Eds spot on - I can confirm that windage between discs and brg structure of a jet engine consumes many KWs of power! (I design em at work). His laser tacho is just what organisers need in their Tech Check box (along with Scales, Tape Measure, Calcuulator and Charts (who thought this was going to be easy :sad

BE VERY CAREFUL about standing by discs and checking their speed at an event (classic situation for injury) - but it would be really usefull if we could set up controlled conditions to plot theoretical disc speed (ie, motor free revs x ratio) against measured disc speeds - and create a rule for correction factor(s) based on an assessment of windage.

Then we measure motor revs (while the disc is off for weighing) and can more acurately asses your potential disc speed
Kev.