Category Archives: Section 47, Spinner And Cowling

Although I have not posted in quite some time, work has continued on the airplane.  Primary attention has been given to cowl preparation and engine baffling.

The propeller used for the aircraft will be a Hartzell HC-C3Y1R-1N/N7605C three composite blade, hydraulically controlled model.  The recommended gap distance between the rear of the spinner and the forward face of the cowl is  to be no greater than 1/8″ to 1/4″.

The propeller hub measures 2.5″ from the forward face of the flywheel to the rear face hub bolting location of the spinner retention flange.  The flange spacers are 1/4″ long, the flange itself is 1″ wide, and finally the 1/4″ gap between spinner and cowl means the distance from flywheel face to cowl front should be 1″ apart.  Fine adjustments up to about 1/8″ can be made to the final gap by shimming the spinner flange with washers.

The cowl gap distance seemed difficult to measure accurately without a clearly fixed reference point. I decided to use the face of the flywheel as the starting point for all measurements. Here a plywood disc is measured and cut to match the spinner diameter.

The rough-cut spinner disc was rounded smooth using a router. The 1″ cowl gap was simulated with smaller discs made of plywood and plastic sheets.

The mounted reference disc shows where the forward face of the cowl should be.  All cowl alignment measures were taken from here.

This photo shows aligning the center of the spinner disc with the crankcase split.

To properly align the cowling, the fuselage must be leveled laterally.  The leveling points are between the door bulkheads in the center of the cabin.

Next the forward face of the upper cowl is positioned behind the spinner disc, then leveled left-right with the floor using plumb bobs.

Level references are marked at the front and rear of the upper cowl.  These are used during the trimming process.

The first trims are for the firewall gaps on the upper cowling.  Anything between 1/16″ to 1/8″ is considered acceptable.  Too tight is not good due to paint chipping and difficult fit, while too wide impacts airflow and does not look good. The key for appearance is uniform width along the length of the gap.

Next action is position the piano hinges and match drill for fit.  Unfortunately, the upper cowl fiberglass had reinforcing honeycomb too close to the edge for the hinge to lay flat.  That honeycomb had to be sanded back and layered with fiberglass to provide sufficient edge strength for piano hinge rivets.

On the left the final upper gap is shown – a bit less than 1/8″.  The right photo shows initial reference line for the cowl side gap.

Once marked the upper cowl side edge was sanded flat with a 24″ sanding block.  This edge became the reference for the lower cowl adjustments.  Four inch spacer blocks were attached between the upper and lower cowls, then measurements from the side reference line to the lower cowl.

The initial cowl side gap is shown here.

The hole pattern was used to layout locations 1″ apart for the firewall side piano hinges.  The resulting pattern established the permanent gap a bit less than 1/8′.

The same patterning process was performed on side piano hinges.

A spare piece of .030″ aluminum with one side covered with self-adhesive sandpaper was used to create a uniform distance from the hinge web to the fiberglass edge.  The lower cowl was drilled from the inside out, then clecoed in place.

The forward section cowling was roughly cut for the outline of the Aerosport pin retention plate.  The right photo shows the first smoothing steps on upper cowl.

The forward edges of each cowl did not align properly, so resin/glass bubbles were applied to provide a smooth transition.  Next the final fitting of the pin retention template was completed.

The Rod Bauer air scoop was positioned above the Van’s default scoop and correct dimensions for the cutout were made.  The forward face of the scoop must be about 1″ behind the swinging propeller arc, so very careful measurements were required.

Puckered and pulled the trigger on the pneumatic cutting wheel to remove the Van’s scoop.  This action left a hole just big enough for the K+N air filter housing of the Rod Bower system.

A test fit of the air inlet ring was made, followed by an initial resin/glass bubble layer around the scoop perimeter.

Multiple sanding and filler applications built up the edges between the original cowl surface and the newly introduced air scoop. Clecoes roughly 2.5″ apart securely held the scoop in position.

With the external portions of the scoop under way, the internal locations of Camlok fasteners were determined.  As with earlier described issue on the honeycomb webbing, the fastener bases needed relief and then reinforcing for a flat base.

Small base templates were used during the build-up stages, but a real Camlok served as the real template for fastener attach holes.

This week started the fitting of cowls together.  Supplemental elements were fabricated which will later provide rigidity when splitting the lower cowl into two sections.  Two lower halves will greatly facilitate the cowl removal after a three bladed propeller is installed.

First measure and trim the lower cowl such that the height from center is 1/2 the diameter, and that the inlets are equally spaced above the centerline.  In retrospect I should have measured from the inner ring instead of the forward surface as shown.

The  primary stiffness for holding the two halves together will come from an .063″ aluminum plate fabricated while the cowl is still in one piece.  This was sandwiched with a layer of West Marine resin to ensure a tight fit without any gaps.

The jig in the right photo was used to scribe a consistent line on the outer cowl surface. Pilot holes of #40 size were then drilled through the cowl and the stiffener bracket.

These photos show the drilled bracket from two angles.  The additional fiberglass piece in the left photo will provide overlap (without fasteners) for overlap at the split location.

Air inlet ramps were the positioned and drilled into the upper cowl.

This week a stainless strike plate for the oil door camlock latches was fabricated, as well as a backing plate for the Cleaveland Tool hidden hinge.

These  photos show the application of West Marine resin / colloidal silica mixture to the upper cowl in preparation for riveting the hinges and oil door in-place.

Backing plates for the oil door hinges and the latches were also fabricated from .032″ aluminum pieces.  The plates were bonded to the fiberglass substrate with resin adhesive, followed by cleces in place during the curing process.  The whole door was then sprayed with heat resistant aluminum paint.  Finally all the parts were fastened together with AN426A (soft) rivets.

Here the oil door is dry fit in cowl opening.

SuperFil was applied to even the profile differences between the cowl and the door itself.  After this dries, all parts will be sanded smooth and prepared for further process of cowl fitting.

Here is the interim result ready for prime/paint.  Note the push buttons are slightly recessed.  Depending on the final paint job, I may address making these more flush with the surface.

The oil door on the upper cowling will be modified from the original vans plans to accommodate a hidden hinge and camlock latches.  Also work on miscellaneous components of for the cockpit were started.

The first step was trim the fiberglass door part to fit snugly into the preformed pocket on the upper cowling piece.  This involved initial shaping with a Dremel cutting wheel, followed by hand sanding.  As with all the fiberglass work done so far, the default fits were just approximations – nothing was square, smooth or level.  While I tried to get the door as square as possible, a bit of distortion remained.

Next was measure the inner opening to the engine compartment.  I wanted the cut line as thin as possible, so the removed piece could be used to reinforce the outer door itself.  At left are the trim lines, at right are the first cuts with the Dremel tool.

After the inner cuts, the hidden hinge was aligned and drilled. The initial fit was good.  Note the pink inner piece attached to the door.

The trickiest part of this build was aligning the camlock latches to get proper clearance with the cowling, while still being tight enough to firmly fasten the door when closed.  The left photo shows a backing plate made to strengthen the door from pressure applied by the latch spring.  At right is an outer view of the finished latches.

Two more views of the camlock latches.  Next to complete is a steel strike plate at the latch lever locations to keep the fiberglass rim from cracking or splitting.

INSTRUMENT PANEL

Holding the upper dashboard while fitting the instrument panel and electrical system can be a problem.  A jig to hold the panel in either vertical or horizontal position was created.

Here is the finished jig ready for work on the panel.

DASHBOARD GLARE SHIELD

A template for the upper dashboard area was created on a sheet of construction paper.  This was then covered in wax paper (for release purposes) and covered with West Marine resin and two layers of fiberglass.

After the template has set for a few hours, but definitely before the resin hardens completely, the sheet was placed on the dashboard and sheet pressed into shape to match the curve.  Later the hardened sheet will be covered with trunk liner to provide an anti-reflective surface.   More on that in later posts.