I understand that the boot repulsors provide the lift/thrust and the palm repulsors provide stabilization but how is it capable of turning without a tail ruder of some sort? I know that there are flaps at various points for air braking but wouldn't he have to use them for doing a maneuver like the corkscrew/barrel roll?
If you watch his flight during the Avengers movie, his acrobatic flight technique is as follows:
As he is approaching the tesseract opening trying to stop the Chitauri he is flying at his maximum speed using his left hand as a stabilizer, his boot jets to direct his vector and his two thrust units on his back, to support his rolls.
He has his left hand turned palm down and uses his right hand repulsor as a weapon. It also creates drag when he uses it and he will take that drag and hard rotate into that drag.
He uses his two hands directed out in front of him to push himself out of the way of the falling debris.
That rotation makes his movement unpredictable to anyone targeting him but more importantly allows him to recharge his right repulsor and use his left as a weapon.
- On his back and shoulders he has some small control surfaces in addition to his jetpack units. It is the coordinated effort of his backpack jets, his planar surfaces, his repulsors, and his boot jets that allow him the advanced maneuvers he uses.
- The two backpack jets appear to have the ability to be directed independently.
Most of the flight scenes with Iron Man do not explain how he maintains a flight vector without an actual flight surface. The only way we can explain it is if we consider he is using himself as a flight surface.
Sure, his arms are outstretched, creating a wing-like surface but it is entirely too small even with the thrust provided by his output surfaces. There is only one other possibility. Antigravity.
I theorize he is utilizing some of the antigravity technology created by his father in addition to his repulsor technologies. See: How much did Iron Man's Suitcase weigh?
Using antigravity would allow him to only have to worry about thrust and fly without the need of a wing-like surface or pesky physical rules like Bernoulli's Principle.
If you watch the scene where the suit flies down to save Tony after being thrown from Stark Tower, you will see the suit is covered in small flight surfaces, at the shoulder and hips. Are they sufficient to support a human in flight? Probably not, but they ARE highly visible in this clip.
That's the least of his concerns. His entire surface is capable of helping him maneuver... at those speeds pulling one arm inward or outward just an inch will almost certainly cause his course to deviate, and quickly. My very limited understanding of unusual flight form factors / cross sections is that they're incredible maneuverable but also incredibly difficult to maintain stable flight. So it's not a matter of "how does he turn" so much as "how does he fly straight".