The 'request' filter is a small peice of code that you put in 'functions.php' that tells the RSS feed what posts to grab. The default is 'post', but if you are using a custom post type that that needs to be specified.
The following should do the trick - it will keep the old 'posts' as well as display the new 'projects'.
* Adds custom post types to the the query when a feed is requested
$post_types = array('projects', 'post');
if(isset($qv['feed']) && !isset($qv['post_type']))
$qv['post_type'] = $post_types;