How YC-14/YC-15/C-17 got their vertical tail planform?

[flateric]

There was a nice Bill Sweetman's article titled "How 747 got its hump".
Now another question haunting me - Shape that became a trademark of AMST and C-X/C-17 - a migration from carrot/bullet to that pronounced endplate fairing copied more or less by Airbus/Ilyushin/XAC - do anyone knows the story behind? I assume NASA Langley or Ames were involved heavily. Bill Norton I think would say something (if he was here).

 

[chuck4]

The rear end of fuselage of commerical passenger jets also saw a partial migration from a bullet shaped or blunt shaped fairing to a knife edge fairing between mig-1970s to early 1990s.

 

[Kiltonge]

The disappointingly pragmatic reason why the YC-15 / C-17 tail looks that way: identical ribs make it cheaper. Flight, 30 Jan 1975 and AvWeek 01 Sep 1975.

So why did the A400M and KC-390 adopt the same style...?

 

[flateric]

This is kinda well known fact. Initial post question was about top fairing shape.

 

[Kiltonge]

This is kinda well known fact. Initial post question was about top fairing shape.

The fin-tip fairing? No particularly sophisticated reason except it was the form required to envelop the tailplane pitch screw-jacks and tracks, hence the kink in the vertical stab leading edge near the top. A consequence of design choices.

You can see the same arrangement in this scrap of a KC-390 cutaway. The tailplane position fore-aft on the vert stab is dictated by the rudder position, and trimming the tailplane assembly using leading-edge jacks is most efficient due to leverage. Hence, the mechanism projects beyond the leading edge of the vert stab and needs the big fairing. It doesn't use a traditional bullet fairing around the mechanism because a slab-sided fairing is nearly as efficient but much cheaper and also lighter, since the port-starboard tailplanes don't need to be cantilvered as far from the datum.

On the C-5 and Il-76 the entire tailplane assembly sat atop the vert stab and was trimmed externally by translating over a "chin" fairing on the leading edge of the stab. Hence its tailplane fairing could have a more aerodynamic bullet shape but at higher weight and perhaps with more risk being jammed by external factors like ice.

 

[flateric]

The fin-tip fairing? No particularly sophisticated reason except it was the form required to envelop the tailplane pitch screw-jacks and tracks, hence the kink in the vertical stab leading edge near the top. A consequence of design choices.

It was not so in the case of YC-14/15 with their short chord horizontals, neither C-17 (if you look at cutaways). Pitch screw is much further back there to affect forward kink shape. So it's obviously aerodimically driven, here we return back to question in post #1.
Opposite example is Challenger or Global Express tails that have no such kink.