I-Z ("Z", TsKB-7) fighter by D.P.Grigorovich

In the early 30's pair of 7.62mm machineguns (typical armament for most fighters) became insufficient. All-metal aircraft (specially heavy bombers) with air-cooled engines and protected fuel tanks were practically invulnerable to such a small-caliber weapons. Designers faced a necessity to arm their aircraft with larger guns firing explosive shells.

But at the time rapid-firing small caliber cannons were not available. Large caliber guns firing heavy shells produced too much recoil for light fighter airframe to withstand. One of solutions (pursued by aircraft weapons designer L.V.Kurchevski) was a recoilless large caliber cannon. In the USSR theory of a weapon using gas recoil compensation was developed by Professor B.S.Stechkin. In 20's number of experiments were carried out with recoilless guns (caliber from 1 to 12inches), including in-flight firing of such weapons installed on Ju-13.

Despite small series of the APK cannons were manufactured, it was still far from being considered a practical weapon. In addition, there were obvious limitations:

• low firing rate: most even single-shot!
• low projectile speed;
• large weight and size;

Nonetheless, the idea of taking a bomber with single shot gained popularity, and in Summer 1930 D.P.Grigorovich received an order to design and build a fast monoplane fighter armed with pair of 75mm recoilless cannons. Team working on the 'Z' project (Latin 'Z', not Cyrillic '3') was isolated from all other team due highest secrecy level set for the aircraft. Here are names (not all) of the I-Z team: A.N.Sedelnikov, V.L.Korvin, A.V.Nadashkevich, E.I.Majoranov, V.D.Yarovitsky, G.N.Chupilko, S.N.Shishkin.

The front fuselage was that of the N.N.Polikarpov/ D.P.Grigorovich I-5 biplane, including engine, helmet-type cowling, cockpit and fuel/oil tanks. Only one (port side) of two I-5 synchronized machineguns remained, loaded with tracers for aiming purposes.

But all the rest was made from scratch. Tail section was an duralumin monocoque of oval cross-section, carrying large tailfin and rudder. Tailplane (also duralumin) was fitted high on the tailfin in order to keep it safe away from the reward blast of cannons and following turbulence. Controls had a fabric skinning, replaced during trials by duralumin.

Straight wing with rounded tips had stainless steel wielded structure with fabric skin. 75mm APK single-shot cannons were installed out of the propeller arch, their mounts integrated into the wing structure. Near the cannons fabric skinning was replaced by aluminum. APK muzzles were just in front of the wing leading edge, and recoil gases were ejected through 0.6m cylinders attached to the rear end of the cannons.

Fixed pyramid-type landing gear had rubber plate shocks in faired front struts.

Many novelties of the I-Z caused number of problems, with the APK guns on the top of list. Aircraft was also substantially overweight.

Flights of the first prototype began during Summer 1931 (pilots B.L.Bukholz and Yu.I.Piontkovskij). Spin problems were solved quickly by tailplane repositioning. Second I-Z prototype was ready in 1932, and pre-production batch of 21 aircraft was assembled in 1933 by Moscow factories.

Series aircraft differed by Townend ring replacing helmet-type engine cowling. Metal wing structures were replaced by wooden. Fifty more I-Z were assembled at Kharkov in 1934 and 1935, armed with latest available APK cannons (normally of smaller caliber than the prototype).

Soon more advanced designs by D.P.Grigorovich (IP-1 etc) became available, and interest in the I-Z faded.

Few were used in 'Zveno' research program. The I-Z occupied the underfuselage 'position' in 6-plane 'Aviamatka' group. Low speed of the I-Z, its agility and good handling allowed pilot to 'hang on' and detach in flight, using special hook.