I was troubled by the helicopter analogy because I thought for it to be apt you'd have to be pulling the boat backwards through the water. But because the pilot has changed the pitch, in effect isn't that analogous to a propeller freewheeling while the boat is moving forward through the water?
Maybe there is some hope that this thread will continue until the spring.
Dick
Quote of the Day: "I feel sorry for people who don't drink. When they wake up in the morning, that's as good as they're going to feel all day." Lyndon B. Johnson
Freewheeling propellers: the chopper theory
Moderator: Jim Walsh
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Dick Barthel
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M. R. Bober
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Even more troubling about the helicopter analogy:
Imagine you are running your Cape Dory, under power alone, at hull speed at an altitude of 5200'. Suddenly your engine fails............wait for it......wait for it....
Now: Your attempt to autorotate fails. Prepare for a serious grounding. Perhaps a larger prop?
Mitchell Bober
Sunny Annapolis (where helicopters are just like sailboats, but different), MD
Imagine you are running your Cape Dory, under power alone, at hull speed at an altitude of 5200'. Suddenly your engine fails............wait for it......wait for it....
Now: Your attempt to autorotate fails. Prepare for a serious grounding. Perhaps a larger prop?
Mitchell Bober
Sunny Annapolis (where helicopters are just like sailboats, but different), MD
CDSOA Founding Member
Do you see that light?
Dick,
you almost have it. in your last question you asked "because the pilot has changed the pitch, in effect, isnt that analogous to to a propellar freewheeling while the boat is moving through the water?
the answer is a resounding NO
it is the pilot changing the pitch on the blades of the falling helicoptor that makes the analogy false, because we cant change the pitch on our propellar as it freewheels in the forward direction with the pitch unchanged.
in order to change the pitch on our propellar we have to turn on the engine and put the transmission in reverse (we are no longer freewheeling at that point.
an analogy is like a math formula. it says A is to B as C is to D. for example 3 is to 4 as 12 is to 16, or 1 is to 2 as 3 is to 6.
does anybody think that (A) an autorotating helicoptor blade which has had its pitch reversed is to (B) a falling helicoptor as (C) a freewheeling propellar which has not had its pitch reversed is to (D) a sailboat under sail moving forward?
the answer is NO, it is not. these are apples and oranges. the correct analogy would be (A) an autorotating helicoptor blade which has had its pitch reversed is to (B) a falling helicoptor as (C) a propellar on our boat spinning on reverse gear is to (D) a sailboat under sail moving forward.
we are so close to a breakthrough, somebody please say you understand......
p.s. Mitch, that was pretty funny.
darrell
you almost have it. in your last question you asked "because the pilot has changed the pitch, in effect, isnt that analogous to to a propellar freewheeling while the boat is moving through the water?
the answer is a resounding NO
it is the pilot changing the pitch on the blades of the falling helicoptor that makes the analogy false, because we cant change the pitch on our propellar as it freewheels in the forward direction with the pitch unchanged.
in order to change the pitch on our propellar we have to turn on the engine and put the transmission in reverse (we are no longer freewheeling at that point.
an analogy is like a math formula. it says A is to B as C is to D. for example 3 is to 4 as 12 is to 16, or 1 is to 2 as 3 is to 6.
does anybody think that (A) an autorotating helicoptor blade which has had its pitch reversed is to (B) a falling helicoptor as (C) a freewheeling propellar which has not had its pitch reversed is to (D) a sailboat under sail moving forward?
the answer is NO, it is not. these are apples and oranges. the correct analogy would be (A) an autorotating helicoptor blade which has had its pitch reversed is to (B) a falling helicoptor as (C) a propellar on our boat spinning on reverse gear is to (D) a sailboat under sail moving forward.
we are so close to a breakthrough, somebody please say you understand......
p.s. Mitch, that was pretty funny.
darrell
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tartansailor
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here We Go Again
First off I would like to commend all the sailors who gave allot of thought to this subject, on both sides of the debate. It is healthy and good.
Now, the critics of the locked wheel side do not understand the math, so what other conclusion can one draw but that all their arguments are opinion, not fact. And thats OK too.
Comparing a falling helicopter to a propeller is a stretch mathematically because of the order of magnitude of the velocities. That is important, as in the equations, velocity is squared, additionally one is operating in a compressible medium, where as the other is operating in a non compressible medium.
And finally, take a propeller with say 48 square inches of surface area perpendicular to the shaft.
You are dragging 48 square inches through the water, and regardless of how fast it is spinning (as long as it is being dragged) you cannot reduce the surface area drag unless you change the pitch.
Now where does the increased drag come from in a free wheeling prop? From lift in the cup in the backof the prop, the same cup found under moldboard plows which pulls the plow down into the earth.
If you did the math, you would see that at 4 knots there is negligible difference, but in our velocity squared example above if you are doing 20 knots, thats drag! Big time.
Do not let me dissuade you from your opposing views however.
Dick
sp.
Now, the critics of the locked wheel side do not understand the math, so what other conclusion can one draw but that all their arguments are opinion, not fact. And thats OK too.
Comparing a falling helicopter to a propeller is a stretch mathematically because of the order of magnitude of the velocities. That is important, as in the equations, velocity is squared, additionally one is operating in a compressible medium, where as the other is operating in a non compressible medium.
And finally, take a propeller with say 48 square inches of surface area perpendicular to the shaft.
You are dragging 48 square inches through the water, and regardless of how fast it is spinning (as long as it is being dragged) you cannot reduce the surface area drag unless you change the pitch.
Now where does the increased drag come from in a free wheeling prop? From lift in the cup in the backof the prop, the same cup found under moldboard plows which pulls the plow down into the earth.
If you did the math, you would see that at 4 knots there is negligible difference, but in our velocity squared example above if you are doing 20 knots, thats drag! Big time.
Do not let me dissuade you from your opposing views however.
Dick
sp.
Last edited by tartansailor on Jan 5th, '07, 21:22, edited 1 time in total.
Viam Inveniam Aut Faciam
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Matt Cawthorne
- Posts: 355
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- Location: CD 36, 1982
Hull # 79
Darrell,
Yes, the analogy applies. Think of it this way. On a helicopter gravity is always trying to pull it down. With a pile of horsepower you can fight the direction that the vehicle naturally wants to go. When the power is removed you can reverse the pitch and slow it's natural tendency do go down, but not stop it (at least for very long).
The sailboat analogy assumes that there is a sail up and driving it in the forward direction. The forward direction is the equivalent to the DOWN direction in the helicopter. If you want to keep the boat from moving forward you can put the boat in reverse and apply lots of power. Remove the power (take it out of gear) and allow the direction of rotation to do the equivalent to changing the pitch and you can slow the boats natural progression forward. Put some sort of break on the shaft and draw some power out of the shaft and you can slow it even more. If you stop the prop from spinning you may or may not be moving faster than the freewheeling case (depending on many things).
If you are sailing and put the boat in forward (the same direction as the freewheeling direction) you can make the boat go faster in the direction that it was already tending to go. In the helicopter, if you pitch the blades down you will help it go faster in the same direction that it already wanted to go as well.
Steve, I stand corrected about nobody practicing a full autorotation through to a wheels-down landing. Do your friends in CH-53s or CH-46s do a wheels-down autorotation?
Matt
does anybody think that (A) an autorotating helicoptor blade which has had its pitch reversed is to (B) a falling helicoptor as (C) a freewheeling propellar which has not had its pitch reversed is to (D) a sailboat under sail moving forward?
Yes, the analogy applies. Think of it this way. On a helicopter gravity is always trying to pull it down. With a pile of horsepower you can fight the direction that the vehicle naturally wants to go. When the power is removed you can reverse the pitch and slow it's natural tendency do go down, but not stop it (at least for very long).
The sailboat analogy assumes that there is a sail up and driving it in the forward direction. The forward direction is the equivalent to the DOWN direction in the helicopter. If you want to keep the boat from moving forward you can put the boat in reverse and apply lots of power. Remove the power (take it out of gear) and allow the direction of rotation to do the equivalent to changing the pitch and you can slow the boats natural progression forward. Put some sort of break on the shaft and draw some power out of the shaft and you can slow it even more. If you stop the prop from spinning you may or may not be moving faster than the freewheeling case (depending on many things).
If you are sailing and put the boat in forward (the same direction as the freewheeling direction) you can make the boat go faster in the direction that it was already tending to go. In the helicopter, if you pitch the blades down you will help it go faster in the same direction that it already wanted to go as well.
Steve, I stand corrected about nobody practicing a full autorotation through to a wheels-down landing. Do your friends in CH-53s or CH-46s do a wheels-down autorotation?
Matt
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Dick Barthel
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Re: Removed message
n/m
Last edited by Dick Barthel on Jan 6th, '07, 10:39, edited 2 times in total.
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Dick Barthel
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Re: Do you see that light?
Darrell,darmoose wrote:Dick,
you almost have it. in your last question you asked "because the pilot has changed the pitch, in effect, isnt that analogous to to a propellar freewheeling while the boat is moving through the water?
the answer is a resounding NO
it is the pilot changing the pitch on the blades of the falling helicoptor that makes the analogy false, because we cant change the pitch on our propellar as it freewheels in the forward direction with the pitch unchanged.
in order to change the pitch on our propellar we have to turn on the engine and put the transmission in reverse (we are no longer freewheeling at that point.
I'll give it one more try. I realize from the participating engineers that a helicopter blade is not analogous to a sailboat propeller. I also realize as an old lawyer friend of mine used to say "that all analogies are bad."
My point is that a helicopter falling with the pitch changed and the air flowing up through the blades is similar to a propeller while the boat is moving forward through the water. In other words it seems that the direction of the pitch and the flow of air or fluid would be the same. My point may well be wrong, but it has nothing to do with the inability of the skipper to change the pitch of the propeller. You would only need to be able to do that to keep the analogy if the boat were moving backwards similar to a falling copter.
This is more fun than putting away Christmas decorations which is what my wife thinks I''m doing.
Dick
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tartansailor
- Posts: 1527
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- Location: CD25, Renaissance, Milton, DE
Dick, You're right.
Yes, sorry about that, I edited it out, but added an example of the moldboard plow to further explain drag.
Thanks for keeping the debate civilized.
Dick
Thanks for keeping the debate civilized.
Dick
Viam Inveniam Aut Faciam
reply to Dick (TS)& Dick B & Matt
oh, wow, now i got two dicks to deal with, i always had trouble just dealing with one. and then theres you also matt.
for the moment, i would like to not argue about the relative virtues of either a freewheeling or locked propellar.
i would like to get everyone on the same page as to what constitutes an analogy, and decide if J.V.s offered analogy, in the beginning makes any sense. i would also like to do it without rewriting my last post about the ABCD and so on, so as needed would you please just refer to that post.
Dick B and Matt....... it is not the blades vs propellar, nor are analogies bad (but there are definitely bad analogies.) i understand that the helicoptor is falling, that the air is rushing up as it falls due to gravity. in johns analogy, the force of gravity equates to the wind and sails pulling our boat forward (we must think of the helocoptor as moving forward (towards the ground)
what makes the analogy wrong is that while the helo is moving toward the ground, the pilot reverses the pitch of the blades (remember his blades will only go in one direction same as our propellar when it is freewheeling)
in order to try to increase drag or lift and bring the helos forward motion to a stop. this is analogous to us putting our transmission in reverse (while the engine is on), and spinning our propellar backwards to bring the boat to a stop. this is not freewheeling. when we are freewheeling, our prop is spinning in the forward direction, and, yes i know it still has drag, but we are not talking about that right now.
i have never talked about the boat going backwards.
you have got to get this guys. clear your mind and think about the analogy.
TS..... hard to know what to say to you. you seem to want to maintain an air of (dare i say) superiorty. i am and have been an engineer for 40 yrs, i understand the math, and you shouldnt be so dismissive.
i may have missed something, but i dont recall seeing any math from you. your statement in your last post "you are dragging 48 square inches through the water, and regardless of how fast it is spinning (as long as it is being dragged) you cannot reduce th surface area drag unless you change the pitch." is (to be kind) pure folly. are you now saying that drag is a constant, no matter our propellar speed. we must go from "no drag" to "max drag" instantaneously. there is no progression, linear or otherwise according to you and your math. drag doesnt lessen as we increase our rpms toward achieving thrust.
as i and others have said before, this is just an entertaining way to excersise our noggins and discuss a hypothetical mathimatical engineering problem. we all realize that there are other reasons you may want to lock you prop.
like you, i would be interested to see "the math" and a hard proof. you are free to offer it up for scrutinity at your convenience. perhaps you can trully put this question to bed.
in closing, i have always hated email (similar to this) because it in no way approximates a conversation. folks dont read, or they read things into a message that is not there, or they dont address the question, or they get off on a tangent, (you get the point)
good luck
darrell
[/quote]
for the moment, i would like to not argue about the relative virtues of either a freewheeling or locked propellar.
i would like to get everyone on the same page as to what constitutes an analogy, and decide if J.V.s offered analogy, in the beginning makes any sense. i would also like to do it without rewriting my last post about the ABCD and so on, so as needed would you please just refer to that post.
Dick B and Matt....... it is not the blades vs propellar, nor are analogies bad (but there are definitely bad analogies.) i understand that the helicoptor is falling, that the air is rushing up as it falls due to gravity. in johns analogy, the force of gravity equates to the wind and sails pulling our boat forward (we must think of the helocoptor as moving forward (towards the ground)
what makes the analogy wrong is that while the helo is moving toward the ground, the pilot reverses the pitch of the blades (remember his blades will only go in one direction same as our propellar when it is freewheeling)
in order to try to increase drag or lift and bring the helos forward motion to a stop. this is analogous to us putting our transmission in reverse (while the engine is on), and spinning our propellar backwards to bring the boat to a stop. this is not freewheeling. when we are freewheeling, our prop is spinning in the forward direction, and, yes i know it still has drag, but we are not talking about that right now.
i have never talked about the boat going backwards.
you have got to get this guys. clear your mind and think about the analogy.
TS..... hard to know what to say to you. you seem to want to maintain an air of (dare i say) superiorty. i am and have been an engineer for 40 yrs, i understand the math, and you shouldnt be so dismissive.
i may have missed something, but i dont recall seeing any math from you. your statement in your last post "you are dragging 48 square inches through the water, and regardless of how fast it is spinning (as long as it is being dragged) you cannot reduce th surface area drag unless you change the pitch." is (to be kind) pure folly. are you now saying that drag is a constant, no matter our propellar speed. we must go from "no drag" to "max drag" instantaneously. there is no progression, linear or otherwise according to you and your math. drag doesnt lessen as we increase our rpms toward achieving thrust.
as i and others have said before, this is just an entertaining way to excersise our noggins and discuss a hypothetical mathimatical engineering problem. we all realize that there are other reasons you may want to lock you prop.
like you, i would be interested to see "the math" and a hard proof. you are free to offer it up for scrutinity at your convenience. perhaps you can trully put this question to bed.
in closing, i have always hated email (similar to this) because it in no way approximates a conversation. folks dont read, or they read things into a message that is not there, or they dont address the question, or they get off on a tangent, (you get the point)
good luck
darrell
[/quote]
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Dean Abramson
- Posts: 1483
- Joined: Jul 5th, '05, 11:23
- Location: CD 31 "Loda May"
Record?
I know this is off-subject, but I am wondering if there is an existing record for maximum number of posts on a subject, or visits to a thread.
As I type this, this thread has had 4037 visits! Are we going where personkind has never gone before?
Maybe Cathy can illuminate us on this.
Anyway, back to props and choppers...
As I type this, this thread has had 4037 visits! Are we going where personkind has never gone before?
Maybe Cathy can illuminate us on this.
Anyway, back to props and choppers...
Dean Abramson
Cape Dory 31 "Loda May"
Falmouth, Maine
Cape Dory 31 "Loda May"
Falmouth, Maine
A Proper Analogy (final try)
John Vigors analogy, properly phrased, should have read:
a falling helicoptor with its blades autorotating (pitch has been reversed from normal flight setting) is like a sailboat moving forward, under sail, with its engine on and its transmission in reverse gear.
this is not a freewheeling sailboat.
another beautiful day in so fla., but i understand it is warm all over (hurray for global warming, not)
darrell
a falling helicoptor with its blades autorotating (pitch has been reversed from normal flight setting) is like a sailboat moving forward, under sail, with its engine on and its transmission in reverse gear.
this is not a freewheeling sailboat.
another beautiful day in so fla., but i understand it is warm all over (hurray for global warming, not)
darrell
Re: Freewheeling or locked prop
Let's go back to page 1. John's original statement read:
"....a stopped, stalled, propeller will cause less drag than a spinning propeller, which is gaining lift and trying to drag the boat backward."
For the benefit of all sailors, maybe the answer to whether the locked prop or the freewheeling prop allows a faster sailing speed is beneficial. The previous thought exercises are entertaining but let's advance an hypothesis and then design an experiment to find some data. This wouldn't be the first time an answer was found using science to settle a difference of opinion.
Therefore, to keep this thread flowing along, I offer the following:
1) A locked 2-bladed prop in a vertical position and aligned with the rudder and keel will offer the least drag while sailing, say, at 6 knots.
2) A locked 2-bladed prop in a horizontal position and perpendicular to the rudder and keel will offer the greatest amount of drag while sailing, say, at 6 knots.
A freewheeling 2-bladed prop will offer varying amounts of drag; minimum as in #1 and maximum as in #2 while sailing at, say, 6 knots. Here's the contention: Is the average drag of a freewheeling prop while it spins 360 degrees less than the drag in #1 above??????
My hypothesis would be that a locked vertical prop offers the least drag in a sailboat moving through the water at 6 knots.
I'll leave it to someone else to come up with a hypothesis for a 3-bladed prop.
"....a stopped, stalled, propeller will cause less drag than a spinning propeller, which is gaining lift and trying to drag the boat backward."
For the benefit of all sailors, maybe the answer to whether the locked prop or the freewheeling prop allows a faster sailing speed is beneficial. The previous thought exercises are entertaining but let's advance an hypothesis and then design an experiment to find some data. This wouldn't be the first time an answer was found using science to settle a difference of opinion.
Therefore, to keep this thread flowing along, I offer the following:
1) A locked 2-bladed prop in a vertical position and aligned with the rudder and keel will offer the least drag while sailing, say, at 6 knots.
2) A locked 2-bladed prop in a horizontal position and perpendicular to the rudder and keel will offer the greatest amount of drag while sailing, say, at 6 knots.
A freewheeling 2-bladed prop will offer varying amounts of drag; minimum as in #1 and maximum as in #2 while sailing at, say, 6 knots. Here's the contention: Is the average drag of a freewheeling prop while it spins 360 degrees less than the drag in #1 above??????
My hypothesis would be that a locked vertical prop offers the least drag in a sailboat moving through the water at 6 knots.
I'll leave it to someone else to come up with a hypothesis for a 3-bladed prop.
The trouble with trying to introduce math here is manifold.
The first part of the problem is the lack of hard data. None of us has a figure for the rotational resistance bearings and transmission offers, none of us has any more information about their propellers than the common pitch and diameter, none of us has information on the wake field for the propeller, etc. Heck, we don't even have a single data point for the thrust vs rpm curve for open water, let alone for operation under the boat. So in light of that, introducing math into the discussion would serve absolutely no useful purpose.
The second part of the problem is the math itself. There are two ways the solution to the problem might be determined: actual testing and computational fluid dynamics. The first approach is clearly the more practical approach for us even though probably none of us has access to very accurate/precise instrumentation or the ability to set up controlled conditions such as you would find in in a hydrodynamics lab. The second is something that only becomes viable through the use of sophisticated and expensive software.
The third part of the problem is that this forum is definitely not conducive to displaying math in a readable format, even if the math is limited to relatively simple formulae. Generally, if I am discussing math in such a format, I do my math in a math program I have (MathCAD) and then capture my screen, save it as a .jpg, upload it, and then post the picture. It looks like what you would see in a textbook. (the one below is scaled way down to fit the NPWSB width requirements and will probably not be readable):
<img width=540 src=http://www.todspages.net/images/TrailerTongueWt.jpg>
The second part of the problem is the math itself. There are two ways the solution to the problem might be determined: actual testing and computational fluid dynamics. The first approach is clearly the more practical approach for us even though probably none of us has access to very accurate/precise instrumentation or the ability to set up controlled conditions such as you would find in in a hydrodynamics lab. The second is something that only becomes viable through the use of sophisticated and expensive software.
The third part of the problem is that this forum is definitely not conducive to displaying math in a readable format, even if the math is limited to relatively simple formulae. Generally, if I am discussing math in such a format, I do my math in a math program I have (MathCAD) and then capture my screen, save it as a .jpg, upload it, and then post the picture. It looks like what you would see in a textbook. (the one below is scaled way down to fit the NPWSB width requirements and will probably not be readable):
<img width=540 src=http://www.todspages.net/images/TrailerTongueWt.jpg>
Tod Mills
Montgomery 17 "BuscaBrisas", Sandusky, OH (with trips elsewhere)
Tartan 26 project boat
Cape Dory admirer
Montgomery 17 "BuscaBrisas", Sandusky, OH (with trips elsewhere)
Tartan 26 project boat
Cape Dory admirer
reply to ed
dear ed,
you are right in #1
you are right in #2
as to your question: who cares!!
your hypothesis: obvious
get your own thread(ha-ha)
helicoptors have more than two blades (most of them) and we are discussing 3 bladed props to eliminate the deadwood scenerio (again ha-ha)
ananymous
you are right in #1
you are right in #2
as to your question: who cares!!
your hypothesis: obvious
get your own thread(ha-ha)
helicoptors have more than two blades (most of them) and we are discussing 3 bladed props to eliminate the deadwood scenerio (again ha-ha)
ananymous
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Dick Barthel
- Posts: 901
- Joined: Feb 5th, '05, 10:29
- Location: Dream Weaver, CD25D, Noank, CT
Well said Tod
Tod is another engineer in the know. I think you've said a mouthful. Perhaps if there is enough interest we can pool our money and purchase all that expensive stuff Tod has mentioned. (Just kidding!)
I marvel at how long this thread has gone on and how much information has been exchanged with consistent civility.
I certainly know a lot more about autorotation than I did. And personally, I will keep my propeller locked for reasons that have nothing to do with performance. I need to get a lot better at handling the sails and other things before I have to worry about fine tuning items.
This has been (and I guess will continue to be) a wonderful thread. Thanks John.
Dick
I marvel at how long this thread has gone on and how much information has been exchanged with consistent civility.
I certainly know a lot more about autorotation than I did. And personally, I will keep my propeller locked for reasons that have nothing to do with performance. I need to get a lot better at handling the sails and other things before I have to worry about fine tuning items.
This has been (and I guess will continue to be) a wonderful thread. Thanks John.
Dick