The different viscosity of water and air, perhaps? Also, do you leave the spinning propeller turning the diesel engine (or steam turbine in an older ship), or do you declutch it so that it's just the prop spinning? If you could declutch an airplane propeller from the engine, the drag from windmilling would be negligible.
A thin, compressible medium versus a viscous, incompressible medium, with the blades on the former a tiny fraction of the relative area of the blades on the latter?
If it's a single engine piston with a hydraulic governor, no. You'll need oil pressure.
There are springs to pull the prop blades to a default position, which the governor pulls it away from:
I thought we were trying to calculate work.
Yes, work. My mistake.
It can be computed to be a disk of "some size" but the "size of the prop arc" is nonsense. Aerodynamicists talk about an equivalent "flat plate area" but they don't mean that's necessarily equivalent to size of the prop arc.As pointed out in Post # 14, Fig 15 may infer the logic: It all depends on the propeller pitch. At below about 40° AoA, the power required to windmill rises dramatically. If the prop loses its low pitch stop, I'd suspect the "disk theory" is real, but I'll defer to an aerodynamicist.
So then, it could even be more than the the flat plate of the prop arc?
I think this is wrong. I think the whole “turning the engine requires work therefore it causes more drag” is wrong. It’s very easy to increase drag without producing any “work”. I don’t think the engine rotating is relevant. But this whole subject has always been a mystery to me so I admit I might be totally wrong.
Not if you're using the PHYSICS definitions of those words. Drag is a force. That drag acting through a displacement is work.
I can create drag without resulting in any parts moving. Ie like a rotating an engine, but yes, technically you are correct, drag is a force.
Work is the amount of energy transferred by a force acting through a distance. If the aircraft is moving through the air, then the drag force is causing the aircraft to do work on (i.e., transfer energy to) the air. If the propeller is turning a dead engine, then the propeller is doing work on (i.e., transferring energy to) the engine. Assuming a dead engine, that energy in both cases is ultimately coming out of gravitational potential energy, so if a constant glide speed is maintained, then the total rate of energy transfer dictates the rate of descent.
The propeller is designed to convert the relative wind to rotational velocity in most circumstances. The energy comes ultimately from gravity. What you want to do is convert as much of that energy as possible to lift rather than drag.
Yes, a free-spinning propeller would still provide some drag even if it weren't turning an engine, but the drag would be considerably less.
Meh. The NACA report I linked to is 4 years newer, just sayin'.
I guess it went over my head the first and second times!
That’s the one that convinces me without a doubt that windmilling is more drag, but that doesn’t mean it’s the engine rotating that’s the main factor.
No need to guess any more — see my earlier post quoting the NACA report. https://www.pilotsofamerica.com/com...dmilling-the-truth.132499/page-2#post-3132908
Yeah but that just means that the numbers would be bigger at higher speed. Until you reached a point where compressibility became a factor I don’t think the relationships in the 75 knit data would change. But I’m not an engineer. Where is @nauga and company to weigh in on this one?
I chose 75 mph because I assumed that was more typical of the glide speed for an airplane at the time (1930s), but as I mentioned, the report measured at 25 mph, 50 mph, 75 mph, and 100 mph. The numbers for 100 mph are higher, but their relationships are the same:
Since even the smaller wetted area of the airplane propeller stopped caused far more drag than the airplane propeller spinning free (declutched from the engine), I'd think that would apply even more so to the ship's propeller.
That the document you reference is from the 1930's does not necessarily mean it's valid. There are a lot of things that were accepted as gospel at the time which are now proven to be incorrect. I recall that cigarette smoking was considered a treatment for asthma at one time.
