Wish it were true, Tail Spike, but the S-70 family will not maintain level flight in this situation, even at a large slip angle.
The original US Army requirement was for the UTTAS to be able to accomplish this, with no more than a 20 degree slip angle at max endurance speed ( ~80 KIAS ). Thus the huge, and highly cambered, original tail that was flown on the prototypes for most of 1974. Someone will ask about why we didn't use a fenestron in order to meet this requirement ( we had already flown a large fenstron made by Hamilton Standard on the S-67 original Blackhawk ), but there were a number of factors at play:
- The T-700 engines were already selected by the Army so the power available was a fixed number.
- The Army had a vertical climb requirement stemming from UH-1 issues in Vietnam, and everyone knew that this requirement had to be met or exceeded.
- The cabin size, crashworthy structural requirements, fuel/range requirements etc pretty much sized the machine.
- The Army had strong directional control thrust requirements.
Given all this, there wasn't room, from a performance standpoint, to accept the fenestron performance penalty. One can envision a fenestron that would provide the same thrust as the present 11' diameter tail rotor!
In spite of a few very elegant pieces of aerodynamic structure* , instrumentation and data analysis wizardry, and after a lot of flying, we could never satisfy ourselves ( not to mention anyone else ) that we had achieved the goal. On top of that, that original tail brought on terrible, speed related, directional control trim change requirements for the pilot and in themselves, a violation of the US Army handling requirement specification.
* The UTTAS competition had a fixed schedule, with a head to head fly-off with Boeing. So anything that was suggested to fix problems brought with it heated discussions about risk to making the fly-off on time . One of the suggested tail changes to meet this requirement/problem was an entirely new vertical tail with a triple slotted arrangement ( new tail structure from the intermediate gearbox up ). Talk about sophisticated aerodynamics! The manufacturing director told us that if this was the answer, he would have to build not one, but three, one for each of the competition aircraft. Approval was granted and he had three totally new tails built. I likened the effort to that which I've read existed at Supermarine in coming up with Spitfire advances in the middle of a war ( see the Jeffrey Quill book " Test Pilot" ). Anyhow, we finally received and installed the first one and went out and flew it.....one time. None of the aero predictions were accomplished and we stacked all three brand new tails in a corner of the hangar.
So, faced with taking possibly two spec non-compliances, and being hardly certain of the zero tail rotor thrust results with the big tail, we changed the vertical tail to its present geometry, solved the trim issue, and took the deviation on the other issue.
There was no lack of effort, though.
Thanks,
John Dixson