[LAMPSguy]

Question from lack of familiarity:

If you have electric trim, and you have an autopilot, do they use the same servo? Or do you have an aileron trim servo and an aileron autopilot servo?

For those who might think my question strange, my overly complex work machine has multiples of both with a triple redundant hydraulic system.

 

[MarkW]

Not strange as I wondered the same thing in the beginning. Why can't you use a autopilot servo to trim the control surface.
However they are very different. The pitch autopilot servo controls the bell crank which controls the pushrod. The pitch trim moves a small tab which only influences the elevator.

 

[BobTurner]

Not strange at all. In fact I thought that some certified autopilots did just run the trim motor. But the TruTrak and Trio autopilots (most popular for RV's ) as well as the EFIS driven ones all drive the elevator bellcrank directly. My guess is the electric trim motor isn't fast enough to use as a control in turbulence.

 

[Tandem46]

If you have electric trim, and you have an autopilot, do they use the same servo?

Not speaking for everyone, but on my -7 I have a separate servo for each. The trim servo is completely different from the autopilot/roll servo. My trim servo came as an option from Van's and then I bought the roll servo separately from Trutrak. Wouldn't surprise me if there are systems in other RVs that share a servo, but I don't know.

 

[Ironflight]

I don't know of any case in an RV where the trim servo is used for the autopilot - and the key reason was mentioned above - it just wouldn't be fast enough at the speeds we fly, and with the tabs the size they are. Big and fast airplanes have considerable differences in their control systems, and can share the functionality.

 

[dhammer]

Another reason

Trim servos generally have brakes and lock in one place. Autopilot servos do not. Some autopilots will run the trim to take the load off of the autopilot as the CG changes in flight. That's probably a big airplane thing though. Others give you a re-trim message if the autopilot can't handle it.

 

[SteinAir]

It's been tried - and there are some threads here in the archives which detail that trial and error. The short story is that on some planes (canards specifically) you can kinda/sorta get it to work. On RV's and related planes, not so much.

The crux of the issue is the slow moving trim motor is moving a rather ineffective secondary flight control (sort of), coupled with the fact that the "autopilot" has to sense the airplane nees to move somewhere, then send that command to the trim motor, which then slowly moves the the trim servo (some fixed amount), then waits for feedback to see if it's correct, then starts all over again. It's always behind the plane just based on the way it works. The certified stuff you hear about is either very old, or it's on big planes which fly with servos tabs entirely (like a DC9) - and that's a wholly different story and flight control system than a trim tab...do not confuse the two because trim tabs and servo tabs serve different purposes.

Cheers,
Stein

 

[Neal@F14]

My RV-6 has independent servos for aileron trim and autopilot.

The autopilot is a TruTrak, with a normal-sized autopilot servo mechanically connected to the aileron control linkage.

The aileron trim has a small Ray Allen trim servo (connected to the aileron linkage via a couple of springs to merely provide biased pressure to one side or the other). It's mostly ineffective and unnecessary. It basically only provides an extra LED bar graph display on the instrument panel to look at, and a need to add wires to the side-side section of the hat switch on the stick grip.

 

[BobTurner]

As mentioned above, both Trio and TT offer options for the autopilot to drive the electric trim motor as well as a servo connected to the elevator. It drives the trim motor to relieve the force, just as a pilot would do manually, not to control the aircraft (that function is for the main servo on the elevator). I have the Trio with trim control and it works- you can change speed with the autopilot engaged, and when you release the autopilot it's still in trim, no surprises.

 

[LAMPSguy]

thanks

Thanks for the input. Getting really close to the build and trying to consider/calculate costs for the wings (don't flame spray me...planning on a -14 now, just waiting).

I fly fling wings and with no aerodynamic trim tabs to dynamically adjust in flight, we use multiple different methods for doing all this from a magnetic brake and spring system for artificial feel to hydraulic and electric actuators for both trim and the "autopilot". We have altitude, attitude, heading holds but that is the extent of "inflight" autopilot modes. Although some may find it interesting that we have an "auto approach to and depart from hover function" which is a pretty cool thing.

 

[RV8R999]

One reason but not the only....

Most GA airplanes have Reversible Flight Control systems - meaning the aerodynamic loads are transmitted from control surfaces back through to the pilot. Trim in GA planes is about relieving these forces and usually controlled through trim tabs to balances those forces at the control surface itself - although some RVs utilize a spring system for ailerons to balance the forces at the stick. Reversible flight control systems are not generally suitable for combined trim/AP servos because to function as a trim servo it would necessarily need the requisite force to hold the control surface throughout the entire speed band of the airplane - which isn't difficult. However, since a great deal of hysteresis exists in reversible control systems using the trim tab to control the elevator for autopilot functions often results in severe oscillatory dynamic longitudinal stability. The combined trim/AP servo is therefore relegated to controlling the elevator directly - which isn't a problem either and how most AP servos work except if the AP servo is also the trim servo and the trim servo is connected directly to the elevator control a conditions will exist in which the aerodynamic trim tab will almost always be severely out of trim - which can be a major problem when disengaging the servo. You can certainly trim those aerodynamic forces with the standard trim tab as most of us do now but if you have to do that then what is the point of having a combined trim/AP servo?

The helicopters you fly are non-reversible flight controls as are most larger commercial aircraft. Aerodynamic loads are not transmitted to the the control system to the pilot. Forces are generated artificially - less sophisticated systems use simple springs while more advanced aircraft have Fly By Wire (FBW) unique trim controllers capable of shaping the forces based upon flight conditions. Trim in non-reversible control system is about holding the stick fixed in a position set by the pilot vice relieving forces - this is why when you depress the Force Trim Release button on the cyclic, all forces go to zero and you can move the stick around until you let the button go, which resets the trim point to that location and all artificial forces are referenced to this new position.