers
It is governed by weight, inertia, size and location of flight controls, structural strength, and powerplant
The flightpaths and attitudes an aircraft flies are limited by
The aerodynamic characteristics
If the maximum utility is desired, it has to be able to be safely controllable to its limits without exceeding the pilot’s strength
There are two types of stability
Is the initial tendency of an aircraft to move, once it has been displaced from its equilibrium position
This type of stability has three subtypes:
Positive static stability is indicated by initial movement back to the original position
Neutral static stability is indicated by initial movement to stay in the new position
Negative static stability is indicated by initial movement away from the original position
Dynamic stability refers to the aircraft response over time when disturbed from a given AOA, slip, or bank.
This type of stability also has three subtypes:
Positive dynamic stability—over time, the motion of the displaced object decreases in amplitude and, because it is positive, the object displaced returns toward the equilibrium state.
Neutral dynamic stability—once displaced, the displaced object neither decreases nor increases in amplitude. A worn automobile shock absorber exhibits this tendency.
Negative dynamic stability—over time, the motion of the displaced object increases and becomes more divergent.
The oscillations made during the progression are called periodic motion
Amplitude is the measurement of the movement of each oscillatory period
Aperiodic motion is non-timed motion
The airplane may have positive static stability but that does not mean it has positive dynamic stability in every circumstance
Outside forces may act in such a way as to increase the amplitude
Static and dynamic stability
If the airplane has positive static stability normally an oscillation will exist
However, if acted on by an outside force, the dynamic stability may be neutral or even negative
The oscillations may stay the same or become greater
This may happen to the point of structural failure
If the airplane has neutral or negative static stability no oscillation will exist
The movement may be to a new direction or diverge from the original direction at a faster and faster rate
We can categorize stability along the 3 axis:
Longitudinal or pitch stability
Pitching occurs about the lateral axis
Lateral or roll stability
Rolling occurs about the longitudinal axis
Vertical or yaw stability
Yawing occurs about the vertical axis
Longitudinal stability is the quality that makes a plane stable about it’s lateral axis
A plane without this may pitch into a dive or climb and into a stall
Static longitudinal stability is dependent on 3 major factors:
Location of the wing with respect to the cg
Location of the tail with respect to the cg
Area or size of the tail surface
The center of pressure moves aft with a decrease in a
The center of pressure moves forward with an increase in a
This means that a pitch up moment causes a unstable condition because lift is increasing and moving forward at the same time
This causes the a to further increase
In order to counter this problem, the cg must be forward of the center of lift
To make this condition stable, tail down force is needed
There are two forces in play here:
a is set to a negative value
Downwash from the main wing
The faster the plane flies the more tail down force from downwash (except for T tails)
On elevators the manufacturer sets the tail down force to optimum for cruise speed and power settings
On stabilators, camber of the airfoil and trim is used to achieve the same result
On average a stabilator only needs to deflect about half the amount of an elevator
As the speed decreases the dynamic pressure is decreased on the tail allowing the nose to pitch down
In addition the downwash is also reduced causing a lesser downward force on the tail
This places the plane in a nose low pitch allowing speed to increase
This in turn causes the nose to pitch up but not as far this time (in positively dynamically stable aircraft)
This oscillation continues until it levels out
A power change has the same effect
Power is considered to have a destabilizing effect on stability
Generally addition of power causes the pitch to increase
This all depends on the thrust line built into the aircraft design, however
Below the cg, addition of power will give a pitch up
Through the cg, addition of power will give no pitch change (other than downwash on the tail discussed earlier)
Above the cg, addition of power will cause a pitch down
Loading effects on longitudinal stability
With an aft cg, over-rotation may become a real problem
With a forward cg, the plane may be so stable as to resist any rotation until a very high airspeed is reached
Directional stability is mostly influenced by the vertical structures
In order for positive stability to result, more surface area must be behind the cg than ahead
When displaced the aircraft is still moving in the same direction with the longitudinal axis offset
This results in a momentary skid which is corrected by more force on the side of the plane in the direction of the skid
This force causes the plane t
o return, however a new heading will emerge
So a yaw force will always require a course correction from the pilot
Sweepback may be employed to enhance yaw stability
Wing drag increases on the forward moving wing which results in the nose yawing back to the original position
Dutch roll may be encountered when quickly depressing the rudder peddal and releasing
As the plane yaws, more lift is produced on the forward moving wing which causes roll and drag
As the drag pulls that wing back the other wing now moves forward creating more lift and again roll
This may continue until structural failure results
The degree of directional stability is proportional to the size of the vertical stabilizer and the distance from the CG
Increase either or both and an increase in directional stability will result
Lateral Stability and Control
Lateral stability is the stability displayed about the longitudinal axis of the airplane or specifically the stability in the roll.
There are 4 main design factors that make a plane laterally stable:
The different thing about Lateral stability is that there is really no force in a roll that will cause the airplane to right itself
There is really no aerodynamic force created in rolling that tends to restore the wings to level flight
In addition there is no force that will continue the roll once it has begun
Most airplanes are neutrally stable in the roll
Overbanking tendency in a turn
Dihedral or Anhedral
Dihedral is a stabilizing design, whereas Anhedral is a destabilizing design.
The stabilizing effect of dihedral occurs when a sideslip is set up as the result of turbulence or gust displacing the plane.
The side slip results in the downward wing having a greater angle of attack than the upward wing. The extra lift then rights the airplane.
The most common way to produce lateral stability is to use dihedral
Manufactures build in a 1 to 3 degree angle
Dihedral involves a balance of lift created by each wing
If a gust causes roll, the aircraft will sideslip in the direction of the bank
Since the wings have dihedral the air strikes the lower wing at a much greater a
This causes more lift to be generated on the lowered wing making it rise
Once level the lift is equal again
Dihedral How Does It Work?
As you can see in this exaggerated diagram, the sideslip that sets up causes an increase in a
There is a change in the relative wind due to the slip
The lowered wing has a higher a due to the relative wind changing from directly 90 degrees to an angle off the wing tip
In addition the lowered wing has a greater vertical lift component
The raised wing has a greater horizontal lift component
This causes the imbalance in lift between the two wings
If we look at the force vectors for a wing with dihedral we see that some of the lift the wing generates is tilted into the horizontal
This horizontal vector requires more lift from the wing than if it had no dihedral
This concept however is slight, the main reason dihedral works is due to the sideslip and increase in a
There are some penalties that go along with too much dihedral:
Less vertical component of lift
More drag (higher a to make up for loss of lift)
More aileron force to roll
A high wing sets up a pendulum type of situation
This can result in the equivalent of a 1 to 3 degree dihedral.
So not as much dihedral is needed.
In some planes, negative dihedral is needed.
The low wing however the reverse is true.
Still other airplanes have both dihedral and anhedral
A greater portion of the keel is above and behind the cg
When a slip occurs airflow pressure against the upper portion of the keel rolls the wings back to level
When a side slip is set up in a sweepback wing, the upwind side wing will have a greater angle of attack because of the more favorable relative wind.
One of the best examples of this is Adverse Yaw
Because the yaw is produced in the opposite direction of the turn it is referred to as adverse
When rolling into a turn, the upward wing’s lift vector is tilted aft because of the change in the relative wind components being up and parallel to the flight path
The downward wing’s lift vector is tilted forward because of the change in relative wind components being down and parallel to the flight path.
These two forces oppose the turn entry and cause adverse yaw.
Aileron drag is another common cause of adverse yaw.
Frise ailerons and differential aileron travel are common ways of offsetting the effects of aileron drag.
Using spoilers to turn solves this problem.
A spin may be defined as an aggravated stall that results in what is termed “autorotation” wherein the airplane follows a downward corkscrew path.
As the airplane rotates around a vertical axis, the rising wing is less stalled than the descending wing creating a rolling, yawing, and pitching motion.
The airplane is basically being forced downward by gravity, rolling, yawing, and pitching in a spiral path.
A stall occurs when the smooth airflow over the airplane’s wing is disrupted, and the lift degenerates rapidly.
This is caused when the wing exceeds its critical angle of attack.
This can occur at any airspeed, in any attitude, with any power setting.
If recovery from a stall is not achieved in a timely and appropriate manner by reducing the Angle of Attack (AOA), a secondary stall and/or a spin may result.
All spins are preceded by a stall on at least part of the wing.
The angle of the relative wind is determined primarily by the aircraft’s airspeed and attitude.
Factors to consider are
center of gravity
the amount of acceleration used in a turn
2 aerodynamic conditions support a spin:
1. a stall
2. some kind of yawing force
The primary cause of an inadvertent spin is exceeding the critical AOA while applying excessive or insufficient rudder and, to a lesser extent, aileron.
Insufficient or excessive control inputs to correct for Power Factor (PF), or asymmetric propeller loading, could aggravate the precipitation of a spin.
There are 3 types of spins as defined by the FAA:
An incipient spin is that portion of a spin from the time the airplane stalls and rotation starts, until the spin becomes fully developed. Incipient spins that are not allowed to develop into a steady state spin are commonly used as an introduction to spin training and recovery techniques.
A fully developed, steady state spin occurs when the aircraft angular rotation rate, airspeed, and vertical speed are stabilized from turn-to-turn in a flightpath that is close to vertical.
A flat spin is characterized by a near level pitch and roll attitude with the spin axis near the CG of the airplane. Recovery from a flat spin may be extremely difficult and, in some cases, impossible.
The first step in recovering from an upright spin is to close the throttle completely to eliminate power and minimize the loss of altitude.
If the particular aircraft spin recovery techniques are not known, the next step is to neutralize the ailerons, determine the direction of the turn, and apply full opposite rudder.
When the rotation slows, briskly move the elevator control forward to approximately the neutral position.
Some aircraft require merely a relaxation of back pressure; others require full forward elevator control pressure.
Forward movement of the elevator control will decrease the AOA.
Once the stall is broken, the spinning will stop.
Neutralize the rudder when the spinning stops to avoid entering a spin in the opposite direction.
When the rudder is neutralized, gradually apply enough aft elevator pressure to return to level flight.
Too much or abrupt aft elevator pressure and/or application of rudder and ailerons during the recovery can result in a secondary stall and possibly another spin.
If the spin is being performed in an airplane, the engine will sometimes stop developing power due to centrifugal force acting on the fuel in the airplane’s tanks causing fuel interruption.
It is, therefore, recommended to assume that power is not available when practicing spin recovery.
As a rough estimate, an altitude loss of approximately 500 feet per each 3-second turn can be expected in most small aircraft in which spins are authorized. Greater losses can be expected at higher density altitudes.
PARE is a good acronym to remember these steps
Normal category airplanes are required to pass a 1 turn spin or a 3 second spin whichever is longer
However they are still placarded against spins.
Spin must be controllable
Utility category airplanes can be certified in the normal or acrobatic categories with appropriate placards and markings identifying which is which
Acrobatic category the plane must recover after 6 spins or 3 seconds whichever is longer
Must be able to recover from any point in the spin
A cross controlled airplane will generally rotate in the direction of the rudder
A deflection of aileron at slow airspeeds may cause the upward moving wing to stall because the aileron has demanded an increased angle of attack (down aileron
The only way to be fully sure of an aircraft’s tendencies in a spin is to test them in a spin
In a spin deflection of the ailerons may increase or decrease the rotation rate
An airplane may be able to spin in the utility category but when loaded in the normal category it may not recover
The spiral mode is an autorotation mode similar to a spin.
The center of rotation is close to the centerline of the airplane but the airplane is not stalled.
Many airplanes and gliders will not spin at forward CG locations but will spiral.
Many airplanes will enter a spin but the spin will become more vertical and degenerate into a spiral.
It is important to note that when the spin transitions into the spiral the airspeed will increase as the nose goes down to near vertical.
The side forces on the airplane build very rapidly and recovery must be effected immediately before exceeding the structural limits of the airplane.
Release the back pressure on the stick (yoke), neutralize the rudder and recover from the steep dive.
As in stall and spin recovery, avoid abrupt or excessive elevator inputs that could lead to a secondary stall.
Some airplanes may exceed Vne if allowed to continue past 1 turn in a steep spiral