Lateral Stability Questions and Answers

Answer: B

The aircraft will roll with the starboard wing up.

An aileron deflected down increases the camber, and so the lift, of its part of the wing; one deflected up reduces it. With the starboard aileron down and the port aileron up, the starboard (right) wing gains lift and the port (left) wing loses it. The starboard wing rises and the port wing drops: a roll to port.

The port wing goes down, not up, so 'roll with the port wing up' has the direction reversed. Ailerons deflected in opposite senses produce a rolling moment about the longitudinal axis; pitching nose up or down is the job of the elevator, so neither pitch option applies.

Answer: B

The deflections are left aileron down, right aileron up, rudder right.

To roll right, the right wing must lose lift and the left wing gain it, so the right aileron goes up and the left aileron goes down. The down-going left aileron increases the left wing's lift and also its induced drag, which yaws the nose to the left, away from the turn: this is adverse yaw.

Right rudder counters it and keeps the turn coordinated (no sideslip). The options with left rudder would add to the adverse yaw, and the options with left aileron up would roll the airplane to the left.

Answer: C

The destabilizing feature is a fuselage with a low wing.

Static roll (lateral) stability comes from the dihedral effect: in a sideslip the airplane should develop a rolling moment that lifts the wing towards the sideslip. With a low wing, the cross-flow around the fuselage in a sideslip pushes air down onto the upwind wing root and up behind the fuselage onto the other, reducing the upwind wing's angle of attack. This produces a negative dihedral effect.

A high wing does the opposite and adds to roll stability. Positive dihedral and wing sweepback are the standard ways of increasing the dihedral effect, so they are stabilizing, not destabilizing.