Chapter 20: Installing the Internal Bellhorns


 

 

I'm using the internal rudder bellhorns, which are approved by Nat.  The RAF plans, available through the RAF website, are really meant to provide the installation step to change over from external to internal bellhorns.  The retrofit involves forming a pocket in the rudder.  There is a fair amount of cussing, throwing of tools, and head scratching involved trying to get the geometry correct, not to mention trying to lay up plies in such small confines.

 

I avoided all the cussing, tossing, and head scratching by simply installing the internal bellhorns BEFORE installing the lower winglets and BEFORE cutting out the rudders.    I read all of John Slade's miserable escapades.  It didn't work for him, but I promise I'm much craftier than he is! :-)

 

I did this as part of Step 7.  Here's my process:

 

1.  With the wing right-side up, draw the rudder cut lines onto both sides of the winglet.  These cut lines will help you determine the geometry for the bellhorn movements and more importantly establish where NOT to put the glass and flox.

 

2.  Now turn the wing upside down.  With the wing upside down you have plenty of access to the installation area for the bellhorn.  You should be able to see the rudder cable conduit in the wing tip.

 

3.  Locate your rudder cut line reference marks on the inside and outside surfaces, place a straight edge across them, and draw a line on the exposed foam.  All layups and flox will be aft of this line!  If the glass and flox go forward of that line you will have a hard time cutting the rudder out later.  I even cheated just a bit and kept all glass and flox about a quarter inch aft of that line.  Keep track of the mark because you'll be drawing this line each time you carve more and more foam from that area when forming the pocket.

 

4.  Place the bellhorn into its approximate location on the exposed foam.  Place a mark onto the outside edge of the bellhorn at the point where the outboard rudder reference line intersects the bellhorn. This becomes your mythical "hinge" point.   Pretend you are deploying the rudder by rotating the bellhorn in and out around your mythical hinge point.  (Keep the mark on the bellhorn directly above the rudder reference mark.)  Pay close attention to the arc the tip swings through.  What you're trying to do is figure out the best aft position for the bellhorn.  You want the best aft position because the further aft the bellhorn, the shorter the arm, the greater the deployment angle of the rudder.   But it's a compromise!  If the bellhorn is located too far aft, the tip will end up too close to the trailing edge of the wing tip.  You don't want to run the risk of jamming the rudder cable's swage fitting into the trailing edge of the wing!   Do this several times and keep moving the bellhorn fore and aft until you find the best location.  Mark that best location.

 

5.  Now it's time to carve out the pocket for the bellhorn.  There are two things you need to know.  If you think about it for a minute, you'll realize that when the wing is right side up the tip of the bellhorn actually swings downward by about 3/8th of an inch as the rudder is deployed.  This is because the winglet is canted inward toward the plane.  Thus, the tip of the bellhorn starts out at 3/8th above the rudder conduit when the rudder is in (no feet on pedals).  With the wing upside down, you must reverse this.  You will carve and shape the pocket so that the tip of the bellhorn is located approximately 3/8th of an inch BELOW the height of the rudder conduit.  You will carve the pocket to be deeper than that so the bellhorn can't snag on anything.  The second thing to know is that you want the tip of the bellhorn to end up at the same height as the rudder conduit once the rudder is fully deployed.  Thus, the pocket is shaped at a slight angle fore to aft and side to side.

 

6.  Okay, you didn't understand a word of that, but let's start making the pocket anyway!  You will do this by VERY CAREFULLY cutting away some of the existing fiberglass and sanding away the foam until the bellhorn is located at the proper angle.   You can see how much fiberglass has to be removed in the pictures below.  It's all the fiberglass within the arc that the bellhorn swings through, plus about 3/8th inch ahead of the arc for good measure.  Yes, it does require removing part of the outer 9-ply layups.  Yes, it does require removing a very small portion of the internal spar/web.  There's no way around it.  If the thought of cutting through structural layups raises the hairs on the back of your neck (as it should!), rest assured these are very strong layups and there are plenty left.  If that doesn't do it, keep reminding yourself this is an approved mod. :-) 

 

      

 

7.   With the glass removed, start making the pocket by removing the foam ahead of the rudder cut line.  At this point the depth of the pocket is easy to estimate because you can see the rudder cable conduit.  Start out by just aiming to get the pocket about 3/8-th inch deeper than the rudder conduit.  Continually fit-check the bellhorn into the pocket.  Once you get the bellhorn positioned properly, don't remove any more foam aft of the rudder cut line, but  do remove more foam forward of the line to make the pocket a bit deeper so the bellhorn won't snag on anything. 

 

8.  From there it is a simple matter to glass the bellhorn in place.  I put down 2 layers of BID, imbedded the bellhorns with flox, then glassed 1-BID over that.  I'm sure it's overkill, but I've lost my bellhorn plans and I'm sort of winging it (ha, ha).  Note that my layers of glass extend on the rudder side only.  The layups end just short of where the rudders will eventually be cut.  This will make it much easier to cut through the foam and remove the rudders from the winglets.

 

 

9.  Okay, the battle's half done.  Time now to form the other half of the pocket.  If you think about it, the lower winglets are supposed to form the bottom half of the pocket.  The following pictures show the sequence of installing temporary spacers that can be easily removed once the lower winglet is glassed in place.  The first picture shows a small piece of foam that goes in aft of the rudder cut line.  It is part of the rudder and is permanently micro'd in place.  Forward of that are two spacers to form the bottom half of the pocket.  One is shaped to fit the "triangle area" to the inside of the bellhorn.  The other is shaped to fit over the bellhorn.  (you can make it as one piece, but it's easier to do as two pieces.)  Both are held into place with two VERY SMALL drops of 5-minute glue.   In the final picture I simply sanded the two triangles flat and flush with the winglet/wing skins.  I applied box tape over these two triangles just before microing on the lower winglets.  The box tape serves as mold release to keep the triangles from being micro'd to the lower winglet core.  I applied only a few drops of micro to the winglet cores for the area that would be over the box sealant tape.

 

   

 

10.  Once the rudders are cut out, give the rudder a slight twist to break the 5-minute glue bonds and the rudder will easily pull away from the winglet.  From there it is a simple matter of slipping a hacksaw blade in there to release the box tape and pull the triangles out.  This picture shows what the pocket should look like after the rudders are cut out.  (Note:  This picture was taken when I had winglet stubs instead of lower winglets.  I added the lower winglets later.  But the pocket process remains the same.)

 

 

 

11.  Congratulations, you've won the battle.  You can tweak the exact position of the bellhorn tip later by very gently bending the arm.

 

A Note about the Rudder Cable Conduit

 

As you remember in Chapter 19, we used dabs of 5-minute glue to secure the nylaflow tubing in place before glassing the wing skins.  Well guess what?  You really want to remove the last 2 inches of that conduit so you have room for the swage fitting that attaches the rudder cable to the bellhorn.  This is especially true if you're going to use a forked-shaped fitting.  So after the fact, I cut out part of the wing skin to expose the conduit.  I snipped off the last two inches and hogged out the foam in and around the end of the conduit.  I made sure the swage fitting wouldn't snag on anything.  I glassed the "cover" back on with 1 BID.  It later gets covered with the BID tapes used to glass the lower winglets onto the wing.

 

 

 

 


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