Stability 😱 Flashcards
What increases longitudinal stability
Larger tailplane area
Further forward cg
What does reduced stability do to manoeuvrability and controllability
Increases manoeuvrability but reduces controllability
Where does the aerodynamic centre need to be for it to be stable
Behind the cg
What does forward cg do to drag
Increase trim drag
What is absolute AOA
Angle between zero lift line and the raf
What is the most significant factor for pitching coefficient
Dynamic pressure
What type of gradient will a longitudinally stable aircraft have
Negative
What is the most important contributor to longitudinal stability
Tailplane
What is the area of the tailplane and the distance from aircraft cg collectively known as
Tail volume
What effect does the fuselage have on the stability of a aircraft
Destabilised
Do the engine nacelles have a stabilising or destabilising effect
As AOA increases air catches on the lips of the intake = upwards moment = less stable
How does cg position in relation to engines affect stability
Closer cg is to engines = less instability
Further away cg is to engines = more instability
What is a static margin
Safety margin between the neutral point and aft cg
What affect do trailing edge flaps THEMSELVES have on the stability
Stabilise
What effect do flaps have on the overall stability of the aircraft
Increase in downwash
= reduced effectiveness at tailplane
= less stable
How does flaps affect the neutral point
Flaps extended results in neutral point further forward resulting in reduced longitudinal static stability
What is stick position stability
The position you expect the stick to be in for a certain phase of flight
What would be a unstable stick position stability
Push forward when you slow down and want to increase AOA
Does trim affect stick position stability or over all stability
No
What is stick force stability
Size of force required is greater for further distance from trim position
What is manoeuvre stability
Having the right amount of control force to produce a given pitch rate
What are the risks of light manoeuvre stability
Overstress
What is stick force gradient
The amount of extra stick force that must be applied per unit increase in load factor
Do airliners have a high or low stick force gradient
High
How does cg affect stick force gradient
Bigger if cg is forward
How does aerodynamic damping affect stick force gradient
More aerodynamic damping at low level = more stickforce required
How does aerodynamic damping work
During manoeuvre = reduces effectiveness of tailplane
How does being stick free of stick fixed affect the manoeuvre point
Stick fixed = manoeuvre point further back
What is the manoeuvre point
The neutral point adjusted for aerodynamic damping (only during the actual manoeuvre)
Always aft of neutral point
Is the aircraft more or less stable during a manoeuvre
More stable due to damping
Can a aircraft be dynamically stable and statically unstable
No for dynamic stability it must be statically stable
How do you work out the degree of dynamic stability
Time taken for oscillations to reduce by half
What are short period oscillations
Little variation in height or airspeed
Large variations in AOA and load factor
What is pilot induced oscillations
Pilot trying to correct for short period oscillations but being behind the aircraft and making it worse
What is long period oscillations (phugoid)
Little change of AOA or load factor
Large variations in height and airspeed
How does altitude affect dynamic and static stability
Altitude decreases static and dynamic stability
For directional stability what does a positive gradient mean
Positive directional stability
What will a aircraft with positive directional stability do
Yaw into the direction of sideslip
What is the biggest contributing factor to directional stability
Fin
When will the directional stability yawing moment be stronger
Cg further forward
Fin is larger
Higher IAS
Size of side slip angle
How does the shape of the fin affect cl and AOA
Straight fin with high aspect ratio
Higher lower AOA and higher Cl
What does a dorsal fin do
Increases directional stability
Greater sweepback
Lower aspect ratio
What does a ventral fin do
Increase directional stability
What do strakes do
In slow flight rear of aircraft dips into air = creates ventral fin
How does sweep back affect directional stability
One wing is more straight on to airflow = more lift (but more drag) therefor yaws towards raf = more stable
What does a fuselage do to stability
Destabilises
What does weak lateral stability mean
Slow to recover from wing drop
What will happen if you have positive lateral stability during a sideslip
Roll in opposite direction of sideslip
What gradient does positive lateral stability have
Negative gradient
What affect does a dihedral wing have on lateral stability
Increases it
What effect does sweepback have on lateral stability
More stable
What effect does High wing have on lateral stability
More stable
Centre of pressure moves slightly down the downgoing wing creating a moment
What affect does High keel surface have on lateral stability
More stable
What is spiral instability
When directional stability is strong
Lateral stability is weak
What is a Dutch roll
Strong lateral stability
Weak directional stability
Wing drop = wings level (however more drag with extra lift = yaw in direction of wing drop)
What does a aircraft that suffers from Dutch roll require
Yaw damper which inputs rudder to prevent yaw
Can you fly with an inoperative yaw damper
No
How does altitude affect dynamic stability
Reduces with altitude
For a canard where must the AC and neutral point me
Forward or the cg
What must you remember when calculating stick force per G
To take of 1G for normal gravity
