Apr 18, 2017

Oracle AC50 'Bow Down attitude' explained by Slingby

Video source: Oracle Team USA. -- As I don´t get much from Slingby aussie accent for some words in video above , and Tom also speaking as a 'trade off' to level platform upwind, I went directly with Guru Martin Fischer to get a second thought on the matter.
We covered the bow down attitude here since first pic/video was shown that I remember, also we explained how is possible to achieve a more controlled and faster flight in beachcats with a similar trim.

But AC45T and AC50s are another league so I wanted to confirm again what Groupama Team France lead designer (and flying A-Cat/FP/GC32 pioneer) had to say on how they apply the bow down 'attitude' / trim, he confirms Slingby gral concept but more clear to read/understand:
Martin Fischer: " It's not just a trade off. With the bow down trim you reduce the wetted surface of the rudder. This reduces frictional drag and therefore increases speed.

As of the trade-off Oracle is talking about: we want to sail with maximum elevator differential, which means that the net angle of attack of the elevators with respect to the boat cannot be changed.

Therefore the trim of the boat changes as a function of boat speed (more bow down if the speed goes up). As the result the boats sail downwind with more bow down than upwind. Ideally we would like to have the same bow down trim downwind and upwind, but due to rule constraints we cannot achieve that - if we want to keep the full rudder differential."