Introduction
In rotorcraft and multirotors, Figure of Merit (FoM) is a critical parameter describing hover efficiency. Originally used in helicopter aerodynamics, FoM compares the actual power consumed by a rotor to the theoretical minimum required to generate the same lift.
Definition
FoM is often expressed as:
Or
Where:
- T = thrust
- ρ = air density
- A= rotor disk area
- P = Measured power
- n = rotation per second
- Q= measured torque
Values closer to 1.0 (or 100%) indicate high hover efficiency. Practical UAV propellers typically achieve 0.7–0.85. Advance designs can go up to 0.9.
IMPORTANT NOTE: In Dynotis products, FoM is calculated using the second formula, which takes measured torque into account and gives the propeller-only efficiency value.
Insights from FoM
- A higher FoM means less wasted energy during hover.
- Comparing propellers of the same diameter but different airfoils can reveal which geometry delivers better efficiency.
- FoM trends with propeller diameter, pitch, and blade number can guide design choices.
Role of Dynotis
Because FoM calculations rely on precise thrust and power data, accurate calibration is essential. Thrust stands like Dynotis (calibrated to ASTM standards) provide trustworthy values, reducing error in FoM analysis.
Conclusion
FoM provides a direct way to quantify hover efficiency and compare propellers objectively. With proper ground testing, UAV designers can select propellers that maximize endurance and payload capacity in hovering flight.