Fitting BD-4 Spars
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By: Lance Schlichter

Fitting the BD-4 Spars to the center section is one of the more mysterious parts of building a BD-4.  I searched high and low for a description or procedure for selecting and fitting them, but I didn't find much.   In the end, I wound up inventing my own methods.  This paper describes what I found, and how I did it.


Aluminum likes to stick to itself.  In fact, many transistors are bonded with aluminum wires.  An aluminum pad about the size of a human hair is deposited on the transistor, a aluminum wire about 1/7th the size of a human hair is pressed on the pad, a little ultrasonic energy is added and Viola! You have a wire bond.  If you don't lubricate your center section and spars, when you try to slide them together they will try to stick to each other and gall. 

I tried quite a few lubricants.  First I tried Tri-flow, a Teflon-based lubricant.  Didn't work.  Next I tried motor oil.  Didn't work.  Finally I found a newsletter that said use STP- worked like a charm.  I strongly recommend you use STP as you fit your spars.  Lots of it.

Tools and their uses

Here is a list of the tools I used, and what they were used for:

1. 6 - 7" Micrometer

If I had to pick one tool that would ease the fitting of your spars, it would be a 6-7" micrometer.  It lets you measure your spars and center section and find out which sides need to be sanded first.  You can fit your spars without it, but it will be a blind process.  I tried a big vernier caliper but it didn't fit all the way around the tubes.  The micrometer can be the cheapest one you can find.  In fact some clever people probably can make one out of a C-clamp and a few screws.  You don't need to measure the numbers; you just need to find out which sides of the tubes are fattest, and which are the skinniest.  You sand the fat sides first.

I got mine at Airgas/Rutland Tool (  On sale it was about $80.  It would be cheap at twice the price.

2. Long and short ramrods

Unless you have a jacking system already on your spars and center section, you'll need a way to get them apart when you first fit them together.  Trust me, they'll jam!  I made up a long ramrod to remove them.  The three-fourths inch plywood disk on the end of the 14 foot two by four is sized to just fit inside the spar tube and ram the center section out.  The shorter ramrod was used to install the 11-inch inserts.  Then is was put between the short inserts and center section, because the short inserts get in the way and one of us (me) had a heck of a time getting the center section out the first time he tried to remove it.

3. Modified 1" Belt Sander

This is a cheap 1" belt sander with the belt support removed.  I started with 80-grit sandpaper then used 120-grit to smooth the tubes when they fit.  I got mine at Harbor Freight, ( 

This shows how the modified belt sander fits around the center section when you are sanding.

4. Curved Tooth File

A curved toot file is another secret.  Regular files scratch the surface.  The curved tooth file planes the surface.  Use it to quickly take material off the center section, then as a final finish after everything fits.

5. Center Section Rotating Fixture

This is the basic rotating fixture.  The 3" wheels are from Harbor Freight- the cheapest ones I could find.

To drive the center section, I used a variable speed one-half inch drill and a plain cheap v-belt.  The pulley was a off-the-shelf 2" cheapie with a ½" bolt through the middle.  The drill has a knob on the trigger switch that lets you vary the speed when the switch is locked in the on position. 

This is the way the first setup looked.  When I tried it, the center section promptly walked endwise out of the rolls.  That's what the two short boards standing up are for.

Here is how a board was C-clamped to the sawhorse and stops the center section from walking.  Some of the builders use fancy wheels here, but I found I didn't need them.


This is the final setup.  Works like a charm.  The center section rubs on the end boards, but not to badly. 

6. Hammer and board

DON'T HAMMER ON THE ENDS OF YOUR SPARS!!!  If they need a little "persuasion" put a board against the end and hammer on the board.

7. STP

The Magic Goo that fits everything together

Two Tricks

I fussed with an 88-inch center section for about 6 months and never finished fitting it into either of my spar sets.  I then fussed about a month fitting one end of a 66-inch center section and learning how to do it.  After I figured out the tricks, it only took an afternoon to fit the other end into the second spar.  I wish I knew then what I know now.

Here are two tricks that will greatly reduce the effort you will need to fit your spars:

  1. Lubricate, lubricate, lubricate.  A liberal coat of STP really helps.
  2. Sand and fit the wide sides first.  Use the micrometer to find which sides are the widest on the center section, then using the modified belt sander, sand the wide sides first.  Make a few passes with the belt sander, then try fitting the wide sides of the center section into the wide sides of the spars.  You might be pleasantly surprised.


Step-by-Step Procedure

Step 1-Inspecting and Repairing Your Spars

The first step is inspecting your spars.  Look for defects, scratches and nicks.  Clearly circle and mark each defect with a magic marker.  Each of these defects needs to be gently sanded out and blended in using good aircraft practices.  After the defects have been located and marked, step back and see where they are at, and if there is a pattern to them.   Ideally, all the defects can be put on the neutral axis at the outboard end of the spars.  The neutral axis is half way between the top and the bottom of the spar.  In flight, the wing flexes upwards, compressing the top of the spar and stretching the bottom.  Where these forces meet in the middle is called the neutral axis.  Some spars have a line of lightening holes along the middle.  That's where you want to position your defects.  Failing that put the defects in the outer half of the spar.  The long wing version uses .032 rolled spars at the very end, so you can see the forces decline rapidly as you go out.  If you doubt the spars safety, get some new ones.

Step 2-Measuring and marking the Spars and Center section

Now use the micrometer and measure both ends of both spars and both ends of the center section.  Find the widest part and the narrowest part of each end.  Mark them so you can see how far out of round they are.  Also, and this is important, mark the spars 1 and 2, and the ends of the center section 1 and 2.  If you don't you might swap ends on the center section and try fitting it in the wrong spar (like the idiot author did several times).

Step 3-Selecting the inboard and outboard ends of the spars

Now you have to use some judgment and trade off between the defects you found and fixed during step one, and the roundness of the spars.  Ideally, your spars will be round, they won't have any defects, your center section will be round, and all you'll have to do is knock off a little corrosion, grease them up and slide them in.  In real life, everything will be oval and nothing will fit.  Now a word of caution--you might be able to fit two oval sections together and think you are done.  Not so!  Later, when you finish the wings and put them on the plane, you will have to set the angle of attack.  When the center section is round and the spars are round, it is simple to rotate the wings into position.  If the center section is oval, and the fitting spar is oval, you may have one heck of a time setting the angle of attack.  Worst case, one of your wings could wind up pointing straight down while the other points straight ahead!  Think ahead here.

Step 4-Preparing the 11-inch inserts (skip these steps if you don't have the short inserts)

Measure the short inserts, clean them up and use the belt sander to make them as round as you can.  Their roundness will help round out the spars when they are in place.

Step 5-Marking the 11-inch inserts

Mark the inserts 1 and 2 so you don't lose track of which insert went with which spar.  Otherwise you might swap them, and things will mysteriously quit fitting.

Step 6-Inserting the 11-inch inserts

First, make sure you are putting the right number short insert in the right number spar.  Then grease them up with STP and use the short ramrod to drive them in.  Drive them in about 10 ½ inches.  They will go into the final position later.

Step 7-Sanding the Center Section

            Now comes the hard part, sanding the center section into shape.  Start by putting it in the rotating fixture and knocking off the corrosion for about a foot on one end.  Use the belt sander and keep it moving so you take off an even amount. Now, before you forget, re-mark the end number along with the wide and narrow labels you just sanded off.  Next, use the belt sander and make a few passes on each side of the wide parts. Move back and forth as evenly as you can. Go easy, because you probably only have to take off a few thousandths.  The spars and center sections I measured were only out by about 10 to 20 thousandths, so you only have to knock 5 to 10 thousandths off each side.

Step 8-Trial Inserting the Center Section

If you are using the short inserts, put the short ramrod in first, before you put the center section in.  Put it in so the short two by four hangs out the end of the spar. That positions the disk next to the short insert.  Later, it gives you something to ram against when you try to ram out the stuck center section.  This is important.  I know. The second mistake I made (after not putting the short ramrod in) was putting it in with the two by four inside the spar.  Later I had to try to ram the end of the little two by four because I couldn't get my ramrod by it to ram the disk. 

 Next, liberally lubricate the center section, match wide side of the center section to the wide side of the spar and push away.  With any luck, the center section will slide right in.  In your dreams.

Step 9-Getting the trial fitted Center Section back out of the spars

By now you have a partially finished center section stuck in a spar.  No problem.  Reverse the long ramrod and using the small end, drive the center section out.  Oh, by the way have someone there to catch the center section when it pops out.  Otherwise it makes a wonderful boing-boing sound as is bounces off the floor.  Gives you a whole new oval to sand out.

Step 10-Removing the 11-inch inserts and final trial fitting of the center section

Keep repeating Steps 7, 8, and 9 until the center section fits one spar.  Repeat as necessary for the second spar.  When the center section fits, use the long ramrod and drive out the 11-inch inserts.  This lets you check fit the final inch or so of the center section.  When you are happy with the final fit of the center section, you can re-install the short inserts.  Put them in about 11 ½ inches.  Leave the final ¼ inch or so for installation when the wings are installed on the plane.

Step 11-Final cleanup of the center section

After the center section fits the way you want it, switch the belt on the sander to 120-grit, put the center section on the rotating fixture and clean up the scratch marks left by the 80-grit belt.  Now is a good time to use the curved tooth file to carefully plane the scratch marks out. If you don't have a curved tooth file, use progressively finer emery paper and hand sand the scratches out.  Polishing the center section ends wouldn't be a bad idea.          

Step 12- Wing assembly

Before you do your final wing assembly, be sure you position the defects along the neutral axis the way you planned in Step 3.  Also, make sure the inboard end of the spar is where you want it.  Be careful you don't wind up with the wing assembled backwards!! It can happen.

Step 13-Final installation of the 11-inch inserts

I plan on finally locating the 11-inch inserts as I install the wings on the airplane.  They are still loose (a relative term) in the wings.  Actually, they are stuck in there pretty well now but they haven't been glued or bolted in position yet. 

Step 14-Securing the 11-inch inserts

I plan on leaving the 11-inch inserts loose in the spar.  If you look at the forces acting on a wing, they will tend to flatten the spar vertically.  The bottom will move up and the side of the spar will move out.  Since the center is thicker, the thin sections outboard of the 11-insert will flatten more than inboard of the insert.  This flattening will force the insert inwards toward the center section.  It is kind of like squeezing a tube of toothpaste.  Squeeze the end; the toothpaste pops out the top.

Having said that, I'll measure their location then watch how they stay during the flight tests.  If they start walking outwards, I can always tap them back into position.  After that there two fixes:

  1. Glue them into position with Proseal or
  2. Remove the gas cap, drill a bolthole through the spar and insert using the gas cap hole, and then put a bolt through the hole to secure the insert.

Hope this paper helps

Good luck in fitting your spars