Gnome Monosoupape 100 Rotary Engine

AirVenture must have 500 engines on display... real ones, mockups, and a few which are pure fantasy. Most are the same old thing in a different color.  However, here is one you do NOT see every day, a Gnome Monosoupape 100 rotary.

That's a rotary, not a radial; The crankshaft is fixed to the airframe, while the crankcase and cylinders whirl around with the propeller. Not cheap, but perfect for your trusty Sopwith, or with an Oberursel data plate, an early Fokker. They're available brand new from Classic Aero Machining Services of New Zealand.  For more information, visit

Dan Horton

Dan Horton says he is "just a homebuilder." He has been a pilot more than 30 years and currently flies a custom RV-8.

4 Replies to “Gnome Monosoupape 100 Rotary Engine”

  1. Just a thought but I would think the inertia of the spinning mass would manifest a gyroscopic force felt 90 degrees to any pitch or yaw input requiring some control compensation.

  2. Paul:
    That's correct. Are you aware that many aircraft engines into the 1920s were rotary? The pilots of the early birds were well aware of the gyroscopic effects from the spinning motor.

  3. The engine is called a "Monosoupape", mono (one) and soupape (valve) in French. It is a four cycle engine and the valve works both as exhaust and intake.
    After the cylinder fires and the valve opens to exhaust, it remains open. During the intake cycle, the valve remains open sucking in air, until the piston has traveled approximately 2/3 down the cylinder, whereupon the valve closes. A vacuum is created in the cylinder as the piston continues down to the bottom of its' stroke, where a series of holes at the base of the cylinder allows a mixture of fuel and castor oil to be sucked in from the crankcase. The piston comes up and the cylinder fires, and the cycle starts again. Cylinders fire every other rotation.
    All rotary engines are not the same. LeRhone, Clerget, Bentley, etc., are not of the monosoupape type and do in fact have separate intake and exhaust valves.
    The gyroscopic effect is not as pronounced as widely rumored, but does become more of an issue as horsepower increased, thus leading to their obsolescence.

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