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Best way to trim the rudder?

R

RV-9 Yellow Bird

I'm New Here
At cruise, my RV-9 flies hands off with the ball about half a diameter to the left, requiring left rudder to center it. I'd appreciate some guidance on how big of a wedge I need to glue on the rudder to correct it, and if somebody knows where I can get a plastic wedge kit rather than using a wood wedge.

Thanks,
Brian RV-9
 
About 3/4 of one of these placed midpoint on the rudder addressed my trim needs when the ball was 1/2 out:
http://www.averytools.com/prodinfo.asp?number=9883. Hurry though b/c Avery is closing things up pretty quick.

I recently moved my wedge down to the bottom of the rudder where it is less obtrusive visually. However, since it is probably now partially blanketed by the HS and elevators, I find that I probably need a bit more of the wedge.

Long and short, decide where you want to locate yours and then just trim a little at a time until you find the right size to center the ball.

Good luck.
 
Rudder trim wedge mounting method

To mount the wedge for determining its size, use scotch 3M temporary double side poster tape, the thin clear stuff just like Scotch Tape. It is strong enough to secure the wedge while you figure out how big it must be and is easily removeable for adjustments. Use new tape each time. Once you have finalized the wedge size, switch to the permanent Scotch 3M tape. It sounds hokie but mine has been on there for over 8 years using this tape.
This tape is very thin so the wedge fits flush to the rudder. Cleanliness is important and the area must be free of wax.
 
RV-9 Yellow Bird said:
At cruise, my RV-9 flies hands off with the ball about half a diameter to the left, requiring left rudder to center it. I'd appreciate some guidance on how big of a wedge I need to glue on the rudder to correct it, and if somebody knows where I can get a plastic wedge kit rather than using a wood wedge.

Thanks,
Brian RV-9
Click to expand...

Installing a trim tab on the rudder is treating the symptom rather than treating the cause of the problem. If the skid ball is out you obviously have a rigging problem that is causing asymmetrical drag and hence yaw. I recommend you start by taking a good look at your wheel and gear leg fairings to make sure they are in trail. It has been reported than an error of just 1/8" in the trailing edge of a gear leg fairing can cause a half scale deflection of the skid ball.
 
Another vote here for the Avery rudder wedge approach. Cheap, easy, and effective. :). Mine ended up as noted above ... About 3/4 of the original wedge length located mid span.
 
Gurney flap

Wow! Dan's a shirttail relation and I never knew he did that. Sounds worth a try on my left flap lower (?) surface to correct a 1/3 right ball in cruise. Has anyone on the site given this a try?
 
Bill Dicus said:
Wow! Dan's a shirttail relation and I never knew he did that. Sounds worth a try on my left flap lower (?) surface to correct a 1/3 right ball in cruise. Has anyone on the site given this a try?
Click to expand...

Small world, the person who named it married an engineer who worked for me... :)
 
Gurney flap

Thanks Gil - neat to hear. My maternal aunt was Dan's mother's sister. Got to meet him only once but he is memorable. His accomplishments as a driver, builder and innovator are amazing. Interesting - his dad was a baritone with the Metropolitan Opera, reputed to do limericks and great jokes non-stop for 48 hours; quite a raconteur! Sorry - this is really peripheral. Hope someone enjoys.
 
Use of Gurney Flap

A -4 in Santa Paula (SZP) is using one now on his rudder - very effective, and not very big.

HFS
 
Before you do anything to the rudder, remove the wheel pants and gear leg fairings. Then go fly and see if it flies with the ball centered.

If it does flight with the ball centered, then the issue is with the alignment of those fairings and wheel pants.

If it still flies 1/2 a ball out, check the wing tips, ailerons, and flaps.

If after all that it still doesn't flight straight, then play with the rudder.
 
N941WR said:
Before you do anything to the rudder, remove the wheel pants and gear leg fairings. Then go fly and see if it flies with the ball centered.

If it does flight with the ball centered, then the issue is with the alignment of those fairings and wheel pants.

If it still flies 1/2 a ball out, check the wing tips, ailerons, and flaps.

If after all that it still doesn't flight straight, then play with the rudder.
Click to expand...

Top advice from Bill. A trim tab on an RV rudder is like a sign on the plane that says: "THIS AIRCRAFT HAS NOT BEEN BUILT PROPERLY".
 
The left/right alignment of the vertical stabilizer is critical, it would take VERY LITTLE to be off to require some rudder trim. It would be very easy to inadvertently build in a little skid......
 
Trim

Thanks for information on the -4 using a Gurney flap. If you have any details that come up in the future please let me know. Thanks.
 
Captain Avgas said:
Top advice from Bill. A trim tab on an RV rudder is like a sign on the plane that says: "THIS AIRCRAFT HAS NOT BEEN BUILT PROPERLY".
Click to expand...

Darn it, there is a lot of certified airplanes out there that must not be built properly.
Better get Cessna, Beech, Piper, Boeing, etc. on the phone and let them know.
 
Far North Texan said:
Darn it, there is a lot of certified airplanes out there that must not be built properly.
Better get Cessna, Beech, Piper, Boeing, etc. on the phone and let them know.
Click to expand...

I will reiterate what I said again. A trim tab on the rudder of an RV is a sure sign that the the plane is poorly rigged and is experiencing asymmetrical drag. What else could it mean. :rolleyes:

The problem with a rudder trim tab is that you are simply introducing more "equal and opposite" drag to counter the asymmetrical drag you already have. And adding drag to counter drag just robs you of speed. It's not an ideal situation but it appeals to people who are looking for a quick and dirty fix.

Bill Repucci's advice is still the best plan proposed. Take off the wheel/gear fairings and see if that fixes the problem. If it does then the OP will know where the problem lies and can address it in the correct manner rather than adopting the bandaid approach of using a trim tab.
 
Far North Texan said:
Darn it, there is a lot of certified airplanes out there that must not be built properly.
Better get Cessna, Beech, Piper, Boeing, etc. on the phone and let them know.
Click to expand...

Have you ever seen how Cessna, Beech, and Piper built planes? (The same probably goes for the early Boeings too.)

They have (or had) big templates that they would lay over the raw aluminum, trace it and use it as a hole template. They were built very much like the RV-3's, 4's, and 6's, in a jig and a lot shimming.

The match hold technology that is used in the -7, -8 (later 8's), -9, -10, -12, and -14 make it virtually impossible to build an airframe that isn't straight.

Production techniques for certified aluminum airplanes may have changed and I hope they have but most of those planes flying were built with a lot of craftsmanship and not much technology.
 
Captain Avgas said:
The problem with a rudder trim tab is that you are simply introducing more "equal and opposite" drag to counter the asymmetrical drag you already have. And adding drag to counter drag just robs you of speed. It's not an ideal situation but it appeals to people who are looking for a quick and dirty fix.
Click to expand...

No airframe is perfect and by moving gear leg fairings, wheel pants, flap positions, wing tips, etc.. to correct the yaw in flight is just hiding the correction that an external trim tab provides. Now if you actually find one of the listed items actually out of the proper position that is one thing, but to just move them around until you don't need a trim tab is just hiding the fact. A trim tab does not mean the aircraft is unsafe or even built out of tolerance. Staying within the allowed plans tolerances over the length of an RV aircraft could require a tab to correct for straight flight even thought the aircraft is built per print.

I don't have an issue if someone wants to spend their time rebuilding or reglassing fiberglass parts to eliminate a small trim tab on the rudder, however in my opinion I have elected to add a safe trim tab to my rudder and enjoy flying. Life is too short for me to stay in the building and tweaking phase forever.
 
curtis said:
No airframe is perfect and by moving gear leg fairings, wheel pants, flap positions, wing tips, etc.. to correct the yaw in flight is just hiding the correction that an external trim tab provides. Now if you actually find one of the listed items actually out of the proper position that is one thing, but to just move them around until you don't need a trim tab is just hiding the fact.
Click to expand...

No-one is suggesting that the OP just move his gear fairings around randomly to try to centre the ball. The proposal recommended is a more scientific approach.

Firstly the fairings are removed and the aircraft is flown. That's a pretty easy thing to do. If the ball moves to centre that is a clear indication that one or more of the gear fairings are mis-rigged. In that event the solution is to re-rig the gear fairing(s) to put them into correct trail. Problem solved.

I don't think a rudder trim tab is the end of the world but nor do I think it is an elegant or aerodynamically efficient solution to an obvious misrigging problem.
 
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I agree that flying your aircraft with the fairings removed is easy to do, but that is where the easy portion ends. If you already have drilled the attachment holes for both the wheel pants and gear leg intersection fairings you cannot just reposition them, you have to glass the holes shut and then guess where to put the new attachment holes. My statement to "guess" where to move the fairings is an assumption that they were located correctly to begin with. If one was to find a wheel pant or fairing that was not streamlined after rechecking this would be an easy fix, but if there isn't no "smoking gun" then it is more difficult to figure out which component to rework.

The only fairing that can be easily moved or adjusted without any major rework is the lower portion of the NLG fairing, which can be accomplished by loosening the lower hose clamp and twisting the fairing either to the left or right. I know this because I reinstalled my NLG lower fairing clamp first and then didn't notice when I was moving the upper intersection fairing so I could install the screws I was putting a slight twist into the fairing. This caused me a little head scratching to figure out why doing an oil change caused my ball to not be centered.:eek:
 
When and if you need a rudder trim, trying this idea from Tom Albano:

Rudder%252520trim%252520-%252520Tom%252520Albano.jpeg


(Posted with Tom's permission.)
 
Any issues with it being attached to the plastic?

N941WR said:
When and if you need a rudder trim, trying this idea from Tom Albano:

Rudder%252520trim%252520-%252520Tom%252520Albano.jpeg


(Posted with Tom's permission.)
Click to expand...

This looks like an easy, adjustable solution. How long has it been on? Any issues with it pulling the light loose and stripping out the plastic?

JK
 
Captain Avgas said:
Top advice from Bill. A trim tab on an RV rudder is like a sign on the plane that says: "THIS AIRCRAFT HAS NOT BEEN BUILT PROPERLY".
Click to expand...

Just saw a brand new Cirrus latest generation with a trim tab on the rudder. Must not have been built correctly?
 
I haven't taken any survey but IMHO there are a lot of RV's flying out there that don't fly hands off because the builder doesn't want to put a trim tab on his RV that he spent years building to painstaking detail. Most RV's will fly hands off limping along at 100 Kts but crank it up to cruise speed and a wing might start to get heavy. Put the trim tab on and be happy. Obviously you have to check all the easy stuff first per Vans heavy wing fix it sheet instructions.

Jim
RV9a
 
I didn't want a trim tab in my rudder so I installed one gear leg fairing slight misaligned to center the ball. No one knows until I tell them.

Don
 
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