Halifax MkIII

[Biggles]

Wow are you making 12 engines?

[Model Maker]

Yes - 12 engines will be created. It's not as much effort as one may imagine. The hard part was figuring out how to make the first engine and the relevant fixtures. Once that was accomplished, I've been cutting out the components for all engines at the same time. Wrapping the cylinders takes a little time, but again, I have a production line going for them as well and do a few each night.

Next item on the agenda is the propellers. Wood seemed to be inappropriate due to the scale and the fear of the proverbial "Do they rotate? Sorry I broke it" syndrome, Plastic seemed to be the best option. Resin casting was considered, but deemed too expensive. I pondered rotating propeller discs, but preferred the shape of a stationary propeller. Upon reviewing the various plastic materials, I opted for Lexan which is a polycarbonate material. Although clear like acrylic, it is more ductile and much tougher to break than acrylic. It can also be bent and twisted at room temperature for specific geometries.

A trip to the local big box hardware store revealed a reasonable selection of polycarbonate. The piece purchased was 11" x 14" x 3/32"  thick which was the thinnest material available. A local plastic supplier had thinner material, but the sheet size was much larger. A couple of test propellers were cut using the scroll saw and the propeller twist was imparted without the requirement to heat the material first. However, blade breakage was an issue, not due to the material, but caused by the sharp turns at the base of each blade. So based upon the trial tests, the plunge was taken.

A template with multiple nested propellers was created and glued with a glue stick to the plastic protective sheeting. The nesting minimizes material wastage and provides larger pieces of material to hold onto when cutting out the shapes.

Pilot holes at the propeller time and roots were drilled to allow for easier blade turning to minimize breakage.  A red cut out line was created to separate the two propeller nests creating two sheets of 6 impellers. Pilot holes were also drilled at the corners  of the cut out lines to facilitate cutting. A 15 TPI blade was used to cut the material. No melting occurred when cutting the material on the scroll saw, even at the higher speeds.

To facilitate shaping, a Dremel sanding mandrel with a 120 grit sanding sleeve was chucked into the drill press. I was afraid the high speed of the Dremel tool might melt the plastic. The lower speed of the drill press worked great and I wasn't worried about ruining a propeller if I pressed too hard as this combination of speed and sanding grit did not aggressively remove material. If a drill press is not available an electric hand drill can be used.

With the edge sanding complete, 4 propellers are ready for twisting and tapering. An arrow was placed on each blade to indicate the twist direction.

[Model Maker]

Continuing with shaping the propellers.

The back side of each blade was tapered using a very sharp chisel and a standard bench hook. The polycarbinate material is very tough and a great deal of pressure must be applied to the chisel for it to bite into the material. The chisel tends to slip over the material, rather than bite into the material so one needs to be careful. I had both hands on the chisel (one on the handle and one on the shank applying downwards pressure so my hands were not near the chisel tip in case it slipped).

Two pliers were used to bend the propeller blades at the root. A set of needle nose pliers held the root of the blade while another set of pliers was used to twist the blade. The material twisted without an issues and no preheating was required.

Back over to the sanding mandrel in the drill press to shape the leading and trailing edges of the propeller. A moderate pressure is required to force the material into the sanding drum, otherwise the blade skips over the sandpaper surface with no material being removed.

A picture of the shaped set of propellers.

[Boomerang]

  Well Done Ken!

   I'm enjoying this project of yours.
   Your explanations and pictures explain things very clearly.

  Cheers

  Gordon

[Model Maker]

Gordon ;

Glad you are enjoying the build. I'm certainly having fun figuring out the intricacies of a multiple engine model. The engines were a bit of a challenge to figure out the build approach. But hopefully they will turn out successfully. I thought I was off to the races until Lou upped the level of detail on his Argus by detailing the full compliment of cylinders. That's what I enjoy with all the building boards. There's always new approaches and techniques being presented along with a wealth of information for newbies such as myself.

I'm never quite sure on the appropriate level of detail to capture for a build. Thanks for the feedback.

-ken

[Model Maker]

Starting the engine cowling and nacelle shaping. I'm sure there is an easier way to do this, but I've started down this path, so I'll stick with it. I'll begin with the engine cowling which will retain the faux engine.

The first step was to cut some blanks from a 2x3 with the thickness a little larger than required. I found when drilling the hole to receive the faux engine, if I didn't leave sufficient material at the back of the hole, the forstner drill bit would break through. The engine compartment hole was drilled in the end grain to facilitate shaping the outside diameter and to align the wood grain along the axis of the engine / nacelle assembly.  After boring out the engine compartment with the forstner bit, the dimple of the forstner bit was extended with a small drill  bit to create a centre hole. The small hole was the diameter of a wooden toothpick which was the pivot point on the adjustable jig described below. 

Although I was able to approximate a circle by chiselling the outside diameter, I opted to put together an adjustable jig which allowed me to spin the engine cowling against the disc sander to obtain a more uniform outside diameter. The jig consisted of an adjustable slide with a wooden toothpick as the pivot point. A second adjustable stop was installed on the underside of the jig. To sand the outside of the engine cowling the roughed out item was installed on the jig pivot pin. The jig was then pressed towards the disk sander until the underside stop contacted edge of the disc sander table. The engine cowling was then manually rotated against the disc sander to create the circular diameter.

Because the blanks were thicker than required to prevent the forstner bit from breaking through, a second sanding jig was assembled using a dowel the same size as the forstner bit. The dowel was inserted into a scrap block of wood. The engine cowling was installed inverted onto the dowel to allow sanding of the back to obtain the required length. The block was then simultaneously pressed against the disc sander miter gauge to ensure the end was sanded square and against the sanding disc to remove material. Since this was a through away jig, I didn't take the time to create an underside stop and instead simply sanded while periodically checking the length with the vernier until the required length was achieved.

[Model Maker]

On to the other parts of the engine assembly

The vent section disc was shaped using the sanding jig. Since the vents are angled outwards the disc sander table was set to the appropriate angle. I was intending to notch the grooves for the vent flaps, but once I had created a few vent sections, I became worried about scoring such a thin section of wood and so will simply stay with the flared section.

The sanding jig was modified to incorporate a larger diameter stub for shaping the front section of the engine cowling. The end of a 2 x 3 was again bored and thin slices cut to the appropriate thickness. Shaping consists of two steps. Step one - sand to the correct outside diameter. Step two - set the disc sander table to an appropriate angle to sand a bevel on the outside edge. This bevel will be further rounded when the assembly is glued glued together.

An engine and cowling assembly was dry fitted to check fits etc. Still have some work to do, but so far so good.

[lastvautour]

Fantastic work Ken.

Lou

[Boomerang]

Ken,
I agree with Lou. Fantastic work.

Look forward to more progress reports.

Gordon

[Model Maker]

Thanks Lou & Gordon for the ongoing encouragement.

I'm not sure if I'm going down a "too detailed" road or not or how good things will turn out. But hopefully things will all come together shortly.

- ken

[Model Maker]

Now that I have the engine construction figured out, it's on to the 2nd and 3rd fuselages & wings. The fuselage station lines have been drawn and the wings have been fitted into the fuselage cut out. Not sure yet how to make the rear turret. I'm going to try carving it as part of the fuselage. But if the required spherical shape can't be obtained, I'll cut out the section and glue on a separately made spherical turret.

Several reference lines are drawn on the upper and lower wing surfaces to aid in shaping. The station lines for airfoil templates are drawn. The upper and lower crowns of the airfoil are indicated with a solid line along the length of the wing. The airfoil "crown segments" (the upper and lower arc portions of the respective crown) are indicated by the long dashes. Marking the "crown section" aids in  creating the appropriate tapers to the leading and trailing wing edges. It also assists in the sanding process to obtain a gentle rounding near the top of the airfoil.  The nose and tail chord line centres which are closer to the wing underside are drawn on the edges. These are also used as references to aid in shaping the wings.

[lastvautour]

Great progress. Thanks for the updates.

Lou

[Boomerang]

The three of them together will be sight to see.

Gordon

[Model Maker]

Thanks Lou and Gordon - Progress is slow, but things are progressing.

Wing is fixed in the clamping vice. One half of a wing has been rough shaped using a skew chisel, file and bow sander. A template of the root profile which fits over the wing has been used to facilitate shaping along with the airfoil reference lines. The wing shaping will be fine tuned with the bow sanders and sanding blocks once the wing has been attached to the fuselage as part of the final sanding process.

Santa brought me a set of Narex left and right skew chisels from Lee Valley. They work great for slicing along the edges and back side of the wing profile. The skew requires a little more finesse to sharpen than a standard chisel, but if you have a chance to try one you may be pleasantly surprised with the results. The skews tend to be a unidirectional chisel if you use the backside for support and guiding. It was difficult to find a supplier of left and right skew chisels but the Narex seem to fit the bill perfectly.

[buccfan]

Great detailed update Ken, thank you. Regards Paul J.