|Year||Jan 13 1940||April 14 1940||Sep 18 1940|
|Weights and loads|
|Wing Load (kg/m2)||152||163||163|
|Power load (kg/hp)||2.48||2.67||2.67|
|at 0m (km/h)||?||490||?|
|at altitude (km/h, m)||580, 5000||586, 4800||?|
|Turn time (sec)||?||24||21|
|One Turn (m)||?||775||900|
|Gun Type (Position)||Ammo|
|20mm ShVAK (engine)||1*130||1*120||1*135|
|7.62mm ShKAS (engine)||2*420 2*650||2*380||2*750|
The Spanish Civil War experience proved the necessity to boost speed of domestic fighters from 500km/h of the I-16 family to at least 600km/h at the combat altitude. In 1939 Soviet industry already produced the 1050hp liquid-cooling M-105 powerplant, capable to provide necessary speed. Even more powerful engines (like the 1350hp M-106) were under development.
A.S.Yakovlev visited the Great Britain in 1936, where he had seen the Supermarine Spitfire at first hand, and later in Germany he examined Bf-109R and He-100. Designer's experience with light speedy aircraft turned to be in agreement with general European light fighter concept, and he combined it with maneuverability, simplicity in construction from readily available materials, multiplied by superior Yakovlev's weight culture.
For his high-speed fighter A.S.Yakovlev used construction that proved successful on his light trainer/sport airplanes like UT-1: fuselage strength was provided by wielded soft steel tubes frame, while the skin was supported by light wooden ribs and stringers. Nose section covered with duralumin, while tail section - with fabric. Wing of wooden construction with plywood skin, covered with fabric. Tail surfaces and all controls - duralumin frame covered with fabric.Cannon was mechanically attached to the engine, what allowed dramatically reduce recoil and keep the airframe very light.
Official Governmental order was issued on July 29, 1939. Two prototypes were ordered, one powered by M-106 (to roll out in February 1940) and another by turbo-charged M-105P (March 1940). Both prototypes had to be armed by engine-mount 12.7mm UB machinegun and pair of synchronized 7.62mm ShKAS machineguns. Upgrade to 20mm ShVAK cannon with 160 rounds, firing through the propeller shaft and four synchronized 7.62mm ShKAS (625 rounds per gun) installed was scheduled.
Three flight prototypes were built (designated I-26-1, I-26-2, I-26-3) plus one airframe for static tests. I-26-3 was intended for VK-107 engine, but none of planned engines were ready. As a result, all three prototypes received standard M-105P driving 3m VISh-52P variable pitch propeller. Some sources claim that the M-105P (Pushechnyj, cannon ready) engine was not ready at the moment, and the first prototype was flown without the cannon, with 'basic' M-105 engine. Number of ShKAS machineguns on the I-26-1 also is unclear: 2 or 4. A.T.Stepanets (test-pilot of many WW-II Yakovlev fighters) writes about engine-mount ShVAK with 130 rounds and four ShKAS with 420 rounds per each of upper gun and 650 pear each of the lower ones.
I-26 project started on May 9 1939, construction started on August 15 and first prototype I-26-1 rolled out on December 27 same year. For team of 45 designers and drafters backed by 152 mechanics this is a record pace. Aircraft was painted in Yakovlev's - gloss red with white strips on the rudder.
First flight was performed by Yu.I.Piontkovskij on January 13, 1940 on skis. Without further delay pilot pushed prototype to 550m, circled airfield twice (with landing gear down) and landed. Performance was exceptionally good for an aircraft that early in its development, with maximum achieved speed 586km/h. I-26-1 totalled 13h9min in 43 flights.
It was known that some elements of the aircraft structure have insufficient strength, and I-26-1 was flown without ammo loaded and with fuel tanks half-empty. On 27 April the first prototype was lost in fatal accident, killing long time Yakovlev's test pilot Yu.I.Piontkovskij. The cause was a manufacturing defect (landing gear got loose, damaging wing skin) and pilot's hurry to push prototype to its limits.
But even short lifetime of the first prototype allowed to overcome numerous problems of all-new machine. Propeller was replaced by new VISh-61P. Carburetor, engine controls, radiators, tubes were also revised. I-26-2 differed by strengthened wing, seat armor increased from 8 to 9mm. Oil radiator was moved from the above-the-engine to the 'chin' position, providing I-26-2 with 'beard' typical for early WW-II Yak fighters. Lower pair of ShKAS was removed and ammo reduced to 120 for cannon and 380 per machinegun. Aircraft still had no radio, no landing light and not ready for blind flight.
I-26-2 was ready on April 27, 1940. 31 flights (13h37min) were performed.' Despite the second prototype was intended for flight performance studies, loss of the I-26-1 forced to use I-26-2 for final design refinement also. Maximum loads were restricted, and only low speed aerobatics was performed.
Despite the accident and delays, I-26 was approved for mass production (at Moscow Factory N°301) in the mid- 1940, even prior to completion of the State Acceptance Trials. Sixty four were ready in 1940, allowing to perform 'field' trials in combat units and introduce more modifications in first mass production batches.
State acceptance trials started on June 1 1940. 52 flights (21h11min) were performed with restricted load factor (6.5), impairing aircraft maneuverability. Engine rate also was limited by 2400rpm (instead of nominal 2700) due to overheating problems. Lightweight tail structure resulted in 'family' problem of (most) Yakovlev piston engine fighters - even during engine tests 'parked' aircraft had a habit to raise the tail, and ground crew had to hold it in place manually. High risk of noseover demanded very gentle use of wheel breaks - and excessively long landing roll.
Most serious revealed defects and shortcomings:
All those defects could be easily overcome in production series. Pilots were impressed by the aircraft's aerobatic and spinning characteristics. While 100km/h faster than late modifications of the I-16, I-26 was much easier to fly. Trials continued until November 1940, when I-26-2 was transferred to the Moscow Aircraft Factory N°301 as a reference for mass production (under designation Yak-1).
Heated debates took place around the radio equipment. VVS representatives insisted that experimental aircraft has to be tested fully-equipped for modern combat. Designers and manufacturers consolidated around following points:
I-26-3 prototype had strengthened wing leading edge, increased pressure in pneumatic system, increased ammunition. Were added also cartridge collector, some service access hatches, rubber cord employed for easy canopy sliding at high speed, landing gear position indicators (light and on-wing pins), cockpit lights, pilot tube heater and other changes.
Conclusion of the State Trials (October-November 1940) was positive: despite the weight of the I-26 was increased, its overall performance was improved. New fighter could be flown by service pilots of the VVS RKKA with average and below average skills level. Remaining shortcomings were cured during mass production, and caused no much trouble.
Below is list of I-26 prototypes features (as listed by 'The Problem with Yak Fighters'):
I-26 was first seen on public on October Day (1940) fly-past over the Red Square.
|67k b/w I-26-1 (first prototype) drawing, courtesy of Sergey Andreev
Used for background on this page.