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In-Box Review
Oberursel U.I
The licensed built Monosoupape
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by: Stephen T. Lawson [ JACKFLASH ]

Originally published on:

The Oberursel U.I was an early rotary engine that powered many early German fighter aircraft in the first part of World War I. It was a 9-cylinder air-cooled rotary engine, a licence-built copy of the Gnome Monosoupape that Oberursel was producing before the war. It developed 75 kW (100 hp).

In 1913, Louis Seguin and his brother Laurent (engineers who founded the Société Des Moteurs Gnome [the Gnome motor company] in 1905) introduced the new Monosoupape (Mono-soo-pap) series, which eliminated the inlet valve, replacing it with piston-controlled transfer ports similar to those found in a two-stroke engine. Beginning with the power stroke, the four-stroke engine operated normally until the piston was just about to reach the bottom of its stroke (bottom dead center, or BDC), when the exhaust valve was opened "early". This let the still-hot burnt combustion gases "pop" out of the engine while the piston was still moving down, relieving exhaust pressure and preventing exhaust gases from entering the crankcase. After a small additional amount of travel, the piston uncovered 36 small ports around the base of the cylinder, leading to the crankcase which held additional fuel–air mixture (the charge). No transfer took place at this point since there was no pressure differential; the cylinder was still open to the air and thus at ambient pressure. The overhead valve exhausted directly into the slipstream since there was no exhaust manifold in order to save weight.

During the exhaust stroke, total scavenging occurred as the air moving past the cylinder exterior lowered the pressure inside due to the direct exposure of the exhaust port to the slipstream. The piston continued its exhaust stroke until top dead center (TDC) was reached, but the valve did not close. The piston began to move down on its intake stroke with the valve still open, pulling fresh (presumably un-filtered) air into the cylinder. It remained open until it was two-thirds of the way down, at which point the valve closed and the remainder of the intake stroke greatly reduced the air pressure. When the piston uncovered the transfer ports again, the cylinder sucked in the balance of the charge.

The charge was an overly rich mixture of air, which was acquired through the hollow crankshaft, and fuel that was continuously injected by a fuel nozzle on the end of a fuel line, entering the crankcase through the hollow crankshaft. The nozzle was in the proximity of, and aimed at, the inside base of the cylinder where the transfer ports were located. The fuel nozzle was stationary with the crankshaft, and the cylinders rotated into position in turn. The compression stroke was conventional. Like later B and all N type Monosoupapes, all the Oberursel U.I types had the spark plugs and were connected to high voltage wiring.

Like all Monosoupape engines this Oberursel copy had no carburetor or throttle, and since most of its air supply was taken in through the exhaust valve, it could not be controlled by adjusting the air supply to the crankcase like other rotaries. Early Monosoupapes had a single petrol regulating control used for a limited degree of speed regulation. In early examples, engine speed could be controlled by varying the opening time and extent of the exhaust valves using levers acting on the valve tappet rollers, but this was later abandoned as it caused burning of the valves. Instead, a blip switch was used, which cut out the ignition when pressed. This was used sparingly to avoid damaging the engine, since it was only safe to be used when the fuel supply was also cut. These Monosoupapes were fitted with a selector switch which allowed the pilot to cut out six cylinders so that each cylinder fired only once per three engine revolutions but the engine remained in perfect balance.

The lubrication system, as with all rotary engines, was a total-loss type in which castor oil was injected into the fuel–air mix with a small pump. Castor oil was used because it could not be easily dissolved into the fuel, and because it possessed lubrication qualities superior to mineral oils of the day. Over two gallons of castor oil were sprayed into the air during each hour of engine operation. This explains why most rotaries were fitted with a three-quarters cowl ring, open at the bottom. The cowl directed the spray of castor oil, along with sparks from the exhaust, away from the flammable aircraft structure.

Like all rotaries, it had to be precisely balanced, requiring precision machining of all parts. As a result of the design Monosoupapes were extremely expensive to build, the 100hp (75 kW) models costing about $4,000 in 1916 (approx. $65,000 in 2000 dollars). However, they weighed slightly less than the earlier two-valve engines and also used less lubricating oil. (Text originated from Wikipedia highly modified by me.)

Euler D.I
Euler D.II
Fokker D.II
Fokker D.V
Fokker E.II
Fokker E.III
Pfalz A.II
Pfalz E.II
Pfalz E.III
Pfalz E.VI
Siemens-Schuckert E.III

Taurus Models kit motor
Is a good representation of the Oberursel U.I 100hp used on German tractor type aircraft.

80 resin pieces
02 copper wires
03 section fold out instrucions

Note please! When replacing other kit pieces with the Taurus Models resin items you will find there may be a need to modify the kit cowling to allow a good fit. Taurus Models tends to be exacting when it comes to overall measurements. Plastic kits tend to have overly thick walls neded to get a kit out of the mold without distorting the freshly poured plastic.

While a bit costly it is justified in the additional details seen here (especially in the pushrod connectors and rocker arm assemblies) not seen in any other resin rotaries previous to this.

When contacting manufacturers and publishers please mention you saw this review at AEROSCALE

Click here for additional images for this review.

Highs: Very High level of detail and clean crisp molding. The level of detailing is unprecedented for a 1:32 resin kit. Online instructions are very good.
Lows: Cost is steep. Check your pieces carefully. Some smaller parts may have bubble interruptions in the surfaces. Hard copy instructions are minimal.
Verdict: Details are very impressive. For this level of detail it is worth the price.
Percentage Rating
  Scale: 1:32
  Mfg. ID: #3208
  Suggested Retail: $37.50
  Related Link: Website
  PUBLISHED: Mar 20, 2013

Our Thanks to Taurus Models!
This item was provided by them for the purpose of having it reviewed on this KitMaker Network site. If you would like your kit, book, or product reviewed, please contact us.

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About Stephen T. Lawson (JackFlash)

I was building Off topic jet age kits at the age of 7. I remember building my first WWI kit way back in 1964-5 at the age of 8-9. Hundreds of 1/72 scale Revell and Airfix kits later my eyes started to change and I wanted to do more detail. With the advent of DML / Dragon and Eduard I sold off my ...

Copyright ©2021 text by Stephen T. Lawson [ JACKFLASH ]. Images also by copyright holder unless otherwise noted. Opinions expressed are those of the author(s) and not necessarily those of ModelGeek. All rights reserved.


I've seen it live and have to say that detail level is incredible. Worth every penny but only if you are experienced modeller
MAR 21, 2013 - 02:49 AM
Yes, it's certainly not for beginning modellers. It is a very beautifully detailed replica though, well worth considering if you plan building an extra special version of WNW's Fokker Eindekker. Compliments to Stephen for adding juicy details on the history and use of the U.I in his review. Underlines the difference between WWI fighters and my daily transport Volvo. Can't imagine chucking in pints of castor oil just to trundle off to IJmuiden once a day Julian
MAR 21, 2013 - 06:55 AM

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