Page 4 of 7

Re: electric trim tab actuator

PostPosted: Wed Apr 25, 2012 11:27 pm
by Dave Krall CFII SEL SES
Thanks, I learned some things scanning through its volumes again looking for the data we're talking about too.

Their policy on permissible trim tab "slop" percentages is interesting and should be sobering to builders.

Maybe we can find some more specific info on a drilled hole near the edge of a curved form on a critical control surface component, that is absent of any protective redundancy....


SheepdogRD wrote:
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.

Re: electric trim tab actuator

PostPosted: Thu Apr 26, 2012 12:42 am
by Gil T
Is this what you're talking about? I filed down the front of the tab and the shaft reaches its limits before their is any binding on it. This isn't the Space Shuttle. You don't need 50000 lbs of engineering drawings to get it done.

Gil T

Re: electric trim tab actuator

PostPosted: Thu Apr 26, 2012 1:44 am
by stede52
Gil, you are exactly right, I drilled the new hole at the end because the longer you can keep that arm the slower the trim will move and as you know it doesn't take much movement in that trim to affect the flight.

Re: electric trim tab actuator

PostPosted: Thu Apr 26, 2012 7:06 am
by SheepdogRD
Gil T wrote:Is this what you're talking about? I filed down the front of the tab and the shaft reaches its limits before their is any binding on it ...

That's exactly it, Gil. How did you get the RAC pin to fit the tab hole?

Re: electric trim tab actuator

PostPosted: Thu Apr 26, 2012 7:20 pm
by Gil T
[i]That's exactly it, Gil. How did you get the RAC pin to fit the tab hole?

You're asking me to remember a lot when I can't remember what I had for breakfast. I did this project several years ago and I can't recall any difficulties doing it. Sorry I can't be any more specific. Good luck with the project.

Gil T

Re: electric trim tab actuator

PostPosted: Thu Apr 26, 2012 9:31 pm
by stede52
I could not find a clevis to fit the an3 hole in the control horn of the trim tab, thats the reason for drilling a new hole. Based on what I'm seeing of Gils setup, it looks to me that Gil shorted the horn to just under the hole and drill a new hole to fit the clevis which is provide with the servo.

Re: electric trim tab actuator

PostPosted: Sat May 12, 2012 1:07 pm
by Dave Krall CFII SEL SES
Because the above methods both take away metal from a critical airframe component and weaken it, we decided to just scrap the standard clevis and all-thread supplied with the trim tab actuator and replace it with a steel, custom made conector of large enough square tubing to completly cover the bellcrank's existing hole and allow using its existing bolt or pin size.

Will allow full trimtab bellcrank swing with no jamming and is way stronger than anything shown so far.

Re: electric trim tab actuator

PostPosted: Sat May 12, 2012 3:44 pm
by gkremers
Dave,
It seems like there are several ways to connect the servo to the trim tab. I wouldn't worry to much about the strength of the threaded push rod or clevis supplied with the servo kit. I have the same setup on my RV (along with 1,000's of others) with no issues and the RV is twice as fast. As long as you keep proper edge distance all should be fine.

I don't see anything wrong with what has been shared.

Gary

Re: electric trim tab actuator

PostPosted: Sat May 12, 2012 3:57 pm
by Gil T
IMG_1989.JPG
IMG_1985.JPG
IMG_1986.JPG
IMG_1218.JPG
Gees guys

I thought we had already reinvented this but I will give you my final(?) statement on it. When I installed the Ray Allan motor I riveted it to the original tabs that were on the elevator. I may have had to do a little tweaking on the holes but they worked out okay. I can't remember if the tab from the trim tab already had a hole in it or not which I fitted the clevis pin in. I ran the wire from the servo through a hole in the fabric next to the servo motor and fished it out through another hole in the fabric next to the fuselage. I had to file the tab on the trim tab so the actuator had full movement during actuation with no binding. I just checked this about 15 minutes ago when 3 of these pictures were taken. My servo is the T2-10A which has a full range of movement in 16 seconds. This servo seems to be the one that works best at least for me. As you can see from the pictures there is way more than ample movement. If you need any more than this either you or the airplane need to do some serious dieting. The trim is quite sensitive sometimes only needing a light touch when trying to maintain level altitude which I can't do anyway. That's why I've been trying to convince my accountant I need an autopilot. The fairing that was installed was riveted down at the back along with the back of the servo motor and then sealed with caulk all the way around. There is absolutely no play on the tab hinge which I did have with the original installation.

Gil T

Re: electric trim tab actuator

PostPosted: Tue May 15, 2012 5:47 pm
by Dave Krall CFII SEL SES
gkremers wrote:Dave,
It seems like there are several ways to connect the servo to the trim tab. I wouldn't worry to much about the strength of the threaded push rod or clevis supplied with the servo kit. I have the same setup on my RV (along with 1,000's of others) with no issues and the RV is twice as fast. As long as you keep proper edge distance all should be fine.

I don't see anything wrong with what has been shared.

Gary


I think both the ones shown needlessly compromise metal strength and fatigue resistance by reducing metal in the trim tab bellcrank by narrowing it and/or drilling an extra hole. No issue with the actuator arm assembly itself though, unless material has been removed from the crotch of the clevis to allow for full travel of the clevis against the bellcrank....

In an air only environment even the above compromises might be OK but, for how long? In a seaplane's water environment with sheets of water hitting the trim tab at 100 knots or so up on the step over and over -no thanks. It is too easy to put a much beefier, stainless arm on and attach with a properly sized bolt into the existing bellcrank hole.

Re: electric trim tab actuator

PostPosted: Tue May 15, 2012 8:53 pm
by SheepdogRD
I'm surprised that sheets of water are hitting it at 100 knots, but I'm not a seaplane pilot.

(My total time in seaplanes = 4 hours an Antilles Airboats Grumman Gooses, right seat, as a passenger. It gave me new respect for seaplane pilots.)

If water impact is that much of an issue, maybe you could move the tab onto the top of the elevator and put the actuator and arm on top, well away from the direct impact of water.

Re: electric trim tab actuator

PostPosted: Tue May 15, 2012 9:53 pm
by danerazz
I will agree that the tail catches a lot of spray, but I don't see dragging the elevator on the highlander at 100 kts. If water is hitting the tail on one of these at 100 kts, the trim tab is the least of your worries.

Re: electric trim tab actuator

PostPosted: Tue May 15, 2012 10:57 pm
by Dave Krall CFII SEL SES
danerazz wrote:I will agree that the tail catches a lot of spray, but I don't see dragging the elevator on the highlander at 100 kts. If water is hitting the tail on one of these at 100 kts, the trim tab is the least of your worries.


I didn't say, "dragging the elevator at 100 knots".

The high speed water flying nearly everywhere is routine in seaplanes but, especially at the tail and especially at the elevators and especially at the trailing edge of the elevator where the trim tab is when the elevator is down for pitch change. And it's not just "spray", it's tops of waves and solid splashes, accelerated by the full power of a takeoff slip stream, added with the plane's forward movement for the full impact effect.

Look back out towards the tail next time you take off in a small seaplane, let alone taking one off in rough water.

SheepdogRD wrote:I'm surprised that sheets of water are hitting it at 100 knots, but I'm not a seaplane pilot.

(My total time in seaplanes = 4 hours an Antilles Airboats Grumman Gooses, right seat, as a passenger. It gave me new respect for seaplane pilots.)

If water impact is that much of an issue, maybe you could move the tab onto the top of the elevator and put the actuator and arm on top, well away from the direct impact of water.


The 100+ knot water forces hit the trim tab itself, and that's not getting moved anywhere new, hence needing it to be bomber. Even where it is on the underside of the elevator, the actuator assembly itself is fairly well protected from high velocity hydraulic impact inside its streamlined fairing.

Re: electric trim tab actuator

PostPosted: Wed May 16, 2012 2:51 am
by stede52
Dave,
I'm not trying to be insensitive to your question/comments but there has been alot of proven solutions and information provided here and I haven't seen any positive feedback from you. If you don't believe the experience that has been put forward on this discussion, I'm not sure how much we can help with your issue. If you believe drilling a additional hole or removing some material is "needless compromise" then DON'T DO IT and give the forum a viable solution, not just reasons why you don't think it will work because that's all I've heard :( . I would think if the spray issue against the trim tab is really that bad we would have seen issues by now, and if it was that bad I would be less worried about failures to a welded piece of metal or a proven servo design and more worried about the fabric tearing with these so-called 100 knot forces :? there are lots of amphib Highlanders out there and there has never been and issue as a result of any of your 'so called" needless compromises which have worked well.

Re: electric trim tab actuator

PostPosted: Thu May 17, 2012 2:46 pm
by Dave Krall CFII SEL SES
stede52 wrote:Dave,
I'm not trying to be insensitive to your question/comments but there has been alot of proven solutions and information provided here and I haven't seen any positive feedback from you. If you don't believe the experience that has been put forward on this discussion, I'm not sure how much we can help with your issue. If you believe drilling a additional hole or removing some material is "needless compromise" then DON'T DO IT and give the forum a viable solution, not just reasons why you don't think it will work because that's all I've heard :( . I would think if the spray issue against the trim tab is really that bad we would have seen issues by now, and if it was that bad I would be less worried about failures to a welded piece of metal or a proven servo design and more worried about the fabric tearing with these so-called 100 knot forces :? there are lots of amphib Highlanders out there and there has never been and issue as a result of any of your 'so called" needless compromises which have worked well.


Thanks Steve,

I didn't do it and, I did give you my positive input solutions, (twice). Did you miss them? They're on this page up above.

Lots of Highlander amphibs? Which ones are they, I'd like to talk IF they've done the rough water step work. I've seen no input from actual Highlander amphib installations that used this RAC servo, or their mods on this issue either. Did I miss them?

Proven solutions shown? I certainly do believe the (limited) experiences put forth here but, if you think flying around with the thusfar discussed modifications in an air-only environment for a few hundreds of hours is a "proven solution", think again, even for experimental standards. Particularly much less so with globs of water from wavetops hitting the trim tab at much higher G-shocks than air would ever do.

Also, fabric giving and bouncing under loads is different than metal systems absorbing shocks due to metal fatigue over time.

One thing is for damn sure, nothing in the mods you guys have done with your drilling and grinding has made the trim system any stronger, and they have not kept the strength the same, since they removed metal material, so then....

Thanks again to everyone for their input so far, it's all been useful in one way or an other.