Constant Speed Prop.

[Tinstaafl]

Oh yes. The poor engine shakes like buggery and the time to reach idle is longer. Think of all that mass getting shaken back & forth and the effort the starter motor has to do. More throttle is needed to get the thing to run so higher MP as well.

 

[avbug]

Starting a piston engine from feather doesn't take much more engine effort than out, but getting it out of feather can be an issue of it's own accord. The prop doesn't always want to come out of feather, and without an unfeathering accumulator on some light piston engine installations, coming back out can be a drawn out process. It's not the greater torque loading that's hard on the engine, as there isn't a significantly greater loading, or rather, it doesn't provide a great hinderance. The problem is the excessive starter useage that may sometimes be required before the prop reluctantly comes out of feather...and most of us have seen times when the prop wouldn't unfeather, too. It's not "overtorquing" the engine that's the issue, but starter duty cycle limits.

In a hydromatic installation as identified earlier, the feather button can be held in to drive it out of feather. Going into feather is tougher, because you need to be able to shut it off if it doesn't shut off when feathered, lest the pump drive the prop through feather and right back to turning again. When coming out of feather, one merely lets off once the airstream takes over driving the prop. Introduce spark and fuel, and it's a done deal.

Conversely, I remember trying to relight an engine on a King Air 90 once, and being unable to get it out of feather, or to light off. On another occasion in the same airplane, the prop wouldn't feather, and that produced an enormous amount of drag.

Even on four engine airplanes, I've had difficulty maintaining altitude, even at low altitudes, if an engine failed to feather.

Why do the antifeather pins insert at low RPMs? To prevent the engine from feathering on shutdown under normal circumstances. There's no reason for it to feather on a normal shutdown on the ground. If one is going to feather, it needs to be done at higher RPM's, such as in flight. The engine shouldn't need to be feathered on the ground, though you can...the whole point is that the pins shouldn't insert so long as adequate centrifugal (centripetal) load is on them in flight, and that's a function of RPM...with adequate airspeed, the prop should never get that slow, so it shouldn't be an issue.

The problem is that these are mechanical objects, and they do malfunction. Props don't automatically feather every time with loss of oil pressure. With a governor problem, they can occasionally feather on their own. Aircraft equipped with autofeather have the unfortunate side effect of disabling themselves if the pilot proceeds by habit with the usual identify, verify, feather drill...pulling back a power lever can disable the autofeather system, negating it's value and preventing it from doing it's job.

You want your piston engine to revert to it's low pitch, high RPM setting in the case of a governor failure. If the governor can't control the propeller, then it acts like a fixed pitch prop, and you control it with the throttle. Decrease RPM, merely decrease throttle, just like any fixed pitch prop installation. This allows you to continue using power. A full feathering prop on a single might or might not provide any great advantages, but it does provide more things to go wrong, more errors for a pilot to make in flight, and adds expense that isn't really necessary, not to mention complexity.

In a turbine installation, you have a different animal, with different blade area and energy, and different results to the aircraft in the event of an engine failure or shutdown. The drag from a windmilling prop that fails to feather on most turbine installations can be considerably higher, with considerably more dire results. The same can be true of a runaway prop on a turboprop, vs. a piston installation. The wider blades that operate at lower RPM's and different angles tend to create a lot more drag and absorb a lot more energy. Additionally, these systems don't stop at the low pitch or high pitch; these feather, and go right through low pitch into beta and reverse...with the potential for severe overspeed related consequences, as well as directional and pitch control issues for single engine airplanes using turboprop installations.