Stability Flashcards
there can be no dynamic stability without …
positive static stability
The aircraft (or any other object) will not enter an oscillation unless it has the positive static stability needed to start returning it towards its original attitude
When dynamic stability is positive, the aerodynamic effect that causes the oscillation to die out is known as …
aerodynamic damping
what is a phugoid and what happens to altitude, speed and alpha?
A slow oscillation up and down, like a ship in a swell is long period stability. Height and speed may vary considerably during a cycle (although alpha may hardly change at all) and each cycle could take minutes rather than seconds. Jets at high level have a particular tendency to behave in this way, described as entering a Phugoid
what happens to altitude, speed and alpha if an aircraft has short period stability?
may enter a rapid oscillation pitching up and down every few seconds with no time for significant changes in speed or height, despite rapid changes in alpha
Highly undesirable and so designers must make sure that aerodynamic damping is powerful enough to deal with this tendency, by quickly putting a stop to the oscillation
The coefficient of pitching moment, (what symbol?) is described as positive when … and negative when …
Cm
nose up
nose down
coefficient of rolling moment (symbol?), is positive when rolling …, negative when rolling …
Cl
right
left
coefficient of yawing moment (symbol?), is positive when nose is deflected …, negative when nose is deflected …
Cn
right
left
On the Cm-alpha graph, what gradient indicates stability and what does the steepness of the line indicate?
a negative gradient indicates stability and the steeper the line, the bigger the moment that is created with a change in alpha, so a steeper line indicates greater stability
what is stick position stability?
Upwards elevator deflection required at low speeds, so stick position aft
As we accelerate we need elevator deflection downwards, achieved by moving the stick forward – forward stick movement with increased speed (and vice versa) is normal, and described as stick position stability
what is longitudinal dihedral?
the difference of alpha between the wings and the THS
In cruise the tail should ideally be set at as near as possible to zero lift alpha in order to minimise drag and give a good tail moment response
In cruise the wing will be at near 4 degrees alpha for best lift/drag ratio, so the optimum setup will be to have the tail set at 4 degrees less incidence than the wing – this is longitudinal dihedral
Not always possible due to required forces required for stab to balance pitching moment produced by wing depending on CG position
what is the definition of mean aerodynamic chord?
the chord of an equivalent untwisted, rectangular wing with the same pitching moment and lift characteristics as the actual wing
why does longitudinal aerodynamic damping and stability decrease with altitude?
When an aircraft pitches up, the stab has a downward motion into the relative airflow, changing its alpha, which, in general will increase its effectiveness
This increases stability and give resistance to the manoeuvre taking place
Known as aerodynamic damping and will produce an increase in stick force required to make the manoeuvre
But the change in relative airflow over the stab in a manoeuvre will depend on TAS not IAS, and as TAS increases with altitude the result will be a smaller change in the tailplane angle of attack when performing the same manoeuvre
Therefore, when we climb to high altitude:
Aerodynamic damping decreases, stability decreases, and so stick force per g decreases
what effect does increased weight have on longitudinal stability and why?
Increased weight requires more lift which makes the wing lift/weight couple larger
If the moments are creating a stabilising effect, stability will increase
what is the effect of lowering flaps on stability and why?
lowering the flaps often moves the AC aft – a stabilising move – but the increased downwash behind the wing reduces stab effectiveness and this is a powerful influence in making the aircraft less stable
Overall, flap extension makes the aircraft less stable
A thin, swept fin with a low aspect ratio will create the lowest aerodynamic force, but will also stall at a … alpha – possibly allowing higher …
higher
crosswind landings
how does wing dihedral contribute to lateral stability in a sideslip?
Because the effective airflow is flowing over the wings at an angle, rather than straight from front to back the effective angle of attack is increased on the wing that is in to wind, increasing lift on that wing – a stable restoring force
Same effect if a wing drops
what effect does wing sweep back have on lateral and directional stability?
Will give a marked increase in lateral stability, and an increase in directional stability as well
If the aircraft is slipping right after rolling to the right (perhaps after a wing drop), the new relative airflow is flowing over the right wing as if it were a straight wing, while the left wing has apparently become even more swept
As lift decreases with increased sweep back, the lift from the right wing will be increased, while the lift on the left wing will decrease creating a restoring roll to the left to level the wings
The extra lift created on the right wing will also create increased induced drag on that wing, and yaw the aircraft to the right to the relative airflow – significant contribution to directional stability
why does single engine prop wash have a destabilising effect on lateral stability?
gives an airflow over the inboard section of the wing that wouold otherwise be blanketed by the fuselage in a sideslip, restoring some of the lift in that area and reducing the effect of the extra lift on the into-wing wing
