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
¨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
¨Is the movement over a period of time.
¨This is usually explained in terms of oscillatory movement.
¨Is when the oscillations are getting smaller over time
¨Is when the oscillations are getting larger over time
¨Is when the oscillations are staying about the same size over time.
¨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
¨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 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 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
¨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
¨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
¨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.
¨How easily the airplane stalls may be effected by this as well.
¨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.
¨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.
¨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
¨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.
¨Occur in three flavors,
¤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
¨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
¨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