BModes Theory

Hello everyone,

In the aim of a final school project, I am currently working on dynamic 3D-coupling between different kind of structures and aerodynamics effects. After some research I found your soft BModes which seem to be very complete according to the User’s guide, but I didn’t find any document about the theory (in terms of equations of motions, hypothesis taken, etc…) the program is based on.

Do you have those kind of documents? I was focusing on the method of coupling the blade modes. But only with the source files of BModes I didn’t managed to understand how the coupling between the two directions of flection was taken into account.
If I remember well, BModes supposes that the blades are isotropic, thus no material coupling can occurs in the flexion equations. If I’m correct, only geometric coupling is present through cross inertial moments, isn’t it?

Can you give me a hand?

Thanks for your time,


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Dear Florian,

You are correct in that there has not yet been a theory manual published on BModes. However, BModes is a descendent of the UMARC code. You can find theoretical details in the following publications:

Bir, G., and Chopra, I. (1990). “Development of UMARC (University of Maryland Advanced Rotor Code),” Proceedings of the 46th Annual National Forum of the American Helicopter Society, Washington, D.C., May 1990.

Bir, G. S., and Chopra, I. (1994). “Status of the University of Maryland Advanced Rotorcraft Code (UMARC).” Presented at the American Helicopter Society Aeromechanics Specialist Conference, San Francisco, California, January 19-21, 1994.

Bir, G., Chopra, I., and et al. (1994). “University of Maryland Advanced Rotorcraft Code (UMARC) Theory Manual,” Technical Report UM-AERO 94-18, Center for Rotorcraft Education and Research, University of Maryland, College Park, July 1994.

BModes in its present form does assume that the blade is isotropic. Geometric coupling is induced through structural pretwist, offsets in mass center, shear center, and tension center, etc.

I hope that helps.

Best regards,