Improving battery performance is crucial for increasing drone flight time. However, developing individual parts can also enhance mission performance and extend operating time. By attaching a drone to a wall instead of hovering in the air, the operating time and range of task performance can be extended. This study focuses on the adhesion force of a modular permanent magnet wheel leg for wall climbing drones. The wheel leg comprised several spokes without a rim. It could climb obstacles higher than wheel radius and provide a large adhesion area. An equation for the adhesion force of the wheel leg was derived, considering mechanical factors such as drone size, inclination of the ferromagnetic wall, and drone posture. A simple experimental model was created to verify the validity of the adhesive force equation. The effectiveness of the derived equation was confirmed by experimentally measuring the angle of the ferromagnetic wall that losT adhesion according to mechanical factors and comparing it with the derived adhesion force.