Stability Flashcards
The purpose of a trim tab
The purpose of a trim device or tab is to
- minimize the pilot’s effort when moving a control surface or
- maintaining that surface in a desired position
What are the 3 Axes of Motion and where do they intersect?
Longitudinal
Vertical
Lateral
These axis of rotation intersect at the center of gravity
The center of gravity (CG) is?
The center of gravity (CG) is the theoretical point where the entire weight of the airplane is considered to be concentrated
The center of pressure is also known as?
the center of lift
What is the center of Pressure?
The center of pressure
- (or center of lift)
- is a point along the chord line of an airfoil where lift is considered to be concentrated
What is stability?
Stability is the tendency of the aircraft to return to its original condition (position or attitude) after being displaced
What are the 2 types of stability and what do they mean?
What is positive static stability?
Positive Static: the initial tendency of the aircraft to return to its original position after it has been displaced
What is positive dynamic stability?
Positive Dynamic: the tendency of the aircraft to return to its original position over a period of time through a series of successive, smaller oscillations.
What is positive dynamic stability?
the aircraft returns to its original position through oscillations. Each oscillation is successively smaller until the original position is reached
What is negative Dynamic stability?
the aircraft oscillations increase in magnitude after an initial displacement
What is neutral dynamic stability?
the aircraft continues to oscillate at the same amplitude as the initial displacement
Logitudinal Stability involves what motion along what axis?
Involves pitching motion about the lateral axis
What does longitudinally stable mean?
Longitudinally stable means aircraft will naturally return to same attitude after being disturbed
The Center of Pressure moves _____ with increases in AoA until _____
The CP moves forward with increases in AoA until stall
The further aft of the CP that the CG is located, the LESS _______________
The further aft of the CP that the CG is located, the LESS longitudinally stable
What is a moment?
A moment is a
- force acting at a distance from an axis or fulcrum
- producing a rotation at that point
What is a force?
A force is a mass times an acceleration (F = M x a).
A moment equals force times ___________________
Moment equals force times the distance from the point of rotation (M = F x d)
The _____ is relied upon for longitudinal stability
The horizontal stabilizer is relied upon for longitudinal stability
With a normal Center of Gravity , an airplane will be nose heavy so we need a countering _________
With a normal CG, an airplane will be nose-heavy, so we need a countering tail down force to balance the nose
The horizontal stabilizer is mounted at a negative angle of incidence which provides __________
The horizontal stabilizer is mounted at a negative angle of incidence which provides a tail down (negative lift), nose up force
Longitudinal stability
The _____ the horizontal stabilizer and the _____ the distance the tail is located from the CG, the _______ the horizontal stabilizer’s contribution to longitudinal stability
The larger the horizontal stabilizer and the greater the distance the tail is located from the CG, the greater the horizontal stabilizer’s contribution to longitudinal stability
CG Too Far Forward
What happens?
- Higher stalling speed (bad).
- More stable (good, to a point).
- Slower cruise speed (bad).
- May not have enough elevator authority to flare in the landing (very bad).
CG too far aft
What happens?
- Lower stall speed (good).
- Shorter moment arm from the CG to the tail, and from the CP to the CG,
- reducing longitudinal stability and controllability (very bad) = easier to stall and spin, and more difficult or even impossible to recover (terrible!).
- Faster cruise speed (good)
What is directional Stability?
Stability about vertical axis (yaw), and is provided by the vertical stabilizer
What is keep effect?
- The nautical term “keel surface” is used to refer to the side surfaces of an airplane (fuselage, tail, etc.).
- When an airplane is yawed, keel surfaces are exposed to the relative wind.
- The airflow pushes on the exposed keel surfaces helping to return the airplane to its original directional attitude.
Keel Effect
The _____ the surface area of the keel, or side, surfaces, the ___________________________
The larger the surface area of the keel, or side, surfaces, the greater its contribution to directional stability
Stability about the longitudinal axis (roll) is termed ________
Stability about the longitudinal axis (roll) is termed lateral stability
Lateral stability is built into an aircraft through what 4 things?
Lateral stability is built into an aircraft through:
- Dihedral
- Swept wings
- High keel surfaces
- Low CG
Dihedral
Wings are inclined ______from the fuselage to the wingtips.
When an aircraft is forced to roll one way, the aircraft sideslips _______toward the lower wing.
Sideslip means the aircraft goes sideways…airflow is _____________
Wings are inclined upward from the fuselage to the wingtips.
When an aircraft is forced to roll one way, the aircraft sideslips downward toward the lower wing.
Sideslip means the aircraft goes sideways…airflow is upward & sideways
Dihedral
The low wing has an ________ and, therefore, produces _________.
This allows the lower wing to ___________
The low wing has an increased AOA, and, therefore, produces more lift.
This allows the lower wing to rise up and level the aircraft
What is adverse aileron yaw?
Ask Bill. Has something to do with Lateral & Directional Stability
. Coordinated turning cannot be done without both _____ and ____________
Roll and yaw together
What is a dutch roll?
- A lateral motion that is a combination of rolling/yawing, side-to-side oscillations.
- Dutch roll can be described as a series of uncoordinated turns alternately in opposite directions.
How can a dutch roll be induced?
Dutch roll can be induced by a
- sharp wind gust against one wing or the vertical stabilizer,
- or as the result of a very large control input that is then suddenly released
