The UDF definately saves fuel, of that there is no doubt. The doubts are in getting the fan noise to meet the current very strict Stage IV noise levels around some airports, which I read is possible, and the effect the UDF has on maintenance.
Today's turbofan just keeps on chugging, where these UDFs will be more mechanically complicated because of the requirements for variable pitch blades.
But Pratt&Whitney, GE, Rolls Royce, and others are actively pursuing UDFs as an option for the next generation engine for the replacements for the A320 and B-737.
The other option being explored is the geared turbofan.
Green sky thinking - carbon credits and the propfan comeback?
"Will the emissions guys back off for the noise guys? It could be a case of 'Can you listen to the birds around the airport?' trading against the melting polar ice caps," Clapper adds.