I strongly believe that Turkey shot down the Russian plane as a warning as Russia is really crippling IS (while Turkey wants to crippled Assad) and hitting the Turkey /IS lifeline for oil and troop reinforcement.
But it is impossible that Putin did not know it could happen and he could have send some air superiority fighter to protect the SU 24 bomber.
A few ideas.
Putin could not risk that Russian warplane shot down Turkish plane first.
Putin could want this to happen so to expose Turkey / IS connection
Putin is realigning with France so this create a wedge between France Nato and Turkey US
It could be a distraction for Ukraine
investorpolitico 