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Fitting the Rudder

Here's a great article written by Roger Isackson on rudder assembly. You can download an Adobe Acrobat file of the same article by clicking the following title link.

Fitting the Rudder, by Roger Isackson.

Dear Seawind Builders,

In a recent conversation with Mike Pastelak of Seawind, he told me a control surface, such as a rudder, should extend out beyond the stationary surface in front of it by 1/8 inch. This is so that when the laminar flow of air jumps the gap between the stationary surface (in this case the vertical tail) and the control surface (rudder) it "bumps" into the control surface and re-attaches as a laminar flow. If this relationship does not exist, a turbulent flow begins to spread out over the control surface, and actually out into the slipstream, and creates much drag and loss of control surface effectiveness. I read in the June 2000 Kitplanes magazine, in the Designers Notebook section, that drag could actually be increased by 10% because of this. The Seawind construction manual does not address this fact.

Mike suggested I dry-fit the rudder on the aircraft instead of on the bench as the manual instructs. That is how the rudder is assembled on the high performance Lancair. I know the Seawind is not nearly as high performance as the Lancair, but I want to get as much performance, and as much effectiveness out of my control surfaces, as possible.

To this end, here is how I proceeded:

I first installed the right half of the rudder on the tail. This was not as easy as I just made it sound, because there is a lot of excess material on the trailing edge of the tail. Do not trim to the scribe line or you will be adding back on later! The geometry of the vertical tail and the rudder, coupled with the fact that when you apply right or left rudder everything changes, precluded me from simply measuring and cutting. The only way to do it is to cut and try, cut and try, etc.

By shining a strong light under the hinge receptor pockets, it is easy to see when you are getting close to the point where you can get the two bolts in. Don't use the bolts! Instead, put about a 30 degree point on a piece of 1/4 inch by 3 inch long drill rod. It is easier to get through the bearing, which likes to swivel out of alignment. Don't worry about it not having a head, it won't fall out due to friction. The corner of the rudder swings a greater arc that the front or side, so if you hold right rudder when cutting and trying eventually you will be able get the pins in the bearing holes, but you will not be able to neutralize the rudder because the trailing edge of the tail is still too long-this is good!

Now mark the skin with a felt pen and keep trimming. You will have to remove the rudder every time you trim. Remember there is a fairing that goes above the rudder so don't worry too much about the gap on top. The idea is to keep trimming and fitting until the rudder can be pivoted with no clearance.

Now with the rudder precisely neutral (this is important) you need to find out its relationship to the slipstream. I took a 1/8 inch welding rod and bent it into a U about 2 inches wide and 4 inches long. Then I bent the U end up to about 45 degrees for a handle. Imagine a dinner fork missing the handle and only retaining the two outer tines. This is my depth gauge. By holding it flat near the trailing edge of the vertical tail and placing a straight edge across the "tines" and over the leading edge of the rudder, you can tell if the rudder is sticking out into the slipstream the required 1/8". Mine wasn't, but then I've never been the lucky type!

There are hard point washers installed in the rib of the rudder. I opted to leave them in place in an effort to be as least invasive as possible. I filed the holes very carefully till I got the right relationship between rudder and tail. Be careful here to check the protrusion in line with the bearings. This is because if, for instance, the top of the rudder is moved to the right, the very bottom of the rudder which is way below the bottom bearing will move to the left or in, and vice versa. Drill out 4 washers to .250 inch, scratch them up a bit, and bond them in the proper position. Do this with the rudder in the proper position and use a bolt with nut to keep the washers square to the axis of the bolt. Then you will have a nice landing spot for the head of the bolt and the nut later. Be sure to clean the excess adhesive from around the washers.

Now go to the manual and look at the mandatory modification to the horizontal stabilizer hard points. Bond the glass donuts around the washers and cover in the same manner. Now we're half done, or so it should seem! Now with the access holes cut in the left skin we need to start dry fitting, on the bench. Don't get carried away with the dry fitting quite yet. As soon as you can get a cleco in the leading edge joggle of the rudder, do it. You have to install them from the inside so you can get the rudder installed on the tail. Also cleco the trailing edges together. This is important.

Now we have to start trimming the left trailing edge of the tail so we can get the entire rudder hung. Do it the same way as the right side, only this time hold left rudder till you can get the pins in the bearings. This time, however, you have two things to monitor; the amount of trim on the tail, and the relationship of the left skin to the slipstream. Take it very slowly with the trimming and fitting. It is much harder to put fiberglass back on than to leave it there in the first place. Continue dry fitting and trimming till you get it right. Re-drill the cleco holes as necessary, but be sure to mark the "bad" holes so you don't get mixed up when you drill all the holes for bonding. Pay particular attention during the whole process to how the clearance between the rudder and the trailing edge of the tail changes from top to bottom as you rotate the rudder. When you finally get it right - and I mean finally - put the rudder back on the jig you built on your perfectly flat table and, leaving the clecos in position, drill all the holes on the leading edge joggle for bonding.

Ever see a carpenter's bench dog? If you bend a piece of 3/8 inch smooth rod to less than a 90 degree angle, then drill a hole just big enough through the bench where the access holes are you can make a real neat clamping device if your rudder doesn't want to lay flat on the table. I used 4 of them. You just slip them in the hole with the other end over the area you want to hold down and tighten them up by tapping them into the hole with a mallet. Put something under the end so you don't damage the foam core. Be sure to leave a tail sticking out under the bench so you can release them.

Now that you have a new rudder, you need to make sure it has 25 degrees of travel right and left. Take a piece of plywood and mark a centerline with a line at 25 degrees on either side. Sight down through the bearings for the rudder and put the apex of your lines in the center of your peep sight. Center the gauge with the center of the tail cone. Now it is easy to get the proper travel. Keep cutting your hinge slots until you get the necessary travel. Slow and easy here too! If you shine a strong light from underneath the hinge pockets, you can see if you are running out of pocket. If you run out of pocket, like I did, call Mike Pastelak!

The final clearance between the rudder and the tail is up to your preference I guess. I set mine at 1/8 inch, which will close a bit with the addition of primer and paint. This, I should add, is minimum clearance because it constantly changes as the rudder is rotated. I wish I had counted the times I had the rudder off and on the tail, I'm sure it was in the hundreds! But it sure fits nice, and yours will too!

Roger Isackson

ADDITIONAL NOTES (by John Ricciotti):

I built the rudder just as Roger described and it turned out beautifully.  I hope it flies well!   I did learn a couple of other things that I would like to add in case others run into the same difficulties.

I found I had to move the hinge point on my rudder about 1/4 of an inchto the left to get the right skin to have the proper profile with the vertical stabilizer.  An equal amount on the left skin and suddenly I had a rudder that was about a 1/2 inch fatter.  

With the bonding flange method that we use to join our flight control surfaces I would totally miss the flange with the rib from the right skin.   Therefore  I utilized the cap strip method that I learned at Graham Woodd's shop in Seattle.   This is similar to what we use when we bond the top wing skin.   The only difference is that we have to build the cap strips.   Graham utilizes this method in building all of his control surfaces as it assures perfect bonding and a minimum of adhesive to help with keeping weight down.   As we know, the lighter the entire control surface is, the less lead we need later to balance it.   This bonding method is also known as imprinting in an earlier post (#186) which was graciously submitted by the late Scott Devlin.

I won't go into the whole 'imprinting process', as you can look that up, but I will note that forward and outboard of the left rudder cable attach point I had to add a small piece of fiberglass sheet (same as used in closing trim tab ends) to make up the gap where there is just hollow skins.  Maybe you'll be lucky enough and have the right side ribs a little wider than mine were.

After the small filler skins set I filleted and taped the cap strips.

Additionally, I picked up a tip from Wally Weller (also at Graham Woodd's shop) that saves considerable weight in the rudder.   Three axis trim is nice to have but the weight of installing a servo and trim tab on the back of the rudder needs to be balanced out by lead.  This is a double penalty in one of the worst places to add weight to our Seawind, the tail.   We already fight the aft C.G. battle, this only adds to it.   In lieu of the trim tab and servo I will hopefully be able to use a rudder trim system similar to Wally's which trims the entire rudder from the cockpit.  This is similar to a Lake amphibian setup.   I believe Cessna also uses this design philosophy in some of their single engine aircraft.  Thank you Wally for the excellent idea.

Also, since the bottom of my rudder would not have any attachment to the original water rudder design (I am using the cable/pulley system perfected in Mike Bowes' shop), I closed the bottom of my rudder with another rib which was build as light as possible as it will not be supporting any structure of the rudder.   This cannot be done until you fit the skin that covers the water rudder area as the angle of the rudder hinges determine how much of the bottom of the rudder skins will remain and still be able to install and remove the rudder.   I tried to leave on as much bottom rudder skin on as possible to help obtain maximum rudder effectiveness.  Also I built the bottom rib watertight so as to not allow water into the rudder which would be slow to drain after takeoff.    Any opinions as to the necessity of making the closing rib watertight would be most welcome.

The top of the rudder I did not close with another rib as it would contact the fairing above the rudder during full scale rudder deflection.  I might reconsider and later add some speed tape in this area after I am flying to see if it makes a difference as far as reducing drag.  At that point I could remove the rudder and add a closing rib.

I hope some of this is useful to our members tackling this important part of the Seawind.   I welcome any comments, pro or can, and they can be submitted to the Editor.