Lazarov Laz-7M

 

In the previous post we wrote about the Lazarov Laz-7, however, we didn't mention that, albeit a successful aircraft (relatively speaking) it was prone to various accidents caused by a nimble crankshaft in the Walter-Minor 6-III engine.
In order to cope with that issue the Bulgarian People's Army Air Force (BPAAF), which by the time of these events had already fallen under Soviet influence, ordered the replacement of the  Walter engine with a Soviet-made M-11FR, as they were, theoretically, equally powerful, more reliable and, furthermore, they had constant-speed propeller.
Although the engineers of DSF-Lovech (manufacturers of the Lazarov) were reluctant at first, they were forced to comply with the command of a higher authority, so some engineers got permission (among them prof. Lazarov) to go to Bozhurishte airfield, where, among many others, some Soviet Yakovlev Yak-12 were stationed, to study its propulsion system. 
With that knowledge gathered, a single Laz-7 (numbered as 68) was delivered to the aircraft factory at Lovech, where the Walter engine was replaced by the M-11FR in the Laz-7. 
In this configuration, that Laz-7 was test flown by test pilot Popganchev, with unsatisfactory results as the M-11FR engine, coupled with the light fuselage of the airplane, lost force really quick. 

After this failure, the BPAAF ordered the creation of a complete new aircraft, powered by the M-11FR engine and the features of the Laz-7. For this, they were given a very short deadline; by 1952 that aircraft had to be already being mass produced (all these events took place in 1951). So engineers had no time to make a scale model nor test it in the wind tunnel.

This time, design was tasked to eng. Kiril Karlov who took the same layout of the Laz-7, installed the Soviet engine and made some alterations to the wings to make them lighter. Some further changes were also made, to the point that Karlov asked permission to name this design as ZAK-1, petition which was rejected, as the design was directly based on the Laz-7, so the designation of Laz-7M (the letter standing for "modernized") was given instead.

The first test flight of the Laz-7M, with its characteristic star-shaped nose (similar to that of the Yakovlev Yak-18) took place on 16th June 1952 with satisfactory results. This was the first of a total of 102 flight tests which took place until the next month. In July the prototype was passed on to the Bulgarian 2nd Night Light Bomber Aviation Division, stationed at the town of Stara Zagoda, to undergo military trials.

In that unit it was used both as a day and night trainer but also as a ground attack trainer, as it had the same armament the Laz-7 had. However, in order to make it lighter, most of the times it flew unarmed, as it was easier to fly. 

A total of 150 Laz-7Ms were manufactured by DFS-Lovech and were employed mainly by the BPAAF's School, the BPAAF and some flight clubs until the early 1960s.

Apparently the Polish People's Republic considered ordering 100 machines, but they opted for the Yakovlev Yak-18 instead. Egypt also was one possible customer, as they considered ordering 50 Laz-7Ms in the mid-1950s. We have included hypothetical profiles for both countries.

Some few aircraft also flew with the DOSO (a civilian volunteer organization for the protection of the Bulgarian aerial space).

Sources:

1st http://www.airwar.ru/enc/other/laz7m.html (translated)