Could a trim system conceivably manipulate only primary surfaces?

Question

Most airplanes' trim systems manipulate a tab associated with a primary surface. For example, a pitch trim tab (secondary surface) is attached to the elevators (primary surface) and will change the airplane's pitch characteristics. Could a trim system that only moves the primary surface work?

Note: Such a system would probably work better in a fly-by-wire airplane where there is no physical connection between the control yoke/stick and control surfaces. If they were connected, the yoke/stick would move along with the trim, and that seems unpractical.

Answer 1

I assume by "primary surface" you mean the elevator, not the stabilizer, so we are not talking about moveable stabilizers.

Most gliders with stabilzer/elevators, and a lot of homebuilts and ultralights, (and a few production power planes like the Piper Pawnee ag plane) use bungee springs as the sole method of applying trim forces via the elevator. The spring is linked into the control circuit, like someone holding a clothesline to pull it toward one pulley or the other (or elsewhere in the linkage to do the same thing). When you move the trim control, you're moving the "relaxed" position of the bungee spring.

If you are on the ground, and you move the trim control without holding the stick, the stick will move back and forth with the trim control as it follows the unloaded position of the spring. When you apply, say, nose up trim in the air, the effect is identical to hooking an elastic band anchored to the seat to the stick itself to apply back pressure for you.

In another variation, some power planes, notably the Piper Super cub, use both an adjustable stabilizer moved by a jackscrew, AND a passive, non adjustable spring bungee within the tail assembly, that applies tension that tends to keep the elevator aligned with the stabilizer chord wise. This improves the trimmable range achievable by the jackscrew system without having to make the stabilizer bigger (if you see a Super Cub parked without the controls being locked, you'll notice the elevator sags just below neutral instead of dropping all the way down - that's the internal bungee holding it up).

Now when you see the term bungee, an elastic shock cord you buy at the hardware store comes to mind, but a bungee in an airplane can be made from a rubber shock cord, axial springs doing the same thing, or springs inside a housing with internal linkages and an external lever that provides the springy element to connect to the control system (the Cirrus line uses such spring bungee-box units for trim).

It's not trim per se, but on some jets with hydraulic controls that need artificial pitch feel, a box with internal springs, cams, and levers called a pitch feel bungee unit actually provides the pitch force you feel at the stick.

Anyway,in the several dozen types of power planes and gliders I've flow over the years, I've found that cockpit adjustable tabs always provided more precise and responsive trim control than bungee devices, so I don't think of bungee trims as optimal, but in a design where tabs are impractical, it's a useable alternative.

Answer 2

Yes.

Gliders usually have a coil spring in the pitch linkage, and if the horizontal tail is of the fully flying type, this spring will affect the zero-force position of that surface. Changing the attachment point of the spring to the airplane structure by moving the trim lever will adjust the spring force, so the zero-force incidence of the tail surface can be adjusted.

By adding a bit of camber to the horizontal tail a speed-dependent trim moment can be added such that the spring force will become larger than the aerodynamic moment at lower speed and re-trim the tail incidence in pitch-down direction (and vice versa). This is done to artificially adjust the stick force over speed. By combining spring and camber, this allows to tailor the stick force precisely. Add a bobweight in a vertically-moving part of the linkage (trivial in T-tails) and you can even tailor the load-factor dependent stick force gradient. The spring will cancel the 1g contribution of the bobweight.

These coil springs help to make very simple, but widely adjustable trim mechanisms.

Answer 3

When we talk about airliners they all trim the stabilizer with the primary surface.

See this drawing from US patent US6851648B2:

The ball screw (6) turns moving the collar (8) up and down. This moves the primary surface around the pivot point (4).

You can even see this from the outside when looking at the fuselage in the area of the horizontal stabiliser leading edge:

Forward is to the right. You can see the trim markings on the fuselage indicating the trim range. (Source: Wikipedia)

And no – the yoke doesn't move when trimming the stabiliser. Even on non-fly-by-wire aircraft. You just need to make sure to get all controls routed in or near the pivot point.