# Better understanding of TipClrnc and non-interference validation

Dear Jason,
After understanding the physical significance of the channel Tip-tower clearance (TipClrnc) over the 360º azimuth angle, it is clear that the only interesting region for no interference validation is when the blade points down. My question is, does FAST v8 take into account the thickness of the last airfoil section to calculate this variable? (i.e. if it does not, for an hipothetic 0 pitch, 0 twist, symmetric airfoil the real tip to tower clearance would be approximately: TipClrnc - TowerRadius (at tip height) - (max. thickness last foil)/2, correct me if I am wrong). I know that ElastoDyn does not have any information about the airfoil coordinates at each section but I think that AeroDyn 15 does (but probably is not used with this aim).
Secondly, regarding the critical deflection analysis discussed in IEC 61400-1:3 (2005), it is stated that the deflection in the unfavourable deflection (which will be out of plane) shall be multiplied by three safety factors (material, load and failure consequence) and then the non interference shall be checked. I suppose that FAST does not account for this augmentation of the real out of plane deflection, so, I have written an (in)equation using the FAST output TipClrnc to prove the non-interference condition (considering the above discussed):
min(TipClrnc)-TowerRadius(at tip height)- (distance from tip node to innermost airfoil surface point)-(FS-1)*OoPDefl > 0
where FS is the overall safety factor (materialloadconsequence). Do you think that it is right ?

OFFTOPIC: I am using the SWRT turbine setting and geometry (unless blade) to test an own blade design. From CertTest files I have discovered that the tower (h=34 m aprox )diameter is 1.44 m on the base and 0.355 m on the top. Do you think that doing a linear interpolation to find the tower diameter at the height of the tip (blade pointing down) would be reasonable?

Thank you very much for your attention and time
Very best regards,
Alvaro Olcoz Alonso

Dear Alvaro,

Regarding your first question, ElastoDyn assumes that the blade is a line with no volume in the calculation of TipClrncα (for blade α). Please note the AeroDyn v15 tower clearance output BαNβClrnc (for blade α and node β) is calculated more accurately and also considers the local tower radius, but still treats the blade as a line.

Regarding your second question, your equation seems OK, but will still be a bit off for a rotor with precone or shaft tilt. I’m not sure how users of the IEC 61400-1 Ed. 3 compute this in practice.

Regarding your third question, I don’t know anything about the tower are considering, but towers are often tapered quite linearly.

Best regards,