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the mission calls for Turbo so a T or P (or 310 HP Riley!) is what I'm looking for...some mountainous routes & airstrips...so speed brakes and gear door mods are also high on my list of wants...
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Why speed brakes? Shock cooling is a debatable subject (and the source of much "hangar flying,"), but is irrelevant if proper power management is used.
One shouldn't try to fly a piston airplane like a turbojet, making idle descents and disregarding proper use and treatment of the piston engine, but the piston engine isn't a sacred cow, either.
Generally, if you keep adequate power on the motor such that the engine is driving the propeller and not the other way around, you'll be carrying enough power in the descent. A common rule of thumb is that one shouldn't retard power below the bottom of the green arc on the tachomoeter (about 15" of manifold pressure, for most light airplanes), but one may also want to carry considerably more than that during an enroute descent for thermal reasons and for speed.
I've heard some argue in the past that whereas a 3:1 descent in a turbojet is the norm, a 6:1 descent in a turbocharged piston twin is a better idea. I disagree. Especially if one is cruising at any respectable altitude, trying to descend slowly over a 6:1 plan isn't practical in most places and will only end up pissing off controllers and creating a flying roadblock. Instead, a normal 3:1 descent with a little forethought and planning works wonders.
I've seen a lot of skymaster motors go right to TBO with people doing idle descents in them, however, and those were turbocharged models. In fact, I've seen idle, dirty, diving descents to landing used as a normal part of the daily routine by an operator that rarely sees anything but motors lasting to TBO. If shock cooling were indeed an issue with those airlplanes, one might have expected to see parts raining down off the runway end, but not so. Such operation was never my first choice, but the TSIO-360's in the Skymasters are surprisingly tough, versatile engines, and not the glass wrist-charms that some might think them to be.
I'll also add that they run extremely well lean of peak, though in my opinion that's best reserved for operations at or below barometric pressure, and not in the boosted range of operations (especially on hot days, and especially during a climb).
Mixture management is an important part of power management. I was once told by a former military aviator that he had just experienced an engine failure in his skymaster. When I asked how he knew he had an engine failure, he told me that he was in cruise in the "jet" (this is how he referred t the skymaster), and stated that he saw the manifold pressure "roll back to zero," and that this is how he knew he had experienced an turbocharger failure, and therefore an engine failure. He stated that the aircraft lost 300 feet, and there was the proof.
I was somewhat bewildered by the accounting, as manifold pressure can't roll to zero, nor would a turbocharger failure result in an engine failure (except in catastrophic circumstances which become immediately apparent), nor would a turbocharger failure result in what he experienced, or thought he had experienced. I had never heard of a 300' altitude loss representing an engine failure, but he was absolutely convinced that he'd experienced an engine failure.
I was asked to investigate the matter, consult with the mechanics assigned to the airplane, and then to ferry the airplane a short distance for an inspection. Setting aside the notion that ferrying an airplane with a failed engine might not be the brightest act of the day, I sought out the mechanics associated with the airplane, and these rocket scientists informed me that the airplane had a failed turbo, because the manifold pressure had been reported to roll back to zero.
I tried explaining to them the concept of barometric pressure and that manifold pressure couldn't have gone below that with a failed engine, but they didn't seem to grasp the point. I had a fairly good idea what really happened, so I loaded up both mechanics, and taxied to a run-up area. Exactly as predicted, the airplane started and taxied fine, but had roughness on one mag, and this was easily cleared up during a power run. The ex-fighter pilot had been flying with the mixture rich all the time, fowled the plugs, and had a rough engine and power loss. The manifold pressure never rolled to zero; it was the action he thought he saw because it was what he expected based on his turbojet experience; he saw what he wanted to see. Not at all an uncommon experience.
The point there is that mixture management is just as important as power settings when it comes to issues of climb and descent, and this also applies to the question of "shock cooling."
Continued (due to length)...