stall speed of both the Messerschmitt Bf 109 and the Focke-Wulf Fw 190 was similar to the U-2s maximum speed, making it difficult for the fighters to keep a Po-2 in weapons range for an adequate period of time. The success of the Soviet night harassment units inspired the Luftwaffe to set up similar Störkampfstaffel "harassment combat squadrons" on the Eastern Front using their own obsolete 1930s-era, open cockpit biplanes (most often the Gotha Go 145 and Arado Ar 66 biplanes) and parasol monoplane aircraft, eventually building up to larger Nachtschlachtgruppe (night attack group) units of a few squadrons each. The Polish Air Force used these slow and manoeuvrable aircraft for air reconnaissance and COIN operations against UPA detachments in mountainous area of Bieszczady. Pilots and navigators were dispatched to look for concentrations of UPA forces and if needed, engage them with machine guns and grenades. On several occasions, the UPA managed to bring down some of the Po-2s, but never captured or operated them. The U-2's 5-cylinder engine had an unusual exhaust manifold arrangement that gave the engine a peculiar rattling or popping sound which made the airplane easily identifiable even at night. German soldier Claus Neuber listed in his war diary six different German nicknames for the plane, the most common of which were Nähmaschine (sewing machine) or Kaffeemühle, (coffee mill), both due to the distinctive engine sound. Neuber added that some German troops derisively called it the "Runway Crow" or "Fog Crow." He also cited the nicknames "Iron Gustav," for the belly armor the plane carried to protect it from ground fire, and "The Duty NCO" because the plane almost always came at night at the same time. The fabric and wood construction of the airplane made it extremely vulnerable to catching fire when hit by tracer rounds, resulting in a Russian nickname of Kerosinka, or kerosene lantern. Finnish troops called it Hermosaha (Nerve saw). Korean War North Korean forces used the Po-2 in a similar role during the Korean War. A significant number of Po-2s were fielded by the Korean People's Air Force, inflicting serious damage during night raids on United Nations bases. During one such attack, a lone Po-2 attacked Pyongyang Air Base. Concentrating on the 8th Fighter-Bomber Group's parking ramp, the Po-2 dropped a string of fragmentation bombs squarely across the group's lineup of P-51 Mustangs. Eleven Mustangs were damaged, three so badly that