Infield Engineering P-40C Part Two
Part 2: Wing and landing gear
Welcome back to the second installment of the Infield Engineering P-40C build! Last time, we completed the construction of the tail group and the fuselage. This month, we’ll move on to the construction of the wing and main landing gear.
As mentioned last time, this build is an offshoot of the P-36/Hawk 75 from the October 2018 "Fighter Face-Off" seen here in Model Aviation. Like last month, I’ll focus more heavily on describing things that are new, while providing basic instructions of what was discussed in detail for the similar P-36.
Framing the Wing
To get the wing started, pin a main spar down to the building board. I used 1/8 × 3/16-inch hard balsa, but basswood works well if you feel you need more strength. Shim between the board and the spar with 1/16-inch scrap. This will raise the spar into the ribs so that the sheeting will later cover the spar.
Stand rear spar RS on top of the plans and start gluing in the ribs. Leave root rib W1 for last—it will be tipped at an angle by using the dihedral gauge on the plans. The angle of the root ribs will set the dihedral when the wing halves are joined. Tie all of the ribs together with the upper main spar, the 1/8-inch square balsa stringers, and the trailing edge (TE; part TE). Attach shear webs S2 through S7 to the back sides of the main spars.
Start the ailerons by gluing riblet A1 to the inner side of the aileron pocket. Now add doubler A2 to the back of the rear spar to form the front of the aileron pocket. Set the aileron’s leading edge (LE) A3 against A2, but don’t glue them together—this will be the parting line between the wing and aileron. Complete the aileron assembly by gluing the rest of the aileron riblets to A3 and the TE.
Unpin the assembly and flip it over. Epoxy plywood shear web S1 securely to the back of the main spars and to ribs W1 through W4. Epoxy plywood half ribs G1 and G2 to the main spars and to S1. Fit the LE into place to ensure that the ribs are aligned at the LEs. Glue the LE into place after the gear bay parts have cured. Stack and glue parts WT to form the wingtip.
Pin the assembly back down with the retract pads hanging over the edge of the board. When the feet under the ribs are pressed flat to the board, the proper amount of washout will be set. Lightly sand the assembly between the LE and the main spar on the top of the wing and between ribs W1 and W2 behind the main spar.
Now, sheet these areas with 1/16-inch balsa. Lightly wetting the upper surface of the sheeting will make the installation easier. After the upper sheeting is cured, the wing should be rigid and straight. Unpin it, flip it over, and remove the alignment feet from the underside. Sand and sheet the underside where shown. Double up parts G3 and G4 and epoxy them to the outsides of ribs G1 and G2 and to the sheeting to form the retract mount bosses. Finish the wing by attaching 1/4-inch soft balsa to the front of the LE and sanding it to shape.
Aileron RDS
For most projects, I try to do something new. In the case of my P-40 prototype, this meant using a rotary drive system (RDS) to actuate the ailerons. RDS has been around for a long while in the glider community. In my case, I found it an attractive way to eliminate the exposed control linkages that were used on the P-36/Hawk 75 project.
To be clear, RDS ailerons are not a requirement for this build.
Ready-made RDS hardware is available, but I opted to build my own. The system is simple, composed of a collar that screws down to a servo wheel, a bit of music wire trapped in the collar at one end and bent at a 45° angle at the other end, and a box that traps the bent end of the wire. When the servo rotates the wire, the bent arm deflects at the other end. The deflection can be used to move a control surface, such as an aileron, by trapping the bent end of the wire in the box.
To get started, a box was fabricated from balsa to house the aileron servo. Instead of mounting the servo perpendicular to the aileron parting line, it was rotated 45°, as shown in the photo. A removeable door was made to cover this servo bay.
To make the hardware, a 3/32-inch wheel collar was silver-soldered to a 1/2-inch outer-diameter stainless steel washer. The actuator rod was made by bending a length of 3/32 music wire. The bend was placed at the aileron parting line. When the placement of the rod was known, a hole was drilled through the rear spar for the control rod to pass through.
The box to trap the rod was built from two 3/32-inch plywood plates. The plates were spaced at each end with scrap balsa strips so that the 3/32-inch control rod fit closely but could still rotate freely. The box was then epoxied between two of the aileron riblets and the aileron’s LE.
As a first experience for me, I was surprised at how little slop there was in this linkage. This is really a slick system—I’ll definitely use it again.
Retracts
Similar to the P-36/Hawk 75, the retracts for the FMS 1,400mm P-40 are a perfect fit for this model. To install, set the retract, strut, and wheel assembly in the retract pocket. Push the retract forward so that it is snug against the back of the LE. Mark the wheel position on the lower wing sheeting, and then cut the hole for the wheel.
With the wheel passing into the opening, mark the screw positions in the retract plate on the retract pads in the wing. Drill holes for the screws, harden the holes with CA, and then fasten the retracts into place.
Rigging the Main Gear Doors
Vacuum-formed main door blanks are included in the laser-cut kit and are also available from Park Flyer Plastics. The simplest solution is to just fix the blanks to the wing and cut slots in them for the struts to pass through. Builders who take this option will have a simpler build that looks good.
With a little extra work, the same blanks can be transformed into working doors. This work can be finicky, but the result is well worth it!
For the prototype, this subproject began by replicating the fairings around the struts with scrap balsa. Included in these fairings were rails that would trap the plastic gear door blank. After some careful sanding, the edges of the blank slipped into place. The doors were then marked and cut free from the blanks.
Hinges were made with music wire fitted to bits of aluminum tubing. After thoroughly cleaning and scuffing the inside of the door and fairing, the hinge parts were epoxied to the plastic. The photos that are provided show how a simple tubular hinge is sufficient for the front of the door, but an offset hinge is needed for the back where the door hangs free.
To close the door, small torsion springs were made from a steel guitar spring. The spring pressure can be adjusted by the gauge of the spring (I used .013 inch) and the number of winds (more winds equal less force). To make the spring, a small drill bit was clamped in a vice, and then the guitar string was pulled tightly and wound around the bit. The resulting spring was trapped by the music wire hinge pin to snap the door open.
Closing the door was much simpler. Many full-scale aircraft have used cords to pull landing gear doors closed. This was accomplished by attaching one end of the cord to the door while the other was fixed to the airframe. When tensioned properly, the retracting strut will press the cord into the open bay and pull the door closed behind it. Of course, there are other solutions available to the builder, including attaching a spring between the door and the strut or using servos linked to the door.
For my model, the closure began by drilling a small hole in the middle of the main gear door. A bit of 20-pound test Kevlar line was knotted at one end and pulled through the hole until the knot was tight against the outside of the door.
A hard balsa post was epoxied to the wing on the other side the bay. After epoxying the line to the post, the line will hang across the bay with the strut down. This allows the torsion spring to hold the door open.
When the gear retracts, the strut will press the Kevlar line up into the bay. The increasing tension overcomes the spring and pulls the door tightly closed just as the strut settles into the fully retracted position. These same techniques were used for hinging and closing the tailwheel doors.
That’s All for Now, Folks!
That completes the construction of the wing, but there is still plenty of work left to do. Next month, we’ll tackle the tail group controls and add some details, then we’ll finish this model and get it into the air. Until then, fly low and build light!
SOURCES:
Manzano Laser Works
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