Covers Lost-Foam Fiberglass Casting and Simple Scratch Building

Hawk III Background

The Curtiss Hawk III was one of the many Curtiss Hawk biplane fighters to find that fame is hard to come by. Developed from the Curtiss BF2C naval fighter, the Hawk III was an export version of a not so successful effort. It nevertheless performed admirably in the hands of China, Thailand, and Argentina. Despite the efforts of Curtiss, the BF2C Navy Fighter was not to become a success in US hands due to a design flaw that caused excessive vibration in its all metal upper wing. It was rejected from service after only a year in fleet service aboard the carrier USS Ranger. To meet the quickly expanding export market, Curtiss redesigned the wing to an all wood structure, and re-engined and re-armed the BF2C (a number of minor variations between export orders existed, but the airframe was basically the same). The resulting aircraft was the Hawk III.

In service, 60 Hawk IIIs were to form a large part of the Chinese fight against the Japanese onslaught in the late 30's and into the 40's. Quickly outclassed by Japanese monoplanes, it was to find a niche as a ground attack plane and an advanced trainer. Curtiss Hawk IIIs were excellent aircraft, loved by pilots, and deserve more credit than is given to them.

Kit Options

In looking for a model of the Hawk III, I had three options: In 1/72 scale, MPM makes a decent limited run kit, though the engine is a bit difficult to deal with (It¹s closer to a US version BF2C engine and cowl arrangement anyway, so it isn¹t entirely accurate for the paint scheme on the box). In 1/48 scale, Sierra had just released a beautiful vacuformed kit of the Hawk III, but I had not ordered one at the time. My third option was to convert the 1/32 scale Hasegawa BF2C-1 into a Hawk III. I chose this route, since I wanted to try a simple fiberglass casting skill I had just learned.

The Hasegawa BF2C kit is a relatively old kit, and may usually be found at swap meets or via old kit dealers. Many hobby shops still have some of these kits, since they are not uncommon or rare. Its a nice kit in itself, and it bulds up with very little trouble. The conversion to a Chinese Hawk III is simple too, if you are willing to do a bit of simple scratchbuilding. The major changes are in the engine cowl, and if you build a Thai or Argentine version, some weapons fairings on the forward fuselage sides need attention too. Other changes include minor cockpit details, canopy changes, and minor changes to the rear landing gear.

I began by acquiring scale drawings, very important since there are many variations among Hawk IIIs. I selcted the Chinese version powered by a Wright Cyclone engine, which required a deeper, more enclosed cowl than the BF2Cs narrow cowl. Making a new cowl was the most challenging part of this conversion, but not difficult if planned properly. I used a method called lost foam casting, and here's how:

Casting a New Engine Cowl

Taking a three view of the cowl, I constructed a master form out of Dow Corning blue insulation foam (a dense sheet of styrofoam used by home builders). This is not as difficult as it sounds, simply cut a rough cylinder from two sheets that have been sandwiched together with white glue and then sand to shape. Use templates as you go, or simply judge using the trusty Mk.I Eyeball. Dont worry about the front opening or hollowing it out, we¹ll do that later. Just a solid form is all that is needed. Important to keep in mind is to sand the entire form 1/16² or so smaller than needed, as the fiberglass resin we¹ll be using will make this up with its thickness. Create your master with about 1/2" of additional length in the rear of the cowl, as it will be easier to cut it off to the proper length later. Once the foam master has been completed, mount it on a wood dowel or brass rod (8" long or so) by creating a hole in the rear center. White glue it in place to keep it secure while handling.

As fiberglass resin is to be used as a casting material, the foam must be protected so that it does not melt away when exposed to the fiberglass. I used white glue diluted with water (1:1 ratio) and painted the foam master with two coats, allowing the first coat to dry thoroughly before painting on the second. The glue is impervious to the resin and forms a barrier that protects the foam while the casting cures.

When dry, obtain some very fine fiberglass cloth (RC section of the hobby shop should have some) and a suitable amount of two part fiberglass resin. Mix the fiberglass resin to manufacturer's instructions, too much is better than not enough. (NOTE: If you've never worked with fiberglass, be sure to practice a few simple techniques. Check the RC department of the hobby shop for books that describe in detail the process. It is important to work with proper ventillation and a mask, as fiberglass resin fumes and glass fibers can pose a serious health risk) With a disposable dope brush (also available at the hobby shop), cover the foam master with resin. The back of the cowl can be left uncovered as it would need to be opened on the final piece, so save yourself some work. Cover the front of the cowl fully to provide strength during the final sanding and shaping process. With the resin still "wet", lay up some strips of the cloth to provide strength, overlapping them and working them into the resin until the entire casting is covered in a thin layer of resin and cloth. Work quickly as the resin is curing, and get those air bubbles out if possible! The ideal casting will be a thin layer of resin and cloth, about 1/16" to 3/32" average thickness. Don't worry about lumps as these can be sanded off later.

Finishing the Rough

When the cowl casting has cured, remove the dowl and get some very coarse sandpaper mounted on a comfortable wood block. Use the scale drawings and cardstock templates as a guide and sand, refining the basic shape to an accurate outline. Fiberglass is very hard, so vigorous sanding will be necessary if your casting has lots of bumps or needs a lot of refining. I used an orbital electric sander with great success. Wear a mask, the dust is dangerous! Gradually, your cowl should become an accurate replica, and finer grades of sandpaper should be used to create a smooth finish. Fill airholes and mistakes with supeglue if needed. If you follwed you plans and sanded attentively, your finished piece should be geometically correct: perfectly round and tapered, flat along the top, and properly curved along the forward edge.

Now for the neat part! Take your casting outside, cut into the foam master from behind to open the glue barrier, and pour a small amount of lacquer thinner on the foam master from the rear. The foam will dissolve away. Keep adding lacquer thinner, rinsing the goo from inside. When all of the foam is gone, peel away the glue barrier. Voila! A hollow, one piece casting!

Finishing the casting is tedious, but these steps ought to speed the process along:

The center opening of the cowl front is easy to open. Just mark the absolute center of the cowl opening (where the prop shaft will eventually reside) and using a mathematician's compass trace the opening on the fiberglass. When satisfied with the accuracy of the circle, replace the lead in the compass with a cutting point. I made mine from an old airbrush needle. Using the compass centered in the same spot that was used to trace the opening, scribe along the traced line. Keep scribing a line until the depth is sufficient enough that you can use it as a guide for grinding the center hole opening with a Dremel tool. If your casting is consistently smooth enough, you might be able to scribe right through, but keep in mind that the fiberglass will chip easily if you try to tap the center section out after scribing. Once opened, clean up the cowl opening with files and sanding sticks as needed.

Since the cowl master form was created with about 1/2" additional length in the rear, it is easy to cut the opening in the back accurately. Measure and mark the location of the cut you will need to make at the rear of the cowl. Lay the cowl flat on it's front. Use small balsa or wooden blocks and strips to build up a platform that is the same height as your mark. I used balsa, 1/2" thick, 1" wide, and 6" long. I glued these blocks together and then sanded them down so that the final "stick" could hold an Xacto blade on my mark. I then scribed the cowl line by keeping the cowl and the wood block flat on my work table, roatating the cowl as I went around the entire circumfrence several times. Use the back of the blade as the cutting edge may creep. The resulting scribed line is perfectly the same distance from the front of the cowl. I the used a razor saw to finish the cut using the sribed line as a guide. Clean up by sanding the rear opening on a pice of sandpaper taped to a flat surface (vacuform style).

After all this work, you now have a finished Hawk III engine cowl! Whew! And that was just step 1! Never fear, that's the worst of it.

Engine

Now prepare to install the engine into the cowl. Though your new cowl is deeper than the original kit's cowl, if it was created to proper dimesions, you will have ample room to install the engine on the kit's mounting pin on the front of the fuselage. Tape the fuselage halves together and test fit the engine. Place the cowl over the engine so that your propeller hub, when installed, will be forward enough enough to spin the prop freely. Then, mark a spot on the outside of the cowl that corresponds with the location of the cylinders inside. Do this for at least three cylinders, preferably the three that are at 90, 30 and 210 degrees from horizontal (that is, the top cylinder and two bottom ones). Drill a hole through the cowl at these three locations, and the top of each of the three cylinder heads so that you can pass a plastic rod through the cowl and into the cylinder (See the diagram). This is how the engine will hold the cowl in place. When everything is centered and aligned, glue the cowl and engine assembly together. Cut off any excess rod from the cowl and sand it flush. I added individual exhaust stacks to the cylinders so that they would extend out from behind the cowl as was characteristic of most export Hawk IIIs. I made the exhausts from aluminum tube, bent and cut appropriately. Hold off on mounting the engine and cowl subassembly to the fuselage until after painting, as you'll need to paint the forward fuselage where the engine will be mounted.

The cowl interior and engine should be painted before they are installed. I used Yellow Chromate primer for the inerior of the cowl, but aluminum is appropriate too. The engine is a standard Wright Cyclone model- medium grey crankcase, black cylinders and silver pushrod covers. I painted the mounting rods primer color since they are visible to anyone estute enough to look carefully.

Since the kit comes with a two bladed prop, we'll need to find or make a three bladed one. I used brass tube and plastic rod to make a new propeller hub, and robbed the blades from a Hasegawa 1/32 Spitfire Mk.IX, reshaped appropriately. Mount the whole assembly on a shaft made of brass rod that will fit into a brass tube installed in the engine.

Other Kit Modifications

The rest of the kit is relatively simple. I built a new cockpit for my Hawk, but you could easily use the kit's interior. To make it really stand out though, use the kit's molde in fuselage formers as a guide to make new ones, then sand the interior smooth. I used Evergreen plastic rod and attached the new formers to holes drilled at the appropriate edges of the kit's floor and rear cockpit bulkhead. I left the forward ends of the formers free, and installed them slightly skewed outwards so that when the fuselage halves were closed the former rods would compress, insuring that they were snug against the fuselage interior walls. I added a scratchbuilt seat from plastic card since I didn't care for the padding on the kit's seat, and photoetched seat belts from an old Fotocut pre-war seat belt set. A new throttle, a hand pump, compass, map case, and junction boxes were all fashioned from plastic scrap, wire and miscellaneous bits and added as necessary. New running boards were made with Evergreen channel, and I added fuselage stringers from strip. I opted to use the kit's instrument panel as I could find no drawings of Hawk III Instrument panels. I assumed there are differences, but I was having such a good time I didn't want to break the process up! The kit instruments would benefit from being drilled out and having instrument decals placed from behind on a white plastic card. A drop of epoxy to simulate glass will finish them off nicely. I regret not rebuilding my instrument panel now, but the overall effect of the new cockpit is convincing.

Interior colors for the Hawk III are fairly typical of the era. The cockpit sidewalls and the fusealge tube structure, as well as the floor and foot boards are painted aluminum (dope). Note- the Hawk III had no true floor, just foot boards, so the kit is innaccurate anyway. To replicate the floorless cockpit would have been too much of a hassle for this modeler, so I left it in. It could be painted flat black to give the illusion that it is not really there if necessary. The seat is also aluminum. I painted my seat belts a leathery brown for the lap belts, canvas (off white) for the shoulder belts. The throttle is black, and the instrument panel is a very dark grey. The control column is aluminum with a black grip. I used a mix of unpainted soldering wire and stretched black sprue to simulate the conduit and cable that connect the throtlle to the engine and the priming pump to the fuel system. The priming pump was painted silver. A map case made of plastic sheet and painted aluminum was placed on the left side of the seat, while a leather map case was painted appropriately and attached to the fuselage structure on the right.

Assembly continues by closing the fuselage and sanding the seams smooth. The next modification is the in the rear. There is a large opening that houses the tailhook on the BF2C just forward of the tailwheel- this will need to be sheeted over with plastic. Chinese and Thai Hawk IIIs often had the metal fairing removed from around the tailwheel strut assembly. On the kit, this fairing is molded as part of the fuselage halves. A simple cut along the angled indented line at the rear of the fusealge (where the simulated fabric surface meets the simulated smooth metal surface) is all that is necessary, but do this before you install the fin and rudder unit. If you choose to depict your aircraft in such manner, a new tailwheel strut will be needed. I used telescoping brass tube and parts from the kit tailwheel assembly to make mine. Check the last few pages of Squadron's "U.S. Navy Hawks" for pictures.

The final modification is the canopy. The Hawk III did not feature tha BF2C's sliding rear half-canopy, so delete it from the kit. Fill in the slots and cut off the rail for the sliding glass on the kit's fuselage. Also, the windscreen could use replacement. It's not hard, as the panes are flat. I used clear styrene and cut a new one, folding each side panel along a scored line. I also cut open a hole for the gunsight. Simple enough, but still it took me sevral times to get one that fit well!

With these steps finished, assembly of the rest of the aircraft is straighforward, just as the kit instructions depict. Only one more item of note- Despite what some illustrations of Chinese Hawk IIIs would have us believe, the circular "thing" attached on the lower fuselage between the wheel wells is usually not present on Chinese aircraft. This device was an effort to improve stability and was unnessecary with the late version wooden wing, lighter structure, and new engine.

Final Details

With all this work already invested in the aircraft, why not add some detail where the kit lacks? Drop the elevator and add some control horns to them with some music wire. See the squadron book for details on what they look like. Also, replace the gunsight or drill out the ends for more realism. Painting the Hawk III is best done without the upper wing installed. I used Floquil Brown for the primary color, which is actually a drak olive green that dries with a brownish green finish. I read that many Chinese Hawk IIIs were painted overall red-brown, but since all the color illustrations of Hawk IIIs I found featured the more common Chinese dark green, I chose the latter simply based on aesthetics (Photos seem to back this up, but it's too hard to determine for sure). The undersurface was painted in light grey, but not all Chinese Hawks features this. Some were a solid color overall. Decals for the national insignia came from a 1/32 scale Polikarpov I-16 sheet, and the white fuselage numbers from the scrap box. I also installed a data plate in the left wheel well from a Waldron set, but if you got a magnifying glass out you'd see it's actually for a B-17! I don't think anyone can read it without high magnification, so I didn't fret.

I used monofilament for the rigging, and drilled out the holes to glue the rigging into with a super fine drill bit in a pin vice. The holes are located for you by small circles on the kit surface. Paint the rigging dark steel, and make the stabilizing rod in the wing rigging with a piece of plastic rod rounded at both ends. See drawings and photos in Squadron's book for reference.

Final steps include weathering, navigation lights, and a semi-gloss varnish. Whew! After all this work, you now have a Hawk III from a BF2C. It would really look nice next to a 1/32 scale Nate!

Scott is the editor of the great IPMS/Houston web-site. The site is really big with a lot of tips-well worth a visit!!

© Scott M. Head