Sport Scale Curtiss JN-4 ³Jenny²
by David Fielding
Anyone who loves flying remembers their first flying experience. For me, that means that no matter what else may be on the ramp, my eye will wander to the nearest Piper Tomahawk. Thousands of other pilots first soloed in the modest Cessna 150. As our instructors watched us climb away, they may have been remembering a buttercup yellow Piper Cub in the beginning of their logbook. And the odds are good that their instructors learned the art in a Curtiss JN-4 Jenny.
If flying is an unforgiving discipline, then trainers must be the exception to the rule. When you are learning to dance, it helps if the music is slow and your partner is patient while you step on her feet. The JN-4, built from 1915 on, was one of the first airplanes  stable enough to make pilot training a survivable experience. Built in the U.S. and Canada in huge numbers, Jennies were the first step for many of the first generation of fliers. Jimmy Doolittle, Wiley Post, Ernest K. Gann, and Col. Robert Scott were among the legions who started in Jennies. Lindbergh first flew in a Standard, but bought a Jenny and flew it home before he had quite finished his lessons!
When the Jennies had mustered out of the service after WWI, they turned vagabond as the classic mount of the barnstormers. The young pilots of the 1920ıs ranged across the country, sleeping on the prairie under the wings of their Jennies. Their stock in trade was their love of flying...and thousands of small-town people who had never seen an airplane paid a few dollars to watch the ground drop away from under the wings of a chugging Jenny.

The Model
There were several kits available for the Jenny that I knew of. All were for complex replicas, reproducing the delicate structure and bracing wires. (the full scale Jenny contained over 2 miles of wires and cables!) What I decided to try was a simplified, sport-scale, ³everyday² flier which would be easy to build and transport to the field in my very small car. The subject looked fairly simple; the outlines are mostly straight, the wings have a constant chord, and the fuselage cross-section is a rectangle with a simple turtledeck. I found an article on the Jenny which showed Curtiss factory line drawings of the fuselage and wing outlines. I scanned these into my computer to establish scale outlines, then began to add my own construction details.
I decided that a flat bottom airfoil would provide plenty of lift - with 996 square inches of wing area at 72 inch span, the complexity of the scale undercambered airfoil is not necessary. Besides, the flat bottom makes the structure much stronger, and makes the wings easier to build and cover. It also allows the aileron servo room to hide in the top wing, making the ailerons easy to hook up. I looked at dozens of airfoils in textbooks, compared their lift and drag characteristics, then threw them all out and traced the airfoil from my Sig Kadet plans. (Thank you, Claude McCullough.) The rest of the plans were drawn with simple and cheap in mind, using as few complex parts and as much straight lumber as possible.

Construction
Tail
The whole tail is made of 1/4" balsa, with several straight pieces used to make the curves, and is so simple I wonıt bore you by describing it further. Build it and hang it on the wall for incentive.

Fuselage
The fuselage sides build up over two lite-ply crutches which are the only complex cutouts in the fuselage. The rest of the structure is straight, cut-as-you-go 1/4" balsa. While youıre at the bandsaw (or your buddyıs bandsaw), cut out the firewall and cowl pieces, stack your ribs and false ribs (all 78 of them!), and you will have the whole Jenny ³kit². Build the fuselage sides over the plans; the left side can be built over the crutch, the right side built flat and then flipped over to attach the crutch. When you have the sides built (the plywood goes on the inside), join them at the firewall, F1 and F2 bulkheads using triangle stock to locate and glue the bulkheads. This is much easier than slotting the sides and is plenty strong when the rest of the structure is added.
The fuel tank floor is added next, and a few words about the tank setup: I decided to make the fuel tank easily accessible and removable by building the tank area open, and fuelproofing it back to the T2 bulkhead. There is plenty of room for this, the structure is not substantially weakened, and it allows a short, simple connection over the firewall to the carburetor. I prefer not to use fuelers, because they require another break in the tubing - with a removable hatch, you can fuel the tank directly. This setup has worked for several months of flying my Jenny, and has been the best system I have used; simple, clean, and less to go wrong. You can build the tank in if you like - it should still be accessible through the bottom - but Iıve heard it said, ³a hidden tank is a leaky tank².
Now the rear of the fuse can be pulled together and joined. All of the bulkheads from the cockpits to the tail are 1/4" square balsa - add them now, and the diagonal bracing. Head up front for the cabane struts. Bend them from 1/8" music wire, and use a jig to hold them in position, level with the top rail of the fuselage. This is critical to establish the top wing incidence of 0 degrees. With the jig in place, you can fit the 1/8" ply pieces to the sides to locate the angled lower ends of the struts, and sandwich them with epoxy with the outer 1/8" ply pieces. This comes out level with the 1/4" balsa side members outside the ply crutch, and makes a strong attachment with no interference inside the radio compartment. Before you add any of the top turtledeck, solder on the diagonal wire bracing, necessary because of the forward sweep of the cabanes.
The cockpit floors go in now - they can be thin ply. Be sure to leave 3/32" space at the sides for the top sheeting to fit against. The servo lead for the ailerons plugs into the front cockpit, and I put the charging jack and on/off switch there, too. Cut the holes for them before you add the top structure. The turtledeck is all straightforward; stringered with 1/4" x 1/8" balsa, sheeted with 3/32" balsa from the cockpits forward. After you build the two small stringered segments forward of each cockpit, the whole forward sheeting can be added in two pieces, working from the side rails up and joined on the centerline. After the sheeting is in place, the template on the plans will mark the cockpit cutouts.
The nose section can be built any number of ways to suit your engine; the plans show how mine is set up for the O.S. .48 4-stroke. The sides and front of the cowl are 3 lite ply pieces, epoxied to the firewall with triangle stock and fuelproofed. The bottom of the fuselage is 3/32" balsa sheeting, around the forward landing gear block, which is epoxied into the cutouts in the side crutches. This leaves plenty of access to install or remove the engine. I put a drain at the bottom of the engine well at the lowest point next to the firewall. The hatch is built up just like the fuselage, over a flat piece of ply, which is then fiberglassed and cut out to suit your engine. I found an old cigar box with a perfect latch to allow the hatch to be hinged on the left side of the fuselage. For me, part of the fun of scratchbuilding is finding ways of using found materials (junk) to do the job of storebought components.

Wings
The wing panels go together quickly. Pin the lower spars over the plans (1/4" x 1/4" inch - spruce forward, balsa rear) and glue the full ribs over them. Notch the trailing edge - course emery cloth glued to the edge of a ply scrap works great - and glue in place. Drop the top spars in place, and add the leading edge. I used a single sheet of 3/8" balsa cut into strips for all 4 leading edge sections, and shaped them with a hand plane, but there is probably pre-shaped leading edge stock that would work. With the leading edge in place, add the false ribs, and then a-a-aall of the spar webbing. The webbing adds a great deal of strength to the unsheeted wing, forming a wooden box ³girder² at the main spar. Sheet balsa tips with balsa supports can be added while the wing is flat on the board. For the top wing, add balsa fill pieces between the rear spars at the hinge locations.
The center sections took the longest for me to figure out - especially the top wing, where there is a lot of structure coming together in a small space. But the solution turned out to be easy to build. Construction begins the same for both wings: Place one wing panel on the board, with the tip elevated to the proper distance for the 1-1/2 degree per panel dihedral. (13/16" for the top wing, 5/8" for the bottom, measured at the tip rib. It is important to get these heights correct.)
For the bottom wing, just epoxy all 4 dihedral braces in place, incorporating the stub spars between the main spar braces. The landing gear block is epoxied to the rear dihedral brace, which transmits any landing stresses to the heavy F2 former in the fuselage. The wing attachment screw plate is similarly epoxied to the main spar dihedral brace, and soft balsa fill added above it. A small piece of 1/4" ply (check your firewall scraps) makes a sturdy center piece at the rear spar, drilled to accomodate the mounting dowel. The center two ribs are added last; I used fairly strong, hard balsa although lite ply could also be used. When the center section is cured, slide it on down the workbench, prop the opposite panel up, and epoxy the spars into the braces at the center. Flat balsa scallops at the center finish the lower wing - be sure to leave room to just clear the fuselage between the scallops.
The top wing is similar, except that the rear cabane mount is built into the rear dihedral brace. Cut out the indicated area of the rear brace, and epoxy it in position over the rear cabane mount. Both cabane mounts are made from grooved landing gear blocks. Proceed to join the panels and build the center section. The forward cabane mount is epoxied to the leading edge dihedral brace, and braced with triangle stock; you will need to saw off 1/8" from the forward edge of the cabane mount to get the correct spacing. The aileron servo lives in the center section, on a removable plate accessible from below the wing, and drives the bellcranks via a nice, straight wire. The bellcrank mounts are set up so that a straight line from one bellcrank to the other puts the wire at the top of the servo bay, where it is easily attached to the servo wheel.
The ailerons - big devils, arenıt they - are built up over the plans. All I need to mention is that mounting the control horn at the angle shown makes for much easier pushrod motion, without making a large slot necessary in the wing. There is a little differential built in (more up than down) if the bellcranks are set up at 90 degrees; I have mine set up this way. More differential can be added by turning the bellcranks out a little at neutral, which may ease your rudder workload somewhat by reducing adverse yaw - this is a long wing. The flat balsa scallop at the rear of the center section is the same as the lower wing, without the break for the fuselage.

Final Assembly
Drill the hole in former F2 to accommodate the bottom wing dowel; align and drill the bottom wing and tap the ply plate in the fuse for 1/4"-20 nylon bolts. Drill and tap the cabane mounts in the top wing for 10-24 nylon bolts and steel straps. Fine adjustments to the wing incidence can be made by wrapping tape around the top of the cabane wires, but if your jig was accurate this should not be necessary. Add hardwood blocks in the wings to attach the interplane struts and wingtip bows.
I have heard dozens of ways to make struts, but what I ended up doing was making them from 1/4" x 1/2" spruce. The struts were first cut to length and roughly tapered with a razor saw, then shaped with a plane and sanded. All 8 struts are the same length. A hole was drilled in each end, about 3/4" deep, to allow a brass clevis and a threaded connector to be glued in with carpenterıs glue. I used a simple jig made from two control horns attached to a board at the proper spacing, and attached each strut to the jig while the glue dried. The finished struts were stained and polyurethaned.
It is an easy matter to cover the tail first,  which is then glued to the top longerons at the rear of the fuse. The hinge line for the elevator should be slightly ahead of the rudder hinge line, which is even with the back of the fuselage. I had to sand a small notch in the rudder to clear the wire joiner on the elevator.
I made up a complicated, difficult-to-build shock absorbing landing gear, which looked awful and weighed a ton, which I hid in the closet. Then I made the simple 2-wire version shown on the plans, which has turned out to work very well. There are a couple of articles out there on how to make scale WWI landing gears, any of which could be added by strapping them into the blocks, but this is a simple Jenny, after all.
Once the tail is attached, but you can still get at the inside of the fuselage, install your servos and pushrods as you like. I used old-fashioned balsa sticks for the elevator and rudder, and a cable for the throttle. And hey, isnıt it nice for a change to have no nosegear or tailwheel to hook up? A little payback for building that extra wing.
Now the serious junk collecting begins. All of the strut attachments were cut from the 90 degree lip of a nylon container from the local bargain store, cost $1.79. The radiator grille is made from an aluminum air-conditioning duct cut to size, in a vacuum-formed shell. Window screen would work fine also, and the shell could be carved and shaped from wood. Just make sure enough air can still get in to the engine. The louvered panels on the sides were made from printerıs aluminum litho plates, which are discarded by the hundreds at many printing shops; ask around. I made male and female ply molds, cut the lines in the panels, and squeezed them in my vise to punch the louvers. The litho plates are pretty stiff and tend to crack with this treatment; the panels could also be vacuum-formed, or louvers individually made from wood and attached. Again, try to keep the sides open somewhat as this is a major outlet for cooling air.
The anti-sway wires for the top wing were added at the end of the project, as I felt there was a little too much play in the top wing even with the struts in place. The prototype has wires in this location, too; they run from the side of the radiator to the top wing  at the inside rear interplane strut. 24 gauge wire was used, with ends made from brass threaded wire ends pounded flat, cut off and drilled. (You can buy pre-made wire ends, too, but I was too cheap.) The wires should not be under any real tension, just enough to keep the wing aligned on the narrow cabanes.
The wingtip bows (skids) are made from bamboo food skewers, which I found at the everything-for-a-dollar store, in a bag of 100! You only need two, but spares are a penny apiece! Rest them on the edge of a pot of boiling water under the lid until the steam softens them, then bend them to suit and let them dry out - holes drilled in scrap lumber will serve for a jig. Attach them with electrical wire screw lugs - use the ³blue² size, and pull the insulator off with pliers. These are easily crimped to the bamboo and set with CA, then screwed to the blocks in the wings. Do not omit the bows - they are very effective! More pack-rat detailing: automotive vacuum tubing for cockpit coaming and windshields from a conveniently curved plastic peanut jar. Hey, the barnstormers had to use what they could find to keep their Jennies flying; get into the spirit of the project. Note: this plane is not approved for flight if your pilot figure doesnıt have a white scarf flying back in the slipstream.
Control throws are as follows: ailerons, 1-1/8" up, 1" down; elevator, 1-1/8" up and down; rudder, 1-1/2" left and right. These may seem like a lot - but the Jenny flies very, very slow, and at these airspeeds this setup results in just the right amount of controllability. If you have dual rates (I donıt) you can set up reduced throws for the ailerons and elevator, but donıt take out too much elevator, and leave it on full for the test flight.

Flying
This Jenny, like the prototype, is a light airplane with tremendous wing area, and generous moments. Final weight was 4-3/4 lbs. making the wing loading 11 oz/sq. ft. The day of the test flight, after tuning the engine, I throttled up the engine slightly for what I thought was a taxi test. The Jenny slowly began to roll, and after 10 yards or so floated right off the runway like a soap bubble. I cut the throttle immediately, and she settled back onto her wheels. That was my first clue as to the nature of the little Curtiss; itıs flying at anything over a fast walk. The .48 4-stroke is far more power than is required: I use about 2/3 throttle for takeoff, and cruise around at barely above idle, which makes for very scale sound and tempo. I had to roll in a lot of down trim on the first flight, in part due to too much airspeed. The Jenny retains all of gentle handling of the full-scale plane; it is, after all, a trainer.  Turns can be accomplished with the ailerons only, if you donıt mind skidding around the sky - and the Jenny will tolerate this behavior without complaint. But things happen slowly enough so that coordinating rudder and aileron for smooth turns is easy to learn. The plane can be kept on the centerline for takeoff and landing with the rudder only, as there is very little roll induced with the rudder. All of the barnstormer aerobatics are there for the asking: slow, ³flop over² loops, stall turns pivoting right on the pilotıs belt buckle, and barrel rolls that take all afternoon. I have landed the plane in gusty conditions and crosswinds, which so far has resulted only in grass stains on the wingtip bows. The big rudder allows very positive ground steering for taxiing, with very tight left turns possible to taxi back to the pits.
I have had a great time with my Jenny so far - I couldnıt ask for a better first scratchbuilding experience. My thanks to my model friends Bill Saar, Walter Purdy and Hermann Hoepken for their help and ideas. This summer I managed to make a couple of flights at the Rhinebeck Jamboree. In addition to seeing dozens of great WWI model airplanes fly, I was gratified when, at the beginning of the full-scale show, the airshow crew stopped the museumıs original JN-4H Jenny on the runway and invited my wife Felicia and I over to take a quick photo. It may be too small to sleep under the wings, but at times, a simple model can really transport you.