AAE 556 Aeroelasticity Lecture 7-Control effectiveness Purdue Aeroelasticity

Purdue Aeroelasticity Reading 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 Skip 2.19 for now (will do next week) Read 2.20, 2.20.1 and 2.20.2 Purdue Aeroelasticity

Our next goal learn about control effectiveness Demonstrate the aeroelastic effect of deflecting aileron surfaces to increase lift or rolling moment Examine the ability of an aileron or elevator to produce a change in lift, pitching moment or rolling moment Reading – Sections 2.20-2.20.2 Purdue Aeroelasticity

Purdue Aeroelasticity Ailerons are required for lateral stability They become increasingly ineffective at high speeds 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/flutter.html#flutter Purdue Aeroelasticity

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 aileron deflection Purdue Aeroelasticity

Herman Glauert’s estimators for CLd and CMACd The flap-to-chord ratio is Purdue Aeroelasticity

1 DOF idealized model – no camber Sum moments about the shear center Linear problem (what does that mean?) e Remember Purdue Aeroelasticity

Solve for the twist angle due only to aileron deflection d Lift Purdue Aeroelasticity

The aeroelastic lift due to deflection Compare answer to the lift computed ignoring aeroelastic interaction Purdue Aeroelasticity

Purdue Aeroelasticity The aileron deflection required to generate a fixed increases as q increases Aileron deflection increases as q approaches reversal The required control input is … Is aileron reversal an instability? Purdue Aeroelasticity

The most common definition for the reversal condition 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? Purdue Aeroelasticity

How do I make the numerator term in the lift expression equal to zero? L=0, reversal L=infinity, divergence Purdue Aeroelasticity

Solve for the q at the reversal condition numerator=0 or Why the minus sign? Purdue Aeroelasticity

Purdue Aeroelasticity Understanding what the aileron does Two different ways to compute pressure distribution resultants due to aileron deflection Purdue Aeroelasticity

Force equivalence the same moment at the AC with 2 different models + Solve for the distance d to find the CP distance from the AC = Ld d e A lift force at d produces the same result at the AC as a lift force and moment at the AC Purdue Aeroelasticity

Aileron center of pressure depends on the aileron chord mid-chord Distance aft of 1/4 chord All-movable surface AC Aileron flap to chord ratio, E Purdue Aeroelasticity

Purdue Aeroelasticity Summary Control surfaces generate less lift because the control deflection creates a nose-down pitching moment as it generates lift. 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 Purdue Aeroelasticity