If you want front page radio button in backend of submiting articles be always on front page, then in administrator/components/com_content/controller.php instead of:
$lists['frontpage'] = JHTML::_('select.booleanlist', 'frontpage', '', $row->frontpage);
Write:
if (!$edit) $row->frontpage = 1;
$lists['frontpage'] = JHTML::_('select.booleanlist', 'frontpage', '', $row->frontpage);
Taken from here.
Also, if you want front page radio button in frontend of submiting article be always on front page, then in \components\com_content\views\article\view.html.php instead of:
$article->frontpage = 1;
Write:
$article->frontpage = 0;