Freewheeling propellers: the chopper theory

[bottomscraper]

Does a freewheelin prop gather less barnacles?

[John Vigor]

Does a freewheelin prop gather less barnacles?

Yes, the poor little buggers get dizzy and fall off. If you have any human compassion left in you, you should lock your prop shaft.

Not only will this allow God's little creatures to live their humble lives in the peace they deserve, but it will also make your boat go faster because, despite what the yapping naysayers think, a spinning propeller creates more drag than a stalled one.

If you just think of a sailboat as a helicopter on its side, the whole thing will become immediately apparent. I am right. I am right.

Cheers,

John V.

[M. R. Bober]

When you start comparing sailboats to helicopters, don't forget that running aground in one is not the same as in the other.

Mitchell Bober
Sunny Annapolis (where it's OK to leave your prop unlocked, but you probably shouldn't do the same with your car), MD

[darmoose]

this chart represents the FULL range of propellar motions. to the far left, our prop is rotating at max rpms in reverse. in the middle, our prop is locked and stationary. to the far right, our prop is rotating at max rpms in forward.


........MAX DRAG................................................................MIN DRAG....

. .......MAX REVERSE.............LOCKED PROP..................MAX FORWARD
..............RPM.............................0 RPM..................................RPM........
===============================================
...............^...................................^..............^........................^.........
.........................................................FREE WHEELING........................
................................................................PROP...................................


we all know that in order to stop our boat, we must have a prop rotating in reverse, and that the faster the prop rotates, the quicker we stop. there is no other force in play here other than drag (resistance) to the forward motion of the boat. therefore, the greatest propellar drag is when the prop is rotating in reverse at max rpms.

as we move to the right on the chart toward the center, the drag lessens. as we continue through the locked prop position and into the forward rotating portion of the chart, the drag continues to lessen. at some point on the chart, as we move to the right, we reach a condition we will call "zero prop drag". this condition occurs when our prop rpms match up with the boat speed through the water.

finally, as we increase the forward rpms, we pass through the "zero prop drag zone", and enter a zone where the prop is providing thrust.

the freewheeling prop condition is placed on the chart in its position because it is rotating in the forward direction (complimenting the boats motion). therefore, the freewheeling prop produces less drag than a locked (stationary) prop.

the only way one could believe otherwise, is if one believed that both forward as well as reverse rotating props produced more drag than a locked (stationary) prop.

this matter is most definitely resolved
darrell randolph
s/v mystic rose

[Len]

when a chopper autorotates doesn't the chopper spin as it is falling!Does it spin in the opposite direction of the blades? To persue the analogy - do we (but of course we can't) rotate the boat in the same or opposite direction to the prop??

[Dick Barthel]

this chart represents the FULL range of propellar motions. to the far left, our prop is rotating at max rpms in reverse. in the middle, our prop is locked and stationary. to the far right, our prop is rotating at max rpms in forward.


........MAX DRAG................................................................MIN DRAG....

. .......MAX REVERSE.............LOCKED PROP..................MAX FORWARD
..............RPM.............................0 RPM..................................RPM........
===============================================
...............^...................................^..............^........................^.........
.........................................................FREE WHEELING........................
................................................................PROP...................................


we all know that in order to stop our boat, we must have a prop rotating in reverse, and that the faster the prop rotates, the quicker we stop. there is no other force in play here other than drag (resistance) to the forward motion of the boat. therefore, the greatest propellar drag is when the prop is rotating in reverse at max rpms.

as we move to the right on the chart toward the center, the drag lessens. as we continue through the locked prop position and into the forward rotating portion of the chart, the drag continues to lessen. at some point on the chart, as we move to the right, we reach a condition we will call "zero prop drag". this condition occurs when our prop rpms match up with the boat speed through the water.

finally, as we increase the forward rpms, we pass through the "zero prop drag zone", and enter a zone where the prop is providing thrust.

the freewheeling prop condition is placed on the chart in its position because it is rotating in the forward direction (complimenting the boats motion). therefore, the freewheeling prop produces less drag than a locked (stationary) prop.

the only way one could believe otherwise, is if one believed that both forward as well as reverse rotating props produced more drag than a locked (stationary) prop.

this matter is most definitely resolved
darrell randolph
s/v mystic rose

Darrell,

Your theory seems to be based on the idea that there is necessarily a consistent linear relationship (on a graph) when you move from the left to the right on your chart almost like it is a law of physics. You might be right, but your model just dismisses the idea that a very slow moving propeller might be creating more vortices (i.e. disturbance) than a fixed one (as someone earlier suggested supposedly as the result of actual experiment). If that theory proved to be true that could only mean that your theory must be wrong.

Dick

[S/V Necessity]

Wow this is getting to be a fun thread! Don't forget to put a cavitating prop on that chart! (this should be a clue that the graph is NOT linear)

And to think all John was probably trying to say is neener neener look at what I got for x-mas! Personally, I made a firm decision to never trust the guy after I had to throw out my copy of "20 small boats." Granted I did have choice, the book or my girlfriend.

[S/V Necessity]

I got to thinking about this one some more.... I knew that what Vigor was saying about the fall of the 'copter was correct, cause I've seen it in action. I used to have one of those wooden props on a stick you could spin and make it shoot up in the air. It would also fall slowly to the ground when released. *from a few feet off the ground*

IF you were to drop the copter with the blades fixed it would drop like a rock. And if you drop it with the blades loose it ACCELERATES more slowly.... The difference here is the blades are spinning!!! You don't get something for nothing. It takes more energy initially to ACCELERATE the blades. So the copter *acclerates* more slowly. Eventually it will be falling faster though.

John just needs to let her fall a bit longer to see that she can indeed smack the ground harder with the blades free spinning, as opposed to being fixed.

[Steve Laume]

Mark, you through out a perfectly good copy of John's book for the sake of your girl friend!? That is not just a book. It is fodder for your dreams. So just how long ago did you make this life altering choice? You must have found yourself a pretty incredible woman, Steve.

[Dean Abramson]

"Twenty Crashing Aircraft That Will Take You Nowhere"

Kinda has a nice ring to it.

Dean :-)

[Dick Barthel]

Mark, you through out a perfectly good copy of John's book for the sake of your girl friend!? That is not just a book. It is fodder for your dreams. Steve.

Steve,

You are so right in my case. His description of the 25D turned out to be right on the money. Of course he did own one for a time.

Dick

[darmoose]

what everyone of the "johns helicopter analysis is right" clan is missing here is this.

a helicopter, when it is autorotating in a fall toward the earth, indeed has a spinning propellar, and indeed falls slower than if the propellar was locked in position.

HOWEVER, THE PROPELLAR IS ROTATING IN OPPOSITION TO ITS DIRECTION OF MOVEMENT. IT IS IN REVERSE(as it always is with a helicopter). obviously, if the propellar on a helocopter turned in the forward direction, or toward the ground, it would never get off the ground in the first place.

OUR FREEWHEELING PROPELLAR IS NOT ROTATING IN REVERSE (AS J.V. so excitedly exclaimed himself in an earlier post). OUR FREEWHEELING PROPELLAR IS ROTATING IN THE FORWARD DIRECTION, COMPLIMENTING THE MOTION OF THE BOAT.

FORGIVE ME, but john vigors christmas toy has absolutely nothing to ad to this discussion, and the analogy is totally wrong.

additionally, it has been said that i am ignoring "test results" that have been offered up as proof of the position that a fixed prop causes less drag than a freewheeling prop. i heard one reference to an obscure test from i dont know when, by i dont know who, and a proclamation from one old ships captain who for all i know didnt even sail ships with propellars.

what about the half dozen to a dozen of our own board members who have testified here that they have conducted these tests on cape dorys, some up to a hundred times, and have concluded that the freewheeling prop, indeed, produces less drag and increases their sailing speed. do you totally ignore them? what about the more specific test results achieved by the two graduate students in a controlled test at MIT (massachussetts institute of technology) are you familiar with the reputation that institution has earned?

and what about the other experiment that was done with the anchored boats on a springloaded tether.

there has been no evidence offered here, or anywhere that i have read, that a rotating prop causes any additional vortex resistance than a stationery prop being pulled through the water. all we get from the other side is abstract, vague, irrelevent statements or questions, really!

and john, i love your posts on this board, and i have read your books. but, i think this is why we have specialization in all of the serious professions in this country. writers write, and engineers solve math and science problems(eh-eh-eh)

by the way, in south florida today it is about 80 degrees and the sun is shining. i think i will go sailing this weekend and test my freewheeling propellar for drag.

HAPPY NEW YEARS TO EVERYONE

darrell randolph
s/v mystic rose

[Oswego John]

[by the way, in south florida today it is about 80 degrees and the sun is shining. i think i will go sailing this weekend and test my freewheeling propellar for drag. ] Quote by Darmoose

Cruel. Borderline inhumane.

You really know how to hurt a guy.

Is my enviousness showing? Just kidding. I look out my window and see that the ground is covered with white stuff. The boats are snuggled away in their winter covers and the heating fuel bills are going into orbit.

You just go ahead sailing. Don't worry about me. I'll be alright. Have a good time

Kvetching in Oswego, brrrr
O J

[Parfait's Provider]

Do helicopters with variable pitch blades even have the ability to rotate the blades in reverse or put negative pitch on the blades? Does the mechanism that controls the pitch continue to work even if the engine dies? I would assume it is mechanical and can thus be set to cause forward motion even when autorotating and that would help to "skip" the chopper through the air like a flat rock does when tossed across the water, thus improving the aerodynamics considerably.

BTW, I agree with the experimental data. The prop has less drag if it is turning in a forward direction, but slower than the flow of water relative to pitch. It has positive drag or thrust if it is turning faster than the flow of water relative to pitch. I also believe there are no discontinuities in the thrust vs. RPM curve.

[M. R. Bober]

when a chopper autorotates doesn't the chopper spin as it is falling!Does it spin in the opposite direction of the blades? To persue the analogy - do we (but of course we can't) rotate the boat in the same or opposite direction to the prop??

Leonard, this is an excellent question. Most helicopters have a horizontally mounted main rotor (rotating wing assembly) and a vertically mounted tail rotor. They are coupled. In the event of an engine or transmission failure, in flight, the main rotor will auto rotate during descent due to wind pressure on the blades. As the main rotor spins so does the tail rotor.

The tail rotor provides what is known in the trade as the counter rotational effect. This prevents the helicopter from spinning under the main rotor (instead of the main rotor spinning over the helicopter). The loss of the tail rotor (loss of counter rotational effect) generally results in a unfortunate outcome as the helicopter will spin causing the pilot to lose control.


Mitchell Bober (former SP/5--123rd AVN Bn)
Sunny Annapolis (where not everyone served in the Navy), MD