The Use of Mass Forces to Increase the Efficiency of Jet Propulsion

A scheme is suggested for increasing the efficiency of jet propulsion by combining the jet with an airfoil. The combination is called a jet-foil. From a theoretical investigation of the jet when used in this respect, it is concluded that high propulsive efficiencies are possible under ideal conditions since the action depends upon mass forces and is completely independent of viscosity. Experimental determination of the forces on four model jet-foils, made by measuring pressure distribution on the models: when placed in a wind stream, showed an increase in lift of the model with the jet compared to that of the model alone, and an increase in drag for three of the models and a decrease for the fourth. The lift and drag depended upon the strength of the jet and upon the attitude of the model to the wind. The efficiency of the jet-foil is defined as that fraction of the unused power of the jet which the jet-foil absorbs by doing work. Zero efficiencies are obtained for the three models showing increased drag, and values ranging from 38 percent to zero for the fourth, which showed a decrease in drag. A second set of efficiencies were calculated neglecting increased drag. These range from 40 percent to zero and are much the same for all of the models, decreasing as the strength of the jet increases. An effort is made to account for the low efficiencies on the basis of turbulence.