Fitting a Pitch
THE CONSTANT PURSUIT FOR GAS TURBINE EFFICIENCY ON THE PART OF ENGINEERS MIGHT SEEM A LITTLE… OBSESSIVE. But data from the Air Transport Association puts that obsession in context: in the summer of 2008, fuel accounted for more than 35 percent of airline operating costs. Even in times of lower fuel costs, some 15 percent of airline operating expenses are due to fuel. So even a few percentage points of increase in fuel efficiency can be the difference between profit and bankruptcy for an airline. That’s why a new turbofan engine system being developed by a small start-up company in Connecticut is generating some excitement. Analysis of this new system suggests that it could lead to as much as a 14 percent reduction in fuel consumption relative to today’s turbofans. The key to this greater efficiency is an essential but often unappreciated aspect of turbomachine design: pitch. Designers of gas turbines, wind turbines, and even airplane propellers know that no single pitch angle works best under every condition. But building machines that can change the orientation of their rapidly rotating parts is a challenge.
To an engineer, the pitch of a turbomachinery blade is the angle at a representative blade cross-section between the blade chordline and the plane of the blade’s rotation. Like airplane wings, turbomachine blades are characterized by a chordline, which is a straight line drawn through the foremost point on the blade’s leading edge and the rearmost point on its trailing edge. Getting the right pitch to turbomachinery blading is key to efficiency and successful operation, much as musicians strive to get the right pitch with their instruments. Before the start of a concert, members of a symphony orchestra all tune their instrumen |