Quote Originally Posted by mmarien View Post
Your cockpit may have a slightly higher pressure than static if it's open. It will capture air. My friend had a cub copy and the altimeter was sourced from the cockpit. It lost some altitude when he opened the window.
ITOH, It also may have a slightly lower pressure than static, depending on the fuselage shape and how much air is escaping the cockpit. Extreme example; An ultralight type aircraft with a nose cone completely open to the rear will usually produce a lower pressure in the cockpit.