Argon 916iS experimental light sport pilot reported a loss of engine power during cruise with a forced off field landing.

Narrative

Partial engine power loss in Argon 916iS gyroplane resulting in forced landing in field near ZZZ.Aircraft: Argon 916iSFlight Crew: PIC - CFI-Sport (Rotorcraft-Gyroplane); second instructor onboard for engine data recording.Operation: Phase 1 test flight within 40-hour flight test period (no passengers).Flight Conditions:Day VMC; light winds; clear visibility. Operations approximately six miles west of ZZZ at altitudes between 300-600 feet AGL to remain below the airport traffic pattern altitude.Event Summary:While conducting a local test flight to collect engine performance data; a partial loss of engine power occurred in level flight approximately 18 minutes after startup. The engine continued to run but did not respond to throttle input. A forced landing was executed into an open field. No injuries or damage occurred.Narrative:I was PIC of a newly built Argon 916iS gyroplane operating under the experimental 40-hour test phase requirement. I hold a Sport Pilot certificate and CFI privileges for Rotorcraft-Gyroplane. A second instructor occupied the right seat to record engine parameters using a laptop computer.During preflight and startup; the Lane A; Lane B; and Start lights were illuminated as expected. Both electronic control lanes (Lane A and Lane B) extinguished normally after engine start and run-up; confirming both systems were operating correctly. This aircraft has exhibited the same indication behavior on previous flights without issue; as long as no 'lane crossover' faults were indicated. Engine instruments showed normal readings throughout preflight; start; and run-up.We departed to the west of the field; remaining about six miles out; operating between 300-600 feet AGL to stay below pattern altitude. After approximately 18 minutes of flight time since engine start; while established on a downwind heading at approximately 300 feet AGL; we experienced a sudden; uncommanded reduction in engine power. The engine continued running smoothly but produced limited thrust. Throttle advancement to full power yielded no change in RPM or torque response.The right-seat instructor confirmed the throttle position and power setting but had no access to the flight controls due to the laptop on his lap. I immediately selected a suitable field ahead and initiated a forced landing. Airspeed was maintained at approximately 70 knots IAS through descent; avoiding nearby trees. A flare was performed to minimum airspeed; and the gyroplane touched down upright and rolled to a stop without damage. The engine was still running upon completion of the landing roll.The aircraft was later recovered to ZZZ by the owner. Subsequent high-speed taxi tests were performed; but the issue could not be duplicated. Engine Control Unit (ECU) logs will be downloaded and forwarded to Rotax for analysis.Outcome: No injuries No damage to aircraft or property. Engine remained operational. Power loss could not be replicated postflightContributing Factors: Low altitude during test operations provided limited forced-landing options. Data recording equipment restricted access to right-seat flight controls. Unknown electronic or fuel system anomaly within engine control lanes. Power loss occurred approximately 18 minutes after startup; possibly related to ECU or electronic control behavior.Lessons Learned: Future engine data flights will be conducted at higher altitudes when practical to ensure more landing options in the event of power anomalies. Crew coordination and unobstructed access to controls will be verified before flight. This event reinforced the importance of maintaining suitable emergency landing options during low-level testing.