How to get drag to affect an aircraft velocity but not the gravity of the aircraft?

Hello, I’m writing an arcade flying game with basic physics and I got stuck on a problem which seems so simple but I just can’t figure out the solution.

The rigidbody.drag of the aircraft works fine except it stops the gravity from accelerating! If I set the Drag to 2, Angular Drag to 3, Gravity to -9.8, the aircraft just falls at a constant -4.7. If I set the drag to something low like 0.05, then the gravity works fine but the aircraft goes in one direction and can’t turn properly.

I have tried using AddForce instead of the rigidbody properties but it didn’t make a difference. The drag still doesn’t allow the aircraft to fall and accelerate downwards (considering I left out the lift force for now). I have also tried using the solution from http://forum.unity3d.com/threads/physics-drag-formula.252406/ , but it didn’t work.

Does anyone know how to get drag to affect an aircraft velocity but not the gravity of the aircraft?