The World War I Curtiss Jenny trainer evolved from less-than-optimal JN-1 and JN-2 models of 1915 to the definitive JN-4D that found its stride 100 years ago.
In the spirit of centennial commemorations, a look back at the Jenny is in order.
The earliest Jennies employed control wheels. By the advent of the JN-4D, a more traditional stick and rudder arrangement prevailed.
A nomenclature quirk attends the Jenny, designed before the Army Air Service established its standards for model numbers in 1919. Many written descriptions of the most numerous variant call the aircraft a JN-4D.An original 1918 handbook published by the builder, Curtiss Aeroplane and Motor Corporation, places the dash later in the alphanumeric identifier, consistently calling the aircraft the JN4-D. We flipped an editorial coin for this article and went with JN-4D.
The JN-4D used straightforward wood and wire bracing in its construction, covering the airframe with linen fabric that was typically clear-doped, causing a pleasing warm translucence to the finished product.
But not all Jennies were finished in this manner. Some later carried olive drab coloring, and photos depict at least two that received varied camouflage patterns — unusual for a noncombatant training airplane.
The JN-4D rode behind a Curtiss OX-5 V-8 producing 90 horsepower. The engine relied on an artistically simple round-topped radiator for cooling.
Scale elevation drawings of the JN-4D fuselage as built by Curtiss show a slight downward cant to the engine and radiator. One of the six other companies contracted to build JN-4Ds to meet the World War I need was Liberty Iron Works, whose 100 JN-4D2 variants were built without that downward tilt.
As the United States geared up for major production during World War I, several factories undertook construction of OX-5 engines. The end of the war in 1918 made thousands of OX-5s available as cheap surplus. Even though newer engine developments surpassed the power and reliability of the OX-5, its availability made the engine useful in a number of post-World War I civilian biplane designs.
Curtiss provided a green bound book with the JN-4D to aid in its assembly out of crates, since new Jennies typically were freighted instead of flown to their destinations. The book’s description of the Jenny includes a reference to the wings’ use of the Eiffel No. 36 airfoil — yes, that Eiffel, the French engineer who began studying the effects of wind on his structures, and migrated that work into pioneering aeronautical designs.
The JN-4D also introduced prominent cutouts in the upper and lower wings at the fuselage juncture, which the manual explained were “to enlarge the range of vision above and below.” No doubt this was a useful revision for an aircraft used in the mass training of fledgling aviators.
Spacing between upper and lower wings on the Jenny was 5 feet, something that post-World War I barnstorming wingwalkers found reasonably spacious as they scrambled around flying wires and gripped airfoil-shaped wooden struts for support.
Stagger of the upper and lower wings was 16 inches. The top wing of the JN-4D was substantially larger in span than the bottom wing, measuring 43 feet, 7-3/8 inches compared to the lower wing’s 33 feet, 11-1/4 inches. The wings were intended to be mounted and rigged with 1° of dihedral. From nose to tail, the JN-4D covered 27 feet, 4 inches.
This elegant expression of 1917 training airplane state of the art had an empty weight of 1,430 pounds and a gross weight of 1,920 pounds. The useful load of 490 pounds presumed two aviators weighing not more than 165 pounds each, along with 21 gallons of gas totaling 130 pounds and oil weighing in at 30 pounds. Interestingly, no mention was made of the weight of the radiator water, possibly considered to be part of the fixed weight of the aircraft instead.
That 90 horsepower OX-5 engine swung an elegantly shaped wooden propeller at, typically, 1,400 rpm.
The wing panels and tail surfaces shipped in their own crate or case. The fuselage, with engine mounted, was in a second case. In an era when powered flight was only 14 years old, the nuanced care of airframes was not universally established, evidently. Unpacking a Jenny took more finesse than uncrating a boiler, or heavy machinery. The handbook advised: “The most serviceable tool for unpacking is a nail puller. An axe or saw should never be used.”
That 1° of dihedral was dialed in by adjusting turnbuckles on the wing wires. A properly rigged wing had 2-3/4 inches of upsweep in 13 feet, 6 inches of span.
“An easy method for checking the correct adjustment of the dihedral is to place a block 2-3/4 inches high on the upper surface of the lower wing, at the extreme inner edge,” the manual explains. “A straight edge resting on this block and on the upper surface of the wing (straight edge kept parallel to front or rear beam) should be level.”
Curtiss offered observations about flying the JN-4D, including: “At all times remember that although you are controlling the actions of the machine, it has a considerable amount of stability itself, and be a little free with the controls rather than stiff and rigid.”
The manual discussed the value of attaining altitude before attempting turns, adding: “Now that a safe altitude has been reached, your anxiety diminishes, for with height there is safety; but remember to keep within gliding distance of a landing place.”
The end of World War I produced surpluses of both pilots and planes, and Jenny trainers were offered for sale to the public at prices said to have reached a low of $50. Over the decade following the 1918 armistice ending the war, those prices rose, but it is still remarkable to thumb through copies of periodicals like Aviation and find classified advertisements offering a flyaway JN-4D in November 1928 for $750.
Another vendor offered Jenny wings for $35 in that same issue, and listed all kinds of Jenny airframe parts “dirt cheap for cash.” A complete set of linen covers to apply to a skeletal Jenny cost $125 in 1927.
The Curtiss JN-4D Jenny’s service as a military training biplane followed by its sometimes rag-tag employment in the 1920s civilian market etched its place in the American aviation pantheon.
Now, a century later, Jenny deserves a respectful visit now and then.