Hey Burner, you’re playing with me.
The last post was to try to answer the question of why some operators choose to use ground idle as opposed to feather when starting, for all persons reading the thread – not only PT-6 drivers. It is with sincere doubt that I could provide you with any edification regarding the power-plant which you are obviously very familiar. When referring to the split engine design, I was trying to solidify the point that at low power settings the gas turbine doesn’t really know what the power turbine is doing, as long as they are turning in opposite directions.
“Oil required to drive the prop to fine would require some hp too”
You are correct that an additional amount of energy is required to provide the prop with high pressure oil, in the –65 it was 375psi and in the –67d it was around 750psi. In the smaller Kingairs and the –99 it is less. Sorry to say, other than adhering to starter limits, I never compared the duration of the different types of starts. Is it materially different, and do the advantages out way the disadvantages? Guess that would be another thread.
“Is the temp that critical on the batteries ??”
In the Kingair series one cannot do cross-generator starts and the NiCd batteries are taxed starting two engines in quick succession. An already warm NiCd that is depleted and is trying to absorb as much juice as it can from the generators may get very hot, resulting in a possible thermal runaway.
“The starter gen has its own fan so that as soon as you start cranking it has air going through it.”
In the BE-02 cross generator starts are the norm with min N1 of 72% and this zaps the supplying generator of 400 amps. The maximum continuous load on these gens is 300 amps, hence they tend to be overworked in the start. The generator has its own NACA vent located on the inboard nacelle of each engine, a fabric conduit then runs to the generator section to provide outside air. When in feather there is no airflow to the gen from the outside (in the 02) and overheating may ensue, causing premature failure. Then again, it may not.
“When we start always leave in feather till oil temp in the green starters always make overhaul no problems.”
The green arc for oil temp on the –65B starts at 0 degrees C and at 10 degrees C for the –67D, if you’re in any part of the world that I would inhabit, those would be already below ambient temperatures, so there should be no harm to the prop governor seals if starting out of feather.
As OzExpat pointed out, on unimproved surfaces the large suction vortex that is formed below the props at feather tends to transport FOD in a vertical plane. When the props are at ground idle some amount of horizontal initiative is given to the FOD, sending it to the rear of the aircraft rather than against the fuselage or into the engine. Obviously aircraft such as the Twotter and AgCat don’t suffer from these problems.
So those are my thoughts on why and when to start out of feather, or more accurately why some people do it that way. Oh one last thing Burner, you’re quite smart in my book, you manipulated me into spending an hour in front of the computer typing – something my professors could never do.
Fly Safe - Man