So the SR22 uses the same pitch trim method that was used on the PA18 I used to have!
As I said, the SR22 has NO TRIM TABS :-), just like a Piper Warrior has no trim ab, and a Seneca has no trim tab either.
The trim motor only sets the zero position of the elevators.
I started my PPL in the PA38 Tomahawk which had no pitch trim tab. The “trim” was an adjustable spring which basically pushed against the yoke (at the bottom of it).
Obviously this works if you believe that the purpose of a pitch trim is to take out the force which the pilot has to continuously apply to the pitch axis. And sure enough my instructors told me just that.
I do not think that is correct. It is “correct” in so far as the purpose of a gearbox on a car is to prevent the engine stopping when you go up a hill 
The real purpose is quite different. The classical pitch trim tab mechanism is a stabiliser for pitch which (for a given config etc) is a stabliser for the airspeed., which is what you want.
I guess that a spring pushing against the yoke will have the same effect as using the trim tab (because the trimmed speed depends on the yoke position – all else being equal etc etc etc) but it seems a mechanically perverse way to go about it because you end up with a constant force on the pitch linkage.
Anyway whatever has been done on the SR22 does clearly “work”. It’s just hard to get one’s head around their autopilot solution. The way King did it (using pitch and roll data as primary targets, and driving the pitch and roll with directly coupled servos, and driving the pitch trim servo via a torque sensor on the pitch trim servo) intuitively gives the best dynamic performance because it flies the plane just like an “optimally skilled” human pilot would fly it, and the human technique has been established for the best part of 100 years.
Piper Warrior has no trib tab
Really? I have ~50hrs in PA28s and they damn sure had a pitch trim wheel.
The pitch trim on a Super Cub is by a screw jack that alters the angle of incidence of the whole tailplane, and a conventional elevator. The PA28 has an “all flying” tailplane, or “stabilator” as the Americans once called it, with an anti balance tab, but the tab is adjustable via the trim wheel to give the trimmed neutral position
No, the so called anti servo tab is not moved by the trim wheel, nor is it moved by the servo. The trimming is done by setting the stabilator to a different 0 position, and the tab moves in the same direction. It’s one of my favourite Warrior checkout questions. The tab is directly linked to the stabilator. It is only connected to the stabilator and moves in the same direction to enhance control forces in pitch.
Also the way the DFC90 controls the pitch trim is great. It flies extremely precise, more so than any human (i know). It’s the result that counts.
From the PA28-181 POH page 46
Dual controls are provided as standard equipment, with a cable system used between the controls and the surfaces. The horizontal tail (stabilator) is of the all-movable slab type with a trim tab mounted on the trailing edge of the stabilator to reduce the control system forces. This tab is actuated by a control wheel on the floor between the front seats (see above figure).
Here is a link to it
http://www.glasscockpitaviation.com/MainPages/documents/PA-28-181-POH.pdf
I have a PA-28 Warrior in the family since 1978.
The explanation in this POH you pointed me to is clearly wrong. The tab is actually not independent of the stabilator but it’s movement can be controlled by the trim wheel to change it’s position in relation to the stabilator. The tab is MAINLY for enhancing ctrl forces but it also helps to keep the stabilator in the trimmed position … Maybe i did not express it clearly before – but what’s wrong for sure is that the tab REDUCES control forces
Just that the POH calls it a trim tab does not make it correct… It is an anti-servo tab that also serves as a trim tab.
The actual elevator movement is controlled by cables linked to quite a hefty piece of metal in the fuselage. The little rod that goes from the tab into the tail cone does not move the elevator directly, it only influences the position of the tab relative to the elevator. The position of this rod is what is adjusted by the trim wheel.
Alexis, unless you have a vastly different PA28, you can verify the above by pulling the elevator back and moving the trim wheel, you can clearly see the tab moving as you adjust the trim.
If you look at the PA28 Maintenance Manual Page 2B15 and refer to section 5-23 there is a procedure for rigging the trimmer, which involves checking the position of the anti balance tab at various trim settings. One part of the procedure is to “turn the trim control wheel until the tab streamlines with the neutral stabilator”
Sorry, I am sure you are mistaken alexisvc, because FOR SURE the tab is moved by the trim control wheel
Cobalt, i always thought that moving the trim wheel moves the stabilator – and the tab with it, but not directly. I still think that the 0 position of the stabilator is the trim, assisted by the tab – which mainly enhances control force. I thought that the above procedure meant to move the stabilator with the trim wheel until the tab is flush with the stabilator, but it never occurred to me that it can be moved independently aswell.
Maybe i did not understand this for 20 years? ;-) If it wasn’t 1 am I’d drive to the airport to check that :-)
