When flying a C 172 or C 150/152, try deliberately opening the door in flight. Get it all trimmed up for level flight, and open a door which has an opening window (it'll give you an excellent grabbing point to close it shortly).
As you gently force the door from its 3" or so trialing position, to 4"-6" open (you really won't get it too much farther without excessive force on the door), you'll find that you have some directional control of the aircraft. Interestingly, you will find that opening the left side door will cause the aircraft to very gently roll to the right. This may seem counter intuitive - anyone want to venture a guess as to why? I have my unverified opinion as to why...
I do not suggest this for any other aircraft type, as I have no experience with that.
In addition to the front doors of 100 series Cessnas, I have reclosed doors in flight in an Aztec (difficult), and the back door of a Caravan. The clamshell doors of a C 206 opening in flight is very bad, and will probably cause a $15,000 repair bill for the side of the fuselage behind the door.
The first time I have a door open, it startled me. After that - no big deal. I have approved my 150 to fly with either door removed. Do not fly a 150 with both doors removed
