THS hinge point
 

Back end - front end?

As far as I know (I guess everything has been tried in aviation at some point or other), usually THSs have their pivot (not really a hinge - elevators have hinges, stabilizers have pivots) where the main spar meets the fuselage, since that is the strongest point. The pivot is also the main stabilizer attachment point (the other being the actuator connection).

I.E. with a hinge, one end is fixed and the other end moves up/down. With a pivot, the center (approximately) is fixed, and both ends move in opposite directions, up/down and down/up

Hinge: ( ––––––––––––––o or o––––––––––––––– )

Pivot: ( –––––––––o––––– ) or ( –––––o–––––––––– )

And that is fairly close to the center of the entire airfoil - not at an "end." 25% to 60% of the stab chord. Ideally at/near the center of lift (~ 25% of chord).

T-tails may be a bit different, since there is less internal space for the actuator inside the vertical fin than in the fuselage. But space overall may be a factor in conventional tails as well - geometry so the moving tailplane doesn't crunch into the tapering fuselage, or produce gaps when moved; postioning of a reinforced "box" to mounting the pivot and carry the tailplane loads to the airframe; making room for an APU and/or pressure bulkhead in the same tailcone, etc.

BTW - don't be confused by internal diagrams that show the hinge on the end of the stabilizer center section inside the aircraft - that is truncated, also for space reasons. The hinge point is aligned near the center of the external airfoil.

e.g. https://lessonslearned.faa.gov/North...tem_pop_up.htm

Thanks, hinge wasn't the right word. It was said in the lion air threads a few time that the jack screw working on the front end with at max AND with the elevator deflected to pull up (and vice versa) lead to higher loads on the jack screw (and possibly the inability to manually trim), than if the jack screw was in the back, so I was curious why most design this way..

Because the jackscrew will not fit "in the back."

Remember that the back end of a fuselage is a tail cone - it is getting smaller and smaller with every inch you move backwards.

The jackscrew is about 4 to 5 feet tall. It needs that length to move the stabilizer slowly and precisely, and for the "lever arm" of a long thread to use the drive-motor power most effectively.

If you try to put that 4/5-foot-tall device at the back end of the stabilizer center section:

1) it won't fit anyway
2) it might fit if you impaled it through the APU exhaust pipe, which is in the bottom of the increasing-shrinking fuselage, thus eating into the available vertical space back there even more.
3) it will eventually get in the way of the elevator actuators, which must be near the back of the stabilizer because that is where the elevators are.

Here is a cutaway view of a 737-800. You can click on the image and then click it again, to blow it up to 4K, and then scroll right and up to look into the tail cone.

https://thelexicans.wordpress.com/20...oeing-737-800/

Part "120" is the jackscrew (fills the fuselage almost floor to ceiling at that location), part "121" is the APU (about 2 feet in diameter, with an exhust pipe running all the way to the tail), part "117" is the stabilizer pivot, and part "109" are the elevator controls.

If you can figure out how to cram part 120 into the vertical space at or behind point 117 (don't forget to leave room for that APU exhaust pipe!), and without interfering with part 109, give us a drawing. You'll be one heck of an aircraft engineer! ;)

Thanks for the detailed reply, I know any design is a compromise, and design space wise it definitely makes sense. Perhaps after an accident the engineers/bean counters are scratching their heads, wondering if they should have tried something else (AA DC10 engine swap, Alaska MD80 jackscrew lubrication, B737 MCAS only one AOA at a time, you get my drift). Luckily I just have to fly them....

Yeah, well, as I said, there have been all kinds of approaches to tailplanes and trim.

In most (if not all) Mooneys, the entire tailcone (roughly the back 20% of the fuselage, including all the fins) is tilted for pitch trim. With a horizontal jackscrew pushing and pulling the hinged tail rather than raising and lowering.

I guess one would have to get hold of internal Boeing memos 1964-67 to really know what their reasoning was for the basic geometry of the 737 THS. Or maybe even those from the 707, which had a similar general arrangement - "if something works, let's keep using it."

Not that it necessarily worked perfectly even then - note this 707/720 crash in which aerodynamic forces from manually-set nose-down trim in a storm updraft resulted in a steep descent and high speeds that also apparently made retrimming physically impossible (possibly complicated by a slip-clutch to prevent breaking the system with too much applied retrimming force):

https://lessonslearned.faa.gov/ll_main.cfm?TabID=3&LLID=66&LLTypeID=2

Not forgetting the similarly innovative pitch trim mechanism on the Lockheed JetStar which involved the entire fin, to which the stab was rigidly attached, tilting backwards and forwards to alter the stab incidence:

https://cimg4.ibsrv.net/gimg/pprune....568142ec2c.jpg

:ok: Amazing what the boffins can come up with, especially in early days of a new paradigm. ;)

I fear I raised the pivot 'hinge' point over on R&N without looking here. I will read in tonight - but just for the moment, I feel there is a case for the jack screw to be more or less where it is, but driving a beam in the centre of the two Stab sides.

The big questions are, would it resolve most of the problems (elevator issues I'll address tonight) . The beam does not have to be straight so that with the full down position the front end of such a beam would still be contained comfortably above the tapering tail section.

The argument being that the loads would be so much less that all these mechanisms would be down-sized.

The question is, why was it done the counter-intuitive way in the first place? I can't believe it was just to make it fit, there has to be a more fundamental reason. As I say, I'll read in tonight.

One point that I haven't seen raised up to now: The pivot point of the stab appears to be in the same location (relative to both the fuselage and the stab root chord) on the Classic, NG and Max (not sure about the -100/-200).

The NG/Max stab has a bigger area than the Classic stab, with about 5' greater span. That would result in both the CofP and CofG of the stab (presumably originally roughly in line with the pivot) moving further aft. Not by a huge amount, admittedly, but possibly enough to affect the manual trim forces at full AND/ANU?

Salute!

Good stuff about the hinge point and the moments.

When I first saw a 737 next to my plane I thot it had a all-moving stabilator. The tiny elevator looked like a trim tab, heh heh

But when you start using that big stab for trim versus the "elevator", the pivot point of the stab and its use as a trim control surface is a big deal compared to the old days where it had a fixed incidence. Any really old dinosaurs here agree? In days or yore I knew at least one troop that tweaked the stab incidence in his Yankee to reduce the "trim drag" or reduce the downforce required to keep the nose up. He was leery of changing it more than a degree or two.

I predict the upcoming trials and maybe even FAA and other regulatory agencies will discuss the manual trim problem and bring into the discussion things like pivot points, jack screws, electric power to trim motor, and the beat goes on.

Gums sends...