How must the lower leg be rotated?

We need to:

• Rotate the left lowerLeg around the red axis,
• Causing the blue axis to tilt downward (cause the imaginary airplane to go into a dive)

Therefore we need to:

• Call the turn method on the lowerLeg object,
• Turning FORWARD one-fourth of a revolution.

Figure 13. Red and blue axes of lower left leg showing.