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The merits of this argument mean little to me as my concern is for "sailing aesthetics", if you will.
When my prop freewheels on my CD27 I can hear it. Its an alien sound to the otherwise beautiful sounds of sailing and I take exception to it. So, my prop gets locked. The thumping noise vanishes and my senses are left to enjoy only the water rush past the hull, the wind in the sails and the sound of the hull as it plow into a swell. Heaven I tell ya! Heaven!
Last edited by Warren Kaplan on Jan 3rd, '07, 15:13, edited 1 time in total.
"I desire no more delight, than to be under sail and gone tonight."
(W. Shakespeare, Merchant of Venice)
It was just those same things that you mention that intrigued me about sailing from the get go.
During the Great Depression there were only two main classes, the haves and the have nots. Many of the haves owned sailboats, the have nots only had dreams.
There was no way that we could buy a sailboat, so I built one. But that's another story. Ckeck, make that a story and a half, to be told at some later date. Believe me when I say it was a riot.
Sad to say, there was nothing to lock or freewheel, unless a pivoting lee board counts.
As a very senior engineer in the development of rotor systems for a major aerospace company, perhaps I can demystify the spinning/not spinning thing. First of all, a body (in this case the propeller/rotor blade) can produce drag or lift proportional to the dynamic pressure and the drag or lift coefficient. A decent airfoil can easily generate a lift coefficient of 1.5. A long, flat plate perpendicular to the wind can generate a drag coefficient of almost 2.0 but a more square flat plate can only generate a drag coefficient of about 1.1. The big difference between the case of a rotor spinning and not spinning is the dynamic pressure. The dynamic pressure is proportional to the air density and the square of the velocity of the body relative to the fluid. A typical helicopter has a rotational speed that keeps the tip of the rotor going between 650 and 750 feet per second. Just for example let’s say 700 feet per second. Given the density of air this translates to a dynamic pressure of approaching 600 lb per square foot. Provided one has a large enough power plant on the helicopter and robust transmissions and shafting the lift per square foot can be about 900 lb just before you stall the rotor. Now consider the non-rotating case. For the sake of sensitivity we will consider your toy helicopter. How fast was it moving when it hit the ground? 20 feet per second? The dynamic pressure would have been less than 1/1000 of the dynamic pressure of the tip of an normally operating rotor. Yes, the spinning rotor has the POTENTIAL to generate more drag or lift than a non-spinning rotor. I say potential, because you have to have the correct conditions, including lots of power and the correct blade pitch. So what if you don’t have any power to put in the rotor? You can exchange some potential energy in the helicopter to keep the rotor spinning. That is to say, a skilled pilot can manipulate the collective pitch control on the helicopter while the helicopter sinks rapidly to keep the rotors turning at a reasonable speed to generate enough lift to keep the helicopter from accelerating downward. To clarify a few points, on many helicopters the actuators used to control the blade pitch are powered by hydraulics. The hydraulic pumps are driven off of the main transmission, not the engines. If an engine dies and is de-clutched from the transmission(s), the rotor spinning maintains the operation of the pumps, allowing control of the rotor. The rotor continues to spin in the same direction. If, while at any altitude above a few dozen feet, the rotor slows too much the hydraulic pressure drops too low and control is lost on that helicopter…forever. In order to keep the rotor spinning in the same direction the collective pitch needs to be set to a negative angle. The reason that the negative angle does not tend to drive the helicopter down is that the negative angle is relative to the helicopter, not the air stream. While in powered operation the air is moving down through the rotor. In unpowered operation the air is moving up through the rotor. As one previous poster noted it is like a glider. The flow up through the rotor has to be fast enough to create a positive angle of attack on the airfoil at the rotational speed of interest to keep it spinning and generate lift. The rotor is kept spinning because a portion of the lift vector is tilted forward of the vertical. In a steady condition the forward portion of the lift vector balances the drag. If the pilot lets the rotor spin too fast the blades will rip off. If he/she lets the rotor slow control can be lost and the amount of lift available on the blades drops causing the helo to skink even faster. As previously mentioned if there is enough inertia in the rotor the pilot can, at the last instant, skillfully extract the kinetic energy of the rotor to generate momentary additional thrust to slow the downward motion of the helicopter in order to land in such a way as to walk away from the incident. Almost nobody practices this maneuver through to a landing, since the risk of trashing the aircraft is extremely high. So how does this relate to a boat?
There are many differences. Firstly, the solidity (the ratio of blade area to the swept area) is very different on a helicopter than it is on a boat prop. The increased area means that the drag from a non-rotating marine propeller is proportionally higher. Another difference is in the dynamic pressure due to the boat movement through the water as compared to the velocity of rotation. Water is much denser than air so large dynamic pressures can be achieved with lower tip speed. This means the drag from a non-rotating prop is proportionally higher than for a helicopter. It takes about 750 shaft RPM to make my CD-36 go 5 knots. The dynamic pressure on the tip of the rotating blade is only about 9 times that of the non-rotating blade unlike the helicopter example where it was over 1000 times higher (probably not 1000 for John’s toy). The big difference is blade pitch. On a boat it is typically not changeable. On a helicopter that can autorotate without engines, the pitch control needed to get the most lift while still maintaining rotor speed is critical and very sensitive. On a boat with a fixed pitch prop you might get a great deal of drag or you might not, depending on the blade pitch and boat velocity. As stated in an earlier post, if you really want drag out of the prop you should put a break on the propeller shaft to increase the angle of attack on the blades. Alternately you can purchase props with proportionally lower pitch until you find one that spins fast. The combination of drag on the blades and drag in the shaft log and transmission will eat up energy and effectively increase the angle of attack on the blades, causing more drag on the boat.
Over the years I have alternately shifted the prop into reverse and then let it freewheel and back to look for any signs of which is better on my CD-36. There has never been any trend that is obvious to me. I would never try and convince the owner of another design of boat or with a different prop that the same result is true for their boat. The factors are many and include friction in the shaft log, barnacles on the blades, blade pitch, number of blades, disk area ratio, deadwood and hull shape. If your transmission manufacturer claims that it is ok to freewheel then do the test while sailing in very steady conditions. Do it many times over the next year or two and decide for yourself what is best on your boat. If you are wrong and slow yourself down a fraction of a knot then you will just have to sail longer to get where you are going. That is not such a bad thing.
If your transmission manufacturer does not want it to freewheel then put it in reverse when sailing.
As for pulley sizing, the alternator has been replaced with a generator that generates current at shaft speeds as low as 220 RPM. In the spring I will be generating current while sailing, but at a slightly slower boat speed.
I guess it was only a matter of time before a rotor engineer from our ranks stepped forward. Thanks Matt for a wonderful post that brought more light than heat to our discussion. If I understood correctly, your tests on your boat with your particular prop resulted in no discernible difference on speed.
Given your expertise and your thorough descriptions I guess we can dispense with the helicopter analogy and get back to sailing in whatever propeller state "floats your boat" so to speak.
Thinking of Warren's love of Shakespeare, the title "Much Ado About Nothing" comes to mind. But just like that play, this thread has been most entertaining as evidenced by the participation.
thank you for the explanation regarding a helicoptor in "autorotating" mode and its relevence to a sailboat either freewheeling or locking up its propellar. you were very clear and precise.
a few questions, if i may:
1.)if i understand you correctly, the helicoptors rotors never change direction of rotation. the only reason the spinning blades generate any lift is because the pilot changes the pitch of the blades to "catch" the air rushing up as he descends. by changing the pitch, what we mean is that he reverses the leading edge so that it is facing up rather than down, correct?
2.)what would happen, if when the helicoptor loses power, the pilot did not reverse the pitch of the blades, but kept them the same as we do with our boat propellars when we are freewheeling while under sail?
3.)would you say that the helicoptor falling analogy equates to our sailboats freewheeling under sail as it relates to drag, remembering that the helicoptor pilot is actively altering his rotors as we can not do?
4.)we seem to have established that on a sailboat under sail, a spinning propellar, as it increases its rpms, can reach a point where the speed of the spinning propellar matches the boats speed through the water, and we no longer have any drag or thrust, a zero position.
we also seem to have reached a conclusion that the maximum possible drag on a sailboat in forward motion (either under sail, being towed, or just coasting) is created when our propellar is engaged in reverse gear and rpms are increased. this is how we stop our boats.
because of the above two scenerios being at the opposite ends of our propellar spectrum, (zero drag with adequate forward rpms and maximum drag with adequate reverse rpms), and because if you are reducing your forward rpms towards zero, you must go through the zero rpms position before you can begin to increase your rpms in the reverse direction, i have offered that the graph or chart plotting drag is a linear progression, and that the fixed prop (locked prop) position is right in the middle of the progression, as you go from zero drag to maximum drag.
while i realize, as you said that all boats will react differently depending on conditions, if we are to put this "freewheeling vs. locked prop" question to rest, we naturally have to assume a hypothetical boat, under controlled conditions, in order to reach a conclusion.
so heres the question: if the progression is not linear, how do you plot drag over the full range of propellar positions and go from max drag at one end to min drag at the other end without putting the "locked prop" position in the middle. which is to say "that a propellar spinning in forward causes less drag than a propellar locked (or not spinning), which causes less drag than a propellar spinning in reverse?
any help will be greatly appreciated
of the freewheeling society
darrell randolph
s/v mystic rose
incidentally, my prop, because of it being new, with minimum friction from the new drive train, makes no noise while freewheeling
your comments are well taken, however, this was never a question which was to lead the masses to free up their propellars or lock them based on our arguments. this was always a hypothetical engineering problem, that does indeed have an answer, somewhere in deep in the bowels of mathematics and physics, and fluid dynamics, as so many have eluded to.
if you look back to the original posting that brought up this subject, our friend John Vigor was being "provocative" in his assertions, looking to get a rise out of someone. i would say he has been exceedingly successful, wouldnt you?
there are numerous other reasons that sailors want to lock their propellars (all of us recognize that)
this was always just an idle discussion, in the middle of winter, when some of you less fortunates (ha-ha) had nothing else to do. a very interesting discussion, and yes, very entertaining.
unfortunately, a few took it too seriously and perhaps personally, where no such intention was present.
darn that Vigor and bless that Vigor, all at once.
Of course, the two curves would be "smoothed" out with no real vertices. The purple line would be the "locked is more drag" situation and the yellow line would be the "freewheeling is more drag"
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 smooth 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.
Matt Cawthorne wrote:As a very senior engineer in the development of rotor systems for a major aerospace company, perhaps I can demystify the spinning/not spinning thing. ....
Sonuvagun! Now THAT'S a dissertation.... and a clear one. Much thanks. As someone before me has already said, or almost said or maybe just imagined they said; " This board has some of the most amazing members lurking about."
Best of the New Year to you,
Didereaux- San Leon, TX
last owner of CD-25 #183 "Spring Gail"
"I do not attempt to make leopards change their spots...after I have skinned them, they are free to grow 'em back or not, as they see fit!" Didereaux 2007
Without disputing the physics of moving bodies in water or in air, I have one little but important thing to add to this garment length thread, When I leave my prop unengaged - it spins and squeaks. This annoys me, so I put it in reverse and the squeak stops, which gratifies me greatly.
Darmoose,
A to Q1 : During autorotation the nose of the airfoil is down while in steady state, level, powered flight the nose would be up.
A to Q2: If the pilot does not respond the rotor stops. How quickly depends on the inertia of the rotor and flight condition that the aircraft was in before the power failure. If the aircraft is in a flight condition that is demanding full power from the engine and the engine quits suddenly the pilot has a couple (perhaps a few) seconds before the rotor is moving so slowly that there is no hope for recovery.
A to Q3: Yes the analogy holds, but the variation in a helicopter blade pitch allows it to reverse the direction of thrust and the flow without changing the direction of rotation. A fixed prop requires the direction of rotation to change to change the direction of thrust and flow.
A to Q4: Yes, there is some rotational speed at which there would be neither thrust nor drag, but that condition is not the freewheeling state. The freewheeling state will be a slower rotational speed.
A to last question: The chart would look something like this. At high RPM you get lots of forward thrust. As the RPM is dropped the thrust will drop to that zero drag state in the above question. Drop the RPM some more and the thrust direction reverses and you have drag. Drop it some more and the drag increases. Drop it more and the blades will stall. Stall is not linear, the overall drag on the boat from the prop will suddenly drop (that would be up on the chart previously presented) I don't really know what would happen in the deeply stalled region as the RPM is changed. Now make your RPMs negative. As you continue to increase your negative RPM at some point the blades will un-stall and the negative drag will jump. More negative RPM and the 'drag' increases.
There are some additional drag effects due to the disturbed flow over the rudder. I have no estimate as to the level of this drag, but it would probably be somewhat less than the direct drag from the prop.
The specific details would be affected by all of the things listed in my previous post.
My suggestion to all is to test your own boat and see what works. When you have decided which works best, look up and learn to trim your sails.
I have waited a long time to enter this seemingly unending thread, Matt finally did the hard work of explaining rotary wing aerodynamics, hopefully to everyones satisfaction. My only disagreement was his statement that "few pilots practice the maneuver to a complete landing...". As a Naval Aviator with over 5000 hours of rotary wing time, and 2000 hours of instructor time I, like all Naval Aviators have done literally thousands of practice power recovery and full autorotations. They are really harder to explain than to do, and they are only scary when they are being executed (or attempted) by the student next to you, or your aircraft is very heavy, or the density altitude is very high. Aircaft models vary; it is a piece of cake to do a full auto in an H57, An H1N comes down like a forklift, and an H3, well that is where Matts' statement is correct. Now I know it is cold up north but the weather is great for sailing in Pensacola so that is where I am going, I can always use crew if your down this way, it is nice and quiet on the bay, I keep my transmission in reverse because I don't like the thrumming, if I go a little slower that will mean I get a longer time to sail.
Matt Cawthorne quoted:
[A fixed prop requires the direction of rotation to change to change the direction of thrust and flow.]
I'm glad to read what you had included in your answer to question #3. It sort of vindicates what I had previously mentioned earlier in the thread.
I was thinking in line with John V's model's crashing as a result of loss of power. Also in mind was the change of air rush direction after a loss of power. It is possible, but I somehow doubt that the pitch on John's model's rotor was reversed after the power failure.
With a pinwheel's reversal of direction due to a reversal of wind rush direction in mind, I mentioned that the helicopter's rotor would reverse (without pitch reversal. Inferred, not actually stated.)
Special thanks to you and Steve Kuhar for your timely input
lets talk about your answer to my number 3 question.
Johns analogy says that because the helicopters blades while autorotating, as the helicopter is falling, are creating drag or lift, that it follows that a sailboats propellar which is freewheeling while the boat is sailing forward is creating drag in the same way.
what he misses, is in your answer to question 3. the helicopter pilot has reversed the pitch of his rotors blades in order to catch the wind coming up at him.
i cant do this with a freewheeling propellar on my boat under sail. my propellar while freewheeling spins in the same direction as when i am motoring forward, and the pitch doesnt change. the only way i can simulate the helicopter (with the reverse pitch) is to turn on my engine and put it in reverse gear. this is no longer a freewheeling propellar.
you explained exactly this in your answer, even though you incorrectly said that the analogy holds (with all due respect)
wont you reconsider......and clarify this for the board?
