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Trim and Jack Screw

vj88

Registered User
Anchorage, AK
Hello,

I have a couple questions regarding the stabilizer jack screw assembly on an experimental pa-18 build. Do Javron's and Dakota's jack screw have the same amount of trim adjustment or does one have more? Has anyone reversed/flipped the assembly to get more trim in the opposite direction? Does anyone have weights of the Javron and Dakota assembly?
 
The yoke of both look the same and should give you more nose down trim than stock. Someone posted the thread pitch on the Dakota was different than stock and it took a few more turns, not sure if Javron is the same. In the days before belly pods more nose down trim was required due to the heavy loads and rear CG. The loads are still big but CG is better with a belly pod. What are you looking to change, nose up, nose down, or total travel?
DENNY
 
Dakota Cub uses an Acme thread pitch which is a finer thread. I don't see where you can get any more trim adjustment with a different jack screw, the fuselage frame is the limiting factor. Found plenty of factory Super Cubs that would not meet the TCDS for nose up and down trim. Something to pay attention to at rebuild or build.
 
The original Yoke had a nubbin on top, not flat like the new ones. If it go's away good for 1 1/2 turns or so of trim.
DENNY
 
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Im looking to at maximizing trim. I've heard several "rumors" that I asked above and was looking to substantiate it one way or another. Thank you
 
I could write a long post about trim, trim range, linear actuators, modified yokes, CG's relationship to trim, etc. But that's based on my plane. Your plane has a better empirical knowledge base to establish the required trim range. What's the motivation to change it? Are other planes running out of trim?

One trim mod worth considering is electric trim. Adjusting trim with my thumb on the stick is handy. The mod of the century? Probably not, but I don't miss spinning the trim handle.
 
My airplane when loaded at gross requires almost all trim down. I would position the yoke to allow for max pitch down if that is at all possible. When flying lite the trim position is still beyond the forward center.
What fuel qnt? All has an effect. I'm not sure if there are PA 18s which suffer from the opposite trim condition. Heavy power plant would have an effect I suppose. I recall when building it was a question as to which direction the yoke went. I've seen them installed both directions.
 
Like Steve said the key is to set the plane up and measure everything while you are doing your build. Unless you have some radical cub follow the TCDS. Where did you get your airframe? Are you planning on a belly pod?
DENNY
 
I bought my yoke and jack screw from Javron; he also uses ACME thread pitch. Piper used a very different pitch and Jay liked using standard ACME threaded rod. He machines it to fit the adapter for his electric trim. Nicely done work by Jay.
Marty
 
As long as we're talking -- don't mix and match components. They might not be compatible. I found the Dakota trim gear and yoke, way better than stock, less shake and a nicer product. Every plane is different, most banner planes don't have enough nose up trim.
 
Go talk to Kirk Ellis, he is on the South Mountain side of birchwood. He has more experience than anyone with that airframe and can tell you if stock angles and dangles work.
DENNY
 
I've used dakota Cubs jack screw on at least 4 rebuilds and have gone back to stock/Univair screw and yolk. Dakotas are to perfect and can make it difficult to get a smooth working trim at cruse power, then if you get any gunk in it it binds up.
I have had that issue when installing the Dakota Cub jack screw assembly on 2 flying Super Cubs. Turned out both were tweaked in the tail. Have used them on the last 4-5 rebuilds with no issues. Rebuilt a friends frame and it bound but seemed like it was rectified when the tail wires were rigged. I want the jack screw assembly to last till the next recover and have found the billet, through hardened DC yoke with Acme threads far superior the the cast yoke original style. Steve Conatster in Fairbanks was going through one a season until he switched, last I talked to him he had 4-5 season on his.
 
The binding I had was noted on the ground so never made it in the air till rectified. Have had J3s and Pacers with issues in the air but never a Super Cub.
 
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I tried to swap out a Dakota on my cub when it was covered and it didn’t work. The tail was twisted. That’s the main reason I rebuilt it. The trim didn’t go all the way. When I rebuilt it I put Dakota in, and the oil lite bearings up in the handle. It works really smoothly now. I don’t put any grease on the jack screws. Any style. Dug old solid grease out of too many airplanes


Sent from my iPhone using Tapatalk
 
Go talk to Kirk Ellis, he is on the South Mountain side of birchwood. He has more experience than anyone with that airframe and can tell you if stock angles and dangles work.
DENNY

Kirk is the #1 guy for any questions on a four place.

Web
 
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Yes, the bungee cable mount has been changed to the same position as stck and the forward face that was thicker is now the same size as the original. Mark has always been receptive to my comments and criticisms.

Hi Steve.

That is good to hear.
 
So you can't get more trim with a new jackscrew assembly from Javron or Dakota? I run out of nose down trim even before I get to 2000lbs. PA-18 experimental. AOI changed to 1.84 I believe and tail was adjusted to proper angles but I'm still running out of nose down trim often.
 
So you can't get more trim with a new jackscrew assembly from Javron or Dakota? I run out of nose down trim even before I get to 2000lbs. PA-18 experimental. AOI changed to 1.84 I believe and tail was adjusted to proper angles but I'm still running out of nose down trim often.

Picture of your jack screw installation please


Sent from my iPhone using SuperCub.Org mobile app
 
It would also be nice to know if the elevator is resting on the down stop When you have full nose down trim cranked in. Sometimes if the bungee is too short, it deflects the elevator up which requires more nose down trim. You can run out of nose down trim pretty easily if this is the situation.
 

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Adam, easy to tell. Just start cranking nose down Trim in, when the stick stops moving forward the elevator should be on its stop. Then you go back to the tail And physically see if the elevator is on the down stop. Should hit the stop when you still have 4 to 6 Turns left nose down.
 
That cable runs below the brown pulley guard? Is that what you noted? It's been some time but isn't that for another trim spring on floats or?
 

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