# Inertia properties of Rotor and Nacelle, OC3Hywind

Dear Dr. Jonkman

I want to calculate mass moment of inertia of rotor (bladed+hub) and nacelle about the center of mass of rotor and nacelle, respectively. I have searched most of the topic regarding inertia, on the forum. I found detail information about the total mass moment of inertia of nacelle+rotor, however could not find how to calculate separately for each one. Your help will be highly appreciated.

Regards,
Zahid

Dear Zahid,

The nacelle and hub are modeled as rigid bodies in FAST and their mass, center of mass, and inertias are inputs to FAST (within the ElastoDyn module of FAST v8). A blade within the ElastoDyn module of FAST v8 is specified as a mass distributed along the blade; the integrated mass, center of mass, and inertias of a blade then are calculated when running FAST and output in the ElastoDyn summary file. The OC3-Hywind spar is available as Test24 in the CertTest of the FAST v8 archive, available from here: nwtc.nrel.gov/FAST8.

Best regards,

Dear Dr. Jonkman

I am thankful for quick response. I checked the Elastodynamic input file and summary file. I found the inertia of rotor about rotor axis and nacelle inertia about yaw axis only (attached a snapshot of input file), however the inertia about the other two orthogonal directions is not given there. I will appreciate if you can guide me to determine the inertia of rotor and nacelle about all the three principal axis (Ixx, Iyy, Izz). I have seen in some of your post the Ixx, Iyy and Izz values for the whole system (tower+rotor+nacelle). I am looking for rotor and nacelle.

Thank you very much

Dear Zahid,

It is the hub inertia about the low-speed shaft that is specified within the ElastoDyn input file, not the rotor inertia. While the full rotor inertia (hub plus blades) about the low-speed shaft is calculated and written to the ElastoDyn summary file, the rotor inertia about the two axes orthogonal to the low-speed shaft is not. However, these inertias should be easily calculated by hand from the blade masses, center of masses, and inertias already calculated and written to the ElastoDyn summary file.

ElastoDyn assumes that the pitch and roll inertias of the nacelle about the nacelle center of mass are zero. ElastoDyn also assumes that the inertia of the hub about the hub center of mass and about the two axes orthogonal to the low-speed shaft are zero. These assumptions were established to minimize the number of inputs needed in the ElastoDyn primary input file because in many turbines, these inertias have a negligible influence on the dynamic response.

Best regards,

Heartfelt thanks Dr. jonkman