AAE 556 Aeroelasticity Lecture 12 Flexural axis Control

AAE 556 Aeroelasticity Lecture 12 Flexural axis Control effectiveness Purdue Aeroelasticity 1

Flexural axis (FA) concept an attempt to explain aeroelastic effects FA Definition - a line (locus of points) along which the streamwise (or chordwise) angle of attack does not change when a discrete load is applied there (and the air is off). L plus yo upward load Purdue Aeroelasticity xo minus 2

What is the difference between flexural axis and elastic axis? The air is off flexur al axi s b x upward load Purdue Aeroelasticity y 3

Locate the flexural axis by choosing an arbirary y position and poking around the wing in the x-direction yo xo Purdue Aeroelasticity 4

Step #1 Compute angular displacements Purdue Aeroelasticity 5

Use flexural axis definition to solve for the x, y coordinates The flexural axis location is always forward of the elastic axis if the wing is swept back. Increased sweepback rotates the flexural axis forward and increases the distance between aero load and the flexural axis. The red dot is the position of the resultant lift force. The dashed green line is the flexural axis. The dashed green line is the reference for wing sweep. Purdue Aeroelasticity 6

Flexural axis angle in degrees Example calculation dependence of flexural axis angle on stiffness ratio – notice that when the bending stiffness is infinite then the flex axis and the elastic axis are identical Wing sweep angle in degrees Purdue Aeroelasticity 7

How much wing sweep does it take to exactly place the flexural axis on the aero load position at y = b/2 and x = -e? Purdue Aeroelasticity 8

Switch gears - Control effectiveness rigid wing/springs model aile ron Purdue Aeroelasticity 9

Invert the aeroelastic stiffness matrix Then solve for the two deflections Purdue Aeroelasticity 10

Swept wing lift due to full-span aileron deflection Purdue Aeroelasticity 11

Lift expression algebra Purdue Aeroelasticity 12

Aileron effectiveness measured as aileron lift effectiveness Purdue Aeroelasticity 13

The reversal dynamic pressure solution Purdue Aeroelasticity 14

Example - effects of sweep on aileron effectiveness unswept wing divergence at 250 psf. (qo) Reversal at 150 psf. Purdue Aeroelasticity 15

Summary-swept wing control effectiveness i wing sweep changes the divergence dynamic pressure and lift effectiveness i sweepback stabilizing - slight sweep only i Sweep forward is aeroelastically destabilizing i sweepback reduces lift effectiveness and control effectiveness i Sweep forward increases lift effectiveness and control effectiveness Purdue Aeroelasticity 16

Homework for next week f 2 K 1 f 1 W Purdue Aeroelasticity 13 -17