The Jona that never took off.

“It is certainly German. Its wings bearing the black crosses reveal it; but it’s a very odd bird. It’s running away at full speed. It’s really extraordinary: it has a cruciform empennage, but it looks like it not only has a forward propeller but also a pushing one behind the stabilizers, at the end of the tail.”

Do-335

This is how Pierre Clostermann described his first encounter with the Dornier Do 335 Pfeil, over the Elbe River, in April of 1945. Ten years earlier, Alberto Jona had designed an astonishingly similar heavy fighter, the J.10.

The  drawing of the J.10 that was filed  in 1935.

By Roberto Gentilli  (translated by L. Pavese)

The drawing of the first patent, # 342987, which was filed on June 22, 1935, depicts a large single-seat, monoplane, retractable-gear fighter. The projected aircraft was powered by an air-cooled radial engine, mounted in the conventional position in the nose, and by a fuselage mounted, in-line engine, driving a propeller which was placed behind the empennage. The tail surfaces were of “classical” shape, but the horizontal stabilizer was very ample and was mounted in a low position. (The horizontal stabilizer was big, because of the relative short distance between it and the center of gravity of the airplane. L.P.).

The wing was equipped with flaps and leading edge Handley Page slats, and it was appreciably swept back. The composite angle of sweep-back was necessary to move the center of pressure of the wing rearward, since the placement of the engines moved the center of gravity of the aircraft aft.

A few months later, at the end of the 1935, the design of the aircraft had been modified, but appeared complete. The forward engine was a fourteen-cylinder radial Piaggio P.XI RC.30, and the rear one was an eighteen-cylinder, W type, Isotta Fraschini Asso XI RC.45 that drove the aft propeller. The vertical stabilizer had been doubled in two twin oval-shaped parts and the wing retained the earlier varying angle of sweep-back.

The project guaranteed exceptional performances: a speed of 320 knots, a ceiling of almost 46000 feet (This seems really optimistic. L.P.), four guns; and all of this at a time when the rosiest promises for the Italian Air Force were based on the FIAT G.50.

          Between 1935 and 1936 the project was evaluated, and eventually approved, by the D.G.C.A. (Direzione Generale Costruzioni e Approvvigionamenti. An Italian state agency that evaluated and decided what to build and what to procure) and later by the Italian Royal Air Force General Staff.

Aerodynamic test models of the aircraft were commissioned, which simulated the distribution of the masses for the wind tunnel tests, that were also carried out in the vertical wind tunnel. There was some concern that, due to its unconventional configuration, the airplane could have exhibited dangerous spinning characteristics. For that reason, the horizontal stabilizer was relatively over-sized and mounted in a very low position, not to be put in an aerodynamic “shadow area” by the wing, especially at high angles of attack.

Nevertheless, the wing tunnel tests confirmed the good qualities of the design, also as far as the behavior of the aircraft in the spin was concerned.

The fuselage of the 1935 version was a truss of welded of Cro-Mo (Chromium Molybdenum) steel tubes, covered with DurAL (duraluminium) sheeting and, besides all the equipment that comprised an oxygen system and photographic camera, it also housed four fuel tanks for a total capacity of 1240 liters (327 US gallons) of gasoline. Two auxiliary tanks, with a capacity of 26 and 32 US gallons, were mounted inside the wing roots.

The wing-spars were trusses of welded steel tubes, and the ribs were made of structural DurAl. The wing was covered with DurAl sheeting.

          The portion of the wing close to the root had a sweep-back angle of 12⁰. This part of the wing also housed the retracted landing gear, which was designed to have a stabilizing function and to assist the wing flaps. When the the main landing gear was extended, a slot of a length of three meters (about 10’) opened in the wing, increasing the effectiveness of the flaps.

The outer part of the wing was swept-back at an angle of 30⁰, and it was provided with leading edge Handley Page slats. The ailerons (flaperons, actually) could also be deflected simultaneously to act as flaps (like on the Jona J.6).

The vertical stabilizer, like we said, was split in two, to free the airflow of the tandem propellers, and the entire tail assembly was of metal construction, and covered with DurAl.

The armament consisted of two mm 7.7 (.303 caliber) SAFAT machine-guns in the fuselage, and two mm 12.7 (.50 cal.) machine guns, one in each semi-wing.

1937 drawing of the J.10

In 1937, the project was dragging its feet and the design of the J.10 had evolved further. At the end of that year the drawings showed a 1000 cv (kW 735.5) FIAT A 80 radial engine in the nose, and an Isotta Fraschini Asso XI RC40 in the fuselage.

The composite angle of the leading edge of the mid-mounted thick wing had been eliminated, and the leading edge was rectilinear (and the wing was now straight, because the liquid cooled engine had been moved forward in the fuselage).

The aircraft had a single, tall vertical stabilizer. The aircraft could now be adapted to the fighter or the ground attack role, and it had an internal bomb bay.

The provide the necessary clearance for the aft propeller, the leg of the tail-wheel was very long, although very robust, and the aircraft sat on the ground with an angle of only 11⁰, which afforded good visibility while taxiing.                                                 

Final (blurry, sorry) configuration of the J.10 (1937)

 

With its powerful armament and a great range (the aircraft now held 1450 liters, 383 US gallons, of fuel), the twin engine configuration with an air-cooled unit forward (ideal for ground-attack role), and the capability to navigate economically on one engine, the Jona J.10 could have become one of the most powerful weapons in the arsenal of the Italian Royal Air Force.

The design had been completed, it required only the will to build the actual aircraft, but after Agusta (to whom the Ministry had proposed the project) turned it down, the J.10 fell by the wayside and the funds, that had been already earmarked for it, were destined to something else.

          This article originally appeared on the March 1980 issue of the Italian aviation magazine JP4. Your comments, as usual, will be greatly appreciated.

Thank you,

Leonardo Pavese