Can Hellfire missiles be used effectively as air-to-air weapons?
While I can't say that I know the too much about Hellfires, the aiming capability appears extremely limited for any air-to-air combat.


The range is bad: ~8km (compared to ~35km for a 'proper' Sidewinder). Covering this does not take many seconds for jets travelling in opposite directions. I'd imagine you might be able to get a helicopter if you're lucky.
I'm incredibly doubtful that a sensible lock on any aircraft could be achieved from a Predator. Bear in mind that you only have a camera on the bottom, so you've got huge blind spots.
You got a fair bit of satellite/communication lag, in the range of 0.5 seconds either way to get any response. More likely that your opponent has already got you by the time you understand what's going on.
The majority of the Hellfire missiles appear to require a constant lock to seek its target; some can continue on autopilot with limitations if it were to lose it. Again, sustaining a lock with that setup does not have a good outlook.
Remember that the Su-27 has countermeasures (such as flares) to protect itself.