Thorp Air Command - T18.net

Guys,
I'm libel to ruffle a few feathers here but this is a subject which I feel strongly about and quite frankly I'm concerned for those of you that have altered the Thorp design in bypassing the rugged jack screw for a plastic servo, clevis and threaded rod.

The story ...

Yesterday I received a call from one of our fellow Thorp owners. The short of it was that he and his wife were headed home when his trim went full forward and remained in that position. He stated that he needed "both hands pulling on the stick to maintain control of the airplane" and that he was very concerned the force he was putting on the stick was going to "break something". Assuming he had the stock Thorp trim installation I talked him through removing the rudder and vert stab and resetting the trim to a neutral position, which with the stock install is quite simple. Unfortunately after pulling the fin off he sent me a picture and I was looking at one of those damn RAC servos. I know the airplane and I also know that someone pulled the original bulletproof trim mechanism out and put the plastic servo in there. The rest of the story is not that important other than after pulling the servo out and running it back to neutral on a bench he was able to reinstall and fly home. Keep in mind if that servo had failed he would have been spending the night in BFE waiting on a new servo from ACS. Of course with the stock setup he would have been home without a hitch.

My point here is not to talk about inconvenience but rather the integrity of the servo setup versus the stock jack screw/Grainger motor setup. To be honest I really don't know how we've gone down this path, as the original setup is elegantly simple and strong but that's a topic for another day. Let's talk about safety, recall from above that the pilot reported he needed both hands pulling on the stick with as much force as he could muster to keep the airplane level. First off the trim must have been WAY to far nose down as I've flown my airplane with full forward trim (in fact towards the end of must X-Countries that's where we all wind up) and maintained complete control. So this tells us that the RAC setup allows the trim to end up in extremes that should not be allowed. Think of the strain on that little 3/16" threaded link with that kind of force required at the stick. On the same subject instead of a 3/8" jack screw pushing/pulling against four layers pieces of .032 (576 bulkhead, 582 doubler, fin spar and fin spar doubler), and a bearing cage, your pushing and pulling against a single piece of .032 and whatever you guys are making the RAC "tray" out of. In fact you

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Well put.

I think the Ray Allen servo was an invasive species brought over from RV-land. But you are right... a trim tab and a stabilator anti-servo tab are two different things, with vastly different areas, and presumably vastly different forces.

But I'm no expert. I've never flown a Thorp. Or an RV for that matter.

D

Thanks for airing out this issue. I have concerns about those that deviate from the jack screw design also. You describe the original design as "simple & strong", but there I may disagree just a bit. It is not a simple piece to fabricate. You need good machining skills to cut the treads precisely, make the bearing block, etc. Rigging it up with the original mechanical drive mechanism, trim wheel, etc is challenging. I understand why guys study that system and conjure up what they perceive as a better solution. But, they ain't engineers with JT's credentials. The full flying stabilator, anti-servo tab & trim mechanism is a critical system on the Thorp. I'd want to see a thorough design/stress analysis by a highly qualified engineer before betting my life on that little plastic wickerbill.

Gary Green

I agree with Lee, Dick, and Gary. Guys trying to reinvent the wheel are out to do themselves in. A large servo tab such as the Thorp needs a little more than a plastic over grown RC modelers servo. As Lee pointed out the RAC has to much travel and will over drive the servo tabs. It works on the RV's because it's a trim tab and not a servo. I use one for the aileron trim and like it. It drives a trim tab and not a servo tab, which are two different animals. There is a reason JT limited it to eight turms and pinned it. Even the biggies use a jack screw for trim. I've made several over the years and they are still in service. While I'm at it, the vernier/rod end is a poor design at best. One thing you don't want is the servo loose and flopping around , it will only do it a couple of times before the tail comes off. flutter is not your friend. Like Gary said, if you are not and aero engineer, don't re design JT's controls.

Bill - elaborate on your posit of "the vernier/rod end is a poor design at best" please - I'm not picturing it.

Frase,
Some guys have been running a vernier cable from the cockpit back to the tail and controlling the anti servo tab that way. Just like the rv's trim tab setup. Basically a manual version of the RAC servo setup.

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Lee Walton
Houston, TX
N51863,N118LW
KEFD

Hi Frase, I have the verneer cable trim, the cable is double the size of a rudder cable. It is the same as your verneer throttle cable only the verneer ball threads are approx 1- 1.5 in in Diameter, that is the portion of the cable just below the knob that does the work. It has a resistance nut below the knob also so you can adjust the resistance turning the knob,you can set the resistance so tha you can turn it with just your thumb and fore finger very easily of tighten it up and then it takes some forearmfortitude to move it. The knob being verneer,it has a button in the center of the knob,when depressed this allows for you to go from full down to full up trim by just pushing and pulling on the knob like your standard verneer,it takes some effort to push the center button in. This system is probably as heavy or heavier than the standard Thorp Trim system because of the cable, it is not a light weight my guess is that it has more weight than the standard system. The Verneer mechanism is detachable from the cable for any service it might need, or for replacement. I ran across another version of this system that I had never seen, it has a lever instead of the verneer mechanism. This lever sticks up out of the tunnel just in front of and to the right of the flap handle, it has about 2inches of travel. The cable trim I know is not for everyone and has been met with a bit of controversy and that's ok, I hope this explains a bit......Dan

You are eliminating the screw jack assembly which holds the servo system stable by using The RAC and venier cable. If any of these fail, the servo tabs are free to move. I've seen rod ends come apart and cables break. JT went to great lengths to design the trim system when he could of used a cable ??. I'll stick to the jack screw. There are some things on a Thorp that need to be left alone and built like JT designed it.

Sorry for bringing and old thread back to life. But being a newbie here I'm trying to read most of all the threads to immerse myself in Thorpogy.

Having "grown up" on horizontal stabs and elevators the one thing that "scares" me a little is the flying tail. I realize that this is proven with all the pipers and even moony's out there and of course 50 years of T18 history. But with this system you are betting your complete life on the integrity of the tail and trim system.

Therefore I believe that this tail and trim system is one of the most critical components of the entire aircraft. As a result it would seem logical to me that this system be the most robust and tightly fit with little or no "slop".

If I'm trusting my life to this system I personally do not want to trust it to any plastic components!

It seems mayny people may want to simplify and save some weight. But it also seems fairly common to add lead to the tail for CG reasons. So what did saving 2 lbs or whatever do when you may just have to add 2 extra lbs of lead anyway? Exactly nothing and it may be risking your life! Does NOT seem like worthy trade off to me.

It seems like everyone goes with an electric system. This risks a runaway trim situation. Has anyone put in a manual trim wheel/mechanical system to operate the jackscrew? This would completely eliminate any runaway possibility. Which MAY be worthy of a bit of a weight penalty.

Also when it comes to flap and aileron trim controls. It seems like most want or have these controls on the stick grip. Do you really need these at that location? Do you use them that much? It would seem to me that having rocker switches on the lower part of the panel to operates these within reach of both seats would make sense if only needed ocassionly.

I would think that a flap swiitch or switches pre set to say 5 degrees to start slowing you down at a higher speed. Next when on base hit to go to half flaps and the on final hit again to go to full flaps. Again on the lower panel.

This would be similar to a Cessna I guess.

Sorry for the long post and bling this whole thread back to life.

Just trying to learn and thought I'd air some of my possibly newbie thinking which may stand to be promptly corrected.

Of course. That's the way it was originally designed, and many/most of us did it that way. If you're serious about the T-18, it would be to your advantage to continue reading the old newsletters and studying the design.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

I am thinking of putting in a Ray Allen servo for aileron trim on our new bird. Should I just ditch it? I think I have read about guys putting them in for aileron trim. Would it be uncontrollable if the servo ran away for a aileron trim system?

A trim tab in the aileron the size of a dollar bill is sufficient to handle any asymmetric load in the cockpit. It won't trim out full external fuel asymmetries, nor should it. The dollar bill sized tab is easily overcome at full deflection.

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Bob Highley
N711SH
SN 835
KLAL