A prop makes thrust by applying power to the airflow passing through its disk to accelerate that airflow to a higher speed. Likewise, it creates drag by absorbing power from the airflow.
The key word in the above paragraph is "power". The power has to come from somewhere, and it has to go somewhere. If there is no source of power, or no place to absorb the power, you have the aerodynamic equivalent of an electrical open circuit.
For conventional thrust, the power comes from the motor and is absorbed by the air.
For windmilling drag, the power obviously comes from the airflow, but just exactly where does it go?
Some of it is absorbed by the profile drag of the blades, but unless the airplane's airspeed is high and/or the pitch is flat enough to result in a high windmilling rpm (and therefore a high helical airspeed for the blades), this tends to be small. This is why it's often best for a rubber free-flight model, with its wide bladed, high pitched prop and slow gliding speed, to let the prop windmill freely.
OTOH, for a full-scale piston aircraft, the prop is attached to what amounts to a giant air pump. Pumping lots of air at high pressure can involve massive amounts of power, so that engine being driven by the windmilling prop can absorb gobs of power and make huge amounts of drag while doing so.
For a turbine engine, things get more complicated. There are two general types of turboprop engines, "fixed shaft" and "free turbine".
The "fixed shaft" type such as the Alison T-56 in the C-130 Hercules, P-3C Orion, etc., or the Garrett TPE-331 in the Fairchild Metro, OV-10 Bronco and others, has the power turbine that drives the prop mounted on the same shaft as the compressor. This means that for the prop to windmill, it also has to drive the compressor. Since the compressor can absorb astonishing amounts of power, typically far in excess of even the engine's max rated output power, the windmilling drag of a fixed shaft engine can be astronomical, typically more than sufficient to make the airplane fall out of the sky like a stove. Such engines and their propellers typically have a whole army of backup systems to make sure this can't happen, even with some system failures.
The "free shaft" or "free turbine" type, such as the Pratt & Whitney PT-6 and PW-100 series, has two or more separate, concentric shafts. The outer ones carry the compressor stages and the turbines that drive them. The innermost shaft contains the power turbine that drives the propeller gearbox. If you shut down the engine, the only thing the windmilling propeller can drive directly is the power turbine, which in this situation typically absorbs very little power. However, if the plane is flying at fairly high airspeeds and the variable-pitch prop is at a fairly flat pitch (causing it to windmill at a very high rpm), the profile losses in the blades alone can absorb enough power to make a significant amount of drag. The EMB-120 crash that killed Senator John Tower was caused by a truly freakish propeller control system failure that allowed the prop to go to an abnormally flat pitch. The resulting massive windmilling drag on just one side of the airplane caused the airplane to lose control.
In a gas-powered model, the pumping ability of the engine would create a lot of windmilling drag. However, in most cases the torque required to drive the engine is sufficient to stop the prop. With the prop stopped, the drag is approximately equal to the flat-plate drag of the area of the blades themselves.
In an electric motor, it's a little more complex. If there is no complete circuit to the electric motor, then the motor itself absorbs very little power and therefore generates very little drag. If there is a gearbox, then the friction from that can absorb a little power, but as long as the motor isn't allowing any significant load on the gears, then those losses tend to be small as well. If you have two motors in parallel geared to the same prop, then conceivably there could be some back EMF issues between the two motors. I'll leave it to the electronics gurus on the list to address those.
The big factor in an electric motor system will be the profile drag of the blades. If the prop pitch is fairly high and the gliding speed low, the drag from that could be less than the flat-plate drag of stopped blades, especially if the blades in question are big paddle-bladed types with lots of area. However, especially with narrow blades, with a flatter pitch prop and/or higher gliding speed, resulting in a high windmilling rpm and high helical airspeed for the blades, the profile losses could exceed the flat-plate drag of a stopped prop.
Like I said, no "one size fits all" answer.
Don Stackhouse @ DJ Aerotech [EMAIL PROTECTED] http://www.djaerotech.com
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