Freewheeling propellers: the chopper theory

[John Vigor]

Eureka! I have found it. I feel like the Archimedes of the freewheeling propeller controversy.

I don't need to remind any of you that there has been no definitive answer to the question about whether it's better to let a propeller revolve while you're sailing, or whether there's less drag if you put it in gear to prevent it from spinning.

The answer finally came to me yesterday when I was flying, or attempting to fly, a radio-controlled toy helicopter I found under the Christmas tree.

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.

So Vigor's Principle states that when you translate that effect to an auxiliary propeller working in water, as seems reasonable, a stopped, stalled, propeller will cause less drag than a spinning propeller, which is gaining lift and trying to drag the boat backward. Q.E.D.

So Hiscock was right. Lock your propeller shaft when you're sailing.
Am I right, or am I right?

John V.

______________________

From Vigor's Rules for Life:

25. Farnsdick's Corollary: "After things have gone from bad to worse, the cycle will repeat itself."

[Mark Yashinsky]

when the main rotor blades are freewheeling like that (autorotation mode) they are producing a lot of drag. Autorotation only works if you have a NEGATIVE angle of attack on the blades, to get them to spin. Without engine power, you are going down. When you have an engine failure (or run out of fuel), you can land quickly (crash) or slowly (autorotate). In fact, when nearing the ground, you WANT to build rotor speed, so that at the last moment, you let the blades dig into the air and trade the built up momentum in the rotor for lift to stop the downward movement just as the skids touch ground. Yes, it is a practiced maneuver.
Along the same line, look into the autogyro. Generally a pusher engine for forward movement and a freewheeling rotor above.

I tought this question was put to rest? In fact, those of us that have two blade props can put two lines on the shaft, to hide the prop behind the keel.

[Steve Laume]

So what happens if your little toy helicopter blades started to spin backwards? Would that little sucker pile drive itself into the ground even harder, needing still more glue to ever fly again? I am not so sure you have definitive proof. You might need more helicopters till you rule out all variables. Definitely more glue, Steve.

[Mark Yashinsky]

First, this is when the engine is NOT providing any power to the main rotor (freewheeling). Second, you want the main rotor to continue to rotate in the same direction as normal, but instead of a positive angle of attack to pull the copter up, you have a negative angle to get it to spin (remember, no engine to spin the blades) and produce drag (and control). If the engine were powering the blades, yes, a negative angle of attack would force the copter down.

[Steve Laume]

I think, Steve.

[darmoose]

dear john, (as in dear john letter)

i personally thought the controversy had been put to rest as so. a forward spinning propellar causes less drag than a stopped (locked) propellar, and a stopped (locked) propellar causes less drag than a backward spinning propellar (as in helicoptor).

if your helocoptor spun its rotor forward( like a boat) rather than backward (in order to provide resistance to gravity) in order to fly, your analogy might hold some water, as they say. but unfortunately, helos spin their propellars in reverse to the force that takes over when their prop stops spinning (gravity).

i think the easiest way to understand the propellar drag question is to visualize our boat prop spinning in reverse ( which obviously provides the greatest drag on forward motion, like when we are trying to stop the boat). the slower our prop spins in reverse the slower we come to a stop because we have applied less drag.

a stopped (locked) prop provides less drag on forward motion than a prop spinning in reverse, at the same time a stopped (locked) prop provides more drag on forward motion than a forward spinning prop because the forward spinning prop is releasing some of the energy from the forward motion of the boat against the water. eventually as the prop spins faster in forward is ceases to be a drag and becomes a positive thrust, dont you see??

if our props were truly "freewheeling" and could spin at the same speed as the boat is sailing, there would be zero resistance.

as to the other question you pose, whether. freewheeling or stopping the prop is the "better choice? other considerations make for that opinion, not drag or resistanse to sailing speed.

regards
darrell randolph
s/v mystic rose

from where the only prop that drags more than a locked prop, is a reverse spinning prop

[Angela and Tom]

For those of you who'd like to visually analyze "Vigor's Principle", here are two links to a video of a toy helicopter in action. The first is a Real Player file, the second is a Windows Media Player file.

http://www.hobby-lobby.com/media/AT600-Falcon.rm
http://www.hobby-lobby.com/media/AT600-Falcon.wmv

If they don't work, go to the main page:

http://www.hobby-lobby.com/falcon.htm

and click on the links halfway down the page.

[M. R. Bober]

Practical Sailor (with some help from MIT) performed the tank tests to answer the "to freewheel or not to freewheel" question. All other things being equal--and they never are--speed is a significant variable in this analysis. I will try to locate the original article(s), but if I remember correctly a locked prop is better at low speed (3 knots or less) & freewheeling is better above 3 knots on the hull that was under test. Or maybe it was the other way around?

Mitchell Bober
Sunny Annapolis (where freewheeling may or may not reduce drag, but sure will tax the cutlass bearing), MD

[John Vigor]

Well, reply in haste, repent at leisure, I always say. If all you scientific experts would just take a few more moments to contemplate my momentous discovery I think you'd have to agree that it's right.

I mean, they scoffed at Archimedes, too, didn't they? There are people (even people on this board) who still don't believe Archimedes was right.

No, the issue had not been put to rest before the Great Helicopter Revelation. But it has now.

Listen up, guys, specially those of you who believe the earth is flat. This is the start of a new enlightened era. Quit the spin. Lock up your propshafts. Don't listen to the wicked naysayers. Have faith, darn it. What's life without faith?

John V.

[darmoose]

i think that in the interest of truth we ought not mislead the flocks.

let us picture our boat sailing along at 6.5kts. we have our engine idling in neutral." holycow batman, what is that in front of us, we need to stop asap" or suffer certain disaster.

do we:
a.) throw our trusty tranny in forward and rev it up
b.) stay in nuetral and grab the shaft so it cant spin
c.) put it in reverse and rev it up

i will bet everyone said alltogether now..."C"

we are trying to create the most resistance (drag) possible to stop the boat!

ergo.... the most drag comes from a reverse spinning prop, the next most drag comes from a fixed or locked prop, and the least drag comes from a freewheeling or forward spinning prop.

dont you see??
darrell
s/v mystic rose

[Mike Thompson]

Quite apart from the question of drag, I stop the rotation for the following reason:
So that lobster pot warp will not be wound around the shaft!
Mike

[marilou]

Generally speaking, it is best to put your trans in reverse when sailing to reduce wear on the trans. When trying to go fast (racing) it is best to get your prop straight up behind your keel and your rudder. When your boat is out of the water (or dive under), mark your shaft so you know when the prop is straight up.

There is no advantage for your prop to be freewheeling, only the disadvantage that is putting wear on your trans. (unless your are producing electrictity from the shaft/prop turning).

Depending on your trans. some mfg. (depending on the model) say you can put in reverse or forward...some say just reverse when sailing...the safe bet is to put it in reverse. I know of none that say that is good for the trans. to be freewheeling.

Happy Holidays!

[Bob L]

marilou,

I was scrolling to the bottom of this thread to put my 2 cents in only to find you already did it. My engine manual says to lock the prop (in reverse) while sailing to prevent wear/damage to the transmission.

[darmoose]

all due respect to all contributors and posters, but the question at hand has to do with only drag and resistance, as offered up in john vigors original post.

this is, or has been, a serious controversy for a very long time, and deserves to be resolved. indeed, the introduction of superfulos information has prolonged this controversy.

there are no doubt other reasons to put the propellar in lock position. they have nothing to do with sailing faster and the question being discussed.. and for what it is worth, no less than volvo(2006) when talking about the new d1-20( an engine being installed in several of our boats) recommends freewheeling the prop when under sail(in their manual). and finally, this discussion is not limited to only two bladed propellars, where the deadwood makes this discussion moot.

now to put up the clearest argument i can conjur up at the moment for the freewheeling solution.

if one is sailing along at, say, 4kts, under sail and turns on the engine and revs it up to,say, 3000rpms in forward gear, one will likely increase his speed to, say, 6kts. under sail and engine. everyone should readily agree with this premace, no?

with no change in the wind conditions, one can then lower his rpms to a point where one is again going 4kts, and if one continues to lower his rpms he will begin to slow the boat to below 4kts at some point. the slower rpms are increasing drag.

when one then puts the trans in neutral, so that the prop is freely moving due only to the water rushing by, one has achieved the maximum drag you can get from the propellar in forward gear or freewheeling.

the only way to increase the drag with the propellar now is to 1.) lock it in position, so that now the rushing water cant move it ( and release any energy), or 2.) to put the propellar in reverse and gradually increase the rpms until the boat comes to a stop.

with enough reverse rpms, even with the sails still deployed, one can actually back up the boat.

this explanation is a scale where the absolute maximum drag is represented on the left by a spinning propellar at its highest rpm in reverse gear, progressing to the right, through a fixed propellar position, and then on to a forward spinning propellar, increasing in rpms and thereby reducing drag until the higher rpms actually eliminate drag completely, and produce thrust.

this finding is irrefutable, and so the controversy is indeed resolved.

let the leisurely repenting commence

darrell randolph
s/v mystic rose

[marilou]

Learn something everyday.