Build the Curtiss P-40

Part 1: Design, tail group, and fuselage

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Several years ago, Derek Micko and I challenged one another to a Fighter Face-Off. This involved each of us designing and building a fighter that had opposed the other in combat. The results of our projects were published as a series of articles in Model Aviation, beginning in the October 2018 issue. You might recall that my choice was a Curtiss P-36, also known as the Hawk 75 when in foreign service. A number of people along the way noted that it would not take a great deal of effort to convert my P-36 kit into an early P-40. After all, Curtiss simply swapped the P-36’s radial engine for an Allison V-12 and reworked the sheet metal forward from the firewall when the first P-40 was introduced.

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At a Glance

Specifications Wingspan: 60 inches Length: 50.4 inches Wing area: 591 sq. in. Flying weight: 79 ounces Power: 4258-650 Kv motor; 4S 3,800 mAh LiPo battery; 14 × 8 three-blade propeller What was good enough for Curtiss was good enough for me. After going back to the CAD model, I also lopped off the nose of the P-36 from just in front of the windscreen and lofted a new P-40 nose into place. A bit more work produced a few new formers, a new motor mount, and a 3D-printable spinner. The resulting P-40 prototype will be the subject of this series of articles.

Some P-40 History

Like the P-36, Curtiss’ P-40 was quite a success story. Although it is often described as being obsolete at the beginning of the war, it was actually just entering service at the time of Pearl Harbor. The P-40 was extremely responsive over a wide range of speeds, dove like a bullet, and could take tremendous punishment. What it could not do was operate aggressively at high altitude like a Messerschmitt with its superior higher-boost supercharger, nor could it beat a lightweight opponent, such as a Zero, in a low-speed turning fight. Despite the P-40’s shortcomings, pilots who learned to leverage the P-40’s strengths were tough to beat and many became aces. Curtiss continually improved the P-40, with variants reaching all the way to the P-40R by 1945. Although steadily gaining in weight and power, all models except the P-40Q were readily identifiable as offshoots of that first P-40 that was converted from a P-36. Nearly 14,000 P-40s were produced throughout the course of the war and flew for two dozen nations.

Building the P-40C

Because this model was derived directly from a P-36, it resembles a P-40A, B, or C model. There were a few obvious differences between these variants other than armament and the addition of a drop tank for the C. I plan to add a tank to the prototype at some point, so I’m calling my model a P-40C.

Building the Tail Group

Construction of the tail group begins by laminating the outlines of these parts. Laminations are stronger than built-up balsa parts. Start by making a form for the fin/rudder and another for the horizontal stabilizer and elevator. Forms are often made from foam board or thin wood. Soak 1/2-inch wide strips of 1/16-inch balsa in water overnight or longer. Wrap the softened strips around the form. Use carpenter’s glue to adhere each successive layer to the previous one. After the outlines are fully cured, they can be pinned over the plans. Assemble the tail group inner framework parts in numerical order. Most of these parts have feet, so stand these parts on the board with the feet down.

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This will ensure that these parts are at the correct angle during assembly. Don’t glue the parting lines for the rudder and elevators together! They should only be connected by the outlines. Cut through the outlines after the framework is fully assembled to free these parts. The fin and horizontal stabilizer are both sheeted with 1/16-inch balsa. Sheeting their upper sides while they are pinned firmly to the board ensures that they will be true. After the top sides are cured, unpin them, remove the alignment feet from the tail group parts, and sheet the bottom sides. After sanding the leading and trailing edges into shape, the tail group construction is finished.

Building the Fuselage

The upper half of the fuselage is built on top of the building board. Pin the horizontal keels to the board. Note that K1 is slightly longer than K2. When positioned in the correct spots, the fuselage opening will be aligned with the motor for right thrust and downthrust. Now add the upper former halves. Use the paper angle gauge to set the angle of the tilted formers that make the back of the battery hatch and the back wall of the cockpit. Tie the rear formers together with keel K3. Next on the list is the battery hatch opening. Assembling this hatch is simple if it is done as described here. First, glue the lower hatch rails K4 to formers F3T, F4T, and F5T. Now, prop hatch formers F3H, F4H, and F5H into their positions. Next, tie all of the forward formers together with keel K5, including the hatch formers. Slip the upper hatch rails into position from each side and glue them only to the hatch formers. It helps to put waxed paper between F3T/F3H and F5H/F5T to ensure that they don’t get glued together. After the fuselage is completely assembled, the hatch will be freed with a razor saw by cutting K5 between these former pairs. Install the side window panels next. Use the side view on the plans as a template to cut two 1/16-inch balsa panels slightly oversized to make the inner panels. Dampen the panels and glue them to Formers F7T through F9T. Now, add a few 3/32 × 1/8-inch stringers to keep the fuselage assembly true so that it can be removed from the board. After unpinning the upper fuselage, glue plywood former F1 to keels K1, K2, and K5. Assemble the tailwheel plate by gluing TW, F9B, and F10B together. Build the wing bolt boss by assembling parts WB1, WB2, and F6B. The drawings on the plans help explain these steps. Use epoxy for maximum strength in these critical areas. Now, all of the lower fuselage formers can be glued to their upper former mates. Glue each one at the same angle as the matching upper former. Tie formers F1 through F4B together with keel K7. Dampen the outer sides of wing saddles WS and glue them to F4B through F8B. Tie formers F8B through F11B together with keel K8. Now, add the rest of the stringers, alternating from side to side and top to bottom to avoid warps. The base fuselage construction is finished when the last stringer goes in.

Building the P-40: Chin Scoop and Nose Fairings

Our build will depart from the P-36/Hawk in earnest now as we add the large chin scoop. Start by dry-fitting scoop formers S2 through S4 to center scoop rail S1. Adjust the fit of these parts until this assembly mates closely to the underside of the fuselage assembly. Glue the chin scoop parts together.

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Add the two S5 scoop dividers, followed by the S6 and S7 scoop lips. Now tie the assembly together with the S8 side panels. Remove the scoop assembly to cover the fuselage. Glue the scoop into place before covering it. Now is a good time to talk about infill. It might sound counterintuitive, but filling in some of the openings between the stringers with balsa panels is going to help out the build. Adding infill will add rigidity (the battery hatch), strength (in front of and behind the wing), and curvature (areas with tight radii that would get the "starved horse" look if left open). It’s perfectly okay to skip this, but the benefits are worth the extra effort. Infilling each opening one at a time sounds tedious, but it goes quickly. Using the softest 1/8-inch balsa on hand for infill is a good way to use up balsa scraps and wood that is too light for structural parts. Choose an opening and cut an infill panel to match the length of the opening, but make it a little too tall. Bevel the sides by sanding so that the panel drops into the hole. The long sides should stand above the adjacent stringers. Install another panel opposite the first in the same way. Keep going while alternating from side to side and top to bottom to avoid warps. After the last infill panel goes in, sand the assembly to shape with a sanding bar. Back to the chin scoop. This is an area that will benefit doubly from full infill. As a prominent feature with a tight compound curvature, this is an area that would be prone to wrinkles and sags in the covering if it is left as an open structure. Additionally, an open structure wouldn’t survive many of the inevitable nose-overs or (gasp) a belly landing. After full infill, the scoop will be much more durable, particularly after a layer or two of 1/2-ounce fiberglass. The last thing on the list for today is the fairings for the two nose-mounted, 50-caliber guns and the distinctive, round carburetor scoop between the guns. The gun fairings were made from scrap balsa. After cutting the fairings roughly to size, they were lapped in by wrapping sandpaper around the fuselage and sanding the foot of each fairing in place. A little more sanding filleted all of the sharp corners. A Dremel tool was used to make openings for the 3/32-inch aluminum-tube gun barrels.

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The carburetor scoop started with a trip to the scrap bin. I save many of the little discs and hexagons that pop loose from lightening holes in laser-cut kits. I found a small, plywood disc that was just the right size for the P-40’s scoop opening. This was attached to a long box made from more scrap balsa. Similar to the gun fairings, it was lapped into place until it fit tightly to the top of the fuselage. The front of the scoop was sanded using the plywood disc as a guide. After the shape was right, an opening was made by drilling through the plywood disc. The opening was hardened with thin CA adhesive.

Until Next Time

At this point, the nose and the tail group of the project are screaming P-40! This is a good time to set these parts aside and shift focus. Next month, we’ll tackle the wings and internal hardware. Until then, fly low and build light!

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