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Feb 21, 2020, 03:54 PM
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The same general concept as using one paddle to propel a canoe with the "J-stroke"


How to Paddle a Canoe: The J-Stroke | Art of Manliness (1 min 53 sec)



note: even women can do the J-stroke!
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Feb 22, 2020, 02:03 AM
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interesting yes, i wonder if it actually recovers some otherwise "lost" energy, like insect flight does. the primary goal probably is to paddle straight (i am not experienced in canoe-ing)
Feb 22, 2020, 04:40 AM
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another tale


It is not an exactly parallel analogy of course. Canoe energy is stored as momentum during the J-stroke and "lift" is in another direction coming from static buoyancy. However if the canoe propulsion in this case is considered as a glider with the propulsion as the fwd-tilting lift-propulsion projection of a glider, then it is easily seen that there is a definite "anti-propulsion" drag component of the reverse-hook part of the J-stroke.

In essence a down-pushing stab at neg AoA in a conventional config aircraft (vs plank with reflexed rear) does the same thing, with energy lost due to drag of the stab working against the fwd pitching moment of the wing airfoil. Of course for max L/D due to its ridiculously short pitch stabilizing moment arm, a plank config is not used in high-performance sailplanes (but they can't loop so tightly). And using a swept Nurflügel config (gulls) has a longer stab moment arm than a plank plus reduces induced drag by more efficient involvement with the tip vortex, all further aided by "active stabilization". John McMasters told me that the early flying dino birds had long tails which brain evolution could eliminate and use short tails. However Budgies use these early-design long tails (in addition to a fan-like deployable shorter yellow set), a "de-volution" I suppose for maneuverability -- supposedly these are the only birds that falcons cannot catch in Australia, as they can out-maneuver the falcon as well as having the same speed in level flight, with the same-type high-speed wing planform as falcons & swifts -- plus pounced-on predator-escape, like some lizards can lose their tails: I know this for a fact when I accidentally stepped on young Kiki's long tail feathers and he took off without them, later re-growing them (they anyway drop and grow new ones every year or so to keep them fresh).


Inner yellow tail feathers normally retracted





Kiki airbraking to avoid midair with Avitron orni (note that he also spreads 2 long tail feathers, perhaps to create super vortex-drag ... anyway likely related to fan-expanding yellow feathers)

Last edited by xlcrlee; Feb 22, 2020 at 12:34 PM.
Feb 22, 2020, 07:39 AM
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i agree, but, i think one could perform it in such way that there are only propulsive forces. this might be a bit awkward angled to really put force behind the motion.

cool birds yes. actually i have a x-twin (have had more in the past) and sometimes a group of swallows would follow it, chattering. (in thermal mode, seemingly).
the x-twin is about bird size. this is my current one, modified several things; https://i.imgur.com/b8V6QY2.jpg

Barn Swallows Drinking on the Wing (Narrated by David Tennant) - Earthflight - BBC One (1 min 47 sec)
Last edited by m4rc3l; Feb 22, 2020 at 12:14 PM.
Feb 25, 2020, 08:45 PM
Everything is broken
JimZinVT's Avatar
That video is the best thing to come out of this thread
Thanks for posting it!
Feb 27, 2020, 11:04 PM
Design is everything.
These birds are pushing air downwards or creating high pressure and low pressure regions?

I prefer the latter explanation.
Feb 28, 2020, 07:36 AM
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yes, i'd like to think, flying by pressure difference, by moving air as little as practically possible.

these birds have a funny thermal flight, not nice slow circles but more like a random "cloud" of birds.
they are good indicators for very fresh thermals, as they gather the bugs which initially get carried away with the thermal, but then detect being lifted by the thermal and letting themselfs drop out of the thermal, at which time the swallows leave and look on. the thermal may at this time be releasing and getting stronger, so with a RC glider may still be worth looking for lift there.
Feb 28, 2020, 01:19 PM
Launch the drones ...
Thread OP

Oh the science, how it suffers ...


Wow - what gibberish, posing as science.

Fact - nothing can suck itself up.
Fact - if it's heavier than air - and it's not throwing air down - it's not going to stay up.

Lift, or thrust, is purely a Newtonian reaction to the action of accelerating air in the opposite direction.

Again - nothing in the world, can lift itself - by sucking air (unless aided by a ceiling - but that's not flight, is it now? Not with a ceiling there doing the lifting). No amount of low pressure above a wing - can lift it up - not unless it's also accelerating air downward - enough to account for the force needed to stay airborne.

And no amount of non-scientific drivel will change any of this.
Feb 28, 2020, 01:46 PM
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yes, but pushing less air, faster, is worse (as in, costs more power for a given force, which generally is worse)

the thing which causes this deflection, really is a lower pressure region above the wing. locally, this "bends" the air, while at the same time causing lift, by a pressure differential, between the upper and lower side of the wing. howmuch the air remains moving after this process, is a degree for the effectiveness of the arrangement of wing. the most clean form of lift can look like the pic of MontagDP, although, in practice, such wing will have more profile drag.
Last edited by m4rc3l; Feb 28, 2020 at 06:04 PM.
Feb 28, 2020, 03:10 PM
2 infect U it 1st has 2 find U
Miami Mike's Avatar
Quote:
Originally Posted by Tim Green
Wow - what gibberish, posing as science.
Well then, how about a simple answer to the question you posed in your thread title?

Q: "Should NASA be teaching incorrect version of lift to our kids?"

A: "No, nobody should be teaching anything incorrect to anybody."

Are you happy now?
Latest blog entry: A short message...
Feb 28, 2020, 03:30 PM
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NASA
Not
A
Straight
Answer
Feb 28, 2020, 03:58 PM
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the lost "cause"


Quote:
Originally Posted by m4rc3l
the thing which causes this deflection, really is a lower pressure region above the wing. locally, this "bends" the air, while at the same time causing lift,
Superimposing this old and technically incorrect "Western-way-of-thinking" notion of "cause & effect" in this case while at the same time and in the same breath recognizing that the "bending" and the lift-deflection occur at the same time is unnecessarily confusing and misleading, More simply stated (in the 2-D case)*:

As the air bends there is a deflection of the flow and a measurable force which we define and refer to as "lift" as well as another force we call "drag"


Some years ago this analogy of the meandering flow of a river came to me: simply cut the river flow along its curved middle, swap the halves and it is the same process as lift. The fast river flow at the outer bank edge of its curve is like a high pressure "sand blaster" and erodes the shore there, while the river bank inside of the curve gets material deposited as the slower flow there lets the stuff drop out, building up the inside curve of the bank.

Note that the river/bank gets "pushed" in the direction of the arrows, "up" toward the top of the sketch >






*in 3-D at the same time the flow also twists, etc., in a more complex fashion for finite-span wings
Last edited by xlcrlee; Feb 28, 2020 at 04:04 PM.
Feb 28, 2020, 04:18 PM
Registered User
yes. i don't think the process, can be adequately captured in words (now). i tried to tie apparently separate explanationary routes, to one and the same reason. so that one can get there, coming from any side. i do think, that this thread in the end will contain something, which noone can dismiss, and anyone reading it and having done some thinking about it, "should" agree on. i myself am deeply intrigued by "Lift" so to say, and want to know a short, clear answer, myself. i am not there yet. i do have a strong indication that viscosity, will be in the final explanation.
Feb 28, 2020, 04:42 PM
Launch the drones ...
Thread OP
Quote:
Originally Posted by Tim Green
Wow - what gibberish, posing as science.

Fact - nothing can suck itself up.
Fact - if it's heavier than air - and it's not throwing air down - it's not going to stay up.

Lift, or thrust, is purely a Newtonian reaction to the action of accelerating air in the opposite direction.

Again - nothing in the world, can lift itself - by sucking air (unless aided by a ceiling - but that's not flight, is it now? Not with a ceiling there doing the lifting). No amount of low pressure above a wing - can lift it up - not unless it's also accelerating air downward - enough to account for the force needed to stay airborne.

And no amount of non-scientific drivel will change any of this.
Made a picture … showing that nothing moves through the air by sucking (creating low pressure zones) … It's all Newton. If you guys knew what you were posting about - contraptions like this would move in ways they do not move in today's world.

Sucking on air doesn't do a thing to create thrust - IOW the low pressure doing the sucking - it has no thrust vector - as the U tube experiment demonstrates so ably.
Feb 28, 2020, 05:06 PM
Registered User
yes that is not wrong, but, the sucking in regards to the upper side of the wing, is not a static effect, however has to do with inertia, mass, acceleration. more like, a long straight tube, with a fan sucking on it from standstill. it would be (near) equivalent if the fan sucks or blows air. (until a steady situation is achieved)


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