Arado Ar 240 - Design and Development

Design and Development

The Ar 240 came about as the response to a 1938 request for a much more capable second generation heavy fighter to replace the Messerschmitt Me 110, which was becoming outdated. Both Arado and Messerschmitt responded. Messerschmitt's response, the Me 210, was a totally new design, but thanks to Messerschmitt's experience with the Zerstörer ("Destroyer") concept, it would be able to enter service quickly. Arado's design was considerably more ambitious for the smaller firm, a dream project of Arado's chief designer, Walter Blume, since the mid-1930s. While it would take some time before deliveries of the Arado design could begin, the Reichsluftfahrtministerium (German War Ministry, RLM) was nevertheless interested enough to order prototypes of both designs.

Prior to this point, Arado had invested heavily in several lines of basic research. One was the development of the "Arado travelling flap" which offered excellent low-speed lift performance. Another was ongoing work into the design and construction of pressurized cockpits, which dramatically lowered pilot fatigue for any flight above about 4,500 m (14,760 ft). Finally, they had also invested in a technically advanced remote-control defensive gun system, which they had been experimenting with for several years. The system used a gunsight located in the rear cockpit, operated by the navigator/gunner, which had optics on both the top and bottom of the aircraft allowing aim in any direction. The gunsight was hydraulically connected to well-streamlined "pancake"-shaped turrets on the top and bottom of the aircraft. For the Ar 240 design, the Arado engineers combined all this research into a single airframe.

For outright performance, they used as small a wing as reasonable, thereby lowering parasite drag (at the expense of greater lift-induced drag). Normally this would make the plane have "impossibly high" landing speeds, but this was offset by the use of a huge travelling flap and leading edge slats for high low-speed lift. When the flaps were extended, the upper portion of the ailerons would remain in place while the lower portion extended rearward, essentially increasing the wing area.

The Daimler-Benz DB 601 inline engines were conventionally installed and equipped with a three-blade fully adjustable propeller. The radiators were somewhat unique however, quite similar to those fitted to the Junkers Ju 88 which pioneered them, consisting of an annular block located in front of the engine, but with the Ar 240 partially covering each of them in an oversized propeller spinner, with the air entering through a large hole in the front of the spinner and exiting out the cowl flaps. As with the Jumo inline-powered versions of the Ju 88, this made the plane look as if it was mounting a radial engine, and the Ar 240, like later Jumo inline-powered fighter aircraft from the Focke-Wulf firm (the Fw 190 D, Ta 152 and twin engined Ta 154) also benefitted from the simpler setup of an annular radiator just forward of the engine.

The fuel cells in the wings were provided with a newly developed self-sealing system that used thinner tank liners, allowing for more fuel storage. The liners could not be easily removed as they stuck to the outer surface of the tank, so in order to service them, the wing panelling had to be removable. This led to a complex system for providing skinning stiff enough to be handled in the field, complicating construction and driving up weight.

As with all German multi-use aircraft designs of the era, the aircraft was required to be a credible dive bomber. The thick wing panelling wasn't suitable for piercing for conventional dive brakes, so a "petal"-type brake was installed at the extreme rear of the fuselage which opened to the sides when activated. When closed the brake looked like a stinger, extending beyond the horizontal stabilizer and twin fins.

Finally, the cockpit was fully pressurized. This would not have been easy if the armament had to be hand-operated by the gunner, as it would have required the guns to penetrate the rear of the cockpit canopy. However, the remote control system allowed for them to be located in turrets in the unpressurized rear of the fuselage.

All of this added weight combined with the small wing led to a very high wing loading of 330 kg/m² (221 lb/ft²), compared to an average of about a 100 for a single-seat fighter.