Method for correcting control surface angle measurements in single viewpoint photogrammetry

A method of determining a corrected control surface angle for use in single viewpoint photogrammetry to correct control surface angle measurements affected by wing bending, comprising the steps of:

• providing first and second visual targets spaced apart from one another on a control surface of an aircraft wing wherein the control surface is angularly disposed with respect to the aircraft wing in a direction of expected wind flow, said first and second visual targets positioned at a semispan distance along the aircraft wing, wherein a three dimensional and rectangular X,Y,Z coordinate system is defined with said direction of expected wind flow lying in an X-dimension, said semispan distance lying in a Y-dimension, and a Z-dimension lying perpendicular to said X-dimension and said Y-dimension;
• determining a reference distance between said first and second visual targets by generating a single viewpoint photogrammetry solution using said semispan distance for a zero wind condition defined by no air movement over the aircraft wing;
• determining an apparent distance between said first and second visual targets by generating a single viewpoint photogrammetry solution using said semispan distance for a wind condition defined by air movement over the aircraft wing;
• recalculating said apparent distance between said first and second visual targets by generating a single viewpoint photogrammetry solution using an estimated semispan distance for said wind condition for one of said first and second visual targets, said estimated semispan distance defined as said semispan distance changed by an incremental amount;
• iteratively repeating said step of recalculating for different values of said estimated semispan distance until said apparent distance is approximately equal to said reference distance as defined by an acceptable tolerance therebetween wherein a final apparent distance is thereby defined, and wherein said estimated semispan distance associated with said final apparent distance defines a corrected semispan distance;
• generating a final single viewpoint photogrammetric solution using said corrected semispan distance for said wind condition, said final single viewpoint photogrammetric solution including a difference ΔZ between said first and second visual targets in said Z-dimension and a difference ΔX between said first and second visual targets in said X-dimension;
• and
• determining a tilt angle of rotation ω that said first and second visual targets rotate through as the aircraft wing bends during said wind condition, said tilt angle ω lying in a plane defined by said Y-dimension and said Z-dimension, wherein a corrected control surface angle is defined by tan−1 ((ΔZ/cos ω)/ΔX).