LESSON 16 Chapter 15 Longitudinal Stability and Control ANA Chapter 4

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Flight Theory

Chapter 15

Longitudinal Stability and Control


¨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, pilot input, alcohol, and drugs

¨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

¨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, thrust, and structural limitations

¨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



¨Static Stability

¨Is the initial tendency of an aircraft to move, once it has been displaced (courtesy Joe Lindstrom) from its equilibrium position

¨Positive Static Stability

¨Would be the initial tendency to return to the original position.

¨Negative Static Stability

¨Is the initial tendency to move away from the original position.

¨Neutral Static Stability

¨Is the initial tendency to stop at a random point neither moving farther away from or towards the original position

¨Static Stability

¨Dynamic Stability

¨Is the movement over a period of time.





¨This is usually explained in terms of oscillatory movement.

¨Positive Dynamic

¨Is when the oscillations are getting smaller over time

¨Negative Dynamic

¨Is when the oscillations are getting larger over time

¨Neutral Dynamic

¨Is when the oscillations are staying about the same size over time.

¨Dynamic Stability

¨Combo Platters

¨1. Positive static pos dynamic

¨2. Pos static neutral dynamic

¨3. Pos static neg dynamic

¨4. Neutral static pos dynamic

¨5. Neutral static neutral dynamic

¨6. Neutral static neg dynamic

¨7. Neg static pos dynamic

¨8. Neg static neutral dynamic

¨9. Neg static neg dynamic

¨Airplane Reference Axes

¨In Aerodynamics, for mathematical purposes the axes are labeled:

¤M for pitch

¤L’ for roll

¤N for yaw

¨In order to deal with each of these axes mathematically they are measured in ft/lbs of force about the cg

¨On pages 229 and 230, Dole shows several graphs which represent positive, neutral and negative moment loads about these axes

¨The steeper the line above or below the neutral point the more unstable

¨For identification purposes, the axes are labeled:

¤X for the longitudinal axis

¤Y for the lateral axis

¤Z for the vertical axis

¨Longitudinal axis

¨Runs down the center of the fuselage.

¨This axis deals with roll stability or lateral stability about the longitudinal axis.

¨Stability on this axis determines how well the plane will right itself when encountering gusts

¨Longitudinal Stability

¨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

¨Longitudinal Stability

¨The center of pressure moves aft with a decrease in α

¨The center of pressure moves forward with an increase in α

¨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 α to further increase

¨In order to counter this problem, the cg must be forward of the center of lift


¨Longitudinal Stability

¨To make this condition stable, tail down force is needed

¨There are two forces in play here:

¤α 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

¨Longitudinal Stability

¨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

¨Lateral axis

¨Runs down the wings.

¨This axis deals with pitch stability or longitudinal stability about the lateral axis.

¨This stability is important because it determines the pitch characteristics of the plane.

¨Lateral axis

¨How easily the airplane stalls may be effected by this as well.

¨Vertical axis

¨This axis runs roof to belly

¨This axis deals with yaw stability or directional stability about the vertical axis

¨Stability about the vertical axis is yaw stability or directional stability

¨The prime contributors to directional stability are the tail and side fuselage surfaces behind the CG

¨A symmetrical airfoil

¨Will not experience any pitching because the forces acting on the top and bottom surfaces are located in the same position.

¨A Cambered Airfoil

¨Will experience a difference in forces causing a nose down pitching moment.

¨Negatively camber airfoils experience a nose up pitching moment.

¨The Shape Of The Fuselage

¨May be a destabilizing influence on pitch control.

¨It may have neutral lift characteristics until a pitching up is caused by an gust then the pressure distribution may cause a pitching up moment.

¨Power Effects

¨Are felt in two major areas:



¨Direct effects include the vertical location of the thrust line with respect to the longitudinal axis.

¨If below, nose up

¨If above nose down.

¨Thrust Line

¨Usually a power increase makes the pitch increase but not as dramatically as in the X-5

¨In the case of the PBY, a power increase will cause a pitch down

¨Nacelles And Props

¨Engine Nacelles and props may cause an inherent pitch up force

¨The air has to turn a corner when entering striking the nacelle or prop shaft causing the action reaction.








¨Largest at high power, low airspeed, high AOA, and inlet forward of cg

¨Power Effects

¨Indirect effects are the tail down force

¨This increases with an increase in airspeed or propeller slipstream

¨Which in turn causes the pitch to increase.

¨The tail is the single largest contributor to longitudinal stability.


¨The center of lift related to cg determines a large part of stability.

¨If the cg is lined up with center of lift neutral stability will result.

¨If the cg is behind the center of lift, negative stability will result.

¨If the cg is in front of center of lift, positive stability will result.

¨Dynamic oscillations

¨Occur in three flavors,

¨First mode:

¤Phugoid longitudinal dynamic mode

¨These are long period oscillations lasting 20 to 100 seconds and are easily controlled

¨Airspeed, pitch and altitude may vary widely but α remains nearly constant

¨The motion is so slow, inertia and damping forces are very low

¨It is a slow exchange of kinetic and potential energy

¨Dynamic oscillations

¨The second mode:

¤Short period oscillations

¨Generally lasting 1 to 2 seconds or less that are impossible to control

¨An elevator flapping about its hinge line is an example

¨The pilot should let the controls go and let the inherent stability design take care of the problem if possible

¨If you desire a more rapid recovery hold the controls in their neutral position

¨The third type: (not really a mode)

¨If you try to dampen the oscillations yourself your likely to make it worse

¨This is referred to as pilot-induced oscillation  (PIO) and may destroy the airplane in seconds

¨Helicopter pilots may also encounter a similar problem called Collective Bounce

¤An upward gust forces you and the collective down forcing a decrease in α on the rotor

¤This causes the helicopter to descend causing you and the collective to float up and the process is repeated

¨Vertical axis

¨Runs vertically through the fuselage.

¨This axis deals with yaw stability or directional stability about the vertical axis. 

¨Yaw angle or beta is the primary reference in lateral stability as well as directional stability considerations.

¨Pitching Tendencies in a Stall

¨For low tailed aircraft the tail moves through the air disturbed by the main wing as α is increased

¨As α is increased to the point of stall, the tail is below the disturbed air of the main wing

¨This results in full pitch control as the stall occurs giving the pilot command of the pitch

¨Pitching Tendencies in a Stall

¨For T-tailed aircraft the tail is in the disturbed air at the point of stall

¨At the stall two things happen:

¤A marked pitch up occurs

¤Reduction of tail effectiveness due to being engulfed in the wings disturbed flow

¨This greatly reduces the tail effectiveness in recovering from the stall

¨A high sink rate is established which further increases the α and the problem is worsened

¨Extreme α and deep stall will result

¨Pitching Tendencies in a Stall

¨Swept wings will contribute to the pitch up tendency in a stall

¨Since the tips stall first in swept wing aircraft, this causes the COL to shift forward

¨This reduces the arm between the cg and the COL

¨It is this that causes an inherent pitch up tendency at the point of stall

¨In addition, a long fuselage out in front of the swept wings will cause a destabilizing force

¨This exacerbates the condition of pitching up at the point of stall

¨Longitudinal Control on Takeoff

¨In this particular situation, the rotation is about the gear not the cg

¨Since the angle of incidence is set to only generate lift after rotation the tail must overcome these nose down moment forces to bring the pitch to takeoff pitch

¨The moments the tail must overcome are:

¤The thrust line moment above the main wheels

¤The moment of the cg being ahead of the main wheels

¤The nose down moment of the cambered airfoil

¤The nose down moment of being in ground effect



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