Freewheeling propellers: the chopper theory

[darmoose]

ken,

the defined direction of motion of the blades on a helicopter is in relation to the helicopters direction of travel, same as in a boat.

as the helo rises from the ground, the direction the prop is rotating would be said to be in a direction which causes the rising or a forward direction.

when the helo descends or falls, the direction of the rotation of the prop would be opposing the direction of travel or in reverse, just like a boat trying to come to a stop.

unlike a boat the helos prop does not change rotational direction. there aint no forward and reverse gearshift on a helicopter.

helos will actually fly upside down, and since they dont change the rotation of the propellar to do this, i can only assume they can alter the pitch of the blades to provide lift even when upside down.

O.J.

sorry, i didnt mean to rub that in(heh-heh). was just trying to write a little lighter sentence than this thread has called for to date. but i aint so smart either, as i plan to retire and leave south florida after 25 yrs here this year, and will sail out of rock hall,md on the chesapeake. i am looking forward to that.

and we all await your analysis still.......

darrell

(from so fla where the only prop that provides more drag to a sailing vessel than a locked prop, is a prop spinning in reverse)

[Tod Mills]

I imagine the following two curves to be roughly the two camps in this "problem", the purple line being the "locked has more drag" theory and the yellow being the " freewheeling has more drag" theory.

At zero RPM, both curves have the same value of drag (shown as negative thrust).

However, in making this graph, I noticed some funny phenomena.

With the same prop on the same boat, it would seem that both curves should have the same slope and same peak thrust.

With the purple line, I suppose there is no reason to assume that the straight line would be straight; it could be curved, it's just that it would not have any local maximums or minimums near zero RPM's (as the yellow line does). I would guess that it at least does flatten out in the range of low RPM's where the prop blades are "stalled". It wouldn't be much of a jump, I don't think, for the curve to go from being pretty flat at low RPM's to perhaps taking a dip.

Thanks for the interesting discussion, although I don't think we are nearing a definitive answer. My own <b>guess</b> is that the answer, locked vs freewheeling, may well vary on a case by case basis, with blade area, pitch and drivetrain resistance all playing into the equation.

<img src=http://downloads.c-2.com/photos/1167425641.jpg>

[Tod Mills]

Here's what I was talking about with a flattened section on the purple line. That deviation allows the rest of the two lines to coincide, as they should. So, the debate comes down to "to dip, or not to dip, that is the question"

to reiterate, the reason for a change in the shape of both curves in the low RPM range is that the prop blades are going from having flow over them to a stalled condition.

The jumps at both ends are where the prop is overloaded and stalled and/or cavitating with high RPM's.

Bear in mind that both graphs are rather crude and wouldn't consist of straight line segments.

<img src=http://downloads.c-2.com/photos/1167428692.jpg>

[John Vigor]

With the packs of howling Cape Dory wolves snapping at my heels for my daring to air a revolutionary helicopter theory, I am now beginning to feel more like Galileo than Archimedes.

Galileo, as you'll remember if you were paying attention in Grade 3, was the founder of modern heliocentrism. I can honestly feel a deep connection. I can almost shake the man's hand. Over the centuries, heliocentrism has been proved to be correct but in 1616 Galileo was brought before the Inquisition. The church tried to smother his theory of heliocentrism and ordered him not to "hold or defend" that heretical idea.

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.

[Tod Mills]

and the fan blade DOES in fact change direction when it switches from "power mode" to "falling mode", so the analogy with a boat prop locked/freewheeling doesn't quite hold in that range of RPM's.

If the rotor on a helicopter rotates the same way whether flying or falling, I think it would be because the attitude of the blades is changing (angled up/angled down). If you observe helicopters, you will notice push rods that adjust the attitude of the blades.

<img src=http://static.howstuffworks.com/gif/hel ... -rotor.jpg>

(that from this site: http://science.howstuffworks.com/helicopter5.htm

[Steve Laume]

It's really too bad John couldn't have gotten a nice radio controlled sailboat, without a motor. But no, he had to go and get some silly flying machine. But he couldn't keep his mind off of boats so here we are, Steve. Who really doesn't care about the prop drag but would like to play with John's helicopter.

[darmoose]

john v.

you certainly have a way with words, and are hilarious at times. i very much enjoy this.

however, you must not read very well. if you look back, you will see that i and others have stated several times that a helicopters blades rotate in the same direction, regardless of the direction it is moving. rising or falling, the helo rotor spins in the same direction. the only reason you helicopter falls slower when in autorotation is because when falling those rotors are spinning in opposition to the direction of travel (down). when the helo is rising the rotors are spinning in the same direction (not opposing) of travel.

also at one point, you hailed dean abramson for being the first to realize that our boat prop, when freewheeling, was not spinning in reverse. again, look back to see that no one ever claimed that a freewheeling boat prop spun in reverse.

your revelation that the helo rotors always spin in the same direction is exactly why you analogy is not relevent to a boat with a freewheeling prop. our freewheeling prop never spins in opposition to the direction the boat is sailing, it spins in the complimentary direction, just as if the motor were on and we were under power. this was explained by dedereaux in debunking your theiry, as well as me several times( actually in my first post on this subject)

PLLEEEEAAAASSSEEE stop, clear you head, and reread. i dont wish to see you go deeper. remember, as your helo falls its rotors are spinning in OPPOSITION to its direction of travel. (this is most important)

no helophobia here
darrell

[bottomscraper]

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...

Cheers,

John V.

Isn't it possible that the barnacles are in fact just your tired, your poor huddled masses yearning to "spin" free?

[John Vigor]

john v.
PLLEEEEAAAASSSEEE stop, clear you head, and reread.
darrell

Now, now, Darrell, my dear chap, no need to get your wimmies in a froth. All this shouting and carrying on won't help. Besides, it's giving me a headache.

I can quite understand how you feel. Galileo didn't manage to convert many to his theory of heliocentrism, either. It was simply too radical, too difficult to reconcile with what were thought, at that time, to be the true facts.

Radical theories always take time to assimilate, and I have no doubt that as the years roll by, and the average human brain becomes more developed, my helicopter theory will be more readily understood and universally accepted.

Former Transvaal Republic President Paul Kruger is ridiculed today for having insisted until his death that the earth was flat. You surely wouldn't want to become notorious among future generations as the person who couldn't believe that a spinning propeller causes more drag than a locked one, would you?

Of course not. Just give it time, old friend, and you, too, will see the light.

Cheers,

John V.

[Parfait's Provider]

All we need is some plywood, a wood screw, a screwdriver and a hammer. Try hammering the wood screw through a half inch of ply and then try using the screwdriver to turn it though. This is very similar to pulling a non-rotating prop through the water or pulling one that is free to rotate. Further, if you turn the screw backwards, it will be even more resistant to going into the ply, just like the helicopter resists falling like a rock.

Further, if a stalled prop offered less drag, why would a taffrail log work?

Galileio the scientist was right and the religious fanatics wrong. I think science applies here too.

[M. R. Bober]

we need to get back on the water.

Few things are certain, but here's one: it doesn't matter what you do with your prop; CDs will not plane.

Happy New Year!!

Mitchell Bober
Sunny Annapolis (where pessimism never works), MD

[darmoose]

learn any new tricks......

nice try ken., i already tried that analogy several posts ago, went right over the" yellow line peoples" heads

not that they are in any shape to comprehend an experiment, but an even easier one would be to get your boat going 3,4, or 5 mph and see whether it coasts farther (once you turn off the engine) with the prop locked or freewheeling.

but alas, i think old john is really just pulling our leg now. either that or hes smokin something out there in the land of fruits and nuts. maybe his noggin has gotten soggy from all that northwest rain. or maybe hes taken to writing only fiction.

hard to take this serious anymore. its gotten like asking "slick willie" if he had sex with that woman. i did get a few good laughs out of it though, thanks john.

i wouldnt mind hearin from some of the other yellow liners out there, and we are still waiting for O.J. to deliver his promised analysis.

new years cheers to all
darrell

(from sunny so florida where we dont let the elderly fly anything, thank god)

[Oswego John]

Don't want to fan any flames in regard to the subject. I just wanr to answer some questions that I think that I have an answer for.

Let's face it, there have been a few diverse answers offered. I have to admit that there have been some pretty convincing thoughts, also.

I wonder how many readers have actually let their prop free-wheel while sailing and actually watched their shaft rotate. In my mind, in a slight breeze with forward motion at a minimum, the prop wouldn't spin at all. You'd have to have a good rush of water passing the prop to overcome total drive shaft friction.

Leonard K., you mention autorotation. The Sikorsky layout, which entails about 95% of present day choppers, employs a tail rotor to counteract reverse rotor torque. There is a backup mechanical control, whick includes a transmission that connects the main rotor to the tail rotor. As long as the main rotor is spinning, with or without power, it will spin the tail rotor to prevent the fusilage fron counter rotating.

Both the main rotor and the tail rotor have adjustable pitch, independent of each other. The main rotor's pitch, or angle of attack, can be flat while idling or angled to provide lift. Both rotor angle and degree of throttle control the lift and descent. Ken, I don't know if rotor pitch can go beyond zero degrees to a negative thrust

With enough power, choppers can now barrel roll, loop, and figure eight, moebus or not. Some choppers have an articulating knuckle on the rotor drive shaft where it connects to the rotor hub, to control flight direction, even backwards. The cockpit cyclic controls this squash plate. It is like a joystick.

In case of power failure, if the chopper is high enough and travelling fast enough, it can glide down to a somewhat safer landing. As the fuselage drops, the main rotor reacts to the uprushing air like a windmill to prevent the fusilage from plummeting downward. The rotor pitch remains the same although the direction is reversed.

I'm a little rusty on this subject but I think most of it is okay.

Remember the pinwheel, to and fro.
O J

[mike feeney]

I have crashed twice in helicopters. I can attest that the auto rotate thing really does work but I never knew why until now (English Lit major who flew in the backseat as a medic). You have convinced me that my survival wasn't a miracle but I still don't think that I know where to place the shift lever on our CD30k.

Happy New Year

[Jim Davis]

Here is a link to "Yanmar Help". They recommend leaving the engine in neutral. If you really want to stop the prop it should be done with a shaft brake, not the transmission. Note not all manufacturers agree with free wheeling because of lubrication issues.

http://www.freeboards.net/index.php?mfo ... owtopic=62