Freewheeling propellers: the chopper theory

[Didereaux]

Eureka!

When the rotating wings of a helicopter are stopped in flight, they stall. They offer little resistance, the aircraft plummets to the ground, and you have to spend half an hour with the Superglue putting it all together again. But if the wings are allowed to revolve, they generate lift--the autogyro effect--and the helicopter will glide to a reasonable landing.

Your conclusions re boat props may or may not be true, I do not address the issue....BUT I question the logic that got you there.

A stalled helicopter blade is motionless, therefor only the surface area presented to the air will offer resistance, with a four bladed prop that is precious little and the hull does as you say fall like a rock. So why does an unpowered but free wheeling helicopter prop allow for a glide? Simple it is spinning in the same direction as it would if powered and therefor the blades are acting in their designed function...as airfoils, Which of course provide lift(i,e, resistance to falling). If you were to reverse those blades so the lift was downward you could beat the rock to the ground! Lovely thought that, eh?

oh yeah and do not forget that that lift is generated first at the outer tips and then as the relative speed increases the lift increases towards the center, think angular momentum.

But please NOTE the blades are acting in reverse to the airflow when freewheeling as opposed to WITH the airflow when powered. Which is why the helicopter cannot generate sufficient lift to climb fro the static condition of freewheeling(you could stall the blades, fall faster and theoretically for a short time after releasing thme generate sufficient lift(angular momentum at the tips) to halt the descent, but dammed if I'd try it!)

Helicopters are cool, and complicated critters.
;)
...but not being an aeronautical engineer the above may be just as much hogwash and conjecture as all of the preceeding LOL

Forward Ho, carry on and all that tommy rot for the Sun rose earlier today than yesterday!

[Judith]

Isn't marrying-up the "freewheel vs locked" debate with the "tiller vs wheel" question sorta like (in a Southern phrase I just heard recently: the South is so colorful sometimes. . .) offering to walk through Hell wearing gasoline britches?

[darmoose]

by george, you have explained the flaw in johns original postulation perfectly, something i have failed to get others to understand to my dismay. i applaud you" Vigorously". you are exactly correct. helo blades rotate in a direction which opposes the force of gravity, unlike our freewheeling propellar which when freewheeling rotates in a direction which compliments the force of the sails pulling the boat thru the water.

in order for our sailboat to correlate with the helo theory of johns we would have to be able to put our trans in reverse and have the prop spin in reverse, against the force of the sails, which a trans wont do. it merely locks the prop in a fixed position.

to my tartan friend... i believe my previous post started with defining this question as one of "fluid dynamics", and not a complicated one to understand. we have plenty of anecdotal proof offered in this thread by several people that their boat goes faster under sail when their propellar is freewheeling. i have heard virtually no proof to the contrary theory, other than johns original hypothisis, which is flawed, as so clearly explained by dedereaux

my example of the nail, the screw, and the spinning screw was an attempt to use an example that people might be more naturally familiar with in everyday life. guess that didnt work for some, although it remains very appropos. by the way, i didnt notice you protesting when the author of this thread introduced "aerodynamics" into this "fluid dynamics" question?

lastly, could you please explain how you believe "LIFT" has anything to do with the question at hand.

while i am at this, let me give everyone another example which should be familiar. if you could mount a propellar in a garden hose. and control whether it would spin freely, or be locked in a fixed position when you turned on the water, which do you think would effect the flow of water more?

darrell randolph
s/v mystic rose

from where the only helocopter blade that provides more lift than a fixed blade is a blade rotating in opposition to the force of gravity

[mgphl52]

Well, Yes! Just now reading this entire thread, I noted several things:

1) Nobody mentioned the difference between "air" and "water"... Air can be compressed and water cannot.

2) Most all serious racing boats have folding props OR at least only 2 blades. As previously mentioned, locking a folding prop insures that i folds quicker and stays that way. Racers with fixed, 2-blade props will "hide" them when they the lock the shaft.

3) "My" Volvo manual (MSB) states either neutral or reverse; if left in neutral for extended time, the engine should be idled every 6 to 8 hours to cool the tranny (raw water first goes through the MSB).

My personal observations with KAYLA (3 blade prop) is that locking the shaft DEFINITELY slows her down.

[M. R. Bober]

In an effort to provide some light to accompany the heat, I descended deep into the bilge to retrieve the October 1, 1993 issue of "Practical Sailor" (way back when that publication was still worth its salt; before...[Ed. note: rant withheld at this time.]).

An article entitled "M.I.T. Prop Test" presented propeller research by--then--graduate students Beth Horwich & Todd Taylor, which included the following:

"... Also, Taylor and Horwich tackled the age old question of whether it's better to allow a fixed-blade prop to free wheel or to lock it in place.
Their conclusion is that '...a substantial reduction in drag, and thus gains in boat speed, can be realized from a fixed pitch propeller by allowing the propeller to free wheel. The decision to free wheel or not, though, must also take into account the factors of noise and wear-and-tear on the transmission and bearings.'
Despite some claims that freewheeling works best at higher speeds, Taylor and Horwich found there were 'savings across the whole range of speeds.'"


The thrust of the article (good one, eh?) was a quantification of fixed, feathering and folding propellers' characterists.

Alas, John Vigor, in answer to your question "Am I right or am I right?" The answer appears to me, to be, neither. The discussion that your thread provoked seems to prove Zymurgy's First Law of Evolving System Dynamics, to wit: Once you open a can of worms, the only way to recan them is to use a larger can.

Every best wish,
Mitchell Bober
Sunny Annapolis (where we discuss propulsion, prop-erly), MD

[bill2]

methinks in the spirit of fairness to all contributors a small experiment ( to dispel any discrepancies in prior articles - full versus fin keel versus deadwood versus fixed versus folding versus flat versus pitched versus three bladed ad nauseum ) to be made thusly -

a> a tank ( fixed amount ) of water is passed through a tube ( column ) into which first a locked prop then a freewheeling prop is located

and

b> for a fixed period of time a steady supply of water is slowly injected in the tube ( column ) with a fixed prop and then a freewheeling prop

The water passing the prop is saved in a graduated tank and the amount of water and the time required are recorded. The winner of course will be the one that takes less time/passes more water respectively.

IMHO - my trans requires cooling when the prop is turning ( spell that running the engine ) and I find it cheaper to add sail to get to hull speed ( at hull speed does it not matter whether the prop is free wheeled or locked ? ) than freewheeling the prop - but I am certainly not an engineer and don't even know how to spell flewid dynamics.

BTW - I certainly put a great deal of stock in those old PS articles but as was pointed out earlier - this thread is defined as strictly addressing the question of drag only and a test known to only address drag would seem to satisfy all here.

and Happy New Year to all - regardless of whether we all agree or not

[Neil Gordon]

methinks in the spirit of fairness to all contributors a small experiment ( to dispel any discrepancies in prior articles - full versus fin keel versus deadwood versus fixed versus folding versus flat versus pitched versus three bladed ad nauseum ) to be made thusly ...

Good one.

I found this with a "freewheeling prop" Google search:

"The British journal Yachting World published a series of tests about 20 years ago that compared the drag of freewheeling and fixed three blade props. The experiment was quite ingenious. A boat with a three bladed prop was anchored in a tideway. A spring scale was inserted in the rode. As the tidal flow reached a speed of six knots, the prop was allowed to freewheel and the scale force measured. Then the prop was locked and the force measured again. The process was repeated with several props and several degrees of resistance to rotation as might be provided by a shaft in a stuffing box and turning a transmission. The conclusion was that a freewheeling prop with NO RESISTANCE offered the least drag, followed by a FIXED PROP. The most drag was offered by a SEMI-FREEWHEELING prop rotating about 1/2 to 2/3 the speed of a free spinning prop."

Borrowed from http://www.boatdesign.net/forums/showth ... p?p=112414.

[Paul Grecay]

Which one encounters more resistance? A person walking into a door that is free to swing open or one that is latched shut? The water is ther person, the door is the blade, the resistance is drag. Clearly a free wheeling prop allows the water to go through and causes less drag and turbulence.

[Neil Gordon]

Which one encounters more resistance? A person walking into a door that is free to swing open or one that is latched shut? The water is ther person, the door is the blade, the resistance is drag. Clearly a free wheeling prop allows the water to go through and causes less drag and turbulence.

Except that in your example, you don't have a way to walk around the door. The analogy requires a revolving door. If the door is free to revolve, you push through it to get to the other side. If it's locked, you go through one of the side doors. Which is faster?

[Al Levesque]

A slight change in the analogy would be to visualize a revolving door with open space on either side. If the revolving door is directly in line with the desired path and there is absolutely no resistance then it is the best way. The alternative of walking around the door is only slightly longer. Now consider that our keels make the most direct route off to one or the other side of the prop and perhaps avoiding the prop altogether is the best way. Stalled water ahead of the prop would tend to divert all water to either side.

[darmoose]

i guess we now have to start a new thread to decide if one should freewheel a revolving door or lock it when trying to walk thru.

some of you folks are sooo deep.

darrell

[Dick Barthel]

It seems all that we can say for certain is that an auto-rotating helicopter does not solve our initial dilemma. Einstein used thought experiments in his work (remember the elevator?) and he was a sailor. Its to bad he's not still around. I do remember reading that he ran aground off Old Saybrook at the mouth of the Connecticut a couple of times. Maybe he was pondering the fixed or freewheeling dilemma?

Dick

[Parfait's Provider]

Many thanks for this most entertaining thread. I have no idea how I managed not to post this long.

[Oswego John]

. . . . . don't anyone dare to drive a wooden stake into the heart of this thread. You guys think that Al or Einy (that's what us close knit friends called Einstein)) was cerebral about the relativity of the matter of a revolving door and it's ramifications on life and counter rotating props, I'm burning the midnight oil working of the dynamics, fluid and otherwise, of the effects of a tot's pinwheel on various makes of trannys.

This, of course, is being scrutinized using magnetic north, not true. The tests are moving along smartly and I should have my final, bottom line, infalllible report of the results completed by varnish (Cetol) time

Hang in there, it's coming. You'll be ever so glad that you waited for the issuance of the non-conjectural essay on the Theory of Relativity of The PinWheel. It will certainly be oozing with certitudinous logic.

Patience is a virtue.
O J

[Carl Thunberg]

I've been away for a few days and am just catching up to this thread. I don't dare enter the fray on this, other than to say Darmoose makes a pretty compelling argument. But I can't help wondering what Pucebeard the Pirate would say on this matter. Until then, I'll wait with baited breath for the results of OJ's non-conjectural essay.