edit search.php, find:
PHP Code:
eval("standardredirect(\"".gettemplate("redirect_search")."\",\"search.php?s=$session[sessionhash]&action=showresults&getnew=true&searchid=$searchid\");");
Replace it as:
PHP Code:
eval("standardredirect(\"".gettemplate("redirect_search")."\",\"search.php?s=$session[sessionhash]&action=showresults&getnew=true&searchid=$searchid&sortby=forum\");");
There are 2 instances, change both..
Enjoy.