Geez, you pilots should know this stuff in your bones - F = ma, a is acceleration. No acceleration, no net force. That doesn't mean no force - it means they all cancel each other - they are vectors.
A airplane flying at constant speed relative to ground with a positive pitch will be like any other inclined plane (haha). A cart left alone will roll to the back. In that case there is a net force - from gravity - causing it to accelerate.