### Post by Tom Goodrick on Jul 29, 2009 10:08:11 GMT -5

STABILITY DERIVATIVES

WHERE YOU FIND THEM:

You must use an air file editor such as Aircraft Airfile Manager - AAM, to look at the .air file for an FS aircraft. You will find thes in section 1100 - Basic Aerodynamics where you will also find gear drag, spoiler drag and lift and flap drag and lift increment.

WHAT ARE THEY?

Stability involves rotating the aircraft about each of its body axes - axes fixed to and moving with the body. There are three axes: Roll is the X axis going straight through the fuselage from tail to nose. Pitch is the Y axis going through the middle of the wing perpendicular to the X axis. Yaw is the Z axis going up at the intersection of the X and Y axes. Normally, the origin of these axes is placed at the 25% point on the mean wing chord. Rotation rates around each of these axes are designated p, q and r, respectively.

To rotate the aircraft we need to produce moments using the aerodynamic forces. "Cl" (lower case L) is the roll moment coefficient. "Cm" is the pitch moment coefficient. "Cn" is the yaw moment coefficient. The stability derivatives give the moment coefficients in proportion to a change in some parameter of flight or of control deflection. Damping of a rotation occurs when a moment opposes a rate of rotation.

THE LIST IN ORDER FROM AAM

ROLL

Cl_beta_____Roll mom per radian sideslip_________________dihedral effect, couples roll to sideslip

Cl_da_______Roll mom per radian aileron deflection_________

Cl_dr_______Roll mom per radian rudder deflection_________(small)

Cl_ds_______Roll mom per radian spoiler deflection_________(allows spoilers used for roll control)

Cl_p________Roll mom per rad/sec roll rate________________ROLL DAMPING

Cl_r________Roll mom per rad/sec yaw rate________________Dutch Roll

PITCH

Cm0_______Pitch mom at zero Angle of Attack (AoA)________primary design feature; See also table 573.

Cm_adot___Pitch mom per rad/sec AoA change____________secondary pitch damping

Cm_de____Pitch mom per radian elevator deflection_________

Cm_dh____Pitch mom per radian H stabilizer AoA___________(minor)

Cm_q_____Pitch mom per rad/sec pitch rate________________PITCH DAMPING

Cm_dp____Pitch mom per rad/sec prop rate________________prop effect

Cm_dt_____Pitch mom per radian trim deflection_____________Seconday Pitch Control

Cm_df_____Pitch mom per percent flap deflection____________flap effect

Cmg______Pitch mom per gear deflection__________________gear effect

Cm_ds____Pitch mom per radian spoiler deflection___________spoiler effect

YAW

Cn_beta___Yaw mom per radian sideslip___________________Primary Directional Stability

Cn_da_____Yaw mom per radian airleron deflection__________Adverse Yaw

Cn_dr_____Yaw mom per radian rudder deflection___________

Cn_dp____Yaw mom per rad/sec prop speed_______________prop torque

Cn_p_____Yaw mom per rad/sec roll rate___________________coupling (small)

Cn_r_____Yaw mom per rad/sec yaw rate___________________YAW DAMPING

To find values for these derivatives, look at the file for an aircraft you like to fly and note the values it has for each derivative. They should only be changed when you have an aircraft that does not seem to fly well. Then jot down the original values and try small chnages of one at a time, testing carefully to see the effect.

LONGITUDINAL STABILITY

In FS, calculations of all aerodynamic forces and moments are made several times per second and the results are used to move the aircraft in all directions through a complex process employing the moments of inertia. A basic consideration essential to flight is the calculation of the pitching moment by adition of the effects of lift on the wing, lift on the tail and weight acting at the Center of Gravity. This is known as the longitudinal stability calculation. It determines the extent to which the aircraft oscillates in pitch. It also determines the airspeed at which it flies. If the longitudinal stability is poor, it may be impossible to hold a steady speed and therefore the aircraft is out of control. This can happen when the aircraft is stalled - when one or more lifting surface is operated at an angle of attack too large to enable lift. To assure that the aircraft will fly well in normal circumstances, the CG must be close to 25% aft of the leading edge of the mean chord. You can find this position by reading the CG location in the sim during flight as I do in my CG gauge which can be displayed on the panel. To begin a flight, you should place people and cargo carefully and check the CG position shown on the gauge. As fuel is added this will change slightly. During flight the CG changes as fuel is consummed. In some aircraft, the CG moves aft as fuel is burned. This can cause difficulty during a landing after a long flight.

The pitch rotation of an aircraft has two modes or parts. It is important when setting stability derivatives to distinguish between these two modes. The first mode is the fast rotation in which angle of attack changes in reaponse to a change in the elevator trim or control deflection. The angle of attack moves to a stable value within a second or two. This then changes the balance of forces and induces the second mode which is a rotation of the flight path angle relative to the horizon while the angle of attack remains very constant. This is a slow response. Oscillations can occur with periods of a few minutes. It is very important to avoid locking up either of these rotations in the interest of "stability" or steadiness. Such a lock will prevent proper flight motion and perfomance will suffer badly.

WHERE YOU FIND THEM:

You must use an air file editor such as Aircraft Airfile Manager - AAM, to look at the .air file for an FS aircraft. You will find thes in section 1100 - Basic Aerodynamics where you will also find gear drag, spoiler drag and lift and flap drag and lift increment.

WHAT ARE THEY?

Stability involves rotating the aircraft about each of its body axes - axes fixed to and moving with the body. There are three axes: Roll is the X axis going straight through the fuselage from tail to nose. Pitch is the Y axis going through the middle of the wing perpendicular to the X axis. Yaw is the Z axis going up at the intersection of the X and Y axes. Normally, the origin of these axes is placed at the 25% point on the mean wing chord. Rotation rates around each of these axes are designated p, q and r, respectively.

To rotate the aircraft we need to produce moments using the aerodynamic forces. "Cl" (lower case L) is the roll moment coefficient. "Cm" is the pitch moment coefficient. "Cn" is the yaw moment coefficient. The stability derivatives give the moment coefficients in proportion to a change in some parameter of flight or of control deflection. Damping of a rotation occurs when a moment opposes a rate of rotation.

THE LIST IN ORDER FROM AAM

ROLL

Cl_beta_____Roll mom per radian sideslip_________________dihedral effect, couples roll to sideslip

Cl_da_______Roll mom per radian aileron deflection_________

**Primary Roll Control**Cl_dr_______Roll mom per radian rudder deflection_________(small)

Cl_ds_______Roll mom per radian spoiler deflection_________(allows spoilers used for roll control)

Cl_p________Roll mom per rad/sec roll rate________________ROLL DAMPING

Cl_r________Roll mom per rad/sec yaw rate________________Dutch Roll

PITCH

Cm0_______Pitch mom at zero Angle of Attack (AoA)________primary design feature; See also table 573.

Cm_adot___Pitch mom per rad/sec AoA change____________secondary pitch damping

Cm_de____Pitch mom per radian elevator deflection_________

**Primary Pitch Control**Cm_dh____Pitch mom per radian H stabilizer AoA___________(minor)

Cm_q_____Pitch mom per rad/sec pitch rate________________PITCH DAMPING

Cm_dp____Pitch mom per rad/sec prop rate________________prop effect

Cm_dt_____Pitch mom per radian trim deflection_____________Seconday Pitch Control

Cm_df_____Pitch mom per percent flap deflection____________flap effect

Cmg______Pitch mom per gear deflection__________________gear effect

Cm_ds____Pitch mom per radian spoiler deflection___________spoiler effect

YAW

Cn_beta___Yaw mom per radian sideslip___________________Primary Directional Stability

Cn_da_____Yaw mom per radian airleron deflection__________Adverse Yaw

Cn_dr_____Yaw mom per radian rudder deflection___________

**Primary Yaw Control**Cn_dp____Yaw mom per rad/sec prop speed_______________prop torque

Cn_p_____Yaw mom per rad/sec roll rate___________________coupling (small)

Cn_r_____Yaw mom per rad/sec yaw rate___________________YAW DAMPING

To find values for these derivatives, look at the file for an aircraft you like to fly and note the values it has for each derivative. They should only be changed when you have an aircraft that does not seem to fly well. Then jot down the original values and try small chnages of one at a time, testing carefully to see the effect.

LONGITUDINAL STABILITY

In FS, calculations of all aerodynamic forces and moments are made several times per second and the results are used to move the aircraft in all directions through a complex process employing the moments of inertia. A basic consideration essential to flight is the calculation of the pitching moment by adition of the effects of lift on the wing, lift on the tail and weight acting at the Center of Gravity. This is known as the longitudinal stability calculation. It determines the extent to which the aircraft oscillates in pitch. It also determines the airspeed at which it flies. If the longitudinal stability is poor, it may be impossible to hold a steady speed and therefore the aircraft is out of control. This can happen when the aircraft is stalled - when one or more lifting surface is operated at an angle of attack too large to enable lift. To assure that the aircraft will fly well in normal circumstances, the CG must be close to 25% aft of the leading edge of the mean chord. You can find this position by reading the CG location in the sim during flight as I do in my CG gauge which can be displayed on the panel. To begin a flight, you should place people and cargo carefully and check the CG position shown on the gauge. As fuel is added this will change slightly. During flight the CG changes as fuel is consummed. In some aircraft, the CG moves aft as fuel is burned. This can cause difficulty during a landing after a long flight.

The pitch rotation of an aircraft has two modes or parts. It is important when setting stability derivatives to distinguish between these two modes. The first mode is the fast rotation in which angle of attack changes in reaponse to a change in the elevator trim or control deflection. The angle of attack moves to a stable value within a second or two. This then changes the balance of forces and induces the second mode which is a rotation of the flight path angle relative to the horizon while the angle of attack remains very constant. This is a slow response. Oscillations can occur with periods of a few minutes. It is very important to avoid locking up either of these rotations in the interest of "stability" or steadiness. Such a lock will prevent proper flight motion and perfomance will suffer badly.