Question

An aircraft is negatively stable when ______

(a) its tailplane produces negative lift (downwards)

(b) its tailplane produces positive lift (upwards)

(c) its tailplane does not produce lift

(d) it is flying inverted

I have been asked this question during an internship interview.

This intriguing question originated from Fly-By-Wire Technology topic in section Fly by Wire Flight Control of Avionics

Answer 1

Right choice is (b) its tailplane produces positive lift (upwards)

To elaborate: An aircraft is said to be negatively stable when its tailplane produces positive

lift. The tail of an aircraft without an FBW system must always produce negative lift to maintain stability and to keep the nose up. Since it produces negative lift, it actually acts in the direction of weight reducing aerodynamic efficiency. In an FBW equipped aircraft a positive lift producing tail can be used by using flight control computers and motion sensors to main stability at all times.