The Suffolk side had gone in at half-time a goal ahead after Grant Ward’s scuffed shot found the corner before Smithies made a mess of heading clear a long through ball, allowing Luke Varney to tap into an unguarded net.
The seal on a fantastic second half was sealed by Tom Lawrence who found the bottom of corner in a match that could have easily seen more goals for the hosts.
A poor start to the match was given a lift in the 13th minute when the first shot on target led to the opening goal.
Lawrence’s free-kick was punched away by the under-pressure Smithies and while Ward’s initial shot was blocked, his second effort somehow crept beyond the unsighted keeper and into the far corner.
Ipswich almost doubled their lead in the 32nd minute when Lawrence’s brilliant free-kick was met by Christophe Berra who somehow headed wide from the edge of the six-yard-box.
QPR’s best chance fell to Conor Washington who broke away from Luke Chambers but the striker’s shot was well blocked by the body of the advancing Bartosz Bialkowski.
The next goal was always going to be key and it came via a howler from Smithies nine minutes after the restart.
Jonathan Douglas’ long ball was a little too weighty for Varney but Smithies’ attempts to head clear out of his area only saw the ball slice behind him allowing the striker to stroke the ball into the empty net.
Minutes later, the impressive Cole Skuse fired a volley straight at Smithies after good work from David McGoldrick and Lawrence.
But it was soon three after another long-range shot from Skuse was heading wide before Lawrence controlled the ball and had time in the QPR area to find the bottom corner.
The visitors were offering little although Bialkowski had to be alert to hold onto a powerful volley from Chery in the 65th minute.
At the other end, yet another wicked delivery from Lawrence narrowly evaded both Berra and the far post before the Leicester City loanee burst through and teed up McGoldrick who missed the far post with a curling effort.
The closest QPR came was in the 77th minute when sub Sandro powerfully met Cherry’s corner but thundered his header against the post.
McGoldrick brought out the best in Smithies with a fine angled drive that was brilliantly pushed aside before a fantastic counter led by Freddie Sears saw Ward’s poked shot turned around the post by the busy QPR keeper.
Injury time saw McGoldrick just fail to chip Smithies when played in by Leon Best before Sears also sent the keeper sprawling with a low shot.