No need to keep telling me about the partially braked case, eg. by magnetic resistance of the motor bell. For any given prop the drag when braked (B) will either be higher or lower than the drag when freewheeling (F), and the drag from partial braking (P) will always be somewhere in between. In reality we can only have B or P and we want the lowest possible.
You should have also tested with a motor that has magnets intact because the "freewheeling" test is not equivalent to a real-world situation -- because all the props will be connected to a motor that has magnetic drag. I strongly suspect that even a small amount of magnetic drag will make a
Watching movies of propeller driven aircraft when I was much younger when the hero ordered the prop of the sick motor to be "feathered" I only saw that it stopped and wondered why they bothered. Didn't know at that time that they had variable pitch props and feathering meant aligning the blades with the airflow. "Braking" is a completely new subject to me. Your example of the door in the stream made it much clearer. Thank you.
Thanks for doing this test! This is actually simpler than it appears, once you account for a hidden variable. It turns out, air hitting the prop at 50 MPH always has the same amount of force. What changes is what you do with that force.
In your analogy, you need to remember drag is NOT caused by the leading edge nearly as much as the trailing edges. Air drag of pickup trucks is a common subject for that, also golf ball resistance descriptions. So when thinking about the turbulence caused, the lower pitch creates an air bubble behind it which can be very efficient at letting the high speed air flow back together without much turbulence. The pitched blades will be transforming more of the head-win into rotational energy as well, which also causes more air turbulence because there is no stable low-pressure air zone when freewheeling, but when braked it would have a stable low-pressure zone
what the analogy doe snot take into account is the tip velocity, as with a prop with only a little pitch and a large diameter will create a lot of drag when left to freewheel, becuasse it will start turning very fast, and the tips will be at a very high rate of speed. counter-intuitively, it will be better to brake such a prop. if i understood correctly. great test!
In your river analogy you would also have to change the length of your line (ie length perpendicular to flow) to account for your 2 variables (pitch and diameter) of the props. Great video!
I understand your point of the irrelevance of the prop wheeling resistance but that's not totally true. Fluid dynamics is much more complicated than just simple drag. a few points I siee problems with:
I would have loved to see the test with the motor intact mostly because I'm curious to see if the magnetic portion of the drag vs speed is linear. Thanks for this vid: nice setup with the load cell.
I like how in-depth you went with the data analysis. I wouldn't have thought to relate the ratio of diameter and pitch to anything meaningful at all, but I guess you hit the nail on the head when you did that. The only thing I would do differently would be to add some fins to straighten out the helical airflow from the driven propeller. That might have a small impact on the way the freewheeling prop rotates.
