RAC Servo Question

[Archer39J]

Do the RAC servos lock in place or are they free to move? I'm wondering if I can keep my manual trim with a servo hooked up to the elevator trim horn. Reading discussion of the clutched capstan servos is making me wonder...

Never mind, website says they lock in position when not powered...

[jaredyates]

No, the RAC servos are not clutched. The advantage is of course that they stay where you put them without requiring any further power. The disadvantage is when its position is not where you want it to be.

[Archer39J]

The more I think about it, the more I’d like to have both manual and electric elevator trim. Any suggestions for clutched linear servos similar to the RACs that seem to be popular?

[jaredyates]

Please let us know if you figure it out! I'd love to have both also, but sometimes in building airplanes, as in the rest of life, one must choose one path at the exclusion of another.

[Archer39J]

Just asking if anyone has a lead on a actuator that doesn't lock when unpowered, if I find something myself I'll update here...

[Archer39J]

Found something that looks promising. https://www.actuonix.com/Rod-Actuators-s/1924.htm

The P16 series looks like a good candidate for elevator trim actuation. Looking at picking a gear ratio to provide enough force to move the trim while having a light enough back pressure to be overcome by the trim wheel. I might want the slower speed of the 64:1, but 45 lbs seems a bit much to overcome, I’ll have to wait until I’m back at my plane to get some measurements on what I’ll need.

The L12 series looks like something that could be a good candidate for fitting inside the rudder. It’s smaller than the RAC T4. These prices are quite reasonable too.

[Archer39J]

The L16 has lower back pressures at similar ratios. Hmm...

[jaredyates]

On the topic of using a Ray Allen servo and wanting a clutch, what about an option where the pilot is the clutch? For example, you could mount the servo up near the trim wheel and give it a pushrod that terminates with a 90-degree bend, with a piece of pushrod protruding about an inch past the bend. Label one particular link in the chain, and when you want electric trim, engage the pushrod by sticking that 90-degree bent part into the chain link vertically. When you want manual trim, remove the pushrod.

[Archer39J]

Yeah that could work, though having another thing to manage engaged/disengaged is something I'd rather not have to deal with. The more I'm reading about these Actuonix linear servos the more I'm liking them. Though I'm looking at a simple 2-wire actuator I'm thinking about running 5 wires in case I want to experiment in the future, they have some interesting controller and position reporting capabilities. At $70 a servo they're quite a bit cheaper than the RAC offerings and appear to be much more capable. I have a pull scale on the way so I can quantify what I'll need this servo to do, there's also quite a bit of adjustment in the forces via the trim wheel itself.

The main benefit is that they can be overridden when unpowered. So I think they're a good option for working in tandem with the manual trim system.

[whee]

I’m ordering a couple P16 actuators for my cowl flaps. I think they could work in your application but my concern about using the 22:1 would be if it would hold the correct position when not powered. Conversely, if you increase the friction on the manual portion of the trim so the tabs stay in position is 10lbs of force from the actuator enough to overcome that friction?

[Archer39J]

Yeah that's what I'll have to figure out, they have several options for gearing. Generally the L16 series has lower back pressures than driving forces so that's what I'm looking at now. I also want to see if an L12 will fit inside the rudder but I'll have to wait until I'm back home, it's not immediately apparent from the plans.

[rodsmith]

I’m ordering a couple P16 actuators for my cowl flaps. I think they could work in your application but my concern about using the 22:1 would be if it would hold the correct position when not powered. Conversely, if you increase the friction on the manual portion of the trim so the tabs stay in position is 10lbs of force from the actuator enough to overcome that friction?

I'm interested in how you are doing that. Could you post some pictures or description when you get to that point.

[whee]

I'm interested in how you are doing that. Could you post some pictures or description when you get to that point.

I’ll make a post about my cowl flaps when they are further along.

[Battson]

The more I think about it, the more I’d like to have both manual and electric elevator trim. Any suggestions for clutched linear servos similar to the RACs that seem to be popular?

I posted a thread about this a couple of years ago. There's a plane here which has such a trim system. You can search it.
Long story short, capstans are not sensitive enough for an ideal trim system. They make it very sensitive. You need a gear system or something to reduce the rotational movement.

[Nev]

I’ll make a post about my cowl flaps when they are further along.

whee , did you go ahead with electric actuated cowl flaps ?

[schu]

If you want both electric and manual trim you could possibly get a maule style trim wheel with the gear reduction, then attach some little motor (or stepper) and drive the trimwheel itself. That would give you the cool trim wheel moving effect. Something like this:

s-l1600.jpg

Another option, and what I'm doing is to add a trimwheel that uses a rotary encoder which through a small microcontroller drives the trim servo. This retains the wheel, but it's electric.

Not as cool as the wheel won't move, but it does look nice and I can program it to be as sensitive or insensitive as I want. Some argue that's more complex, but it's not really complicated. Even a modern flashlight has that level of software in it if it has multiple levels of dimming and a flasher.

schu