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A trimotor is an 
aircraft
An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...
powered by three engines and represents a compromise between complexity and safety and was often a result of the limited power of the engines available to the designer. Many trimotors were designed and built in the 1920s and 1930s, when engine power lagged behind the designers' power requirements.
See also
*Trijet
A trijet is a jet aircraft powered by three jet engines. In general, passenger airline trijets are considered to be second-generation jet airliners, due to their innovative engine locations, in addition to the advancement of turbofan technology. ...
References
{{Lists of aircraft, state=collapsed Aircraft configurations Trimotors Lists of aircraft by design configuration