Dave Fitzgerald

I may be wrong, but I am pretty sure it is. Do you know what the original reason was, different?

Flight control sizes are directly related to how much power, or lift you need to maneuver the plane. You take an existing design, modify it or upsize it, the requirements change, usually not for the better. So, now you have to balance the cost of getting fancy to make the existing hardware work, or build something different.

The moment you build something shaped differently, flight test and certification requirements go up dramatically--and costs to certify a new variant. Think 747SP, that was a dramatically different plane than the other 747's.

The other reason is drag reduction. Again, larger plane, different control positions to maintain the CG. Which changes form drag, and overall efficiency. Again, a balancing act between the extra drag, and cost to put in this type of system to reduce drag. Drag and fuel efficiency are huge when calculated over the life of the airframe.

That's in large part why the 727 rudder did what it did. Boeing screwed up and didn't have enough rudder available for engine out, so they changed the rudder balance tabs, unboosted, to servo tabs, boosted, so they could get the force required/needed to engine out with larger engines. It only operated at large rudder deflections, so it was only really used in abnormal situations where you don't really care about fuel efficiency anyway. With all that structure in the tail, engines, T-tail, aft air stairs, they were in deep trouble, need to redesign, or band aid, they chose band aid.

Back to the HLCF on the 787-9. Longer fuselage, different elevator positions related to stab neutral, required for the CG, in other words, deflected, increasing drag. You can redefine the lift line of the stab, and you have some latitude with the stab trim, but over the wide range of CG you need, the drag rise is unacceptable. So, make the existing stab, elevators, rudders more efficient with the HLFC system, you avoid all the above.

A couple of years ago, Boeing took a used 757 and made what they call, an "Eco-demonstrator." They used it as a research test bed for a lot of these systems. Including a HLFC system on the rudder, exactly for the 787 and 777X programs. They could calculate the efficiency gains pretty easily, but the wear and tear, and maintainability was an unknown. Anyone ever built a Rutan Quickie or Varieze? Ask them what effect bugs on the wing leading edge does to their planes. Same sort of problems with the HLFC. What I don't know, is what changed Boeings mind on using the system. It's already built, certified, and in use, so what changed. Line experience? Maybe.

Flying in the clean smog free air of Beijing?

Tonyrome

The 787-9 and -10 have millions of small holes in the leading edge of the Vertical fin
(Most -9's in service will have the system on the Horizontal Stabilizers also)
This is coupled with a system to manage the air sucked through these holes to delay the transition of the boundary layer from laminar to turbulent. The real question is whether the system is achieving fuel savings or not. Maybe more a question for the bean counters?

Dave Fitzgerald

Exactly! The farther back the laminar flow can be maintained, the less drag. The question is how to do that and how much it will cost vs. maintainability.