LESSON 30 Longitudinal Stability 1 Aerospace Engineering Lesson

LESSON 30 Longitudinal Stability 공군사관학교 항공우주공학과 1 Aerospace Engineering

Lesson Outcomes § Review § Neutral Point § Static Margin § Contributions to Longitudinal Stability 2 항공공학개론

Review : 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 3 L’, M, N w 항공공학개론

Review : Definitions v Static Stability: § The initial tendency of a system after being disturbed from equilibrium (trim condition) v Dynamic Stability: § The time history of a system’s response to a disturbance from equilibrium (trim condition) v Trim – The condition where ∑Mcg = 0 – 항공기 각 축방향으로 회전운동이 없는 상태 4 항공공학개론

Review : Longitudinal Static Stability and Trim Criteria CM CM a a trim (Trim angle of attack) a CM o 5 항공공학개론

Neutral Point (n. p. ) The “aerodynamic center” for the entire aircraft CM = constant as changes CM = 0 : 모멘트 곡선 기울기가 0이 되는 지점( Neutral) 종축 정적안정성을 유지할 수 있는 항공기 무게중심의 후방한계 Tail a. c. Wing a. c. Aircraft Neutral Point (n. p. ) 11 항공공학개론

Static Margin (SM) • SM = distance from c. g. to neutral point SM>0의 경우(CG가 NP 전방위치) 안정성 확보 xn xcg SM = xn – xcg 12 항공공학개론

Static Margin (SM) Static Margin Stability Criteria v Non-dimensional difference between Neutral Point (n. p. ) and Center of Gravity (c. g. ) where: If S. M. > 0 (c. g. ahead of the neutral point) If S. M. = 0 (c. g. at the neutral point) If S. M. < 0 (c. g. behind the neutral point) 13 - aircraft is stable - aircraft is neutral - aircraft is unstable 항공공학개론

Criteria for Longitudinal Static Stability Positive CM α SM c. g. location Neutral CMα < 0 SM > 0 c. g. forward of n. p. CMα = 0 SM = 0 c. g. = n. p. Negative CMα > 0 SM < 0 c. g. aft of n. p. (stable) (unstable) 14 항공공학개론

Tail Effects on Longitudinal Stability v Tail aft of cg is stabilizing (cf : canards are destabilizing) Lw Ma. c. Lt cg xac xcg xt v 안정성을 증가시키는 방법 - xt ↑ : Longer moment arm (꼬리날개를 뒤로 이동) - St ↑ : Larger tail (큰 꼬리날개) - 꼬리날개의 CLat et 증가 - 꼬리날개를 주 날개 후류영역(또는 downwash)에서 이격 17 항공공학개론

Tail Effects on Longitudinal Stability v Trim Control 방법 - 꼬리날개 붙임각(Tail incidence angle, it ) 조정 - it is the angle between Chord Line of the tail and aircraft Zero-Lift-Line. sometimes fixed—sometimes moveable. it = 0 it > 0 it < 0 Tail leading edge down is Positive 18 항공공학개론

Changing Variables* C M cg CMo _____ change CM No _____ trim _____ Vtrim _____ C M cg CMo _____ CM _____ trim _____ Vtrim _____ a a Slow Down (c. g. constant) Move c. g. Aft 20 항공공학개론

Stability vs. Maneuverability (Control) v Stable Aircraft—not very easy to move § Not very maneuverable § C-5, C-17, B-52, Passenger airplanes v Maneuverable Aircraft—very easy to move § Not very stable (unstable in many cases) § Require Flight Control Systems − Stability Augmentation System (SAS) − Fly-by-wire FCS § F-16, F-22 21 항공공학개론

Stability Comparison Typical SM values: Cessna 172 - Transports & Commercial AC: Learjet 3 5 0. 05 to 0. 20 Boeing 747 – Fighters: 0 to 0. 05 – Fighters - FBW . 19. 13. 27 P-51 Mustang. 05 F-106. 09 More Stable More maneuverable F-16 A (early) -. 02 Maneuverable F-16 C. 01 with other X-29 -. 33 benefits 22 항공공학개론

Benefits of Canard Trim : Optional Conventional Fighter Subsonic Lw np X-29 np SM > 0 (small) Lt Subsonic Lw Supersonic Lc Lc np Supersonic Lt SM > 0 (large) SM < 0 Lw Lw np SM > 0 (small) Requires more wing lift 23 항공공학개론