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Rudder Trim Geometry Question

UnPossible

UnPossible

Well Known Member
Hey- having fun figuring out how to get the cutout for rudder trim to work.
I am having a small issue with the geometry. I am using the RA servo with the shortest throw.

However, in order to get this to work, the control horn needs to be pretty tall. If I shorten it, the rod runs out of travel in slot in the skin.

I can't extend the slot any further, as it will hit the rib I made to stiffen the opening.

Am I missing anything obvious?

One more questions, I'd like to install a fairing over the slot and rod, but the Avery rudder fairings aren't tall enough... any ideas where I could find a taller version? Worse case, I'll fabricate one out of AL or fiberglass.

Below are a couple of pics.....

Thanks,
Jason

2edxh06.jpg


1pi1c3.jpg
 
Jason
From your pictures it looks your problem is where the pivot point is on the trim surface.
You need To move your trim horn pivot point as close as possible to the hinge center line.
It is to far back from that pivot point. This should help

Geoff
 
Rudder Trim

Hello Jason, I did the exact same rudder trim that you are doing. I also put on one of the Avery fairings. To make it work, it was cut back a little bit so give the push rod some more travel. Also put a slight bend in the pushrod to keep it from hitting the skin.

One thing to remember, the trip tab really only needs to go to the left so you can adjust your travel from center and to the left.

Been flying with it for about 70 hours now and have no issues, works great.

Here is a link to my pics
https://brian-steeves.squarespace.com/rudder-trim/

Brian
 
Aerosport1 said:
Jason
From your pictures it looks your problem is where the pivot point is on the trim surface.
You need To move your trim horn pivot point as close as possible to the hinge center line.
It is to far back from that pivot point. This should help

Geoff
Click to expand...

Exactly.......

Here is mine, note the push rod clevis pivot is right over the hinge line.

PB030048.jpg
 
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flion said:
Not only that, but with the hinge on the left side, your gap on the right will grow as you apply more trim. When I realized this, I rebuilt my rudder so the hinge was on the right. My control horn is on the inside of the tab so it protrudes on the left side only about 3/16", with a much shorter slot and easy to fair. I don't have a picture yet, because I just rehung the rudder and haven't made a new horn and finished the linkage. But below is a picture of the original tab showing how low the protrusion is. Also note that I carved foam ribs and spars for the tab, to be glued in similar to the elevator trims and then surface glassed and finished later. This horn geometry (through the surface) is used in several R/C sailplane designs I have flown; it is a popular way to reduce drag and bury linkages.

DSC01071.jpg
Click to expand...

By the way, I know I have way too much throw. I will depend on electronics for rate and throw limiting. Flight test, too, will be approached cautiously; I have trimmed enough R/C models, not to mention my -6A, to know how to approach new control systems. Most competition R/C sailplanes have extreme throw for slow flight and reduced rate/throw for speed runs, electronically controlled through the system, so the technique is nothing new.
 
True. Mine were cut from the original skin with the idea that the tab would swing to the right, so I didn't have enough tab skin to roll under like that. Besides, it would interfere with my horn. Now, with the orientation correct, the tab actually closes the gap in mine and a piece of flexible gap seal on the rudder at the tab will completely cover it. You did a very nice job there, Mike.
 
flion said:
You did a very nice job there, Mike.
Click to expand...

Thanks, but Randy Thorne (SportAirServ here at VAF) really deserves most of the credit----he is the one who actually cut and bent the metal, I only designed it and told Randy what I wanted.

Tom Brink (GAHCO) did his aileron trim tab like this, and I "borrowed" the detail from an old photo in his build thread. http://www.vansairforce.com/community/showthread.php?t=14993

Not much new under the sun, lots of good ideas get re-cycled however:D
 
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Rudder trim

For those who build the rudder trim do you cut it out before riveting the rudder together or while it's clecoed? My concern is that I think the trailing might not come out straight when you do it before trailing edge is glued and riveted
 
[email protected] said:
For those who build the rudder trim do you cut it out before riveting the rudder together or while it's clecoed? My concern is that I think the trailing might not come out straight when you do it before trailing edge is glued and riveted
Click to expand...

The other option is to install a spring based rudder trim, like the one from Aerosport Products. It's much easier to install and less complex. I've found that once I've trimmed it for cruise, I seldom have to change it from one flight to another. On landing and take off, you can easily over power the springs with your feet.
 
rleffler said:
The other option is to install a spring based rudder trim, like the one from Aerosport Products. It's much easier to install and less complex. I've found that once I've trimmed it for cruise, I seldom have to change it from one flight to another. On landing and take off, you can easily over power the springs with your feet.
Click to expand...

Hi Bob

You have a point. From a practical point of view for the guys who did install the aero rudder trim spring system versus the servo operated rudder trim, whats the pros and cons apart from the work involved?

Maybe the guys who build the servo operated system can chirp in here. My plan is to use control approach rudder pedals. Will the spring system work with mentioned pedals?
 
Control approach rudder pedals

Are you sure you can get the aftermarket rudder pedals? Last time i spoke with Paul, they were not available and he was working on a new version. That was around december of last year, and i have heard nothing further...

You might want to check...
 
Mike and Patrick - looking at your pictures I have a quick question about your rudder trim tabs...

Did you use some sort of a rib at the front edge of the tab to keep the correct shape? I am not seeing any rivet holes at the forward end of the tab. I have spent hours making one the is the correct shape and am now afraid that there is not enough space on the narrow end for the rivets, unless I make a new rib and stagger the rivets front to back.

Thanks,
Jason
 
There are ribs top and bottom of the tab, and the open ends of the cutout for the tab.

They are made of phenolic board, and glued in with proseal. The leading edge of the tab is open, the bend on the right side stiffens the R. skin, and the hinge stiffens the L. skin.

The opening in the rudder for the tab has a "C" channel stiffener riveted in.

Look at the photo, you can see the brown of the phenolic board, and the rivets holding the "C" channel ahead of the tab.

PB030049.jpg


You could also use a setup like Tom did in the linked thread I posted earlier, where he has a center spar, and no ribs------or do both if you want.

Mine has been flawless for 3 1/2 years now, 170ish hours.
 
I'm using foam ribs and spars, glued in with proseal like the elevator trim ribs. I'll finish by sealing the foam with microballoons and epoxy before painting. The same technique will close the opening in the rudder, except for the cutout for the linkage.
 
[email protected] said:
Hi Bob

You have a point. From a practical point of view for the guys who did install the aero rudder trim spring system versus the servo operated rudder trim, whats the pros and cons apart from the work involved?

Maybe the guys who build the servo operated system can chirp in here. My plan is to use control approach rudder pedals. Will the spring system work with mentioned pedals?
Click to expand...

It's been a long time since I've seen Paul's pedals. I suspect they will if the tops are the same as Vans on the cross bars.

If you go to the Aerosport Products page, there is now a video installation posted that will show you how they are installed.
 
rleffler said:
It's been a long time since I've seen Paul's pedals. I suspect they will if the tops are the same as Vans on the cross bars.

If you go to the Aerosport Products page, there is now a video installation posted that will show you how they are installed.
Click to expand...

I spoke to Paul at control approach and he's working on new design which should come available in 2014. The point I'm trying to make is it worth the work for construction or does the spring system does the same job? Not interessted in the wedge as it's for 1 speed. The servo operated one will obviously work on a switch either on the panel or on the stick. Never seen a stick with 3 trim buttons for ail, ele and rudder. The spring system can be quickly adjusted specially for final approach...
 
[email protected] said:
I spoke to Paul at control approach and he's working on new design which should come available in 2014. The point I'm trying to make is it worth the work for construction or does the spring system does the same job? Not interessted in the wedge as it's for 1 speed. The servo operated one will obviously work on a switch either on the panel or on the stick. Never seen a stick with 3 trim buttons for ail, ele and rudder. The spring system can be quickly adjusted specially for final approach...
Click to expand...

They both accomplish the same thing, which is centering the ball. They just do it in different ways. The spring based system can be installed in under an hour, even in a flying aircraft. No cutting or fabricating required.
 
[email protected] said:
Never seen a stick with 3 trim buttons for ail, ele and rudder. The spring system can be quickly adjusted specially for final approach...
Click to expand...

The Tosten grip could be easily setup to run 3 trims. Cooley hat does two, and top side buttons would do the other one. I do not have aileron trim, so my Cooley had does elevator and rudder, my other buttons on top are for xpndr squawk, and Sl 30 freq flip flop. It would be simple to use Cooley hat for elevator and aileron, and buttons for rudder.

cs6.jpg


As to adjusting the spring trim system on final----------I fly final, in fact, almost the entire pattern with one hand on the stick, and the other on the throttle, dont have a third hand to twist a knob for the rudder trim. I even located my flap switch so I could thumb it while my hand was on the throttle.

I also dont normally need to adjust rudder trim on final.............
 
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One nice advantage with the spring bias trim is it keeps the rudder from
slapping around in the wind while parked. It really works well for that and in the air.
So it offers another advantage over the electronic/servo operated trim tab.
As for all 3 options I would say the spring bias offers the most advantages.
I have a trim tab like Mike does on my 10 and I tested the spring bias for 2 months.
I really missed the spring bias when on the ground and it did worked great in the air. However I really like the electronic one a little better in the air. Either one works great but you get more bang with the spring bias and way easier to install.

FWIW
Geoff
 
Geoff, I haven't followed all the discussion and may have missed it, but why could you not use some type of motor to run the biased springs thus being able to operate the trim via a switch on the stick?
 
It's been a while but I finally got back out to the hangar and cut my trim tab free and installed the linkage. It's not quite finished but I've got a couple of good pictures of the tab including one that shows the linkage at the buried control horn. I don't think I mentioned anywhere before, but I am using a T2-7A servo so that the short throw helps compensate for the short arm length of the control horn. It works well and moves at a reasonable speed, though I am prepared to compensate further with a rate limiter. Techniques learned from R/C sailplanes are paying off.

Here's the link to my KitLog entry with some pictures: link
 
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