Freewheeling propellers: the chopper theory

[Didereaux]

What my critics overlook, in their eagerness to conduct their own auto-da-fé and have me publicly burned at the stake for my heliheresy, is the fact that a helicopter's blades do not reverse in direction whether it is rising under power, or freewheeling in the autorotation mode. They spin exactly the same way. The very same thing happens to a sailboat's propeller. It does not spin in reverse when you put the gear in neutral and the boat drags it through the water.

It may take a few centuries more for this to sink through some skulls, and I don't expect to be around at that time, but I have no doubt maritime history will accord me due recognition. St. John the Heli-Eccentric. Yes, I quite like the sound of that.

Cheers,

John V.

Well THIS skull realizes that in the first place the prop of a boat is BEHIND the hull and pushes forward, and a helicopters blades are above((read in front of) the hull and pulls(or technically lifts)yes, they both spin the same in realtion to their load WHEN UNDER POWER.

When a helicopter is unpowered the energy comes from BELOW the prop(i.e. REVERSE) whereas the boat prop does indeed continue in the same diection...BUT the energy comes NOT from the engine now, but from the water whose movement (energy) comes from the sails; hence the boat at some speed lower that that required for hull speed must give up some power to turn the prop. The hull therefor has lost some of its 'steam'.

Continuing with the poor beknighted copter analogy the resistance to falling directly when the rotor is freewheelling is literally the drag created by the wind rushing past the hull UPWARDS. The spinning rotors all the time extracting the energy of that airflow in order to make the rotor turn which in turn through the airfoils of the blades provides the 'lift'(read resistance when un powered).

Look one thing that you seem to have overlooked is that a prop on a boat is more of a SCREW than an airfoil, as a result if you will simply look at any prop directly on its axis the blades fill the view: locked they produce a solid two dimensional wall, and on a three dimensional plane an inclined plane(s). Now look at any aircraft propeller, including the blades of a helicopter and you will see NOT a screw(inclined plane, but a winglike(airfoil) shape quite smaller than the area it traverses.

Will a screw of 1 foot diameter passthrough something easier than a solid 1 foot diameter disc? I believe highschool physics would answer that question. You're not comparing even apples and oranges, more like mangoes and coconuts...and all the nuts ain't on the tree! heheh Further the airfoil, because it generates lift(force) at a much more efficient rate than the simpler screw produces its efficiency(simple mechanical advantage), it offers even less resistance.

Want to argue about how many angels can dance on the head of a pin? Why? Simply go out, set a few bird snares to catch a couple of the heavenly sparrow types and put on a little Creedence Clearwater Revival....or Lawrence Welk if they have a tinge of grey to the plumage. You'll have your answers.
........in other words stuff that copter back in the box and go to hauling some props at madding speeds about the Sound and measure the resistance.

(in all the above gobbledygook is a STRONG clue to the answer of the free or locked BOAT propeller question, which I take it is still the prime issue of curiosity in this snowdrift we have produced! but not being many things, physicist not the least, I will leave that for the genuine articles to unravel for us!heh)

;)
One thing is for certain this is a lot more entertaining than another Cetol or Fortress wrangle!!!!!

[darmoose]

O.J.

gosh, i hope i misunderstood. it was making sense to me, and then i got to the third to last paragraph and i thought i heard you say that the rotor blades on the falling helicoptor actually changed their direction of spin.. i know you couldna said that, just clarify, and we will all pretend it never happened.

even the little girl with the pinwheel will recognize that no matter which direction she waves her little toy, the blades spin in opposition to the direction of travel (like the falling helo). they never spin with the direction of travel cause the poor lil girl got no motor.

can anybody tell me when a freewheeling sailboat propellar spins in opposition to the direction the boat is going under sail?? hhhmmm, can you, i dont think so.

another good dissertation dedes, but i fear we are not gettin thru.

all hope may be lost.

darrell

[bill2]

While traveling thru PhillyTown in an old tavern after closin' time an old bearded salt with a peg leg and a tricorner hat started mutterin and twas hard to understand but here's the gist of what he had to say -

"
Ah m'laddies all this jaw flappin about the devil's own invention - the prop - is something us privateers will not have nothing to do with. We have our hands full keepin the scallywags passing themselves off as crew just to handle the sails and galley - let alone when one o' them spain headed galleons comes inta view. Then all hands best be loadin the cannon with anything that'll put a hole in a man before they get their own saber and their smoke pole loaded. Now iffen we could get more speed from them props to outrun the crowns own man-o-war I'd be sleepin with the devil herself but no self respecting pirate 'll be seen in the company of such demon devices. We be tryin to cut all drag offen the hull since we took ta sea and no freeloadin prop - even iffen locked up tighter than the queen in her chastity belt - will get this full keeled lady ta fly through the water. O' course there's them that'll say times are a'changin and you gotta go wit th' tide but I be DanG'd if ay be screwed by a prop.
"

Certainly inconclusive proof that real sailors don't have props . . .


Entertaining thread - I can't quite tell whether its nonfiction or fiction. Maybe another understanding reader would elaborate upon the subject.

[S/V Necessity]

I could be wrong, but I am going to guess from johns original post, that his copter is NOT auto-rotating. It is simply falling to the ground with the blades left free to spin, the pitch is constant. One thing to keep in mind here is that it's a TOY not the real thing.

What is going on here is John has observed his toy falling from the same height with the blades free to spin, and fixed. And he knows it falls slower when they are free to spin... You cant change his mind, cause he's looking right at the real thing! The observation is valid. However john is observing a difference in rates of acceleration, not ultimate velocity.

Lets assume that in each scenario the helicopter falls from the same height. Then it is beginning with the same potential energy in each instance.

However in one scenario it's landing on the ground with the blades spinning, AND THERE IS ENERGY IN THESE SPINNING BLADES. This energy has to be accounted for... It's subtracted from the fall. This is what he's witnessing. Because the blades are so long and spin so fast they take a great deal of energy to ACCELERATE.

Given enough time and distance the blades would quit accelerating and the toy would fall even faster than if the blades were fixed.

The free spinning blades do pass through the air with LESS difficulty than the fixed ones. BUT IT TAKES ENERGY TO ACCELERATE THEIR MASS. This needed energy doesn't exactly slow the 'copter down, it slows down it's rate of acceleration.

Ever wonder why drag racers try to just barely lift the front wheels off the ground? It's not to look cool, and it's not to overcome the friction of the wheels. It to overcome the Inertia of the front wheels. It requires alot of energy to accelerate a wheel.

Go get a bicycle and hold the back wheel off the ground. Give the pedals a good crank, crank some more. Do it for awhile and you will realize that it's not just normal friction you are feeling. It's awful hard work just to get that wheel to spin even when NOTHING is stopping it. Nothing except inertia.

It's important to understand that it requires alot less energy to accelerate a small light wheel with crappy bearings than say a large heavy wheel with outstanding bearings. The big wheel will be a real Bi&@# to get up to speed. Initially it will look and feel SLOWER. But which one will offer less resistance on a long ride down a hill? The one with the good bearings. While it's slower to accelerate it offers less frictional resistance.

Personally I'd worry more about my transmission than any perceived efficiency that I cant measure. And if you can measure it then what the heck are we talking about all of this for? Perhaps if you were racing you could just leave the prop loose and hope she falls off...

[Steve Laume]

So if I am engaged in a tacking duel with a worthy opponent I want to keep the prop locked. No energy sucking prop acceleration is going to lose the battle for me. Then once you settle in on a long leg you let her spin! Ha those shiny fin keelers don't stand a chance next year, steve.

[Al Levesque]

Mark, I would suggest that the rotor blades also overcome the friction in the shaft system in addition to the inertia. In this way, the rotating blades actually dissipate some of the energy through the heat of the friction. I believe that the friction eventually takes over after the blades have reached some level of speed and the momentum in the blades remains constant. On the other hand, if the rotor is locked, the blades are no longer able to convert airflow to energy in the same way and the energy created in the turbulent air does not help the helicopter.

If we reverse all this we can conclude that dragging a spinning prop is more productive of drag than is a fixed prop.

Happy New Year to all.

[eshanabrook]

The autorotation of a helicopter is a very good analogy to help understand this problem while it may not give us the right answer. First you have to understand the difference.



First understand the rotor is spinning in the same direction but the pilot controls the pitch of the blades with the collective. The rotor “free wheels” from the air passing UP through it. The Helicopter pilot adjusts the collective (blade pitch) so that the rotor doesn't spin either to fast or to slow so they stall. If we had an adjustable pitch propeller on our boat I suspect we could adjust it (under sail) so that it created either more or less drag than the stationary propeller. The least amount of drag would be analogous to the helicopter blades pointed down (collective down or more pitch on our sailboat propeller) with little drag and high rate of decent for the helicopter. More drag would be with the blades closer to parallel to the ground (collective carefully adjusted for optimum glide/less pitch on our sailboat propeller). This is not much different than a glider pilot finding the right glide slope by adjusting attitude/air speed.



The other factor is the simple drag of the stationary propeller or helicopter blades. Clearly the propeller blades take up more area of it’s circle than do the helicopter blades.



Still I think it would be possible if you experimented with an adjustable propeller on your sailboat you’d be able to find a sweat spot where the propeller actually created more drag than when it was stationary. If you adjusted it with lot's of pitch and had a free-wheeling shaft that would create the least drag. I'd guess many of our boats are closer to this. However, by accident some boats may be near this sweat spot with it’s combination of propeller geometry and shaft drag.

The analogy is good but in the end it's the skill of the pilot to control the right pitch to create the lift for a controlled landing. The cool part is the landing. The pilot flares the helicopter by pulling up on the collective at just the right moment using the inertia in the rotor to reduce the decent rate just before touching down.

Very interesting dynamics,

Eliot

[darmoose]

ok all you yellow liners(reference to Tods graph). after sleeping on this overnite, i have seen the light of a clearer explanation(not that the previous explanations were anything less than clear, but you gotta work with the audience that shows up).

we all know (from postings on this very board) that when a propellar is designed for a particular boat that PITCH is one of the most significant considerations. when calculating to determine the proper pitch, those propellar guys run a formula(i have seen it, yes ,i have). one of the most significant parts of this formula is called SLIPPAGE. did you know that SLIPPAGE is expressed in a decimal percentage, and can represent as much as 50%(in some cases more) of the energy being created when your engine is turning your propellar to propell your boat.

the reason SLIPPAGE is important is that when you are trying to push your boat thru the water, you are encountering resistance from the water. a propellar, not being 100% efficient, is spinning like crazy, and it is SLIPPING, in the water as you progress. this SLIPPAGE is also what causes vortexing and cavitation, without SLIPPAGE (a 100% efficient prop) you propellar would move through the water silently and calmly as it propells you.

SLIPPAGE only exsists when the engine is turned on, in gear, and you are pushing the boat with the propellar (thats a law, by the way).

now, when we are under sail or being towed, the propeller is not pushing the boat. the propellar is being dragged along with the boat, and as the boat accelerates the propellar will spin faster. the propellars rate of spin(RPMS) will only be encumbered by two things. 1.) the built in inefficiency of the pitch of the propellar, which will never let it spin as fast as the boat is moving (so as to be silent and calm), and 2.) any built in friction in the drive train (shaft, stuffing box, cutless bearing, etc.).

the inefficiency of the pitch is a built in factor, it is consistant at any given RPM, and is not our concern here. the drive train friction, however, is a variable that we can tweak to increase or decrease through maintenance and repairs. none of us would want a drive train, given our druthers, that was overly bound up, and not allowing our propellar to freely turn.

so, how can we tell if the drive train is bound up or freely turning?

the answer to this question is manifested in how fast our propellar spins when freewheeling under sail or when being towed. and if we see that the propellar spins freely, we know all is well. but if we see that our propellar spins very slowly, or heaven forbid it is stopped (LIKE A LOCKED PROPELLAR) without being purposefully locked in place, we know we are seeing MAXIMUM friction or resistance at work. and so, the effect of maximum friction or resistance on a turning propellar is to bring it to a stop.

ergo: if we purposefully lock our propellar, when sailing or being towed, we are creating maximum resistance or drag opposing the force that is propelling ou boat (the wind or the tow boat)

those of you who subscribe to the yellow line theory, would have us believe that a bound up drive train is better than a freely turning, or even a slowly turning drive train. how silly.

when sailing or being towed, the slower you propellar turns, the more drag is in effect, and the ultimate drag possible to create with a propellar is when you lock it in place!! as someone said earlier in this thread, there are no inconsistancies in this progression. it is linear from maximum drag to minimum drag.

man, i hope this is gettin thru.....i dont know any more way to explain it..think i will go back to bed..

darrell
s/v mystic rose

[Steve Laume]

I think one of the greatest differences encountered with the helo blades is that they are air foils. Doesn't a boat propeller work on an entirely different principal? The boat propeller is much like turning a wood screw that pulls though it's medium. The major difference with the wood screw and why it just doesn't work to explain this problem is that it is very hard to displace wood. Water moves out of the way with relative ease. That is the slippage. So the biggest difference is that as the helo blades turn they generate lift the same way a fixed wing does when it is moved through the air. As far as I know a boat prop works more like a screw though avery soft medium. There was a human powered hydrofoil that I remember reading about that had a wing shaped prop blades. That prop generated lift just like the helicopter. A good kayaker with a wing blade also generates lift as he paddles. Most paddlers just pull the boat towards the paddle and there is some slippage. With a wing paddle in the right hands the paddle leaves the water forward of the entry point. boat propellers are pretty primitive when you start to think about it. How about those little vertical winglets that many of the commercial aircraft are sporting these days. Those were designed to help eliminate slippage and wing tip vortexes. Tunnel drives? Sticking a screw on the end of a shaft motor sandwitched between the keel and rudder just doesn't seem like a very good way to propell a boat. I guess that is why we sail our boats and really don't care how effecent the prop is. There does seem to be some concern about the properties of drag. I want a wing bladed prop that I can feather to generate lift while I am sailing, Steve.

[Parfait's Provider]

"I also think there is a reason most propellers are painted black, and that's because of the magic they contain."

Maybe bronze props contain no magic and work like screws?

A turning screw gathers no moss or barnacles, AND has an effective diameter close to that of its shaft when turning at the same rate as the material flowing past. I think this applies to our bronze props too.

What was that soap opera? As the Screw Turns?

[darmoose]

i am going for simple now. i have come to beieve that as long as J.V.s helicopter is hovering around this thread, its just too complicated. too many are suffering from brain overload (B.O.)

here goes....

the only reason a helicopter glides when the engine stops is because the pilot can control the pitch of the blades. if he didnt change the pitch when the helo starts to freefall the leading edge of the blades would dig into the air and offer no additional resistance. in fact the blades would not even turn. remember that the blades must rotate in the same direction they were when he was flying along or they wont rotate at all. in order for the air rushing up into the blades to do him any good, he has to reverse the pitch(the leading edge) to create resistance.


on our boats, when we are motoring along our prop is spinning in one direction. when we sail and freewheel the prop, it still spins in the same direction, but we cant change the pitch or the leading edge, so there is little resistance offered.

the only way we can increase the resistance on our boats is to stop the prop (so its like dragging a bucket) or turn the engine on and put it in reverse.

a helicoptor is of no use in trying to understand the question of drag on our boats when freewheeling or locking our prop.

gosh dog it.....

darrell
i am going sailing

[Lew Gresham]

I agree with you Darrell, let's go sailing. This topic could go on til next year.

[RC James]

I make'a the helicopter to go up, I make'a the parachuta to come down, everaryabody know there is no sucha thing as a engina to turna da screw that youse senores hava stolen from mya helicoptera to pusha yu silly boats thru the aqua.....whassa a little drag through the aqua if your sails is a'drawing wind and the little vessel itsa moving forward........if you wanted speed, you woulda invented the internalinfernalcombustion'ay engina..spawn of the devil, evil smelling. nasty sounding, gondola shivering, arab supporting, maintainencia inefficiencia, weight robbing, son of a pope (tithes to those in power)....in the meantime, I will continue to watcha your views on this interesting but unsolvable problema.

Mya solutioni ? I will take my new inventioni, la snorkel, and immersa my body into the aqua and remove my helicopter/propeller/lift device from my little shafta and sail silently into my sunsetta until the wind turns aginna me.... and then I shall revisit this board and see what the Saints of Wisdom hava decided is the besta coursa of actioney.

This has been a wunnerfulla thread, but it makesa my head hurt.

Favorable breezes, and a quick end to this devilish conundrum. As for me, I'm locking my 3Blade w/2blades up near the hull and 1 blade down in the deadwood. Besides, I'm just a short distance daysailier.

Leonardo Serenata 25D #10 Norfolk Va

[tartansailor]

First, Please explain to me how you can justify comparing a "screw" to a "prop" in a fluid medium.

Is the screw cupped?
Is an airplane wing shaped like a "screw"?
Does a "screw" impart LIFT?

A free wheeling prop is not "free wheeling" its rotation is inhibited by friction.

The rear (concave) surface of a locked prop exhibits "turbulent flow" at the wall; just the same as an airplane that is spiraling out control, laminar flow is lost.

Laminar flow is critical to lift.

The forward surface of a locked prop is imparting a drag proportional to its area.

A rotating prop (shaped like a wing, convex forward, concave rear)allows water flow over the surface which produces "LIFT"

If the prop is moving faster than the medium, the prop has "POSITIVE" lift, and drives the boat forward.

If the prop is moving slower than the medium You have "NEGATIVE" lift or drag, which slows the boat.

Now at low speeds, under 5 knots, there is an insignificant difference in the drag between a locked and a free wheeling prop, but as speeds increase, the NEGATIVE LIFT of a free wheeling prop increases almost logarithmically (up to a point) because there is NO TURBULENT FLOW, only Laminar Flow.

Vidor is correct.

Dick

[darmoose]

dear TS

it is time to put up.............

you wrote:

"A free wheeling prop is not "free wheeling" its rotation is inhibited by friction."

obviously there is friction. the term freewheeling is just used to describe something other than a locked down prop.

you wrote:


"The forward surface of a locked prop is imparting a drag proportional to its area".

"A rotating prop (shaped like a wing, convex forward, concave rear)allows water flow over the surface which produces "LIFT"

"If the prop is moving faster than the medium, the prop has "POSITIVE" lift, and drives the boat forward."

"If the prop is moving slower than the medium You have "NEGATIVE" lift or drag, which slows the boat."

i will take your word for the above. there is no disagreement so far.

you wrote:

"Now at low speeds, under 5 knots, there is an insignificant difference in the drag between a locked and a free wheeling prop, but as speeds increase, the NEGATIVE LIFT of a free wheeling prop increases almost logarithmically (up to a point) because there is NO TURBULENT FLOW, only Laminar Flow."

heres where we part ways. throughout this thread i have offered anecdotal evidence, logic, reasoning, tests conducted by respectable institutions, and testimony offered by our own cape dory sailors to support my conclusion that a "freewheeling" propellar causes less drag than a locked (stationery) propellar when sailing or being towed.

do you have anything at all to back up the above statement. are we to just take your word for it.

and if , as you say in the above quote, at low speeds (5kts or so) there are no significant differences, just how fast do you think our sailboats go. i dont know about yours , but mine dont get much over 5kts very often. we are not talking about speed boats here.


"Vidor is correct."

i dont think he is, but if it turned out to be, it wouldnt be supported by a falling helicoptor for all the reasons i have stated several times earlier, which by the way goes unchallenged to this point except for the occasional nanananah!

if you cant offer up some evidence, logic, proof, voo-doo beads or something, it is as someone said earlier, this thread will go on forever.

on the basis of science and the evidence i refer to above you are wrong. a locked prop causes the maximum drag on a sailing vessel. the only way to create more drag on a sailing vessel is to engage the engine in reverse gear.

i await your proof and answers, sir.

CHEERS
darrell




Dick[/quote]