Henry walked into the parlor holding the scale model search-and-rescue helicopter he had carried in his canvas backpack all the way from Colorado Springs.

“The tail rotor on this aircraft is mounted on the left side of the vertical fin,” Henry said, stopping three feet from Ethan’s knees.

“Do you know why?”

Ethan set his cup down on the saucer with a small porcelain clatter.

“I assume it helps with hover stability in crosswinds,” Ethan said.

“It counteracts the main rotor torque,” Henry said, his feet planted shoulder-width apart on the rug.

“If the main blades spin clockwise, the fuselage wants to spin counterclockwise.