LESSON 29 Longitudinal Stability 1 Aerospace Engineering Lesson

LESSON 29 안정성/조종성 개요 Longitudinal Stability 공군사관학교 항공우주공학과 1 Aerospace Engineering

Lesson Outcomes § Course 개요 Review § Aircraft Control § Stability 개요 - Static Stability - Dynamic Stability § Longitudinal Stability 2 항공공학개론

과목 개요 Review 1. Airflow (Fluids) 2. Airfoils (wing cross-sections) 4. Performance 3. Wings/Aircraft 5. Stability/Contr ol 3 항공공학개론

Aircraft Axis System Right Hand Coordinate System: L’ x : 기수(nose) 방향 x u y : 오른쪽 날개 방향 z : 배면방향 Velocity components: u, v, w M y Moment components: N z v 4 L’, M, N w 항공공학개론

Axis System Summary Axis x y z (+ out nose) (+ out belly) Velocity U (+ out right wing) V Rotation Roll Pitch Yaw Moment L’ M N (+ IAW rt hand rule) (+ rt wing down ) (+ pitch up) (+ nose right) Control Surface Aileron x Elevator W x Rudder y y y z z 5 z 항공공학개론

Aircraft Axis Sytem v An aircraft has 6 degrees of freedom § 3 in translation (movement along each axis) § 3 in rotation (movement about each axis) v Performance is the study of translations § Aircraft is treated as a point mass v Stability and Control is the study of rotations § Aircraft is modeled as a rigid body v Rotations are produced by moments generated from the deflection of control surfaces 6 항공공학개론

Traditional Aircraft Controls All moving Elevators (Also called stabilator or tailplane) Ailerons (Flaperons) Rudder 7 항공공학개론

Aircraft Motions - Roll v Roll: Motion about the longitudinal (X) axis produced by the ailerons 8 항공공학개론

Aircraft Motions - Pitch v Pitch: Motion about the lateral (Y) axis produced by the elevators 9 항공공학개론

Aircraft Motions - Yaw v Yaw: Motion about vertical (Z) axis produced by the rudder(s) Rudder 10 항공공학개론

Definitions Stability – The tendency of an aircraft to return to its equilibrium condition (“trim condition”) Control – Ability to maneuver the aircraft (using control surfaces) Trim – The condition where S Mcg = 0 – 항공기 각 축방향으로 회전운동이 없는 상태 11 항공공학개론

Definitions v Static Stability: § The initial tendency of a system after being disturbed from equilibrium (trim condition) § System has positive static stability if the initial response is to return towards equilibrium § System has neutral static stability if the initial response is to remain in the disturbed position § System has negative static stability if the initial motion is away from equilibrium 12 항공공학개론

Static Stability Examples 13 항공공학개론

Definitions v Dynamic Stability: § The time history of a system’s response to a disturbance from equilibrium (trim condition) § System has positive dynamic stability if, over time, it returns to equilibrium § System has neutral dynamic stability if, over time, there is no tendency to move towards or away from equilibrium (it may oscillate through equilibrium) § System has negative dynamic stability if, over time, it moves away from equilibrium 14 항공공학개론

Positive Dynamic Stability Example 15 항공공학개론

Neutral Dynamic Stability Examples 16 항공공학개론

Negative Dynamic Stability Examples 17 항공공학개론

Moment Balance : Optional Aircraft is in equilibrium when the sum of the moments is zero (definition of trimmed flight) Sum moments about the center of gravity Assume the following : SLUF : SF = 0 Trimmed : SMcg = 0 W, T and D act through the CG (M = 0) Tail is symmetrical (has no moment) Drag of tail = 0 18 항공공학개론

Moment Balance : Optional 19 항공공학개론

Longitudinal Static Stability v 먼저 항공기 종축(세로방향, Longitudinal axis)에 대해 정적으 로 안정하기 위해서는 어떤 조건이 만족해야 하는지 알아보자 v Definitions : 일반적인 CM vs. α curve CM CM a a trim (Trim angle of attack) a CM o • CM at zero lift (zero α) • CM vs α curve의 기울기 20 항공공학개론

Longitudinal Static Stability Unstable (CMα > 0) CM +CM atrim -α α decreases α increases +α - CM Neutral (CMα = 0) a Stable (CMα < 0) Steeper Slope = More stable (stronger restoring moment) 24 항공공학개론

Longitudinal Static Stability v 항공기가 trim condition을 확보하기 위해서는 어떤 조건이 만 족되어야 하는가? 양의 받음각에서 trim이 이루어져야 필요한 양력을 발생하여 비행이 가능함 v 따라서, Balance(Trim) criteria: In summary, the stability curve must have a negative slope and a positive intercept if the aircraft is to have longitudinal balance and static stability 25 항공공학개론