Subject: Re: [baidarka] Lift and Drag
From: James Mitchell (baidarka@earthlink.net)
Date: Mon Oct 11 2004 - 23:08:06 EDT
A propeller or a foot. Wow, what a great way to put it.
The answer, of course, is "either, depending." If you hold the paddle
in line with the flow of the water ("flat" from your perspective as
paddler), I think it is mostly a propeller (actually more like a wing,
with the "lift" vector pointed toward the sky). If you hold the paddle
cross-ways to the flow, as most of us do most of the time, I think it
is mostly a foot (but a very special foot, one for which stability is a
huge issue).
As regards 1934 and laminar flow, I am a fan of older things (being one
myself). The photos in that book are amazing, totally applicable to our
paddle discussions. The title is "Applied Hydro and Aeromechanics" by
Prandtl und Tietjens.
John Petersen, Werner, and I have been having a conversation regarding
these issues. John came up with an ingenious design for a slotted
paddle -- I am getting WAY ahead of myself here -- that will open a new
chapter on stability.
See, the flow around any object will generate eddies. Think of a
cylinder suspended in a flow. At a low rate of flow, the eddies will
remain small and stable, forming behind the cylinder and remaining in
place. As the rate of flow increases, the eddies begin to expand and
cross into each other. Then they oscillate, one side becoming larger
than the other. Finally they break free, forming a large whirlpool
first on the right and then on the left. We can see this easily with
our paddles. Suspend it vertically and pull forward slowly. The two
eddies will form off the back of the blade. Now increase the speed of
your pull. The eddies will begin to cross, and then to alternate
left-right. The paddle will be drawn toward one side and then the other
-- FLUTTER is caused by the oscillation of the eddies, which increases
with increasing flow velocity.
Even a broom handle will be drawn into flutter. I am interested in
forestalling that flutter as long as possible. Our narrow ridged Aleut
blades do that by accelerating the flow off the power surface so as to
move the eddies further away from the centerline. But still, it is easy
to reach a point where we exceed the stability. Is this a little like
the kayak equivalent of breaking the sound barrier? Perhaps. At least,
it is a useful analogy.
Here is where Werner says, drill a hole in the paddle. John says build
slots along the length of the blade. Our guess is that that should
cause a second eddy to form behind the blade, rotating counter to the
primary eddy and pushing it outboard. Result: more stability (we
think). Now, it is time to head for the shop and try to build one.
I am on my way to Alaska for a couple weeks, but will try this
experiment when I get back.
On Oct 12, 2004, at 3:43 AM, William Nettles wrote:
> Hi James,
> Thanks for this post James. I only just read it. Somehow it got below
> the
> window and I missed it until I plowed through the stuff on top.
>
> All this sounds much clearer than what I wrote.
> Couple of thoughts.
> A 1934 book. Will it cover laminar flow? Maybe not. Is this important?
> I
> don't know.
>
> Angle of attack or airfoil. Which is doing the lifting? The question
> I've
> seen raised is if the airfoil is so essential, how can planes fly
> upside
> down? And there seem to be some modern wings that are almost
> symetrical.
> The northrop guys say they can flya barndoor using computers, its main
> problem is not flying but stability.
>
> My basic paddle question I boil down to: Is a paddle a 'propeller' or a
> 'foot'. does it fly or does it stick?
>
> Is it mostly just a friction device that sticks in the water and you
> push
> off of it to push the boat forward? A high stroke rate for high speed
> indicates this approach. Think of your feet, when you walk pushing
> harder
> backward isn't going to move you any faster, you'll just 'slidewalk'.
> Larger
> shoes aren't going to make you go faster. (Maybe Euro-paddles are just
> joint
> destroying Clown Shoes?)
> Is it a propeller, can the shape of the blade create some 'magic'.
> Think of
> a square rigged sail. It can only move as fast as the wind. A modern
> sail
> with battens can move a boat faster than the wind speed. Can a paddle
> work
> this way?
> I don't know (I should shut up and finish my kayak and go see for
> myself)
> Will
>
>
>
>> From: James Mitchell <baidarka@earthlink.net>
>> Reply-To: baidarka@paddlewise.net
>> Date: Fri, 8 Oct 2004 21:09:25 +0900
>> To: baidarka@paddlewise.net
>> Subject: Re: [baidarka] Lift and Drag
>>
>> You are getting close to where I have been thinking, William. Your
>> comparison of a paddlewheel to a propeller -- yes, and the paddlewheel
>> is just that, a PADDLEwheel. But the propeller, let's talk about that
>> since both you and Peter have mentioned it. Here is what I observed.
>>
>> The airflow on the surface of a propeller is AXIAL, it is generated by
>> the spinning of the propeller. To the blade of the propeller, the
>> airflow is coming from FORWARD and is essentially aligned with the
>> propeller. The lift vector is perpendicular to the axial flow, which
>> just happens to point straight forward from the airplane's
>> perspective.
>> The propeller spins, generating lift on the blades. That lift pulls
>> the
>> airplane forward.
>>
>> Let's back up a bit... Get in the car on a nice straight stretch of
>> freeway. Set the cruise control to 60, and roll down the window. Hold
>> your hand flat, parallel to the ground, and extend your arm out the
>> window. Tilt the front (leading edge) of your hand slightly upward. If
>> your arm is stiff, the lift being generated on your hand will lift
>> your
>> entire arm up. OK, we have all done this as kids, but try it again.
>> Small variations of the angle of attack will make huge variations in
>> lift, easily felt.
>>
>> Drag is the force of the wind pushing your arm back.
>>
>> OK, now to continue our experiment: rotate your hand so it is straight
>> up and down. Did it break your arm? THAT is drag.
>>
>> You can do the same thing with an airplane propeller if you have a
>> variable pitch prop, but you had better be sitting on the ground.
>> Flatten the pitch to the airflow, and the motor just revs up. You go
>> nowhere. Pitch it up slightly, and it begins to pull. Pitch up a bit
>> more, and it reaches its cruising pitch, then its climbing pitch. Now
>> if you keep going and pitch it up 90 degrees, that motor will load up
>> like a car climbing a steep hill. And the pull that the prop was
>> generating and climb and cruise settings is now gone -- as in ZERO.
>>
>> Now I have been in the left front seat of that airplane. And I have
>> been around lots of marine propellers, from fishing outboards to
>> unlimited class hydroplanes. And I have designed and built a lot of
>> paddles... a WHOLE lot of paddles. Propellers use their spinning
>> (axial) motion to generate lift in the perpendicular plane, just as a
>> wing uses the parallel airflow over its surfaces to generate lift at
>> 90
>> degrees. But I cannot understand how a lifting surface can generate
>> ANY
>> lift when it is oriented normal to the air (or water) flow. Therefore,
>> I say that a paddle is only using drag. And yes, Michael, we do need
>> to
>> optimize our paddles for drag (and stability for ease of use).
>>
>> Oh. Aspect Ratio. I think the discussions of aspect ratio are not
>> applicable to situations where we do not have a lifting body. And, I
>> still contend that a paddle held normal to the water flow is not a
>> lifting body.
>>
>> Could a paddle become a lifting body? Yes, but only if it is held
>> nearly parallel to the water. Yes, if it were paddled much the same
>> way
>> as when you held your hand out the window of the car. If a paddle is
>> oriented so that it is pitched just a little up from being parallel to
>> the flow of the water, the lift that is generated could be used to
>> propel the boat forward. There is a stroke I have played with using a
>> Greenland paddle that seems to approximate this. I don't have enough
>> experience with this stroke, or with Greenland paddles in general, to
>> say more about this. Is there also a stroke with the fancy new wing
>> paddles where the paddle is presented to the water as if it were a
>> wing? Ahh, but then the tennis-racket profile of those paddles would
>> not make much sense. It would make more sense then to talk about
>> aspect
>> ratio, to use a true foil-shaped blade. But this whole discussion
>> gets
>> a lot more complex than I intended for this email.
>>
>> I will try to get to Jonathon Winters' article this week, but for
>> right
>> now they are kicking me out of Starbucks again. Am working long hours
>> on the new boat at Corey's, hope to skin it tomorrow. The frame is
>> coming out pretty good. OK, for a modification of an old hull. I am
>> really looking forward to checking it's performance.
>>
>>
>> (((Peter and Michael, I have a book here full of differential
>> equations
>> on hydro and aerodynamics that I barely can read much less understand,
>> but I need to look at this in simple terms. So please bear with me for
>> now, OK? Same book, it is a text published in 1934, has many pictures
>> of flow tests that clearly show the eddies we have been discussing.
>> This comes via Werner, one of his Swiss engineering texts, an heirloom
>> and a gem, portions of which have clearly been influential in the
>> development of the Werner Paddles product line. I am going to take it
>> up to Juneau next week where I will have a bit more reading time. )))
>>
>>
>>
>> On Oct 7, 2004, at 3:34 AM, William Nettles wrote:
>>
>>> You guys are too smart for me.
>>>
>>> LIFT
>>> Place a piece of paper on your table. blow on it.
>>> the air moving across the top of the paper moves faster than the air
>>> under
>>> the paper, according to the Bernoulli Principle this reduces the air
>>> pressure on top causing the paper to rise.
>>>
>>> On a wing the curved upper surface causes the air to moving across it
>>> to
>>> travel farther than it has to on the flatter under side. Again
>>> according to
>>> Bernoulli, the pressure of the air above the wing becomes less than
>>> that
>>> under the wing and the wing is lifted.
>>>
>>> DRAG is the resistance of an object in a fluid (like air). if you
>>> blow
>>> on
>>> the paper, or a ball of paper, and simply push it with your breath
>>> that's
>>> simple drag. If you stick your hand vertically flat out your car
>>> window the
>>> push backwards is drag. As you turn your hand tp the horizontal to
>>> the
>>> wind
>>> it pulls upward-that's lift.
>>>
>>> In designing an airplane wing engineers look for the maximum lift
>>> with
>>> the
>>> least amount of drag. You can tilt any wing up say 20 degrees and it
>>> is
>>> likely to have a lot more lift than the same wing at 2 degrees,
>>> however drag
>>> also increases often to the point where it is inefficent or stalls
>>> (no
>>> more
>>> lift).
>>>
>>> On a kayak paddle design (I'm guessing) the discussion seems to be:
>>> Just how important is lift versus drag. Someone here made the
>>> insightful
>>> comparison to a paddlewheel boat versus a propeller. A paddlewheeler
>>> is pure
>>> drag force a propeller utilizes lift. A propeller is much more
>>> efficient.
>>>
>>> When our source of power is a human being instead of a motorized
>>> shaft, the
>>> issue becomes what is the most versitile and efficent use of human
>>> power to
>>> move the kayak, brace it in turns, etc. . . .
>>> WHat happens when the paddle moves through the water? Its very
>>> complex
>>> and
>>> fluid dynamics is one of the really tricky frontiers of physics.
>>> But have faith, Tom or someone will come up with the ultimate paddle
>>> design
>>> and we'll move on to a discussion of water skiing behind our kayaks
>>> or
>>> so I
>>> hope.
>>> -
>>> Baidarka Mailing List - All postings copyright the author and not to
>>> be
>>> reproduced outside Baidarka or Baidarka archives without author's
>>> permission
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>>>
>>>
>> James Mitchell
>> Email: baidarka@earthlink.net
>> US Mobile Phone: 425-273-0884
>> 15917 Waynita Way NE, E103
>> Bothell WA 98011 USA
>> -
>> Baidarka Mailing List - All postings copyright the author and not to
>> be
>> reproduced outside Baidarka or Baidarka archives without author's
>> permission
>> Submissions: baidarka@paddlewise.net
>> Subscriptions: baidarka-request@paddlewise.net
>> Searchable archive: http://rtpnet.org/robroy/baidarka
>>
>
>
James Mitchell
Email: baidarka@earthlink.net
US Mobile Phone: 425-273-0884
15917 Waynita Way NE, E103
Bothell WA 98011 USA
-
Baidarka Mailing List - All postings copyright the author and not to be
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