In-Box Review
Gnome rotary
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by: Stephen T. Lawson [ JACKFLASH ]

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.

The on the early B-2 100hp version the single park plug was installed horizontally into the rear of the cylinder at the top. but had no connecting high-voltage wire. An internal-tooth ring gear mounted on the engine drove a stationary magneto mounted on the firewall, whose high-voltage output terminal was in close proximity to the spark plug terminals as they passed by. This arrangement eliminated the need for points, distributor, high-voltage wiring and capacitors found in conventional mechanically timed ignition systems. But this necessitated use with tractor design with a firewall behind the motor. Pusher aircraft had none, so a type with external wiring leads had to be developed. This ring gear also drove the oil pump, which supplied oil to all bearings, and through hollow pushrods to the rockers and valves and also drove an air pump which pressurized the fuel tank. On later B and all N types the Monosoupapes the spark plugs were connected to high voltage wiring.

The Monosoupape 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. On later B and all N types the 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 $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.)

Gnôme Monosoupape 9 Type B-2
(1916) 9 cylinder rotary engine, 100 hp (75 kW). Bore and stroke: 110 x 150 mm (4.3 x 5.9 in). 2,188 units produced under license in Britain. One spark plug per cylinder. External wiring only for pusher types. See description above. This was used by, but not exclusive to a whole production series of
Avro 504
Airco DH.2
Airco DH.5
Alcock Scout
BAT Bantam
Blackburn Scout
Blackburn T.B
Blackburn Triplane
Bristol-Coanda T.B.8
Bristol Scout
Coventry Ordnance Works Biplane No 2
F.B.A. C Flying boat
Nieuport 12
Royal Aircraft Factory B.E.8
Royal Aircraft Factory F.E.8
Short S.70
Short S.80
Short Type C
Sopwith Circuit Seaplane
Sopwith Gunbus
Sopwith Tabloid
Sopwith Sociable
Sopwith (Admiralty Type ) 807 Folder Seaplane
Sopwith Two-Seat Scout
Sopwith Schneider
Sopwith Pup
Vickers Gunbus (FB.2, 3, 5, 6 and 7)
Vickers E.S.1 Bullet
Vickers F.B.12
Vickers F.B 19 Bullet

Gnôme Monosoupape 9 Type N
(1917- 1918) 9 cylinder rotary engine, 150 hp (112 kW). Bore and stroke: 115 x 170 mm (4.5 x 6.7 in). Two spark plugs per cylinder. It had typical external wiring for spark plugs. This was used in Nieuport 28 and some license built Sopwith Camels (especially for AEF contracts).

(1915 -1916 ) Oberursel U.I
Was the German license built copy of the Gnôme Monosoupape 9 late type “B-2”. This was used in Fokker E.II & III types.

(1916) Oberursel U.III
Was the German license built copy of the Gnôme Monosoupape 7 in a dual format to make a twin row 14 cylinder rotary of 160hp. It had typical external wiring for spark plugs.

Vector Kit motor
Is a fair representation of the late “B-2” used on pusher type aircraft. It could physically pass for the tractor type “N” if you added a second spark plug. To pull off a tractor early type “B-2” you would have to move the sparkplug 4-5 degrees back toward the rear face of the cylinder. About the 4-5 O'clock position when viewed from above. (Remember no wiring for this version).

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

When finishing this kit you need to add your own pushrods and wiring.

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

Click here for additional images for this review.

Highs: High level detail and clean crisp molding.
Lows: Needs modification to be exact for a specific type.
Verdict: Not cheap but the quality is of the highest level. I do not regret my purchase.
Percentage Rating
  Scale: 1:32
  Mfg. ID: #32-010
  Suggested Retail: $19.88
  Related Link: website
  PUBLISHED: Mar 04, 2012

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 AeroScale. All rights reserved.


Excellent review Stephen ! Perfect for the Roden DH 2 as a better replacement option over the kit engine.
MAR 04, 2012 - 08:57 AM
Here is a bit of fun on Gnome & Le Rhône motors in general. Click here.
JUN 29, 2012 - 01:24 PM

Click image to enlarge
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