Drag: Windmilling vs Dead Prop
 

Lots of good discussions and arguing on various sites over this. But, oddly, not much good, distilled data/info.

Scenario: two single-engine prop planes. Engine quit and seizes in one. Engines quits but prop rotates in the other. Everything else equal. Which has more drag?

Without generating more argument and opinion (yeah... like that will happen), can you post a link to a definitive source for an answer? And that will explain it without formulas that take up 7 pages. I'd like something that can be understood by a History major.

Well it probably isn't exactly what you're looking for, but we secure the failed engine in 2+ engine for a reason. Check out Jeppesen Multi-Engine Ch. 3 Section B and Aerodynamics for Naval Aviators P. 376. A windmilling prop is like a barn door out there when I comes to parasite drag.


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No such thing as a free lunch, and a prop is an airfoil just like a wing...

A windmilling prop is a essentially...a windmill. It's extracting energy from the slipstream and turning it into compression heat in the pistons and internal moving parts friction in the motor. The slipstream energy being consumed by the turning (but dead) motor has to come from somewhere, and that somewhere is the kinetic energy of the airplane. You can trade altitude (potential energy) for kinetic energy to maintain airspeed, and that would be second-nature for a pilot. So net result is higher descent rate.

A non-rotating prop creates some drag, but it's mainly form drag since the prop blade must have significant flow over the airfoil to transfer energy to or from the slipstream. The blade is stalled, and stalled airfoils do not transfer much energy or generate much lift. A prop blade rotating at speed with airflow along the chord is designed to be very efficient at energy transfer but only if the air is flowing the right direction.

More in depth than that will require some math...

I understand what you're stating. I've also read some places where they show/believe the difference is negligible... and in some cases, opposite of what you say, depending on prop length and chord.
Again, I don't know the answer. But I'm hoping someone knows of a link to a solid source.

It takes energy to turn the prop, crankshaft, pistons (fighting compression in the cylinders!), valves, fuel pump, generator belt, vacuum pump. Energy is not free, it has to come from somewhere. If the prop is spinning, all this other crap is spinning too. The AOA on this slowly spinning prop is probably significantly increased due to this load, greatly increasing the drag. Remember, AOA on a prop is relative to the blade's chord line and flight path, which is a function of it spinning around in a circle relative to it's airfoil chord line-generally a lot faster mph than the airplane is moving forward, and the airplane's path (lots of people think it's just the airplane's flight path).

The difference isn't negligible at all. Anyone who has done any multi engine training in a propeller equipped multi engine aircraft understands it immediately, and it's quite visible in the performance of the aircraft. In light twins, it can mean the difference between maintaining altitude and a descent without the ability to maintain altitude. It also make a significant difference in controllability and minimum controllable airspeeds.

A windmilling propeller is responsible for more drag than the equivalent flat plate area of the prop disc. That sounds counterintuitive, but is true. If one doesn't believe the ability of a propeller to cause drag in flight, fly an airplane that can do beta in flight and see what happens. I used to fly a single engine airplane that was designed with a radial engine, but which had been converted to turbines (PZL Dromader), and the installation was rigged such that retarding to idle in flight would slow the aircraft so quickly it would throw one forward in the shoulder harness. One could follow through with the stick briskly until the vertical to prevent a loss of control or stall. It was dramatic.

Windmilling vs. stopped (or feathered) also makes a dramatic difference. Rudder input required in a multi engine prop aircraft is lessened substantially upon feathering (or stoppage of the prop, such as a seizure). The windmilling propeller absorbs considerable energy, driving gearing, accessories, etc, when the engine is no longer driving the propeller.

Prior to a windmilling state, when the engine is imparting energy to the propeller and the propeller is doing work, the combination produces some degree of measurable thrust. In a windmilling state, the propeller isn't receiving torque from the engine any longer, but is still receiving energy from a source, which is the slipstream, and the process works in reverse. Whereas with the engine driving the propeller, that amount of torque and energy went into moving airflow and creating thrust, the slipstream energy now becomes all drag as it drives the prop and that drag is imparted to the prop, shank, driveshaft or crankshaft, and ultimately engine mount, wing and airframe. In effect, the slipstream becomes the engine driving the propeller, and works against the airplane instead of for it.

A stopped propeller experiences a certain amount of flat plate drag area or form drag, but much, much less than a windmilling prop.

This. Think about it for a bit.

Also, difference between putting your car in gear and putting it in neutral while coasting down a hill :)

I believe the correct answer is it depends, if both aircraft have fixed pitch cruising props say set at 18 degrees, like something like a 172, I'd say the correct answer is there ain't much difference...

if both aircraft are constant speed singles a wind milling prop set to low rpm, resulting in blade angle angle of 30 degrees the wind milling prop, would probably glide a little further than other aircraft with a stuck prop which has moved to flat pitch.

A propeller being dragged through the air at high rpm that results in flat pitch will result in far more drag by far.

I think the pitch that makes the difference is about 15 degrees.

This tallies with what I have observed in flight. The resistance movement of engine/gearbox is a bit of a red herring, and doesn't stand up to scrutiny.

I can't find anything that explains this in simple terms however there a some papers on this, but they are not easy reads.

A stationary prop creates incidental form drag.

A turning prop extracts energy from the slipstream. Its definitely going to create more drag. How much drag? Try turning your car motor at 120 rpm by hand and see how much energy that takes. OBTW, the turning prop still has its form drag in addition to the slipstream load. Rotation does not make the form drag vanish.