Rapp Rp III

Rapp Motorenwerke

Industry	Aircraft engine manufacturing
Founded	27 October 1913
Founder	Karl Rapp
Products	Aircraft engines (e.g. Rapp Rp III)

The Rapp Rp III was an inline six-cylinder aircraft engine featuring a single overhead camshaft (SOHC) and water cooling, produced by Rapp Motorenwerke. Its architecture was derived from the earlier Rapp 100 hp four-cylinder design.

Design and development

Initial version of the Rapp 150 hp six-cylinder engine, intake side, illustrating carburettors mounted at the rear

Early Rapp 150 hp six-cylinder engine, exhaust side view, showing pre-heated air intake routed via exhaust pipes

Following the establishment of Rapp Motorenwerke on 27 October 1913, Karl Rapp promptly focused on developing new aircraft powerplants. Alongside the Rapp 100 hp four-cylinder—an enhanced iteration of the Flugwerk Deutschland FD 1416 engine he had previously engineered at Flugwerk Deutschland GmbH—he also initiated work on a six-cylinder power unit rated at 125 hp (93 kW) intended for the second Kaiserpreis aviation engine competition. This 125 hp (93 kW) model was not completed in time for the contest, leading to further refinement that culminated in the 150 hp Rapp six-cylinder engine introduced in 1914. The initial 150 hp variant reportedly delivered approximately 150 hp (112 kW) up to 160 hp (119 kW) at speeds between 1,300 and 1,400 rpm and weighed roughly 260–265 kg (573–584 lb), while subsequent versions were rated closer to 170-175 hp with a weight near 290 kg.

The 150 hp Rapp Rp III

Its cylinders were arranged in pairs, each pair consisting of two steel liners screwed into a single cast steel cylinder head. These heads incorporated integral cooling jackets and were manufactured in paired castings before machining. The valve seats—exhaust valve seats being water-cooled—were separate components screwed into the cylinder heads to facilitate maintenance removal. Each cylinder pair’s two inlet valves and two exhaust valves were operated by individual pivoted leaf springs pressing them into their seats. Both intake and exhaust valves were actuated sequentially by a single cam on the overhead camshaft using roller tappets and rocker arms.

Initial Rapp 150 hp six-cylinder model showing carburettors mounted at the rear

The camshaft received drive through a vertical timing shaft connected by bevel gears positioned between the last two pairs of cylinders. Two magnetos energized dual spark plugs per cylinder and were also situated between these last cylinder pairs, driven from the same vertical shaft. While this configuration was retained from Karl Rapp’s earlier four-cylinder design, the increased spacing between the final two cylinder pairs on the six-cylinder model introduced asymmetry in both crankshaft and cylinder layout. This asymmetry is widely considered responsible for significant vibration issues and reduced durability experienced by this engine.

This location of the vertical timing shaft was seemingly chosen intentionally to preserve the paired cylinder design while relocating magnetos away from the rear of the engine to comply with military requirements aimed at minimizing compass interference.

The crankcase was constructed from aluminum castings split horizontally into upper and lower halves along the centerline. The crankshaft was supported by four main bearings housed within the upper section of the crankcase, spaced after every two cylinders. Lubrication was provided by a gear-driven oil pump mounted at the lowest point of the engine assembly. This pump circulated oil from a heat-conductive cylindrical brass reservoir located beneath the crankcase to main journals of the crankshaft, which then passed through drilled channels to lubricate connecting rod journals and gudgeon pins. An eccentric pump maintained proper oil levels by adding fresh oil as needed, while an additional eccentric pump removed excess oil from the crankcase during inclined attitudes. A gear-driven water pump for coolant circulation was also installed below the crankcase.

Initial versions featured two Zenith-type carburettors positioned at the rear end of the cylinder bank feeding air-fuel mixture through separate pipes that merged near cylinder three. Later models abandoned this setup in favor of mounting both Zenith carburettors on the left side of the block in a more conventional arrangement, each supplying three cylinders individually. Pre-heated air could be supplied during flight via intake pipes routed through exhaust manifolds.

While initial deliveries of these engines were rejected by the Prussian Army Administration as unsatisfactory, limited orders were placed by both the Bavarian Army Administration and German Imperial Navy. Additionally, due to urgent demand for flying boat engines, orders came from Austro-Hungarian military authorities—particularly their navy. Despite these contracts, persistent issues such as rough running conditions, excessive vibrations, frequent bearing failures, and carburettor malfunctions hindered broader adoption of this powerplant.

The 175 hp Rapp Rp III

Following a request from the German military authorities for 40 units of the Rapp Rp III engines, with a stipulated minimum power output of 175 hp during acceptance testing, Karl Rapp enhanced his engine's performance accordingly. This increase in power necessitated further reinforcement of the engine, which consequently added weight and exacerbated vibration issues. Some sources identify this model as the 175 hp Rapp Rp IIIa, while others continue to use the designation Rapp Rp III.

The January 1916 acceptance testing did not succeed, as the crankshaft failed after multiple test runs totaling 32 hours. Due to these setbacks, both the Bavarian and Austrian Army Administrations decided against placing additional orders for Rapp engines, leaving only the Austro-Hungarian Navy as a remaining client.

Variants

150 hp Rapp Rp III six-cylinder

Produced in 1914, delivering 150 hp at 1,350 rpm with a bore and stroke of 140 x 160 mm

175 hp Rapp Rp III (also known as Rp IIIa) six-cylinder

Introduced in 1916, rated at 175 hp at 1,400 rpm, maintaining the same bore and stroke dimensions of 140 x 160 mm

Applications

• Albatros C.I, powered by the German Imperial Navy using the 150 hp Rapp Rp III
• Lohner Type Warnemünde equipped with the 150 hp Rapp engine
• Lohner Type L utilizing the 150 hp Rapp powerplant
• Lohner Type T fitted with the 150 hp Rapp engine
• Lohner Type Te powered by the 150 hp Rapp unit
• Lohner Type Tl employing the 150 hp Rapp engine
• Albatros Type Weichmann driven by the 150 hp Rapp engine
• UFAG Type Weichmann using the 150 hp Rapp engine
• UFAG Type Brandenburg equipped with the 175 hp Rapp engine
• DFW Pfeil Doppeldecker "Renn-Militär-Doppeldecker 1914", powered by a six-cylinder 150 hp Rapp engine
• Otto C.I fitted with a six-cylinder Rapp engine
• Otto Rumpf-Doppeldecker from 1914 utilizing a six-cylinder 150 hp Rapp engine
• Rumpler C.I, used as a training aircraft with the 175 hp Rapp Rp IIIa engine

References

Bibliography

• Angle, Glenn Dale (1921). Airplane Engine Encyclopedia: An Alphabetical Compilation of Worldwide Airplane Engine Data. Otterbein Press. pp. 402-404. OL 23525261M.

• Pletschacher, Peter (1992). Die Königlich Bayerischen Fliegertruppen 1912-1919 (in Deutsch) (2 ed.). Planegg: Aviatic Verlag. ISBN 3-925505-20-2.

• Huth, Fritz (1914). Motoren für Flugzeuge und Luftschiffe (in Deutsch). Berlin: R. C. Schmidt & co. OCLC 11492610.

• Huth, Fritz (1920). Motoren für Flugzeuge und Luftschiffe. Bibliothek für Luftschiffahrt und Flugtechnik, Bd. 14 (in Deutsch) (3rd ed.). Berlin: R. C. Schmidt & co. OCLC 2116726.

• Vogelsang, C. Walther (1918). Der Flugmotor und seine Bestandteile. Volckmann's Bibliothek fur Flugwesen, Bd. 2 (in Deutsch) (3rd ed.). Berlin-Charlottenburg: C. J. E. Volckmann Nachf. G.m.b.H. OCLC 62113315.

• "Der 150 PS Rapp-Flugmotor". Flugsport (in Deutsch). Jahrgang 1914 (13). Frankfurt am Main: Redaktion und Verlag Flugsport: 543–545. June 24, 1914.

• Schwager, Otto (1918). Motorenkunde für Flugzeugführer, Beobachter und Werkmeister, Teil II (in Deutsch). Berlin-Charlottenburg: C. J. E. Volckmann Nachf. G.m.b.H. OCLC 49459610.

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External links

• Collection of 150 hp Rapp Rp III engines undergoing testing: FF-3380-11 FF-3380-12 FF-3380-13 (BMW Group archives)
• Rapp Rp III engine rated at 150 hp, showing damage and reportedly recovered from a depth of 50 meters near Pula: FF-3380-36 FF-3380-39 (BMW Group archives)
• Members of the marine unit disassembling a Rapp Rp III engine; the aircraft is likely a Rumpler B.I: FF-3380-14 (BMW Group archives)
• Detailed representation of the Rapp III engine, presumably from 1916, with uncertain production volume; featuring Pallas carburettors: FF-3380-35 FF-3380-38 (BMW Group archives)