Bill Bowne has designed this sport scale model of the PT-19, which he has simplified to save weight and reduce complexity. It builds into an easy to fly, four channel model for a 1400 KV motor, with an 18A ESC and a 3S 1000 mAh LiPo
Thousands of WWII US and Commonwealth pilots started their careers flying Fairchild’s PT (Primary Trainer)-19. Despite structural issues with their wooden wings, nearly a hundred of those PT-19s still fly, many dressed in variations of the early war US Army blue and yellow colours. Those cheerful colours made it easier for student pilots to see other novices as they trundled around their training fields. Now, those same bright colours aid those of us with ‘mature’ vision. So when I looked for a new building project the PT-19 came to mind.
Like many of us I built tissue-covered rubber-band models when I was younger. I’ve always been fond of the look of tissue so I decided to do this model along those lines. But, I chose to ‘cheat’ and use Litespan, trimmed with doped gift-store tissue paper. Much easier, yet it still looks like Silkspan.
This is a stand-off scale model, simplified to save weight and reduce complexity (like those rubber-band models of my youth). I’ve left off the wing fillets and, to fit an inexpensive brushless outrunner, I’ve widened the cowling. Whilst the model is recognisably a PT-19 it isn’t a competition scale model.
Construction is a bit more complex than the usual slab-sided model and benefits from using jigs. Those jigs are well-worth the effort, providing a light, straight structure that the Litespan covering shows off.
We’ll start by deciding whether to build the wing one panel at a time (one right and one left, please!) or (as I did it) all at once on a hinged building board. I’ll try to describe the ‘one panel at a time method’ but be advised I haven’t built a wing that way in over 20 years!
Once that choice is made it’s time to start cutting and gluing balsa.
Build the undercarriage/landing gear (LG) blocks as sub-assemblies. Slot the trailing edges (TE) for the wing rib aft ends (two hacksaw blades taped together make a 1/16” slot). Laminate the wing tips and rudder from layers of 1/16” balsa. Glue R2 to R2a and R3a to R3 (make a right and a left-handed version of each). Glue the 1/8 x 1/4″ hardwood strips under the servo trays.
Pin down bottom spar and TE. Criss-cross the spar with pins – sticking pins through a spar weakens it.
Cut bottom sheeting to fit the LG blocks. Leave it a bit long in front of the blocks to accommodate the aerofoil underside near the leading edge (LE) and glue it the spar and TE. Leave that extra under the LE loose, for now. Glue the LG blocks and ribs R2-R4 to the spar and sheeting, then add the rest of the ribs. If building one wing panel at a time use the dihedral guides when gluing on F1.
Glue 1/4” sq balsa LE to the fronts of the ribs, add the 1/16″ shear webbing and 1/16″ ply dihedral brace; the shear webbing grain is vertical. When dry, sand the webbing tops to match the spar slots (a bit of old spar stock with sandpaper glued to it works well), then glue on the top spar, the wing tip laminations and support gussets.
Glue the wing sheeting to the 1/4” sq LE and let it dry thoroughly before the next step.
Moisten the TOP of the wing sheeting with ordinary tap water (a damp paper towel is fine). Spread slow-drying glue on the spar and rib tops, then bend the sheeting down over the spars. Secure it to the spar with clothes pegs or bulldog clamps and pin it to the rib tops.
Trim the sheeting excess along the spar. If doing both wing panels at once, glue and clamp down one side, then bend the other down and cut both along the centre rib. Pin and clamp down both sides and let dry. Sheet the top centre-section. Cut out for, and glue on, the aileron servo mount.
Run a strip of plastic sandwich wrap between the 1/8” balsa TE base and the rear spar. Glue the aileron LE, blocks and ‘ribs’ to the base. Pin the aileron/TE sandwich to the rear spar and the work board.
Protect the rear spar with masking tape and set the TE thickness with a length of 1/8” music wire, then sand the assembly to a taper.
Cut the ailerons loose from the rest of the TE and bevel their LE’s to a shallow ‘V’. Slot the centre-section TE for the torque rods (and their plastic tubing bearings), notch the centre section and rear spar, to allow the torque rods wiggle room, then glue on the TE bits (trapping the torque rods).
Remove the wing from the board, finish the bottom front sheeting, and add the bottom spar extension at the wingtips. Remove all the pins, clothes pegs and other clamps. Apply balsa filler (as needed), then sand, sand, sand! Now, set the wing aside and get started on the tail feathers
Make the forms from corrugated cardboard. Glue the layers together with a glue stick, alternating the corrugation direction, making the composite about 1.5 times the thickness of the desired piece. So, for a 1/8” piece, the form should be at least 3/16” thick. If doing two parts at once, such as the wingtips, make the form 1.5 times the thickness of BOTH pieces, in this case at least 3/8”.
Saw and sand the cardboard just as you’d saw and sand balsa block, leaving around 1/2” extra on each end of the form for easy handling. Then wrap tape around the edges of the forms, followed by a protective layer of plastic sandwich wrap.
Soak the wood strips in very hot (nearly boiling) water. Ammonia is NOT needed! One at a time, dry the strips, apply a thin layer of carpenter’s glue and wrap the strips around the form. Hold the wood strips in place with tape and rubber bands and let dry thoroughly.
Make the tail surfaces as two single units, separating the control surfaces later. Start by pinning down the lettered pieces (i.e. ‘S1’, ‘V1’), then fit and glue on the straight perimeter bits from hard 1/8″ x 1/4″ balsa. Fit and glue on the rudder lamination. Now, do the 1/8″ sq and 1/8 x 1/16″ ‘ribs’ and gussets for the fin and tailplane.
When dry, separate the rudder from the fin (including the aerodynamic balance) and separate the elevators from the tailplane. Sand the hinged areas to a ‘V’ (gently round the rudder balance LE). Drill and fit (but don’t glue in!) the music wire elevator joiner.
Prepare by gluing F4a to F4b and the temporary 1/8″ sq balsa braces to F5 and F6. Pin the 1/32″ balsa forward fuselage side and the 1/32″ aft fuselage sheet to the plans. Note that the aft sheet grain is vertical. Glue the 1/8″ sq upper and lower side longerons to the sheet, followed by the 1/4″ sq stabiliser brace and the wing saddle.
Mark the locations of the bulkheads on the fuselage sides, then glue the diagonals and the corner gussets aft of the wing in place. Use the bulkheads as spacers, but don’t accidentally glue them in yet.
Glue bulkheads F4a/b, F5, F6, F7 and the wing bolt plate to one fuselage side and keep them perpendicular to the side until the glue dries. Glue the opposite side to the first and keep it aligned until the glue is dry. It will be rather ‘floppy’ so please handle the sides carefully.
Unpin the fuselage from the board and align it over the top view. Time for another jig, this one for the fuselage. Sand a bevel in the aft fuselage, join it at the tail and add in all the remaining bulkheads and F11. Before gluing in the firewall make sure you’ve drilled it out for your motor and that the blind nuts are firmly set in the firewall’s rear.
With the fuselage upright add diagonals between the top longerons aft of F7, then the 1/8″ sq top centre longeron from F4 to F10, followed by the top 1/16″ sq bass longerons between F7 and F10 and the 1/16″ sq anchors between F4 and F7. When dry, sand the front half of the stringers, leaving a 1/32″ lip. Add the 1/16″ sq sheet anchors then sheet between F3 and F7 with 1/32″ balsa.
Moisten the outside of the wood and spread slow-drying glue along the bulkheads and the centre longeron. Now, carefully wrap one side in place, then wrap the second side over it. If you can’t see the centre longeron on both ends mark the wood being wrapped.
Holding both in place, use a straight edge to cut through BOTH sections, down to the centre longeron. You may find an extra set of hands helpful at this point – just don’t cut their fingers! Let the bits unwrap, remove the excess, then press the sheets back into place. You should have a neat joint that runs right down the centre longeron. Wrap with masking tape and let dry, then trim the excess. Turn the fuselage over and add the bottom aft longerons.
Centre the fuselage on the wing, sanding off the wing TE at the centre section. Drill through F4 into the wing centre ribs, then install the 3/16″ dowel in the wing. Remove the wing and round off the front of the dowel.
Square the wing with the fuselage, then drill and tap for two 8-32 nylon bolts. Add the 1/32″ ply wing bolt reinforcement plate, then epoxy a 1/2” strip of fibreglass tape around the wing centre section.
Add the bottom forward centre longeron and 1/16″ sq braces, then sheet under the nose with 1/32″ balsa in the same manner as you did for the fuselage top. Glue on the ‘cheek’ reinforcements. Save the 1/16″ bass side longerons until the tail is on.
Bolt on the wing, then add the tailplane and fin, aligning the tailplane with the bottom of the wing. With the Clark Y-ish wing section on the PT-19, having the aerofoil’s flat bottom section parallel to the horizontal stabiliser/tailplane automatically generates the right angle of attack.
Place plastic sandwich wrap over the battery bay opening. Sticking pins in from the outside of the fuse, pin lengths of 1/8″ sq balsa between F2, F3, and F4. Keep the tops of these strips even with the tops of the fuselage side longerons. Now, glue the 1/16″ balsa sheet hatch base to those strips, letting the sandwich wrap keep you from gluing the sheet to the fuselage. Remove the pins and verify the assembly can be removed before proceeding. CA glue can be sneaky!
Build up your hatch with blocks and strips of softwood. I used some 1/2″ triangle stock at the rear of the hatch, with a matching piece in front of F4, making for an instant 45 degree joint. Keep using that sandwich wrap, to avoid making the hatch one with the fuselage!
Drill through F1 to get to F2 and the hatch, making a hole for the 1/8″ hatch front dowel. CA some super-magnets into the top hatch magnet plate, then CA the bottom plate UNDER the side longerons. Now, add the top hatch magnet plate and the dowel.
Put bits of tape, sticky side up on the magnets and press the hatch in place. Remove the hatch and screw flat-head screws through the tape; make sure the screws are ones that the magnets will stick to!
With the hatch firmly held down by magnets and dowel, sand it to shape.
Gently probe with a pin through the fuselage top to ensure you’re clear of the bulkheads. Use a glue stick to hold the cockpit opening template in place, then cut out the cockpit and crash pylon openings with a sharp hobby knife.
Install the servo tray then tape the rudder and elevators to the fin and tailplane, noting which side has the elevator control horn plate. Then install your pushrods. I used .032″ (.81 mm) music wire, for the PT-19, with a ‘Z’ bend at the servo end and a threaded connector soldered to the control surface end. This leaves the clevis easily accessible, plus it adds some stiffness to the pushrod beyond the plastic housing tube. Brace the pushrod housing at F7, F8 and F9, to keep the pushrod from bending under stress.
Glue 1/32” balsa floors to the two cockpits, leaving room in the front for battery and ESC cooling air to exhaust.
Final Assembly And Covering
Cover the nose bowl separately from the rest of the fuselage, then glue it on over the motor AFTER you add washers for down and right thrust. You may be popping the bowl off to service the motor, so don’t glue it down too securely!
All of the white, medium blue, dark green and yellow ‘tissue’ was done with Coverlite (or use Litespan), whilst the red, black and dark blue trim was done with ordinary coloured tissue paper.
The crash pylon was built from some old plastic tubing and a bit of bass, and the pilots were carved from blue foam.
Finished weight came out at 20 ounces. A 1400 KV Turnigy outrunner on an older 3S 1000 LiPo turned an APC 6 x 4 E-prop, generating approximately 90 watts at 10 amps for a power to weight ratio of about 70 watts/lb. Since my ESC was only rated at 18 amps, I wanted to ensure a comfortable buffer for the maiden flight.
Came the day of the maiden the wind was right down the runway, albeit it a very short, but very wide runway – in other words it was a 90 degree crosswind! Happily it was also a light crosswind but one thing I learned from my previous PT-19 models was how much that big tail made them want to weathercock into the wind!
Rudder control turned out to be strong enough to keep the PT-19 close to the centre-line during the take-off roll. Acceleration was good, but it was clear that more thrust would be nice. I can’t say the model flew ‘off the board’ as I had to put in quite a few clicks of aileron, elevator and rudder. The stall was pretty tame but rolls required full aileron and rudder combined.
After stooging around until the nice Spektrum lady said ‘Time Expired’, I made an uneventful landing and took a look at the transmitter. Gee, I’d put in a lot of right aileron, down elevator and left rudder trim… Wait a minute – RIGHT aileron and LEFT rudder? No wonder the rolls were so bad! Once I zeroed out the rudder trim the model flew a lot more nicely, with a decent, more axial roll.
For the next flight I replaced the APC 7 x 4 E with an ancient Graupner 6 x 5. The top speed didn’t increase that much but the model pulled through loops a lot more steadily. Plus, it only pulled a few more amps, bringing the power up to 100 watts total (79 watts/lb).
So, she’s a pretty model, especially during those low, slow (and surprisingly steady) passes, with the sun shining through her covering. She’s rapidly become our favourite for those quiet summer evening flying sessions.
Plus, it’s a lot easier to change batteries than it was to wind up those darned rubber motors!
At a Glance
Wingspan: 45″ (1143 mm)
Wing Area: 288 sq in (18.58 sq dm)
Wing Loading: 10 oz/sq ft
Length: 32.5″ (826 mm)
Weight: 20 oz (567 g)
Radio Functions: Throttle, Ailerons, Elevator, Rudder
Servos: Hitec HS-55 (3)
Basic Construction Materials: Balsa, Ply, Spruce, Bass, Lite-Ply
Covering Material: Litespan/Coverlite and tissue
Motor Used: Turnigy D2826/10, 1400 KV
Propeller: APC 6 x 4 or Graupner 6 x 5
ESC: Castle Thunderbird 18 amp
Battery: 3S 1000 mAh, 25-35C LiPo
C of G: 2.75” back from LE at centre section (i.e. under the main spar)
Rudder: +/- 1/2 inch
Elevator: +/- 3/8 inch
Ailerons: up 1/4 inch, down 3/16 inch
CAR Mixing: 25% rudder with aileron