AAE556Lecture 7Control effectiveness

AAE 556 Aeroelasticity Lecture 7-Control effectiveness Purdue Aeroelasticity Reading i Sections 2.15-2.18 – These sections are painfully worked example problems – read through them to understand principles discussed in class – Section 2.18.2 has a virtual work example – wait to read this until next week i Skip 2.19 for now (will next week) i Read 2.20, 2.20.1 and 2.20.2 Purdue Aeroelasticity Our next goal learn about control effectiveness i Demonstrate the aeroelastic effect of deflecting aileron surfaces to increase lift or rolling moment i Examine the ability of an aileron or elevator to produce a change in lift, pitching moment or rolling moment i Reading – Sections 2.20-2.20.2 Purdue Aeroelasticity Ailerons are required for lateral stability They become increasingly ineffective at high speeds i Many of the uncertified minimum ultralights, and perhaps some of the certificated aircraft, have low torsional wing rigidity This will not only make the ailerons increasingly ineffective with speed (and prone to flutter), but will also place very low limits on g loads – http://www.auf.asn.au/groundschool/f lutter.html#flutter Purdue Aeroelasticity The ability of an aileron or elevator to produce a change in lift, pitching moment or rolling moment is changed by aeroelastic interaction L = qSCLδ δ o + qSCLα θ no α o M AC = qScCMACδ Lift α0 + θ V MAC torsion spring KT shear center δ0 e 6-5 Purdue Aeroelasticity aileron deflection Herman Glauert’s estimators for CLd and CMACd The flap-to-chord ratio is C Lδ = C Lα π (cos CMACδ = − −1 E= (1 − E ) + CLα π cf c E (1 − E ) ( 1− E ) ( 1− E ) E Purdue Aeroelasticity ) DOF idealized model – no camber Sum moments about the shear center L Linear problem (what does that mean?) e ∑M sc = = Le + M AC − KTθ Remember αo = Purdue Aeroelasticity Solve for the twist angle due only to aileron deflection d c  qSe C Lδ + CMACδ e  θ= KT − qSeC Lα Lift   δ o L = qSC L δ o + qSC Lα θ δ Purdue Aeroelasticity The aeroelastic lift due to deflection  CMACδ q c   1 +    qD  e  C Lδ L = qSC Lδ δ o  q 1− qD Compare answer to the lift computed ignoring aeroelastic interaction Lrigid = qSC L δ o δ Purdue Aeroelasticity     The aileron deflection required to generate a fixed increases as q increases  1− q   qR  Lo = qSC L δ o   δ  − q  qD   The required control input is … Aileron deflection increases as q approaches reversal Lo δo = qSC L δ 1− q   qD     − q  qR   Is aileron reversal an instability? 7-10 Purdue Aeroelasticity The most common definition for the reversal condition L flex = Is it possible that I deflect and aileron and get no lift? We usually use an aileron to produce a rolling moment, not just lift What is the dynamic pressure to make the lift or rolling moment zero even if we move the aileron? 11 Purdue Aeroelasticity How I make the numerator term in the lift expression equal to zero?  q  c  CMACδ L=0, reversal 1 +  ÷  qD  e  C L δ  L = qSCLδ δ o q 1− qD L=infinity, divergence 12 Purdue Aeroelasticity  ÷ ÷  =0 Solve for the q at the reversal condition qR 1+ qD  c  CMACδ =0  ÷  e  CLδ numerator=0 q = q reversal = q R e C Lδ qR = −qD c CMACδ or KT qR = − ScC Lα  CL δ   CMAC δ  Why the minus sign? 13 Purdue Aeroelasticity     Understanding what the aileron does Two different ways to compute pressure distribution resultants due to aileron deflection aerodynamic center aileron lift = qSC L δ δo aileron lift = qSCL δδo d MAC = qScC MAC δ δo δ0 e (a) aerodynamic center representation δ0 e (b) aileron center of pressure 14 Purdue Aeroelasticity Force equivalence the same moment at the AC with different models + − Lδ d = qScCMACδ δ o Solve for the distance d to find the CP distance from = d the AC A lift force at d produces the same result at the AC as a lift force and moment at the AC CM δ d =− c CLδ e 15 Purdue Aeroelasticity Lδ δ0 Aileron center of pressure depends on the aileron chord mid-chord midchord position Dist ance f rom aileron ce nt e r of pre ssure t o airf oil ae rodynamic ce nt e r (Glaue rt pre dict ion) Distance distance ngths aftinofchord 1/4 le chord 0.20 AC e xample she ar ce nte r location All-movable surface 0.10 quarte r chord location 0.00 0.0 0.2 0.4 0.6 0.8 1.0 f lap to chord ratio Aileron flap to chord ratio, E 16 Purdue Aeroelasticity Summary i Control surfaces generate less lift because the control deflection creates a nose-down pitching moment as it generates lift i At a special dynamic pressure (a combination of airspeed and altitude) the deflection of an aileron creates more downward lift due to nose-down deflection than upward lift 17 Purdue Aeroelasticity