Siemens - Schuckert R . IV The Siemens - Schuckert R . IV was a bomber aircraft built in Germany during World War I . It was one of six aircraft based on the Siemens - Schuckert R . I that were originally intended to be identical , but which each developed in a different direction and were designated as different aircraft types by the German Inspectorate of Flying Troops ( the Idflieg ) . The Maybach HS engines specified by the " Idflieg " proved unreliable , but with these engines were replaced by Benz Bz . IV engines , the R . IV saw service on the Eastern Front before being relegated to training duties . Design and development . As designed , the R . IV was a large three - bay biplane with unstaggered wings of unequal span and a fully enclosed cabin . The three 180 - kW ( 240 - hp ) Maybach HS engines were mounted internally in the fuselage , and transmitted their power via driveshafts to two propellers mounted tractor - fashion on the interplane struts nearest the fuselage . The main undercarriage consisted of divided units , each of which carried dual wheels , and the tail was supported by a pair of tailwheels . The fuselage was forked into an upper and lower section , which allowed a clear field of fire to the rear of the aircraft . Siemens - Shuckert delivered the R . IV to the German military on 29 January 1916 , and trouble with the engines began almost immediately . Shortly after test flights commenced , it was necessary to replace the original HS engines with new ones . While the engines were being replaced , a number of other modifications were carried out , including replacing the original nose - mounted radiators with aerofoil - shaped radiators fitted in the upper wing and adding new gravity - fed fuel tanks . With these improvements in place , the R . IV flew again in May 1916 . Even then , the engines overheated until one of the cylinders turned blue . Siemens - Shuckert engineers believed that with still larger radiators , the Maybach engines could be made to work on this aircraft . However , the firm lacked the personnel and the experience to undertake engine development . Therefore , while Siemens - Schuckert asked the " Idflieg " to allow them to substitute Benz Bz . IV engines on the R . II and R . III the firm suggested that the " Idflieg " accept the R . IV in its current state , including its Maybach engines , so that they could oversee a development programme for the engine themselves . The " Idflieg " agreed to this suggestion and lowered its specifications for the R . IV accordingly . The R . IV passed the new requirements on an acceptance flight on 27 August 1916 , and was accepted into service with the " Riesenflugzeugersatzabteilung " ( Rea — " giant aircraft support unit " ) the same day . Within a few months , the R . IV ' s wings had been extended by adding an extra bay to them and in November 1916 , the Maybach engines were finally replaced by Benz Bz . IV engines , probably while the R . IV was being repaired after a crash . Before the aircraft flew with its new engines , a fifth bay was added to the wings . All work was completed by 14 February 1917 , and the refurbished aircraft flew on 14 March . The R . IV was assigned to " Riesenflugzeugabteilung 501 " ( Rfa 501 ) at Vilna on 27 April 1917 . It was used operationally until Rfa 501 was redeployed to the Western Front , at which point it was left behind in Vilna for use as a trainer with the " Riesenflugzeug Schulabteilung " ( " giant aircraft training unit " ) . The R . IV continued in this role at Vilna until early 1918 , when it was withdrawn to Berlin . On 22 June 1918 , it was damaged in a forced landing and was not repaired until 28 August . Operational history . Specific details of five operational missions while with Rfa 501 have survived . In each case , the raid was carried out together with the R . VI and R . VII :