# Can the measured thrust of a propeller be greater than the max thrust predicted by Froude's theory?

Can the measured thrust of a propeller be greater than the maximum possible thrust predicted by Froude's Propeller Theory?

Is Froude's theory reliable in practice regarding the upper limit of the thrust delivered by a propeller of diameter, d, turned by an engine that absorbs the power, P?

I get the measured thrust of a 26 inch, 2 blades propeller, higher than what Froude's theory predicts.

No.

The actuator disc theory for Propellers uses some simplifications, which are grouped into a propeller efficiency factor which normally is between 0.7 and 0.9. This accounts for friction and swirl losses, which are both neglected but very real. Also, the theory assumes equal speed over the stream tube and an infinitesimally small hub, both of which increases efficiency over that of real propellers.

If the propeller uses a large duct, Froude's actuator disc theory would be insufficient to explain all thrust. You might want to explain your measurements in more detail to get a better answer.

Can the measured thrust of a propeller be greater than the maximum possible thrust predicted by Froude's Propeller Theory?

It might seem incredible but the answer looks to be: yes it can.

"Stoddard [1977] reports measured values of the thrust which exceed the theoretical upper limit as predicted by the momentum theory by 20% to 50%."

Update

"For the propeller flow the momentum theory underestimates the induced velocity, while for the wind turbine flow this is overestimated. As the induced velocity has a different sign in both flow regimes, the total velocity at the disc $u_d = U_0 + u_i$ is underestimated in all flow states: the mass flow through the rotor is always higher than predicted by the classical momentum theory." Source: the same paper as above.

• The paper is on wind turbines, and "thrust" is actually drag. Of course the drag of a wind turbine is higher than predicted by actuator disc theory in off-design conditions. Where propellers show higher thrust than predicted, the author says "This actuator disc concept bas been developed for rotors making use of concentrators like shrouds or tipvanes. These devices increase the flow through the rotor in an artificial way." Jan 28, 2016 at 2:12
• That paper treats all cases, wind-turbines, propellers, rotors in hover, see the diagram from page 4 (pure.tue.nl/ws/files/3577374/345682.pdf). Jan 28, 2016 at 2:55