Should I upgrade my planes

I have the by blood alone plane designer. I usually make a design for the 1936 planes, one for fighter, and another for CAS. I usually use these designs the whole game so I can maintain high production and because they are normally less resource intensive. I've found though that I think I'm taking disproportionate air loses, although I don't know how I would find this out. Should I upgrade my planes later on in the war? Also should I upgrade tanks too or just keep the basic chassis?