# Why does this calculation show Gustave Whitehead's propellers were more than 100% efficient?

I have tried to calculate the efficiency of Gustave Whitehead's propellers using the information he provided to the American Inventor Magazine in 1902 (see the citation below).

Number of propellers = 2

Engine power per propeller, $P$ = 20 hp

Propeller diameter, $D$ = 6 ft

Static thrust per propeller, $T$ = 254 lbf

Propeller efficiency, $\eta_{prop}$

Gear efficiency, $\eta_{gears}$ < 100%

Air density, $\rho=1.2 \frac{kg}{m^3}$

Doing the calculations: $\eta_{prop} > \sqrt{\frac{T^3}{(P^2 \cdot \pi \cdot \eta_{gears} \cdot \frac{D^2}{2} \cdot \rho )}} = 101.4\%$ which is impossible.

Is there a mistake? If yes, where?