• Wing suits – Modern wing suits allow skilled pilots to ‘fly’ through the air by altering body position and using fabric wings similar to squirrel suits. However, they still require falling with gravity and cannot achieve true aerodynamic flight.
• Feathered wings – Some experimental designs have tried attaching mechanical or biological feathered wings to the human body. While prototypes could achieve short glides, true flapping wing flight remains implausible without much heavier and powerful machines.
• Jetpacks – Modern jetpack designs let users hover and maneuver in three dimensions to an extent. But fuel limits durations to just minutes and they cannot match Superman’s agility, payload, or aerodynamic flight control.
• Augmented physiology – Some scientists theorize that genetic engineering or bioelectromagnetic augmentation of the muscles, bones and respiratory/circulatory systems could theoretically enable aerodynamic flight through strength and low body weight alone. But this remains in the realm of science fiction.
• Exoskeletons – Robotic wing-like appendages directly driven and controlled by the human body may one day allow powered flight assistance similar to Superman. But the required miniaturization and energy density advances are still far off.

So while limited controlled flight is possible now, seamlessly mimicking Superman’s vertical take-offs, changes of direction and long-duration flights may require further scientific and technological progress we have not achieved yet. It remains unclear if true unassisted human flight will ever be feasible.

• Air Resistance – At high speeds, air resistance becomes a significant force pushing against Superman. It takes immense strength to overcome this and maintain control during maneuvers.
• Payload Limitations – Superman has a max weight capacity he can carry or tow while flying based on his own body mass and aerodynamic ability. Extra weight impacts speed, agility, altitude etc.
• G-forces – Performing sharp turns and accelerations at high velocities subjects Superman to strong g-forces. His biology would need to withstand effects like blackouts.
• Oxygen Needs – Prolonged flights at high altitudes could require Superman to consciously regulate his breathing to account for lower oxygen levels.
• Thermal Regulation – Generating lift through flapping or changes in body position produces a lot of kinetic heat. Superman needs to release this heat efficiently to avoid overheating.
• Energy Expenditure – Complex flight techniques are energy intensive. His cells would need to continuously replenish ATP through solar absorption to power flight muscles.
• Impacts and Injuries – Collisions at flight speeds or changes in air pressure could potentially damage tissue, especially if solar charge is depleted.
• Disorientation – Flying at supersonic speeds may induce vertigo if he lacks specialized sensory organs for high-speed navigation.
• Weather Effects – Turbulence, high winds, storms pose challenges to control and stability for even Superman’s abilities.

So in summary – air resistance, g-forces, thermal regulation and available energy would be key physiological limits to consider.