What type of scientist test airplanes?

Answer 1

Either, aerodynamicists, aeronautical engineers, or test pilots. Each has their use at different stages of designing and testing aircraft.

The aerodynamicist, is a fluid dynamics expert who will perform theoretical modelling of the system, testing it with computer simulations and wind tunnel tests.

The aeronautical engineer will design aircraft, based on what their aerodynamics team informs them is the best way to go (they have to calculate stresses and strain on the planned aircraft to ensure that the materials selected are appropriate for the design. They too may do wind tunnel testing of the scale model plane, as well as computer simulations.

The test pilot is the one who "tests" the plane once it is built (prototype) and proves whether or not the design performs as predicted.

A scientist is just someone who applies the scientific method to things. Not necessarily someone with a science degree.

Engineers are really applied scientists. They don't care so much about theoretical things, but implementing the theory to make / design something new.