Heinkel had originally built two HD 24s for the competition ”Seeflugwettbewerb” in 1926. Further aircraft were built for the German Airline Pilot School (DVS). Eight Heinkel HD 24s (HD = Heinkel Doppeldecker), Sk 4 in the Air Force, was ordered in the first half of 1926 by the MFV (the Navy aviation), just before the forming of the Swedish Air Force. The Swedish Sk 4s were ordered from Heinkel’s Swedish subsidiary Svenska Aero AB. The aircraft was designed as a naval aviation trainer, but the floats could be replaced by wheel landing gear or skis. The eight Sk 4s were all based at the naval aviation Wing F 2 at Hägernäs.
The first two Sk 4s were manufactured at Heinkel’s Warnemünde works as pattern aircraft for the rest of the series. The two aircraft were approved by the Air Force in November 1926. Some months later, they were given the Air Force numbers 201 and 202. The six aircraft built in Sweden were approved in July 1927 - November 1928 (Air Force numbers 203-208).
The six first aircraft (201-206) were fitted with Mercedes engines of 180 hp. These engines were unfortunately not very reliable. Therefore, the 207 and 208 got Junkers Jumo L 25 engines. These engines delivered 280 hp (100 hp more than the Mercedes engines). The 207 and 208 got the subdesignation Sk 4A. After some time, also the 203, 204 and 206 got the L 25 engine and got their designation changed to Sk 4A. Aircraft 201, 202 and 205 also got their original Mercedes engines replaced, but this time to the Armstrong Siddeley Puma (240 hp). New designation Sk 4B.
Length: 9,69 m.
Height: 4,15 m
MTOW: 2.150 kg
Max. speed: 168 km/h