Economic factors and environmental awareness are driving the evolution of aircraft engines towards decreased fuel consumption and emissions. One idea for designing more efficient engines is to increase the bypass ratio of the engine, effectively increasing the diameter of the fan. Unfortunately, a larger fan also results in a larger nacelle, which increases weight and aerodynamic drag. There is an alternative approach however, called the Counter-Rotating Open Rotor (CROR), consisting of two, unducted, counter-rotating propellers, which increases the bypass ratio of the engine without the increase in aerodynamic drag. Historically, these engines have been plagued by high noise levels due to the impingement of the front propeller tip vortices onto the rear counter-rotating propeller. These engines lack the outer nacelle found in conventional turbofans, so in modern designs the noise levels have been decreased by shortening the rear, counter-rotating propeller. This comes at a cost of decreased efficiency.
An alternative, potential solution lies with the Boxprop, which was invented by Richard Avellán and Anders Lundbladh. The Boxprop consists of blade pairs joined at the tip, and is conceptually similar to a box wing. This type of propeller could weaken or eliminate the tip vortex found in conventional propellers, thereby reducing the noise of a CROR.
This thesis summarizes advances done in the research regarding the aerodynamics of the Boxprop. Aerodynamic optimization of the Boxprop has shown that it features higher propeller efficiency than conventional propellers with the same number of blades, but lower propeller efficiency than conventional propellers with twice as many blades. It is also shown that optimal Boxprop designs share a common design attribute, namely that the blade halves are swept in opposite directions.
The thesis also derives a method to keep track of the energy transferred from the propeller blade to the flow, which can be used for estimating how much of the energy has been used for propulsion and how much has been lost to different aerodynamic and thermodynamic losses. It can also quantify the amount of energy used to create tip vortices and other non-uniformities in the flow.
Finally, a CROR has been designed which incorporates the Boxprop and its performance at cruise is competitive with other published CRORs, paving the way for future research regarding noise.