Stability Flashcards
Which of the following change with AoA?
Cl, Cd, CoP, AC
Cl, Cd, CoP
What factors affect the magnitude of the pressure distribution/alter it?
Speed only changes the magnitude of the pressure distribution, AoA will actually change the pressure distribution.
How does the moment change in a force couple?
It doesn’t. A force couple has equal magnitude forces acting on opposite directions. Regardless of the pivot point, the moment will remain equal.
So total force is 0, but total moment is not 0.
What purpose is superposition?
To simply move the location of a force without changing the the moment or total force.
List all 3 axis and what control surface controls them, then list the stability about that axis.
Lateral: wingtip-wingtip (controlled by elevator/pitch ) stability about longitudinal axis (roll).
Longitudinal: fore/aft (controlled by aileron/roll) stability about lateral axis (pitch).
Directional: normal (controlled by rudder/yaw) stability about normal axis.
What is the aerodynamic centre of a wing? Where is it?
What does it change with?
It is a point along the chord of an aerofoil for which the pitching moment does not change with AoA (does but insignificant). Although it may change with airspeed. It has a small negative moment vale at the zero lift angle (if airspeed is constant), and is usually located 1/4 of chord from LE.
How does the pitching moment change/moment coefficient change with AoA?
When an aerofoil sits at an AoA, TR is produced which acts through the CoP, which the location of changes with AoA. TR creates moments about LE/TE (LE/TE pivot points, CoP force datum) which can change with AoA as location of CoP changes. The characteristics of the changes are the moment coefficients.
Pitching moment about LE decreases with AoA, increase about TE with AoA, will remain constant about AC and 0 about CoP.
At the zero lift angle, what approximates what?
Is the AC the same as CoP?
At ZLA on a cambered AF, the pitching moment about the AC approximates the zero lift pitching moment (if drag is small) as the pitch moment is determined by the two L forces (force couple).
The two L are equal magnitude but opposite directions at different location along the chord for FC created (0 lift not 0 moment). So is a slight downwards moment that is independent of the pivot point.
CoP=AC on symmetrical AF, but not on cambered.
Explain the pitching moments/coefficient pitching moments about Cop/AC
Coeff/PM is 0 about CoP.
CmAC is a constant value, but the pitching moment may change with airspeed NOT AoA.
Go over finding the AC/CoP
List the equations required
Check notes
Note: +/- value of Coeff will show what direction the moment is in… a “magnitude” is the absolute value and shows no direction.
What two types of stability are there?
Static and dynamic.
Static refers to the initial reaction after being disturbed from equilibrium, dynamic is the subsequent reactions.
Explain stick fixed/free
Stick fixed: controls held in neutral position
Stick free: pilot releases control column and allows control surfaces to take their own position.
What is longitudinal stability? What are the 4 factors that affect longitudinal stability?
The a/c must have an inherent ability to return to the same pitch/attitude after a disturbance in pitch.
CoG position, pitch moment on tailplane (Me), pitch moment on main plane (Mm), pitch moment of fuselage (Malpha).
What equation is used for Cm pitch?
What is the criterion of longitudinal stability? What equation could show this?
PM= Cm(p) x 1/2pv^2S x c
Tailplane restoring moment must remain greater than any unstable pitching moment of the main plane.
Stability occurs when M=0, M=Me+Mm+Malpha
List the features of an a/c that can affect longitudinal stability
Wings, CoG, tailplane, fuselage, longitudinal dihedral, sweptback wings, stick control
Explain how wing design and CoG position can affect longitudinal stability
Wing: due to CoP being aft of CoG, there is a natural pitch down tendency. So if nose up disturbance, AoA increases L increase (although distance decreases) the nose pitch down tendency is enhanced. Will be affected by both airspeed and AoA. Wings too far forward or back will affect the arm between Cop & CoG.
CoG: longitudinal stability is the pitch moment about the CoG. If the Cog is too far forward there is a large arm from tailplane so larger restoring force (nose heavy) whilst an aft CoG reduces the arm so less restoring moment. If CoG is too far aft CoP can be ahead which is an unstable configuration.
Explain how the tailplane is a longitudinal stable feature
Generates +/- lift to offset other moments.
|TP| > |MP|
L x d > L x d D is CoG to respective CoP.
Explain how the fuselage and longitudinal dihedral work to produce longitudinal stability
Fuselage: total force is 0, but moment is not 0. Moment is nose up due to force couple when at positive AoA. Refer diagram
Dihedral: incidence angle of TP less than that of the wings… will only work if MP/TP are in the same level
Explain how sweptback wings and stick control help longitudinal stability
Sweptback wings: in subsonic a/c the CoP position is more aft so greater distance between CoG and CoP that is sufficient to provide a restoring force. Wing can act as tailplane if CoP rear enough due to heavily sweptback wings.
“Decreases angle of incidence at wing tip”.
Stick control: stick fixed is more stable, stick free can cause oscillation even though it avoids excessive force on control column
When is an aircraft in equilibrium?
When total force is 0 and moment about the CoG is 0.
Draw and explain the longitudinal stability diagram
Refer to notes for drawing.
States tailplane produces a -ve Cm as nose down, while a rectangular wing is unstable as Cop ahead of CoG so +ve Cm as nose up. But overall a/c as a whole is longitudinally stable.
What is lateral stability? What must occur for lateral stability? Explain this further…
Lateral stability is an a/c ability to produce a restoring rolling moment to return to equilibrium.
For lateral stability to occur, sideslip towards the lower wing must happen, this creates a difference in the RAF/HDG.
Sideslip occurs as in roll/bank, L & W vectors do not act in the same plane (opposite to each other) so completing vector addition there is a side force (Fy) which is sideslip.
Explain the lateral stability diagram.
What is the equation for rolling moment about CoG?
When side slip occurs, there is a lateral component to the RAF which is the sideslip angle.
If the RAF comes from the right of HDG (+ve) or left (-ve) sideslip angle “beta”.
The coefficient of rolling moment is +ve if a corrective roll to the right, or -ve a corrective roll to the left.
Mr= Cr x 1/2pv^2S x b
List the factors that affect lateral stability
Lateral dihedral, shielding, wing position, fin area, sweptback wings
Explain how lateral dihedral and shielding affects lateral stability
Dihedral: an upward inclination of the wings, as the a/c sideslip s the downgoing wing has a larger AoA so will produce more lift and create a restoring rolling moment. -refer diagram (Anhedral wings are opposite and can be used to prevent Dutch roll).
Shielding: the fuselage shields the upgoing wing from RAF so it produces less lift and this creates a restoring rolling moment.
Explain how wing position and sweptback wings create lateral stability?
Wing position:
-High: with a low CoG and high CoP, when in a bank they act through different datum vertically (refer diagram) as to produce a restoring rolling moment. This effect is enhanced by dihedral/shielding as CoP moves closer to the lower wing which increases the arm.
-Low: this affect still occurs but as Cop/CoG much closer there is less of an arm so less restoring force. Also no sheilding.
Sweptback wings: sideslip to lower wing, with the difference in RAF, creates a greater effective AR on the lower wing so produces more lift, as to produce a restoring rolling moment (refer diagram).
Explain the effects of the fin area and fuselage on lateral stability
Fin area: produces drag in a sideslip and as this force acts above the longitudinal axis CoG of the a/c, it produces a restoring rolling moment. A low wing and low CoG, large fin and T-tail will enhance this.
Fuselage: it does not contribute aerodynamic force towards a restoring rolling moment. Drag produced is along the longitudinal axis so no arm so no rolling moment.
What is directional stability? Describe the directional stability diagram.
What is the equation for yawing moment?
After a disturbance in yaw it is the a/c ability to produce a restoring yawing moment to return to equilibrium, also known as WX cocking.
Side slip angle is the angle between the RAF & HDG. If the RAF comes from the right of HDG then +ve or left -ve.
If the restoring yawing moment is to the right +ve or left -ve.
My= Cy x 1/2pv^2S x b
What factors affect the directional stability?
Fin, dorsal fin, sweptback fin, side “keel” surface, CoG position, sweptback wing, rudder fixed
How does side “keel” surface and CoG position affect directional stability?
Keel surface is the side (keel) area when viewed from the side of the fuselage. The side that produces drag whilst in a sideslip is the side facing the RAF. Any area fore of the CoG will therefore produce a unstable yawing moment, whilst area aft of CoG will produce a stable yawing moment. So ideally greater area aft of CoG.
CoG position: a fore CoG will produce a larger yaw moment due to larger longitudinal arm between CoP (tailplane) and CoG.
How do sweptback wings and rudder fixed affect directional stability?
Rudder fixed: just need to accept it just is more stable.
Sweptback: increases drag on the fore (outer) wing facing the RAF as the effective span/AR is increased so produces a restoring yawing moment.
Explain how fin, dorsal fins and swept back fins effect directional stability
Fin: in a yawing sideslip AoA forms between RAF & the fin so lift is produced, which is a side force. So this side force produces a restoring yaw moment about the CoG.
Dorsal fin: can have equal/greater area than normal fin but will have relatively low equivalent parasite area so will delay BLS and increase the stall AoA of the tailplane. (Can also be laterally stable due to increased SA above CoG).
Sweptback fin: will also increase the stall AoA of the fin and increase Mcrit of the fin, so the fin can work more effectively against yaw disturbances. Also will have more aft CoP so greater arm from CoG.
Why is it important to balance (approximately) lateral and directional stability? Which is however preferred? What can occur if they are not balanced?
They have coupled affects.
After roll disturbance, lateral stability wants to correct to wings level whilst directional stability wants to WX cock. OR after yaw disturbance directional stability wants to WX cock but lateral stability wants to correct the roll due to yaw.
If not correctly balanced Dutch roll and spiral instability can occur.
Lateral stability is preferred to be slightly stronger as it is easier to correct a yaw than roll. However a spiral instability condition is preferred as it is easier to correct, Dutch roll should be eliminated by design (yaw dampers).
Describe how spiral instability occurs.
What will enhance this?
Occurs with a strong directional stability but weak lateral stability.
After a disturbance in ROLL, sideslip will occur, so the a/c will want to WX cock and this causes a yaw, and as the outer wing will travel faster in the yaw this causes a further roll which continues this cycle. So a/c will continue to roll/yaw in the same direction. Lateral stability is not strong enough to correct the roll as the directional stability dominates.
An ever tightening and uncontrollable spiral is known as graveyard spiral.
Large side area, no dihedral and low wing… to name some
What is Dutch roll and how is it caused?
What can enhance this? What is an extreme version called?
Dutch roll is caused by too much lateral stability and weak directional stability. It is an unbalanced oscillation of roll and yaw.
After a disturbance in yaw the outer wing will travel faster so will enter a roll, this creates sideslip and changes the RAF and so the strong lateral stability will correct this roll before directional stability corrects the yaw… roll creates yaw (WX cock) due to the new sideslip and this unbalanced oscillation continues.
Dihedral, high wing and small fin.
Snaking is a Dutch roll with a more pronounced yawing motion.
Explain what phugoid mode is.
What are the characteristics?
How to escape it?
It is long period (20-60s) of poorly dampened longitudinal dynamic oscillation about the lateral axis.
AoA along the flight path will remain relatively constant but the pitching moment, ALT and airspeed will change. It is slow and benign so can go unnoticed by pilots.
It can be said Ek & Ep are exchanged, so other than to leave alone, the other way to reduce the intensity of oscillation is to reduce the throttle as you reduce the total energy of the aircraft.
What is short period mode?
How to recover?
Again is a longitudinal dynamic oscillation about the lateral axis, except the period is 0.3-1.5s. AoA will greatly vary but the airspeed and will remain fairly constant.
Oscillation is dampened by design, and usually caused by elevator flapping pitch moment in stick free… so to recover neutralise controls or release controls.
Does airspeed affect L, D, Cl, Cd?
Does AoA affect L, D, Cl, Cd?
1) L, D yes. CD yes as related to Re. Cl no as only AoA.
2) yes x4
Does CoP change with airspeed and AoA?
Does pitch moment about CoP change with airspeed?
Airspeed: magnitude changes (distribution the same)
AoA: location changes (distribution changes)
PM: no as always 0?
Does AC location change with airspeed and AoA?
Does pitching moment/C pitching moment about AC change with airspeed and AoA?
Location:
Airspeed- no
AoA- always constant
Pitch moment:
Airspeed- yes as… Cm x 1/2pv^2S x c
AoA- no
Coefficient pitching moment:
Both no
How does pitch moment about LE/TE change with pivot point?
About LE PM will decrease with AoA (is negative moment)
About TE PM will increase with AoA (is positive moment)
With a negative AoA, PM about LE will be positive
What are the equations for pitch/roll/yaw moments?
C x 1/2pv^2 Sc (roll/yaw)
C x 1/2pv^2 Sb (pitch)
Does the zero lift pitch moment change with a change of pivot point?
No, as ZLPM is a force couple which is not affected by the pivot point position
