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Rudder trim tab ideas seeked

G

GyroRon

Active Member
At cruise speeds in my RV4, I have to hold a slight pressure on the right rudder pedal to keep my ball centered. If I let off with my foot, the ball is at least one full ball over to the right, possibly slightly more than one ball. If I come down on the RPM's and let the plane slow down to 100-120 mph or slower the ball is much closer to being centered.

So I want to put a trim tab on it. I have some sheet aluminum to use, two pieces, one .030 thick and another a bit thinner.

How big of a tab should I fashion, how much bend should I put in it, and how should I install it to the rudder?

My plane is also slightly left wing heavy, so I'll need to fool with that too, but will try adjusting flaps and or ailerons or possibly squeezing the trailing edge first before putting a tab on a aileron. Will likely start a new post when I get to that point.

Thanks
 
I installed an adjustable rudder trim. Forget who made it. It pulls on the rudder pedals.
 
If you're gonna go the route of a fixed tab, go to your local hobby shop and buy something like this:

https://sigmfg.com/products/sigb097-balsa-1-8-x-1-2-x-36-trailing-edge?variant=12305556996174


Cut a 6" section and tape it to the left side of your rudder with the thick part lined up with the trailing edge of your rudder. Experiment with the length to get the ball centered, then remove the wedge, cover it with monokote (or just plain 'ol sign shop vinyl) of the proper color and proseal it to your rudder.
 
Rudder trim tab.....

GyroRon said:
At cruise speeds in my RV4, I have to hold a slight pressure on the right rudder pedal to keep my ball centered. If I let off with my foot, the ball is at least one full ball over to the right, possibly slightly more than one ball. If I come down on the RPM's and let the plane slow down to 100-120 mph or slower the ball is much closer to being centered.
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If you search rudder trim you'll likely find a multitude of ideas on what to do. First thing is to find out how much trim you'll need. Easy way is to fashion a wedge of balsa wood or even pine; something easy to work. Start with a piece 6 inches long or so, thin at the leading edge and, oh, say half inch wide at the trailing edge. That will likely be too much but you will be trimming it down to get the right amount of trim as you go along. Duct tape it in place and go fly. Cruise speed is the place you want good trim as that is where you will notice the need for it. But you can try it at different speeds. Take it off; trim it down, if needed, put it back on and go fly again.

This was my method. Mine is a piece of aluminum bar that I blind-riveted in place. It turned out to be exactly the right shape. Feet off; ball centered!

My plane is also slightly left wing heavy, so I'll need to fool with that too, but will try adjusting flaps and or ailerons or possibly squeezing the trailing edge first before putting a tab on a aileron. Will likely start a new post when I get to that point.
Click to expand...

Search heavy wing for advice as to how to pinch the proper aileron. Usually that will fix it. It may be the flaps, but usually the ailerons are what are in need of adjustment.

Good luck! It means you get to go flying more....:)
 

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Presumably you have already checked other candidates - like, are the wheel and gear leg fairings all properly aligned?
 
I agree with Bob. Is everything else correct? What's making it fly like that? Some people get the fairings aligned with the plane on its gear, but Van's has very clear instructions and guidelines on how to do this properly with the weight off the gear--in a flying configuration, that is. They even mention something like a 1/4" misalignment of a gear leg fairing can throw it off by half a ball width. Or something along those lines.

Of course, you can do rudder trim, and many, many RV's have them. But a full ball sounds like a lot. You may gain a little speed by finding the true culprit.
 
D-Dubya said:
I agree with Bob. Is everything else correct? What's making it fly like that? Some people get the fairings aligned with the plane on its gear, but Van's has very clear instructions and guidelines on how to do this properly with the weight off the gear--in a flying configuration, that is. They even mention something like a 1/4" misalignment of a gear leg fairing can throw it off by half a ball width. Or something along those lines.

Of course, you can do rudder trim, and many, many RV's have them. But a full ball sounds like a lot. You may gain a little speed by finding the true culprit.
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I have no idea why it isn't flying straight at the moment. From the sounds of it, it seems pretty common to have to have some sort of trim tab on the rudder of all RV's, mine has nothing back there now.

Just wanted to see if cutting out a 4-6 inch piece of aluminum sheet and bending it and riveting it to the rudder skin would be ok. Sounds like I should fool around with wedges and stuff first to see if that will work.
 
Hmmm....Picture with the tail-light "trim-tab" is interesting. Would it act to force the rudder left or right? It could act as a simple deflector pushing the rudder left OR it could act the way flaps work, creating (camber) lift, and forcing the rudder right. Could it be acting like a Gurney flap?
 
No - I’d say RVs with rudder trim tabs are the exception, not the rule.

As others have pointed out, first eliminate the other rigging issues. For example a couple of data runs with the wheel pants removed is a good place to start. After that look at the other common rigging issues like:
- Flaps not all the way up and/or different side to side.
- Heavy wing - as in one or both ailerons hanging low/high, etc. Squeezing the aileron trailing edge is the absolute last step in addressing a heavy wing as the other common problems cannot be fixed by squeezing.

Rigging is not “one and done”.

Carl
 
Carl Froehlich said:
No - I’d say RVs with rudder trim tabs are the exception, not the rule.


Carl
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Versions RV-7 and newer had offset mountings of the V-Stab. The RV-6 and earlier DON'T. I wish I had known this when I build my -6A.

this fellow has an RV-4

Yes, check all of the things, however rudder tabs on the -6 and earlier are common.
 
GyroRon said:
Just wanted to see if cutting out a 4-6 inch piece of aluminum sheet and bending it and riveting it to the rudder skin would be ok. Sounds like I should fool around with wedges and stuff first to see if that will work.
Click to expand...

Do remember additional mass back there is generally pro-flutter. No mass balance on your -4, right?

Internal trim adds no control surface inertia. I use a bungee to bias a rudder pedal with a little pressure.
 
helping hand?

DanH said:
Do remember additional mass back there is generally pro-flutter. No mass balance on your -4, right?

Internal trim adds no control surface inertia. I use a bungee to bias a rudder pedal with a little pressure.
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OK so if you are using a spring or bungee to trim the rudder, does this also help with yaw stability in cruise, as opposed to just having ones feet off the pedals with the rudder is free to weather vane?
 
Mconner7 said:
Some folks use a small wedge. Tape it on to test size before mounting it permanently.
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+1 Yep a small wedge, aft edge flush with rudder trailing edge. The length, thickness, width can very. Double sticky tape. Make different sizes. I used balsa wood. Once I got it sorted I wrapped it in one thin layer of fibrglass, primed, and bonded on with epoxy. Once plane was painted it was invisible. Wedge can be made from metal, 3D printer (plastic), wood, foam fiberglass.

The flat peice of metal (0.032) bonded on extending past trailing edge can be bent as needed. It works well but more visible.
 
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