If you are seeking a purely physical reason for Thor's ability to fly, there isn't one. Thor's flight is inconsistently presented and has been so for his entire career. While the mechanism appears to be the whirling throw and release, this mechanism cannot explain the inconsistency in his ability to fly.
In legends, Thor did not fly, he was drawn in a chariot by two goats (Tanngrisnir and Tanngnjóstr) who had the power of flight and the secret ability to be eaten every night and would return to life by dawn if their bones were wrapped in their skin and were unbroken.
When Thor was re-created by Lee and Kirby, they were creating a Superman analog and the power of flight was considered necessary for the character. Their desire was to have his flight have a physical element and once rendered in the Kirby style, the whirling hammer flight was visually stunning.
Flying Thor, Jack Kirby Style
There is no physical explanation capable of explaining it. Thor's flight is purely a magical effect. Depending on the writer, Thor's ability to fly was either:
- Directly related to his throwing-release mechanism of flight or
- A product of being held aloft by the strong winds in the storms he creates.
Neither of these has ever portrayed his method of flight accurately since:
Thor changes direction whenever it suits him to do so. There is no external mechanism beyond him wanting to. The power to change his direction is reputed to be within the hammer, Mjolnir.
He does not stop flying when he chooses to use Mjolnir in battle and he is flying. His flight continues uninterrupted. They have gotten better about this in recent years and instead have him attack at range using lightning or energy blasts from his hammer.
He hovers. He is not whirling the hammer like a helicopter, perhaps it can be said strong updrafts are holding him up, but he has done this when the sky is perfectly clear, so that explanation only works half the time.
Perhaps the best answer to this question can be found in this panel:
- Many are the wonders of Mjolnir, the enchanted hammer of Thor and we'd like to take this opportunity to share a few of them with you.
- Composed of Uru, a magical metallic substance which can only be found in Asgard, the hammer is two feet long and its handle is wrapped in leather.
- Thor can propel himself through the air by hurling the hammer and holding on to its thong.
- By means which are beyond mortal comprehension, Thor can always control the course of his flight.
- We don't know the top speed or distance Thor can attain with a single throw...But it's probably much faster and farther than we'd ever want to travel.