Step 4.6, Finishing the Elevators

The final steps on the elevators are rolling the forward skins and then hanging the completed elevators on the horizontal stabilizer.  To encourage the elevators to cooperate, the first step is to show them the implements that will be used against them.  Iron pipe, duct tape and pliers.  I understand that this alone is usually sufficient to scare the elevators into curling their own skins.

However in my case things must be done the hard way.  The pipe is placed inside the skin and taped securely to the edge of the skin one section at a time. The pipe is then rotated inward using the channel lock pliers while it is also being held down against the table.  The skin then just follows the pipe around its circumference and presto!  A curved skin.  Peel the tape off and go to the next section.

Making sure the top skin overlaps the bottom skin discourages water to enter.  Although I suppose that water would then elect to enter via the hinge cut outs between the sections all the same.

The bent skins are brought into as near as perfect alignment as possible with some judicious bending by hand.  It is preferred to not have any tension (in shear) on the rivets when they are installed.  So after a small amount of wrestling with it, the shape is very close and I proceed with clecos, match drilling, and finally, securing with pull-rivets.

With the closing of the forward edges of the skins, the last step is to hang them on the horizontal stabilizer.  First the rod end bearings are installed using the Sam Buchanan installation tool mentioned in a previous post.  From there the elevators are attached using two very small bolts through the hinge flanges on the stabilizer, continuing through the rod end bearing, then through the opposite hinge flange, and finally secured with a Nylock nut.

This is Carol looking for the hardware in bag 618.  Fortunately for us, she labeled all of our parts bins by rivet type and, for hardware, by bag number.  She also annotated the plans with the bag number where ever hardware is called out.  Thanks to Carol, finding parts is one area where little time is wasted.

While hanging the elevator one discovers there is a step on the horizontal stabilizer that has not been completed.  Notching the skin for clearance with the elevators's counter weight.  "Insert sound effect of record player arm scratching across a record"  Ok, mark and cut the HS and then continue.

In the photo below I'm working the right elevator with my hand on the control horn and checking for clearance with the horizontal stabilizer.  There were no issues.  Moving on.

So now the control horn must be drilled so that a bolt may pass through it and the horizontal stabilizers center bearing.  Remember that?  It was the first part we riveted.  Here it is again.

It's the doo-dad in the center.  The trouble is, you need a bushing in the bearing to protect the bearing surface while it guides a drill through and into the control horn for a perfect match drill.  So what can I use for a bushing?  Nothing.  I have nothing at all.  Unbelievable.  I guess, I need that record scratching sound effect again.  I will have to make another trip to the hardware store.

Yes, I'm back already.  I found a 1/4" standoff that worked out just right.  Now with the holes drilled we continue with the assembly.  Here are a couple of photos of Carol trying to fish the washers through a very tight space stuck to the end of a screwdriver.  One at a time, as we advance the bolt through.  Last one?  Great, we're done.  Oops, I got the wrong bolt out of bag 618, and it's too short.  Arrgh!  Start over.

Once the elevators are hung the last step is a balance check.  Since the elevators are not yet painted, we should have an over balance condition such that the elevators want to deflect upward (counter weights down).  My right elevator does this, but the left does not, probably due to the added weight in the trim tab and servo.  I will have to add some more lead to the left side before closing the elevator tip.

Step 4.5, Trim tab

Well, no more putting it off.  The time has finally arrived.  Time to build the dreaded trim tab.  Quite a lot has been written (on the internet) about the difficulties of constructing this diminutive, yet all important component.  First of all, the build manual instructs one to bend the skin to the final angle.  Then just bend the tabs over.  After all of the drama bending the elevator tabs, I was seriously considering cutting these off and building replacement ribs from the start.  But, I thought I would give the Van's method one more try and so I did.

I had the same difficulties with the bending block moving in, but the results were somewhat less catastrophic this time so I'm going to just live with it.  Since practice makes perfect, I figure that in ten or twenty more planes I could make the Van's tabs look pretty good.

The next step is to attach the tab actuator flange to the bottom of the trim tab skin.

Once that is done then the tab hinge and the tab spar are match drilled and then riveted.

The completed trim tab is then ready for installation on the left elevator.

Here is a view after riveting the tab hinge to the elevator.

Next, the trim tab servo is fitted.  It took a considerable effort to get the servo to fit through the access door and to prevent interference between the elevator actuator rod and elevator skin.  The build manual just barely mentions that it may be necessary to trim the actuator rod egress area.  Out comes the cut-off wheel...

Trim...

Trim...

Trim...

Trim...

And Trim!

What is really annoying is all of the test fitting between each cut-off wheel attack.  It was most of a night's work on this tiny weeny little part.  Finally, the result:

  

Since the build manual fails to give any guidance whatsoever on the proper way to adjust the trim tab servo linkage, I came up with this method:

1.  Using a 12 volt power supply, run the servo out to its furthest extent and measure the arm length.

2.  Run the arm back in to its minimum length.  Take half the difference and then add it to the minimum length to find the exact midpoint of the servo's extension.

3.  Run the arm out to the midpoint.

4.  Cut off the actuator's threaded rod at a length that allows it to be half way in the clevis fork with the trim tab in trail with the elevator.

This method insures the maximum travel possible with the supplied servo.  There is quite a lot of deflection available.

For me, the trim tab was not nearly as horrific at the elevator tabs.  Your mileage may vary, of course.  I did have a minor mishap while fitting the servo access plate though.  One of the #6-32 screws broke off in a nut plate.  Oh, $%@#!

I replaced the nut plate rather than risk damaging the backing plate.  In all, it was a ten minute detour on my way to finishing this airplane.  Not too bad considering I probably still have most of 2000 hours to go.

Step 4.4, Bending the tabs

It been a while since my last post and my progress has been fairly slow due to a misstep in bending the left elevator tabs.  The tabs in question were not photographed before they were originally bent so there won't be any before and after.  Shucks.  The tabs in question, close off the end of the trim tab cut-out on the elevator.

According to the build manual, one simply bends the lower tab up and the upper tab down over the lower and shazzam! An instant mini-rib.  Well this is, no doubt, the least expensive way to go. It is also a sure fire way to induct new members into the club of tab bending goofer uppers.  Fortunately, membership does have its privileges.  For example, I now know what not to do next time.

To join this fine, and apparently quite popular club, one begins by following the instructions to build a tapered bending block.  I used some plywood shown below.  The double sided tape is suggested to prevent the blocks from sliding during the bend process.

   

Then, the blocks are arranged inside and on top of the elevator skin such that the tabs can be bent over the edge of the block, 90 degrees.  In the picture below, the tabs have already been bent over so the lower block is only just visible inside the elevator.

To bend the tabs one is instructed to strike the tab with a hammer using a small block of wood to spread out the force across the length of the tab.  This is where things go south, because the force of the hammer blows cause the bending block to move inward an undetermined amount.  This causes the finished tab to be located too far in.  The undetermined part is especially troublesome as the bending of the opposite tab will most likely be a completely different undetermined value.  The bottom line is that in my case, because of this issue, the tabs were not perpendicular to the surface of the elevator skin.

At this point, I had no choice but to unbend the tabs and apply for membership in the aforementioned club.  Unfortunately, this horror story doesn't end here because unbending the tabs, I knew, would distort the skin.   And the tabs can not be rebent in the same place anyway without significant risk of stress fatigue in the aluminum and in the builder.

Still, the tabs had to go, so the new plan was to unbend it, but then cut those SOBs right off!  I have read online many accounts of intrepid builders doing just this; and so I did.

Good so far, but what about the distortion caused by the unbending?  No problem, I'll just hammer it out flat using the mushroom set on the rivet gun.  Genius!  So, the skin does get flattened, but now there is a little too much of it as the skin has been stretched out by the jackass with the pneumatic hammer.  

Now there is a definite bow to the skin along the edge and the corner is pretty ugly as well.  But, I'm not about to let this elevator die on the table, so I begin to think about what can be done to correct  the stretched out skin.

I was vaguely aware that it is possible to shrink metals using heat, some hammering, and a rapid quenching process.  After a quick consultation with the oracle that is the internet, I'm pointing a propane torch at my elevator.  

Since I had one hand on the torch and the other holding an infrared thermometer, there aren't any pictures of the shrinking process.  Suffice it to say that what is said about making sausage may also apply to airplanes.  Consider yourself lucky you didn't have to see it.   After the quenching process, the skin lays mostly flat and I'm thinking... I can't believe that actually worked!

The next step then, is to fabricate rib to close the elevator in place of the amputated tabs.

After final adjustment and then counter sinking, the new rib was ready for installation.

Finally, I'm installing the new rib which to my utter amazement, fits quite nicely.  I really thought my elevator skin was a gonner, but Roxy who looks on knowingly in the background, is always confident that things will work out in the end.

Step 4.3, Left elevator frame

The parts for the left elevator are assembled and then match drilled with the left skin.  Here is the elevator horn being match drilled to the elevator spar.

Then the parts are scuffed with Scotch brite before dimpling and prior to priming.

Carolina makes an appearance, but hopes to avoid the paparazzi.  Here she is dimpling the counter balance skin.

After priming the skeleton is reassembled.

Roxy standing by.  Or laying by -- to be precise.  Her main concern seems to be keeping the squirrels out of the yard and, by logical inference and deductive reasoning of her superior canine brain, to be certain that the furry rodents are not interfering with the construction of this airplane.  Good dog!

The next order of business is to fabricate the trim tab.  For the non-pilot, the trim tab is a control surface that is adjusted from the cockpit that allows the pressure felt through the stick to be zeroed out. This plane will have electric trim, which means that a switch in the cockpit will be used to control a servo motor in the left elevator, moving the tab. A nonelectric trim tab uses a control cable instead of the servo.

The trim tab is the most feared component of the entire tail assembly.  One builder's blog I read recently suggested that one should buy two trim tab skins before starting because it is a near certainty that the first one ends up in the scrap bin.  We'll see.

While I procrastinate on starting the dreaded trim tab, I'll assemble the trim servo access plate.

Servo motor and trim access plate