Page 27 - Volume 11 Number 1

The first complete prototype PT6 underwent initial runs in a test cell during February 1960. (PW&C ARCHIVES)
propeller rotates at about 2,000 rpm, resulting in a reduction gearbox ratio of 15:1. The compressor section can be operating at a high rpm with the propeller idling, resulting in much less noise. By contrast, a fixed shaft turbine and its propeller rotate at the same speed and create a much higher noise level (compare a PT6­ powered King Air to a Mitsubishi MU­2 powered by the AiResearch TPE331).
The free turbine was selected, but the next question centered on location of the air inlet. Earlier free turbines featured an inlet at the front of the engine with a long, concentric shaft running through the center of the engine to transfer power from the rear section to the front. Because the new design (now designated DS­ 10) would be small and lightweight, size constraints were an important factor. The team elected to use a reverse­flow configuration whereby air entered at the rear of the engine and passed through the compressor section on its way to the power turbine. By placing the air inlet at the rear, components could be mounted on the reduction gearbox (RGB), which could be removed and replaced without demounting the entire engine from the aircraft.
A basic description of the DS­10 was provided to airframe man­ ufacturers including Beech Aircraft and included a number of key points:
 A 450­shp, free­turbine turboprop and turboshaft engine suitable for fixed­wing, helicopter or VTIOL aircraft.
 Size based on airplane design studies and surveys of light aircraft manufacturers.
 A pressure ratio of 6:1 should be attainable.
Chief engineer on the PT6 project was Allan Newland, shown here with the first PT6 that was heavily connected to instru- ments for data gathering. (PW&C ARCHIVES)
JANUARY 2017