The MU-4 (Russian: Михельсон/Никитин МУ-4) amphibious flying boat was designed and built by NG Mijelson at Leningrad Factory No.23. The initials MU correspond to Morskaya Uchebnaya or Marine Trainer.
The MU-4 was designed as a flying-boat with a single hull and amphibious capacity. The hull was made entirely of wood and was made up of five watertight departments.
The wing configuration was sesquiplanar with the wing box braced by V-shaped struts made of duralumin. The wooden wing featured a double spar structure with a slim profile and constant chord. The covering was fabric. The lower plane was inserted into the fuselage, while the upper one was located high above the cabin, from where the engine supports started. The drop-shaped stabilization floats were located in the lower wing.
For operations on land, the MU-4 had a conventional landing gear with oleo-pneumatic shock absorbers and a tail skid. The main units retracted forward on the sides of the hull by means of a manual drive system. To allow winter operations the wheels could be replaced by skis.
The tail unit was of the conventional monoplane type with the stabilizers located high on the empennage and fixed by parallel struts in its lower part.
The MG-11F engine was installed on the upper wing in a tractor configuration protected by a Townend ring hood and moving a two-blade propeller. For winter, a special “winter” cowl was designed.
The MU-4 could carry two people, seated side by side in an enclosed, dual-control, glazed cabin.
The first prototype was finished in August 1937. Tests showed that the plane had good flight characteristics and could perform aerobatics. During the manufacturing tests this example was destroyed due to a construction defect. The engine fixing brackets to the gunwales were glued with a poor quality glue that ended up coming off. A short time later Mijelson would be imprisoned so the responsibility for further development passed to VV Nikitin.
The second prototype was submitted for testing in May 1938. Model tests were carried out at the NII GVF hydrostation located in Konstantinovska Bay in Sevastopol. The flights were carried out by test pilot GI Sergueyev. The MU-4 was presented as a simple plane to fly and capable of taking off and landing in small aquatic spaces.
It was proposed to build a series of 30 examples at Factory No.23, but after Mijelson ‘s conviction this decision was reversed. The Shavrov Sh-2 was produced in series, covering the same needs.
MU-4
Powerplant: 1 x 190 hp MG-11F
Wingspan of upper plane: 12.00 m
Wingspan of lower plane: 9.00 m
Wing area: 33.00 m²
Length: 8.70m
Height: 3.70m
Empty weight: 989 kg
Normal takeoff weight: 1255 kg
Wing loading: 36.4 kg/m²
Power load: 4.2 kg/hp
Fuel + lubricant capacity: 80 + 20 kg
Top speed: 173km/h
Cruising speed: 154 km/h
Service ceiling: 3400 m
Accommodation: 2
All Aero 