I think that the biggest difference is that with a piston engine, a windmilling prop has to drive the whole engine against it's own compression. A windmilling fan-jet engine normally only turns the N1 section, which produces surprisingly little drag. You have to be going really fast, like above 250 KIAS, to even notice rotation on the N2 gauge.

If you have ever tried to hand prop a moderately large piston engine, you know how much effort it takes to get it to turn. You can walk up to the front of a 737 and start the engine rotating with one hand. If there is any wind, it will probably already be rotating. If there is a tailwind, it will just as easily rotate in reverse.

Joe

It's probably already been answered in one of the replies. But if you could "feather" the fan section, there's still A LOT of metal behind that that would cause drag. The rotors/stators, combustion chamber, and every accessory attached to the core of the engine creating a profile contributing to the drag.

BAE-4100

Earlier in the thread, some of you were talking about "P" factor, or the existance of a critical engine on tubofan engines, and I would like to point out that this is no-existant (for practical purposes anyway) due to the air being directed through the nose cowl. Of course, some things will affect airflow at slower speeds (strong crosswind, or high reverse power at slow speeds, for ex.), but there is no differance in power output due to the pitch attitude of the aircaft as the air that is going through the C1 disk is being "straightened" by the nose cowling. Hope this is more helpful than confusing. Some of the bigger engines have 34-48 blades on the C1 compressor, and it would be nearly impossible to build a hub that would withstand the forces imposed on it and accomodate this number of blades.

Cheyenne 400LS is counter rotating.

P-38 was counter, but both spun outwards.

I didn't even think about the Jetstream 41, even though I see them all the time. Also, for the OP some t-props (usually the ones made in Europe) have engines that turn counter-clockwise such as the Jetstream 31/32. Critical engine failure in one of those means lots of right rudder, and then start trimming.

Isn't the Piaggio P180 a counter-rotating propeller?

It might need it, with the props so close to the tail the rudder arm would be very short.

Hi Rick.

I'm trying to visualize whether the location of the props makes a difference. My understanding is that the rudder arm acts thru the C.G. This would seem to mean that the fore and aft position of the props should not make a difference, except in that the weight of the engines would tend to give the aircraft a relatively far aft C.G.

Joe

Correct, look at where the wings are on that thing....waaaay back. That implies that the CG is back there too. You don't HAVE to place the wings near the CG, but it would terribly inefficient not to. Your tailplane (or canards in this case) would have to carry a high load to maintain level pitch attitude.