Ch 23 - Longitudinal Stability And Control Flashcards
Longitudinal Stability and CG Position
The behaviour of an aircraft in response to a disturbance in pitch.
CG is always forward of the Aerodynamic Centre
Therefore a tail must produce a tail down moment and a canard must produce a nose up moment
The Effect of an Initial Disturbance on AOA (Upgusts and Downgusts)
Upgust leads to an Increased AOA on the Aerofoil (nose up moment) and a decreased AOA on the tailplane. This means less downforce and therefore a nose down moment.
Downgust leads to a decreased AOA on the aerofoil (nose down moment) but an increased AOA on the tailplane which means more downforce and a nose up motion.
In both instances, the tailplane has a restorative moment on the Aircraft
The Absolute AOA
The angle between the RAF and the Aerofoils zero lift line.
The zero lift line cancels out the influence of camber (graph goes through the axis. (Will always be above the chord line on a positively cambered aerofoil)
Longitudinal Static Stability Coefficient (M) equation
Cm = M(moment) / q(dynamic p) x S (surface area) x c(mean chord)
Pitching moment is much greater at higher airspeeds
Static Longitudinal Stability Graph (against AOA)
Positive Static Longitudinal Stability is shown by a negative slope on a Cm/AOA graph. The steeper the slope the greater the amount of longitudinal stability
Neutral Static Longitudinal Stability (Cm/AOA) will be shown as a horizontal line. Any change in angle of attack results in no pitching moment
Negative Static Longitudinal Stability (Cm/AOA) will be shown by a positive slope. If a disturbance creates a pitch up moment, the aircraft will pitch up further deviating further from the equilibrium position. The steeping the line, the more unstable the AC is.
Static Longitudinal Stability for a swept wing aircraft
Positive longitudinal Static Stability at most (moderate) AOAs but this reduces to neutral and then negative at high angles of attack.
Helps show the tendency of a swept wing aircraft to pitch up at the stall
Design Influences on Static Longitudinal Stability: The Wing
Usually designed so that the AC or CP is behind the centre of gravity. This is so that if there is a pitch up disturbance, the AOA will increase which leads to the ac will pitch nose down due to the imbalance between the lift and weight forces.
The wing is restorative (usually for all training and transport ac)
At high angles of attack, the drag moment becomes significant which produces an overall destabilising moment
Design Influences on Static Longitudinal Stability: The Tailplane
The most important factor in longitudinal Static stability and is a restorative moment.
Usually trimmed to produce a small downforce which counterbalances all of the other pitch down moments. Size of moment changes by changing the deflection of tailplane.
The bigger the disturbance, the more restorative moment is made.
Depends on two things; the size of the tailplane (volume) and the distance from the ACs COG
Design Influences on Static Longitudinal Stability: The Fuselage
Destabilising due to the asymmetrical surface pressure distribution. If ac pitches up, the difference in surface pressure will result in a larger pitch up moment.
Design Influences on Static Longitudinal Stability: The Engine Nacelles
Usually a destabilising moment created.
Whole Aircraft Stability
The Cm/AOA graph will have a negative gradient reflecting the natural positive static longitudinal stability designed onto the aircraft.
The Effect of CG on Longitudinal Stability
It’s position has a strong influence on total longitudinal stability as it determines how large the moments are from the other forces.
The Aerodynamic Centre is usually behind the CG to provide the necessary restorative moment following a pitch up.
Must be sufficiently strong to provide the minimum stability but not too strong that it becomes excessively longitudinally stable.
Wishing forward and aft limits; maximum restorative moment and minimum acceptable restorative moment
The Neutral Point and Static Margin
The neutral point is the furthest aft position of the CG for which the AC is not statically unstable. This is the point where there will be not pitching moment as the CG and the aerodynamic Centre are collocated.
An aerodynamic consideration which is fixed at the design stage.
Static Longitudinal Stability is proportional to the distance between the CG and neutral point. Regulations require the AC to have positive static stability at all times so the CG aft limit must be set some distance forward of the neutral point.
The distance between the neutral point and the aft CG limit is the static margin
Effect of CG position on elevator Effectiveness
Further forward CG means that the moment produced by the tailplane. This increases the longitudinal static stability but means that larger control inputs are needed to pitch the aircraft.
Fwd CG = Increased Longitudinal Stability, Decreased pitch control
Aft CG = Decreased Longitudinal stability, Increased pitch control
Effect of Flaps on longitudinal Stability
Flaps increase downwash and reduced dynamic pressure at the tail, both destabilising and reducing the elevator effectiveness.
Reduces static longitudinal stability
Stick-Position Stability
As force produced by tailplane is proportional to IAS^2, a larger elevator deflection angle is required to generate sufficient trim force at the slower speed
Trim Angle of Attack will be smaller at faster flights.
Independent of longitudinal stability
Elevator angle must increase at slower speeds to ensure the same trim force is provided by the tailplane
A statically stable aircraft is also stick - position stable
Stick-Force Stability
The magnitude of stick force increases with elevator deflection and IAS. Stick force must be higher for faster speed so that is is clearly perceptible to the pilot.
Aircraft which don’t have enough stick force stability will be equipped with an artificial feel system.
Elevator and Trim Tab
AC equipped with conventional tailplane with elevator and trim tab. The trim tab is moved up relative to the elevator which moves the elevator down as speed increases. As speed reduces, we trim to move the trim tab down which moves the elevator up.
Trimmable Horizontal Stabiliser
More aerodynamically effective, less drag for required lift force
Enables equal pitch up and down authority
Allows control over a greater CG range
Less sensitive to flutter
Provides no feel, must be provided artificially
Effect of CG position on THS
Fwd Cg = Leading edge trimmed down (large) as there is more stick force needed to keep nose up
Aft CG = Less leading edge down (relatively higher) as the force required to keep the nose up is less
THS Position at TO
Fwd CG = Large nose up trim so that the force felt at the TO will be normal (helps get nose off the floor)
Aft CG = Minimum nose up trim otherwise pilot can tail strike or even stall by rotating at too large a angle
Manoeuvre Stability - Stick Force Per G
Amount of control column force required to produce a given load factor.
Too easy = might lead to over stress of the AC
Too hard = pilot may tire quickly when manoeuvring the AC
Stick Force Gradient
How much force is required per G
Airliners have a relatively high stick force per g Gradient because they can only cope with a small load factor (2.5G)
Aerobatic aircraft has a much shallower gradient to allow the pilot to pitch the AC more easily
Easiest Way = BOB WEIGHT / SPRINGS
Factors Affecting Manoeuvre Stability
CG Position - Further fwd the CG position, the greater the stick force per g required. Further aft = smaller stick force per gradient. Aft CG makes spin recovery harder
Aerodynamic Damping - Occurs because aerodynamic forces are produced which oppose input from pilot (pitching (reduced downforce) / rolling (less lift on down going wing))
Effect of Altitude on Aerodynamic Damping
At high altitude, TAS is higher for the same IAS. Less aerodynamic damping at altitude. This reduces the longitudinal stability of the AC - stick force reduced per g/manoeuvre gradient
Longitudinal Dynamic Stability (2 types)
Short period oscillation - Large changes in Aircraft attitude, larger changes in AoA, period of a few seconds (1-2 seconds), large changes in Load factor, small changes in height and airspeed. Pilot induced oscillation can make it worse - let is sort itself out. (PIO)
Long Period (Phugoid) Oscillations - little change in AoA, little change in load factor, large change in speed, large change in height, long period (1-2 minutes), can be stopped by pilot, weakly damped
Effect of Altitude and CG on Dynamic Stability
Static and dynamic stability decrease with altitude because of the reduced effect of aerodynamic damping
Fwd CG, greater dynamic and static longitudinal stability due to the large restoring moments created.
