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Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 5:33 am
by SheepdogRD
The guidelines from RES Mfg. are for metal stamping, not all fabrication methods. For instance, there's a minimum hole diameter guideline because it's hard to punch a deep, small-diameter hole; it's best -- but more costly -- to do that in a secondary operation by drilling.

AC 43.13 is the bible for aircraft.

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 7:14 am
by JackLeroux
How did you guys route the wires from the servo through the elevator and into the fuselage. The servo harness is about four inches long so you have to splice wires at the servo, did you run straight lengths of wires to the cockpit or did some of you use disconnects at the tail end. If anyone has pictures showing the wire routing from elevator to tail cone they would be appreciated.

Also, has anyone looked into slowing the servo down? Jak tells me that these servos run on for a bit after removing power which can make it difficult to find a neutral trim position. I am wondering if by stepping down the voltage or by applying a load, a momentary input would result in less control surface travel thus giving more control. Is this necessary? Are you guys having to trim up and down to find a neutral trim position?

Jack

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 9:57 am
by danerazz

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 12:39 pm
by Gil T
I just fished a wire through the elevator and into the fuselage. Yes, it is a bit sensitive. Anyone that uses the Ray Allen speed device, let us know how you like it. You do have to tap the trim control very lightly but one gets used to it.

Gil T

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 3:08 pm
by scubarider2
Just curious....is there such an advantage to having an electric trim? I have the standard and it works beautifully. :?:
Seems like a lot of work.
Dennis

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 3:11 pm
by danerazz
Mostly seems like there can be some issues with the cable breaking on the manual trim. If it is supported properly then it should not be an issue, but the electric trim seems like it would eliminate these problems.

My only issue with the electric trim is the $300 price of entry for a servo, indicator and switch. Another $50 if you want a speed control.

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 3:24 pm
by SheepdogRD
Here's an interesting article on one way around the high price of servos: http://www.vansairforce.com/community/showthread.php?t=64641.

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 4:29 pm
by Dave Krall CFII SEL SES
SheepdogRD wrote:The guidelines from RES Mfg. are for metal stamping, not all fabrication methods. For instance, there's a minimum hole diameter guideline because it's hard to punch a deep, small-diameter hole; it's best -- but more costly -- to do that in a secondary operation by drilling.

AC 43.13 is the bible for aircraft.


Do you have the chapter and verse in AC 43.13 for trim tab bellcrank hole placement and drilling?

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 6:37 pm
by jak
Dick the Vans link was cool brought back a lot of memories. Jak

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 10:19 pm
by danerazz
That servo is pretty nice, if you got the 100/1 ratio it would take what looks like 2 seconds to travel from stop to stop. Pretty fast, Need to find a way to slow it down. Also, stroke is a little over 3/4", which should be doable? It looks like if you splurged for the pc control board at $40 you could use a potentiometer to control your trim position and it allows you to dial the speed back. With just over $100 and a little tinkering you could have a slick setup.

Re: electric trim tab actuator

PostPosted: Tue Apr 24, 2012 10:33 pm
by danerazz
Now that I look at it, I like the L-12 actuator for $80 better for the highlander. Get the 30mm stroke and 210/1 ratio And it will take a minimum of 6 seconds end to end, and hold position well. The 30mm one should be 82.5mm (About 3.75") hole to hole compressed with a travel of 30mm (about 1.18"). Easy wiring. Says dust/water resistant, but not waterproof. With a simple fairing over it, it should be good.

Re: electric trim tab actuator

PostPosted: Wed Apr 25, 2012 7:05 am
by JackLeroux
The RAC servo is protected from pitch trim run away, before considering other options I would make sure that the same is true. Try a quick Google search for pitch trim run away and you will see why.

I purchased the RAC servo and actuator arm as a result of the failures of the mechanical system reported on this forum. A friend flying his Highlander on floats is on his third cable in two years. I plan to fly on floats so the cost of cables, time to replace and piece of mind is worth the cost of the servo to me.

Gil, if you were to do it again would you exit the wires from the elevator in the same manner? Is that a drain grommet I can see under the fabric? Would you consider using a piece of conduit to exit the elevator closer to the hinge point?

Jack

Re: electric trim tab actuator

PostPosted: Wed Apr 25, 2012 9:25 am
by Gil T
That's a rubber grommet with a patch over it. Since the tail was covered the simplest way was to put a small hole next to the motor and next to the fuselage and just say come on you s** o* a b**** and its done. Simple is better.

Gil T

Re: electric trim tab actuator

PostPosted: Wed Apr 25, 2012 10:53 am
by danerazz
They are Protected from runaway the same way the ray Allen ones are: the switch you use to control it turns the power off completely when you are not actuating it. They are wired the same way.

Re: electric trim tab actuator

PostPosted: Wed Apr 25, 2012 4:20 pm
by SheepdogRD
Dave Krall CFII SEL SES wrote:Do you have the chapter and verse in AC 43.13 for trim tab bellcrank hole placement and drilling?

Please understand, Dave, I wasn't taking a shot at you with my comment; I was pointing out that the RES guidelines are for punching, not drilling. And I was reiterating advice that's been given to me by several aircraft builders whose approach I respect: "Check AC 43.13; it's the bible." This is my first aircraft project, and I learn a lot every time I open the "bible".

So I looked in the bible, and I find AC 43.13 is oriented to inspection and repair, not design. Initial design is up to the manufacturer. AC 43.13 says, in 4-60a(2), when repairing aircraft after an accident or in the course of a major overhaul, to "inspect all highly-stressed main fittings, as set forth in the manufacturer's instruction manual." So, even for repairs, it returns the repairman to manufacturer specs.

In fact, I was surprised to read last night that welding over a worn bolt hole isn't approved. In 4-60a(4) it says "Elongated or worn bolt holes in fittings, which were designed without bushings, are not to be reamed oversize. Replace such fittings, unless the method of repair is approved by the FAA. Do not fill holes with welding rod." Still, I don't think that comment was aimed at simple tab holes; instead, it's focused on "elongated or worn bolt holes in landing gear, stabilizer, interplane or cabane-strut ends".

After thinking about it, I suspect they're concerned about embrittlement, because brittle metal can crack and then fail suddenly, rather than showing wear, which is easier to spot in a preflight or condition inspection.

For something like the tab, where we need to redesign because the hole is simply too big and/or too far from the edge to fit the non-standard clevis, we become aircraft manufacturers. We have to do what we think will work.

Over the years, what works has been codified in engineering guidelines. For instance, boiler design guidelines kept getting tighter and more specific until boilers stopped blowing up, but there were a lot of boiler explosions until they figured out what worked.

While AC 43.13 may be the bible, it seems to be mute here, so it appears that I've referred you to a dead end. No chapter, no verse. But Steve said the magic words:
stede52 wrote:I'm just telling you what will work for that thickness of steel, you can do whatever you wish.

Right now, if I can't find a clevis that works better, my plan is to trim the front of the tab a bit so it fits the RAC clevis, and see if I can find -- or machine -- a bushing so the RAC pin fits the hole. Ideally, the tab would be L-shaped, so the end could be rounded. We're the designers, now, so we can do what we think will work.