Well after one very long X-country and another one in the planning stages I'm adding a trim tab to my Rudder before I go. I made one up and used 3/4" wide 3M double sided tape to attach it. I went to test flight and it was really gusty out up to 21kts. I lifted off almost instantly and something did not feel right at all. The Rudder felt stiff. I pulled the power and floated down the runway with the biggest handful of airplane I've had in a while. I did not stop chirping my tires till I was at slow walk of a speed. The damn thing would just not stop flying. Anyways note to self don't test fly with 12kt gusts up to 21. I quadruple checked that I was not getting any interference with the tab location. I talked to a guy on the field that is fairly knowledgeable and he said it was way too big so I cut it almost in half. The wind kept howling so I never did get to test fly it again. My theory is that it was big enough to add some surface to the rudder...could of just been the wind who knows. Normally those conditions would not have bothered me.

Does anybody have any input on this... does where it's located vertically make any difference? Cessnas put them on the very bottom. I wanted it under the elevator simply for visual reasons. What do you think of the size? Any rules of thumbs on like a percentage compared to the control surface size? Never measured it after I hacked on it but it's only about 3" x 2" now. It was probably about 7" long. Once I get it dialed in gonna just rattle can it white.

The initial tab I started with



After hacking it down





I also upgraded my 2" snap vents to 3 1/4 ones that can be rotated to be completely closed.

Tip: A travel coffee mug is exactly 3 1/4 OD on the top. I traced it onto cardboard, made a template, centered it over the existing hole and transcribed a line. I then just used a sanding wheel on my dremel and went after it. The instructions have you use a hole saw which is a pain and you will end up with a hole that is too big and your vents will rattle. Gotta love experimentals!

Avidflyer
If you would like to get rid of the sight of a trim tab you can attach one of those
small bungie cords to( Looks like you ride the right pedal)the right rudder cable
and make some kind of provision to attach the other end then fly and adjust tension
as needed , you can always over power it Works great! but if you like the tab set it low
To stay stay out of the elevator wash

Thanks Raven Flyer.. I use a bungee for pitch trim..one bungee in the cockpit is enough... Yes takes right rudder all the time..ball is about 3/4 a width off and it will noticeably yaw left if you put your feet on the floor...get's really annoying on a 3-4 hour leg...Last time I flew a 4 hour leg my right knee started to hurt really bad.

Thats a pretty big trim tab. For comparison sake here is a picture of the tab on my 170. You can hardly see it but it is the very trailing edge of the rudder that is not corrigated. Probably no larger than 2 square inches if that. A slight bend of 5 degrees or so dramatically affects flight. I would cut yours down in size. Maybe just make a vertical cut so it only sticks out less than an inch.

Joey, maybe think about rounding off the corners of that trim tab, or make a "half-moon" shaped one the same area.

Doing a preflight or working on the tailwheel one fine day, you might move your head the wrong way and that 'knife blade' can stick you right in the eye. It will complicate your AF medical for sure

Or, as many of our mothers said when we were kids... "Don't make that square trim tab like that - You'll put your EYE out!"

." My theory is that it was big enough to add some surface to the rudder...

Does anybody have any input on this... does where it's located vertically make any difference? Cessnas put them on the very bottom. I wanted it under the elevator simply for visual reasons. What do you think of the size? Any rules of thumbs on like a percentage compared to the control surface size? "

The size of the rudder trim needed will vary according to the speed rather than a percentage of the rudder surface. It takes less tab size to correct at cruise speed than it does at slower speeds even though the size of the rudder is the same. Since the rudder is a self balancing force, equal pressure on each side, all that is necessary is enough trim to change the position of where rests at cruise and where it again finds its new balance point. So really with a fixed tab, you only try to trim for one speed and it is most likely wrong for others unless you enjoy a lot of recreational engineering and testing.

The photo of the trim on the Cessna above puts it at the furthest point rearward on the curved tail. At that point the trim has the advantage of being at the longest leverage point and there it takes less deflection force to accomplish the same thing so it can be smaller there than at a higher location as the leverage would shorten. Your rudder is more equal shaped, being rectangular, so you don't have this consideration.
Normally the pressures on the two sides of your rectangular rudder are spread equally over the rudder surface, except for when you step on a pedal, then the pull is on the cable and there is torsional load through the hinges trying to twist the frame of the rudder to force the tail over. During normal flight after adding the trim tab there will be a constant slight torsional load on the rudder frame and hinges with the added force applied from the trim point, but it is a small enough force compared to any slipping maneuver you would do. The considerations for interference with the elevator, down wash or even taking your eye out are much bigger deals. So don't post any pictures where you have a patch over one eye, or both, like the girl in EZ flaps picture... or are those sunglasses?

AvidFlyer wrote:Tip: A travel coffee mug is exactly 3 1/4 OD on the top. I traced it onto cardboard, made a template, centered it over the existing hole and transcribed a line. I then just used a sanding wheel on my dremel and went after it. The instructions have you use a hole saw which is a pain and you will end up with a hole that is too big and your vents will rattle.

Ya know, that's also the perfect size to chuck empty beer bottles out too. Just sayin' is all....

Gump

I rig Cessna single engine airplanes -for cash , and ANYTHING you ADD to a control surface it adds DRAG ~~~~ ! Correct the original problem and these little "trim tabs" will be gone. Since were dealing with engine torque (Opposite of rotation) you could check Center line adjustment of engine . All Cessna's have a down and left adjustment-depending on model .Also your Vertical fin could be off center line. Also check your wings are in same plane -digital protractor required. Flight controls in trail for least amount of drag in straight and level flight hands free and feet on floor . I've found 1 wing paint vs rough on other side to be a factor .If you start with everything set 00 and add for engine torque you shouldn't need "trim tabs".

There is a way to get all this stuff ironed out the "old-school" way. Very appropriate on an experimental but illegal on certified probably. You can have an airplane that flies straight at all speeds and power settings, all it takes it time.

Set everything at zero, straighten out any warps or twists in the structure, rig all the controls straight, make sure there is no offset in the vertical fin, etc. etc. so the airplane glides perfectly straight power off with prop stopped.

Then adjust the angles of incidence, decalage, etc. until you have about the amount of stability and natural glide angle you want, still power off.

Then run the test glides again to make sure those pitch-related changes have not screwed up the straight glide.

Then finally adjust the engine thrust line up-down / right-left so it compensates for the engine torque and power "pitch-up".

In theory, this will give you an airplane that flies straight at any speed. The only thing yuou will need to adjust on a daily basis is the pitch trim to get whatever climb or cruise attitude you want.

Of course in order to do this right you have to have an old broken down "free flight" model airplane builder (not a modern R/C "semi-modeler") hanging around. Not many of us old-school trained modelers left these days

Most manufacturers found it easier and cheaper (man-hours) to put offset in the fin because it is easier to not have to adjust thrust line to fine tune the airplane's state of trim. But as dirtstrip pointed out, that can by definition only work at one speed and one power setting, so it's a bad bargain.

If you're lucky enough to be able to adjust thrust line easily on your engine, you can make it work really well with just spending time fiddling with it.

Drag from a tab is pretty miniscual when your talking about a plane that is about as aerodynamic as a brick and cruises at 80mph. This thing was hand built in someone's garage not in a factory on a jig. I'm sure if I took it all apart and spent hours upon hours of tweaking on it I could get it to fly straight...or I could spend 20 minutes to slap this tab on and go fly for all those hours and hours Thanks for all advice.

The lighter the plane the less it takes to throw it off. The slower the plane the less drag "being off" will create. I think that's why people choose to put on the tab to make it fly straight if its a low and slow there is not much penalty for the tab. If its an easy fix with the adjustments on the centerline or you find unequal rigging say, one flap slightly lower or mismatched tires then by all means correct it, but if you have something really involved structurally, then go for the trim and live with the little bit of extra drag.

I have a question for someone who cares to answer. In the case of an engine with a non adjustable centerline, is it possible to achieve the same result by placing a shim under the rear leg of one side of the engine mount to tip the centerline to the right or left, then shim at the rear upper mount on the same side to match the change? This is a theoretical question and not intended as a suggestion.

dirtstrip wrote: is it possible to achieve the same result by placing a shim under the rear leg of one side of the engine mount to tip the centerline to the right or left,

I believe this is possible and has been done.

However, there is a very limited amount of shimming you can do in this area, before the axis of the bolt hole in the engine mount... and the axis of the bolt hole in the airframe... start to get misaligned too much to re-install the bolts.

We tried to re-install an engine mount on a Taylorcraft, and even without ANY shims it was difficult to get the bolts to go in. There was hammering and cursing and holding tension on the engine mount and goobered up threads on the bolts. So if we had put in a few degrees worth of shims, I think it would have been a real pain to get the mount back on. I'm sure the bolts would have been curved and bent a little, which is not so good for the structural integrity of the engine mounting.

Hey AvidFlyer,

You can spend days going through all your engine & airframe rigging but I agree with you that it's not needed on a lightweight, slow and dirty airplane. And, yes your tab is way too big.

What we do on some of the Rans I played with including my on S-7 is just take a plain old soda straw, about 6 inches long and glue it on the rudder near the trailing edge with with a little contact cement. That way you can remove it and experiment with just how much you need. We've found that sometimes as little as a 6 inch coffee stir straw was enough for small trim adjustments, it doesn't take much.

Most people just paint the straw and just glue in on but if you want a tab it'll give you some idea of the size you'll need. Also the small bungee on the rudder pedal idea will work also but it'll change as the bungee wears.

I've noticed some folks use a small tapered wedge fastened to one side of the rudder, instead of a trailing tab. Sorta like Baja's soda straw. Just experiment with different thickness of wedge until you get the effect you want. I agree, the added drag of a rudder trim tab nudging the rudder over is not a big deal on a 100 knot or less airplane. My C150/150 was factory built to fly correctly behind a 100 horse engine. With the 150 on the nose, it cruises about 20 mph faster & had quite a bit more p-factor. It needs quite a tweek on the rudder trim tab to fly straight. I also shimmed the engine mount at the firewall to change the thrust line, both left-right and up-down. Flies great now.

Dirtstrip,
Yes you can, have you ever looked at the engine off set of an Aercoupe ? the engine is pointing way off center they do this due to lack of rudder pedals .
But I have to agree with easyflap you dont want to do that.
On the Raven I built in right hand off set in the vert. fin in cruse you can take your feet off the pedals and the ball stays in the center .

Bill! Didn't know you were on here - how's the S7?

Joey, go to the hobby store and by a little balsa strip used for trailing edges on RC planes. Attach that like Bajapilot suggests with the straw. its a common fix as well. Then you can also just paint it white!

Ended up with about 1" extending past the rudder edge and it's about 3" long. The bend is about 30 degrees from center. It was bumpy out while I was playing with it but it's definitly better. I'll fine tune it on my X-country to Idaho on Friday. Thanks for all the replies

ravenflyer33 wrote:.... have you ever looked at the engine off set of an Aercoupe ? the engine is pointing way off center they do this due to lack of rudder pedals ......

I never noticed that looking at Ercoupes on the flight line, but I walked by one uncowled in the local mechanic's shop & it's very obvious that the engine is WAY offset.