Center Wing
Construction the center section of the 25-foot wing
Started 7/24/2015 and Finished xx/xx/xx
Updated 11/1/2015
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This webpage will follow the example set by Charles Snyder with his popular Blog; wherein, he updated the Blog almost daily (well not really in my case) indicating the operations completed usually supported by a photo; plus, the hours required.  I won't report hours but I will provide HD images in contrast to the Blog which had pixel limitations that were often frustrating.

Weight of Center Wing:    32.1 pounds
Weight of Wing Panels:    28.6 pounds each (include ailerons but not wing tips)
    Charles Snyder reports  29.0 pounds for a full wing panel
    Dennis Lithgow reports 25.5 pounds for a clipped-wing panel (minus one bay)

Short Cuts to this webpage:
Rear Spar Cutout 
Spar  Support Angles 
Bulkhead C1 
Trailing Edge 
Wing Root to Fuselage

Links to other webpages:
I have uploaded Center Wing Interior images by several builders onto SmugMug.  Especially interesting are the photos of Dennis Brooks' UltraCruiser:
Center Wing Photo Gallery 

Also, I have dumped construction images on SmugMug, you must move your cursor to the very bottom of the window to see the comments for the image.not included in the description below.
Center Wing Nose Skin 

NOTE: On SmugMug, you must move your cursor to the very bottom of the window to see the comments for the image.

The first step in constructing any part is the review every page in the plans to determine where all the drawings and notes are located in regard to the part you are building; PLUS, determine if there is any need-to-know information on an adjoining part.  <---- THIS IS KEY!

Plan Pages Involved:
(Note: Imagine the page layout to be A thru E down the left edge / 1 thru 5 across the top)
Page 21: Full scale Fuselage Cutout Pattern for Spar
(Note: My older plans ... last page was 18, now it's 29.  Hummel Aviation sold me a new set of plans including the latest pages at a very reasonable price.)
Page 1,A4: Small dwg of Bulkhead C1 using .025 6061T6.  The rear spar angle is attached to the rear face.
Page 8,A4 thru A5: Top view of Center Wing
Page 8, C4 thru C5: Rear view of Center Wing Spar.  An3-5A bolts to bottom rails.  Spar Attach Strip.
Page 8, A1 thru D1: Center Spar Attach Strip (skin to spar reinforcement).  Formed from .020 6061T6
Page 8, E1 thru E2: Dwg - Side view of Center Wing and fuselage
Upper Angle 3/4 x 3/4 x 1/16th inch. 
Middle angle formed from .063, 1 x 1 inch with 65 degree angle. (Revision G)
Page 7, Full scale dwg of the Carry Thru plate formed from 3/16th inch 2024T3 plate
Note: 1 x1 x 1/8 inch Support angles now on both sides! (Revision F)
Revised shape since my plans ... to save weight ... (no Revision number)

The start:  Cut-out completed ... trial fit of the Center Wing Spar

 I cut out the Template directly from the plans (vs tracing the drawing).

Checking to see if the template was 14-inches from the bulkhead and square to the center line.  Turns out the template was shifted by 1/8 to one side ... more later.


Test holes were drilled to ensure that no cut would pierce the Side Rails on the bottom of the fuselage.  The extra clearance on one bottom rail and lack of clearance on the other rail hinted that the template was slightly off center relative to the side rails.

Initial cuts made with 4-inch angle grinder with thin metal cutting blade.  Great care required since the grinder has a tendency to "grab" and jump forward.  Cutting backwards avoids this problem.  Note: holes drilled at all corners.  Usual tin snips and files required to finish the cut-out.

Red line at right indicates the furthest safe distance for the cut to avoid cutting into bottom rail angle.

Testing fit of Center Wing Spar in rough cut-out.  Before the spar would fit, the top edges of the left and right cutout had to be filed for an enlargement of 1/4-inch and 18th inch respectively.

The tops of the two slots are a 4 and 9/16ths from the Side Rail.

Testing fitting ... bottom view.

The very bottom cut-out slots for the lower spar cap angle were widened slightly for easy of sliding the Center Wing Spar through the cutout.... see next image.

Another view of t

Fit at the bottom of the spar.  Notches are for bolt heads  Plans show the skin to be flat.  Should tapered spacers be inserted where the AN bolts go through the lower spar cap into the bottom rails?

Used many levels to check the positioning of the Center Wing Spar.

Checking to ensure the spar is perpendicular to the side rail.  Note: this test is not the best because the bending of the side rail introduced some distortion that prevents the rail from being flat on top.

Eye-ball check proves OK.  The wing spar barely peeks above the Horizontal Stabilizer Spar at all points.
Checking for Center Wing Spar alignment or parallelism using a digital level:
HorzStab:    .0 degrees
Side-Rails:  .1 degrees
WingSpar:   .1 degrees
Motorbox:   .2 degrees
At one time, the Horizontal Stab and Rudder were
off by 3.5-degrees (tilting to the left).  I replaced the C-to-D skin to resolve the problem.

When checking the center wing for alignment, I discovered a problem.  The rear fuselage was canted to the left by an angle of .69 degrees.  That doesn't sound like much, but it translates to 1 inch off at the "C" Bulkhead.  It's hard to measure that small of an angle.  To determine the angle, I solved the right triangle using trigonometry ... 82-inches, 1-inch, and 90-degrees (two sides and one angle).

At every stage of building, I ran a line from the rudder to the center of each section as it was added (and it was always straight), I'm at a loss how this lack of alignment suddenly appeared.  I have countless photos attesting to good alignment.

The solutions rejected:  reinstall the rudder, shift the center wing ... leave as-is and add trim after the first test flight.

Simple solution: slide the fuselage D-to-C skin further into the C-to-B section while shifting the rear fuselage to the right.  The left side was slid forward 7/8-inch and the right side 9/16-inches.  Then back-drill new holes into the C-to-B skin. This adjustment leaves sufficient clearance from the old holes so that the C-to-B skin can be trimmed leaving only the new holes.

The only impacts: Slightly shorter control rods and some trimming to fit the canopy; and, a slight forward shift in CG which will be more than offset by moving the seat backwards.

The effort was extremely easy to do.

Center Wing on Workbench

The Center Wing Spar was removed from the fuselage to begin the build process.  The workbench is a level and flat surface for precision work.  Once the installed skins need to be trimmed ... then it's back to the fuselage.  That will require disassembly of one side ....  NOTE: small machinist's square center bottom.  This was used to square up the flanges to 90 degrees.

While I built-up ribs for the outer panels, I decided to buy factory ribs from Hummel:
- Better uniformity (very nice)
- Faster
- Large lightening holes will ease access

The edge holes for mounting have been punched EXCEPT the upper and lower holes.  This allows one to match drill the holes in the rib to the spar cap angles by back drilling from the spar to the rib.  After that, you back drill the holes from the rib to the spar web.

Blue Sharpie pen marks center line.

Note: I built the outer wing panels using built-up ribs.  I created countess drill guides for the connection between the built-up ribs and the spar before I was satisfied.  It was interesting to find that the Factory holes exactly matched my holes on the outer spar web (I tried a test fit just for the heck of it).

A small angle with a .016 spacer is clamped to the top spar cap.  Also, a scrap of sheet metal is slide under the spar.  The rib (note blue center line) is ready to be slid into position.  The angle on top will ensure the rib is flush with the top of the spar.

A small LED flashlight from Sears (pink in upper left) was used to center the rib before drilling through the spar cap holes into the rib.

A 4.5-inch angle grinder with a thin cutting blade was used to cut off the top-center spar cap angle.  Care must be taken that the angle grinder doesn't "grab" and run into areas you don't want to cut.  As the aluminum heats, the gap of the cut tends to close and pinch ... care is needed.  A jigsaw would be safer.

A laminate cutter is used to complete the cut out of the spar web.  The highly tempered aluminum is easily cut.  One rib and angle mod was removed for knuckle and elbow room.

I left the spar cap in place to use the alignment mark on the top of the spar cap.  Once the alignment was complete ... I could remove that section.

Almost done.

I bought a new set of plans at a very reasonable price from Hummel Aviation (my old plans stopped at page 18) ... there are 29 total pages.

One mod removed a small portion of the 3/8ths-inch Carry Through Plate.  I traced the edge pattern on some scrap mylar from the mylar plans also purchased from Hummel.

Sharpie pen was used to trace the outline ... which has been adjusted slightly once the rib was installed.  The cutting is a job for tomorrow.   Wonder how much weight this will remove?
Center Wing Nose Skin

In the meantime, check this link:

On SmugMug, you must move your cursor to the very bottom of the window to see the comments for the image.


Rear Spar for Center Wing
Warning: a temporary angle was used which shall be replaced before flight.


 The mid ribs for the Center Wing have been installed with two short scrap angles for location of the fuselage cutout.

The drawing above is from Page 1 which shows the side view of Bulkhead C1.

At the point where the angles touch the fuselage, a small hole was drilled (both sides).

Then a tempered steel rod was inserted (piano wire from any hobby shop).

Based on the drawing above, measurements were made to ensure proper location of the rear spar.  All dimensions are approximate

Drawing   Actual
3.25 in.     3.0 in. =Height above fuselage
4.60 in.     4.8 in. =Distance from Bulkhead C
Tomorrow, I will enlarge the two holes and install the actual rear spar.

Bulkhead C1
While the plans called for "Tabs", I decided to use "Flutes"

Bulkhead C1 temporarily resting in place ... CenterWing on workbench
Note: click for enlarged view ... temporary handle Cliqo'ed and Clamped

Finished bulkhead resting in place against the Rear Spar

Final trim, bending adjustments and riveting will be completed after the skins have been installed and the Rear Spar is firmly fastened in place.

It is expected that adding the skins will move the Rear Spar backward .016 inches and may its height may vary from it's correct clamped position.

Scrap aluminum was used to form a pattern.  A light-leakage test was made to confirm a tight fit.  The indentations for flutes in the form were not required as the bend angle is modest.

Once the flange reached the "wavy" state, the bulkhead was removed from the vice.  Flutes were formed used fluting pliers.  Back to the vise for more forming ... removed ... then the flange was bent to the correct angle by fitting in place and using pliers and fluting pliers as required.  Final light test.

Spar Support Angles

Not having any .062 on hand, I bought the set.
NOTE: Extra length for a repositioned wing.
On the original plans Morry had just one support from .062 angle.  It attached at the top of the spar and you had to twist it to get it to fit or almost. When we bowed the side rails, it made this a wider angle. The lower support was added with the carry thru stiffener. Both of these are available from Hummel Aviation.

Terry Hallett
Hummel Aviation

Carpet protects the aluminum from dents and scratches due to dropped tools.

Interior, left side looking forward from the Spar Carry Thru to Bulkhead B. 

Note how flush the Hummel produced angles are to the mating parts.  Also note that the where the angles overlap Bulkhead B, they just manage to miss the flutes!

On the far left below the upper angle is the Center Wing SparAttachment flange where I used flutes instead of tabs.

Exterior view of right side of fuselage showing the Clecos holding the Spar Support Angles in place.

At Oshkosh 2015, Hummel Aviation had a set of construction photos for the UltraCruiser.  This photo shows the Spar Support Angles were either trimmed just short of Bulkhead B flanges or they were sandwiched between the bulkhead flange and the skin.  By not overlapping the flange, the angle will lay flush to the skin.

Most builders are overlapping onto the flange.

I considered allowing a friend with a mill to remove .020 from the angle where it overlaps ... that would still leave .042.

Some nice work by another builder on the Spar Support Angles and the elevator controls.

An alternate method for the upper Spar Support Angle is shown in this image ... it appears that flutes at 2-inch spacing were made to enable the angle to match the curvature of the fuselage.

I was satisfied with a straight angle ... it only pulls the skin in about 1/8-inch ... not noticeable ... and this area will be hidden behind the wing fairing.

Trailing Edge
New Plans include small drawing for the Trailing Edge
Almost too simple of a part for inclusion on this webpage ...
New Plans:

Old Plans:

Initial Bend by 4-foot Bending Brake:

Calibrated Pressure for Final Bend:

The bend must be adjusted to exactly match the width of the rear spar.  Failure to do so will result in a convex or concave surface on the trailing edge.  (If the bend is incorrect, on can easily spread or narrow the opening to install; BUT, the surface will be distorted.

I made the part slightly over size.  After sliding into place, position the top so that it matches the aileron.  Drill and Cleco.

Then adjust the bottom so that the top is in line with the surface of the wing and the bottom is flat.  Drill and Cleco

Build the ribs AFTER completing the installation.  They can be custom fit to any slight anomalies from the plans.

When the Center Wing is installed back on the fuselage, the inner end will be trimmed per the plans and fit to the fuselage.

The upper and lower forwarded edges will be trimmed while ensuring proper hole-to-edge distance.

Aileron folded up vertically.  Ribs not installed.

Wing Root to Fuselage
(Alternate to Wing Fairings)
I decided to save weight by eliminating the beautiful wing fairings.   Instead, I am connecting the wing skin to the fuselage using .016-inch angles.  This is a viable option if the wing skins have been carefully trimmed to fit the fuselage.  The factory designed wing fairings can hide a lot of sins :-)
The arrow points to the skin overlap of the Trailing Edge on top of the small fairing that I have designed.

Note: I expect this change to impact drag and the handling.  The junction of a wing to the bottom of a round fuselage may cause undue interference of wing and fuselage.

This change replaces many very large fairings with small 3/4 x 3/4-inch .016 angles.

vs.  The wing root fairing per the plans.  Image from Snyder's Blog.  Nice work!

It's a huge piece.

Hummel Aviation  notes that the wing fairings stiffen the fuselage. 

Trial scrap aluminum piece cut to fit fuselage.

Under view of cardboard trial piece taped to the bottom of the wing.

It extends nearly to the center line.

Gaffer's tape holds parts in place.

Rough patterns.  Actual piece was rough-cut even larger to allow positioning and marking for final trim.

The Support Angle is formed using pliers.  Very little bend is required.  Trial piece lies on the table just to the left.  Very easy to do.
The trial piece demonstrated the need for a narrower and wide flanges on the angle.  The wide side keeps the flange flat while the narrow flange minimizes distortion of the other flange.
Below: two views of installed angle.

Trial fit.

Tape tabs were used to indicate position of the Support Angle. 

Inside view of installed Wing Root Fairing. 

(Holes along edge will be trimmed off when the fairing is trimmed to size.)

Root Fairing nestles nicely into Trailing Edge.

Bottom view of the trailing edge, new added Root Fairing and fuselage belly looking towards the rear.  Perhaps this added part should be call the Trailing Edge Extension.

Trimming still required.

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