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