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Old Mar 06, 2012, 05:11 PM
Launch the drones ...
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Quote:
Originally Posted by Steve Anderson View Post
Yes, quote below is from page two of your link... what am I taking out of context?
He wrote a whole book about how it's Newton.
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Old Mar 06, 2012, 05:29 PM
Launch the drones ...
Ashtabula, OH USA
Joined May 1999
3,122 Posts
Remember fellas - Feel the Physics.

But you aren't. Oh well.

So, after reading all of your wondrous and wordy posts, I wonder where all that air comes from that I feel when I stand behind a prop?

Because according to you guys, downwash or propwash, is an accidental byproduct of Bernoulli lift. It's incidental, and irrelevant, according to many of you posters.

And even more magical, nothing physical reacts to the action of having produced all that downwash or propblast.

It's just there.

But don't tell Newton that - he died thinking there's an equal and opposing reaction to every action.

Goodness - without you guys, I would not have had the pleasure of reading so many cockamany reasons why lift isn't Newtonian. Geeze - where do some of you come up with this stuff?

Feel the Lift. I've lost pleasure in this - some of you are plain mean. This thread is closed for me now. Enjoyed most of the posts, though. Had some fun. Changed no minds.
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Old Mar 07, 2012, 12:50 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
Socorro, NM
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Amen, at last!

Tom
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Old Mar 07, 2012, 03:00 PM
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i agree that tim changed few minds. mainly because most of the silent majority already had faith in newtonian physics. i think most of the vocal opponents are just disgruntled former equal transit theorists trying to dress up that old belief system so it dont look quite so ridiculous.
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Old Mar 07, 2012, 03:43 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
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Joined Jul 2004
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Naw - it really is little guys in football helmets throwing molecules off of the TE to push down on the other molecules.

The Air Force doesn't lie!

Tom
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Old Mar 07, 2012, 06:17 PM
B for Bruce
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Dave, I never was either way. And I moderated a couple of similarly heated debates on this topic over at one of the other RC forums where I learned a lot from folks that were actually educated in aerodynamics and worked in research departments at a couple of the big airplane makers.

There's a good reason why there's we have a lot of highly theoretical concepts about lift such as the Prandtl lifting line and the circular airflow ideas. It's because the stuff happening around a wing IS complex. There's no doubt that Newton plays a part in all this. But what sets up the stage for Newton to have a role? How does the Newtonian defelction of the air come about? There's a lot going on around a wing's airfoil that comes together to push and pull the air. And the very acceleration of the air is what produces the other factor that cause the push and pull.

Many years ago NACA set up wings with pressure sensing manometers and showed that the pressure distribution and magnitude around the airfoil all added up to show that the pressure difference was equal to the lift. Years later some group apparently showed that the displacement of the air (Newton and downwash) was also able to account for the lift at the wing.

But it's not like one proved the other wrong. Instead both are happening at the same time. So what does this mean? It shows that however you measure it, pressure difference or air momentum, that they are different faces of the same lifting process. This implies that each depends on the other to exist and that they are tied very much together.

Note that at no time have I suggested the "equal transit time" nonsense. And in fact the reality is that the lift would not be as great as it is if the air really did produce equal transit times since the speed of the air over the upper surface would actually be LESS than real life.

It seems like folks just have to have it all black or white. It has to be all pressure and Bernoulli or all mass reaction and Newton. No one seems to like the idea that Bernoulli, Newton, Coanda and a few others are all on the team and working equally hard to produce our lift.

It's a good thing that Mother Nature doesn't believe the same as some of us. If she did then none of our airplanes would actually fly. Lift is a "team sport" and not a popularity contest.
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Old Mar 07, 2012, 06:33 PM
I bail out, anywhere, anytime
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In several posts people did write about Daniel Bernoulli
(born 1700, Groningen, the Netherlands)
One of his quotes and related to this thread was:

"...there is no philosophy which is not founded upon knowledge of the phenomena, but to get any profit from this knowledge it is absolutely necessary to be a mathematician "

"Essays in the History of Mathematics"
Daniel Bernoulli
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Old Mar 07, 2012, 06:35 PM
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Old Mar 07, 2012, 07:01 PM
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Very well put Bruce. Excellent post.

I may have come across as one of the "Bernoulli Only" holdouts in some of these discussions, my apologies. That is not the case, as I have also learned over the last few years that what is happening around a wing is complex, and a lot is going on. I have somewhat pushed the "Bernoulli" concept in this discussion, not so much because that is the only thing I think to be correct, but rather to try and help fellas like Dave and Tim understand that there is so much more going on that air being pushed, and that the downwash you can "feel" is simply the result of the work done by the wing seconds before.

Interesting fact, you can calculate lift by using either Bernoulli's equation OR Newton's, and arrive at the SAME result. One just calculates the pressure variations around an object using the local velocities (Bernoulli), and the other uses the velocity variations themselves. The neat thing is, by adding up the calculated pressure variations or by adding up the velocity variations, one can determine the total aerodynamic force on an object. The math doesn't lie. The math in fact proves both theories (when properly applied!!!) to be correct.

There are some well known theories that are incorrect, and very few have tried to spout them in this discussion. Here are the biggest ones.

Equal transit time-we all know now that this is bogus and why.
Skipping Stone theory-basically says that it is simply the impact and deflection of air on the bottom of the wing that lifts. This has been proven wrong by many.
Half Venturi Theory-clearly wrong because a wing is not a venturi nozzle, and as such works differently.

Now, I will admit, I have been around long enough to have run the gamut of what I believe to be the right theory. I was initially taught the equal transit time/venturi theory. Then I was taught that it is simply the downwash "pushing back up" (Tim's favorite). None of these really made complete sense to me. For example, I realized that the downwash is behind and below the wing, and is no longer touching the wing so how can it apply a force to the wing anymore. I saw wind tunnel testing with smoke pulses which showed the "top airflow" going way faster than "equal transit" would suggest. So, I went off to do some more learning, and have since learned about flow turning, the correct application of Bernoulli's theorem, circulation. Now I know where the pieces all fit, and the elements I was missing for so long.

Am I a disgruntled former equal transit guy desperately hanging on....no. I am happy to have opened my mind enough to see beyond that. I am happy to have opened my mind enough to accept that my original belief was flawed. I am happy to have opened my mind enough to LEARN.

Thank You
Ron.
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Old Mar 07, 2012, 07:14 PM
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beveled-leading-edge airfoil

I was thinking about this some more-- re the second figure in post 725 -- the one with the bevel oriented in the way that appears disposed to push air upwards--

Strictly speaking, if we draw the chord line as running from the sharp pointy leading-edge back to the middle of the thick trailing-edge, then when the flat top and bottom surfaces are horizontal, the angle-of-attack as indexed to the chord line is slightly negative, as was already pointed out. (Assuming a horizontal free stream or relative wind.) However, the actual flow around the airfoil may not care so much about our somewhat arbitrary definition of the chord line / angle-of-attack. I initially guessed that in this orientation, the airfoil would produce some downlift, due to the "wedging" action of the front of the airfoil deflecting air upwards. But could it be that the air initially forced upwards at the leading-edge would produce a vacuum above the upper flat surface so that the airfoil would actually produce net uplift? If no, then never mind the next question. But if yes, then is there still eventually a downwash somewhere behind the airfoil? Maybe the inability of the primitive trailing-edge shape to generate a downwash is part of what precludes this airfoil from generating upward lift in this orientation relative to the flow? Yet, reflexed airfoils generate uplift-- and a downwash-- even with their reflexed trailing edges.

I'm curious what other people think about whether this airfoil--the second figure in post 725-- might actually generate some uplift in the orientation where the top and bottom surfaces are completely parallel to the free stream or relative wind, due to the way the up-deflected air would curve over the flat top surface and possibly create suction and then possibly curve back down again in some sort of downwash.

Of course, if we just had the wedgy front edge with no flat surface behind, just an immediate truncation to a vertical trailing edge, so that the airfoil looked like a right triangle as viewed from the side, then it seems obvious that the airfoil would generate downlift not uplift when the flat bottom surface was parallel to the free stream or relative wind. But maybe adding the flat top and bottom surfaces behind the wedge would change that around so that the airfoil does generate uplift?

Has anybody seen actual results from anything like this explored with a wind tunnel or airfoil modeling program? Just curious....

Steve

Quote:
Originally Posted by aeronaut999 View Post
??

You think?

Even at the angle-of-attack shown in the picture?

Can you sketch the streamlines you think would go along with this?

Hang on, obviously you can't change the forces generated just by flipping the picture upside down. That doesn't make any sense at all. The forces have to just flip along with the rest of the picture.

A variation on this:

Start with foil #2. Assume for the sake of this discussion that we define angle-of-attack strictly in relation to the flat top and bottom surfaces-- arguably not completely rigorous (what is the chord line of this airfoil?) , but let's do it anyway.

What lift do we predict at zero AoA (angle-of-attack)? Positive lift (because it looks sort of like a right-side up airfoil, vaguely), or negative lift (because the beveled front edge is shoving air upward?)

How does the lift curve change with increasing positive angle-of-attack? If (if) the lift was negative at 0 a-o-a, at what point does the lift become zero and then positive?

How does the lift curve change with increasing negative a-o-a? If the lift was positive at 0 a-o-a, at what point does the lift become zero and then negative?

At whatever positive angle-of-attack the uplift force is largest, is this bigger or smaller than the downlift force at whatever negative angle-of-attack the downlift is largest?

How about L/D ratio? At what positive angle-of-attack will we get the highest ratio of (positive) lift to drag? At what negative angle-of-attack will we get the highest ratio of (negative) lift to drag? Which L/D ratio is higher, the positive one or the negative one?

These are all questions about whether the shape in figure 2 is acting more like a right-side-up airfoil shape, or more like something that is "shoving air upwards" with the front beveled edge.

This would be a really interesting experiment....

Steve
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Old Mar 07, 2012, 07:20 PM
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Quote:
Originally Posted by stardustertoo View Post
Am I a disgruntled former equal transit guy desperately hanging on....
For a second there I thought you were saying you were an angry bus driver, yikes...
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Old Mar 07, 2012, 07:24 PM
I don't want to "Switch Now"
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Toronto (Don Mills), Canada
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WAG #1 -

The section will have hysteresis in the CL versus Alpha curve as the "TE" moves back and forth between the upper and lower rear edge.

WAG #2-

There will be a range of angles where the section makes little or no lift as the "TE" bounces between the upper and lower rear edge. But outside that range it will have the normal slope of Cl versus Alpha.

I would lean toward WAG#1 at low Reynolds numbers, but #2 at higher Reynolds numbers.


Pat MacKenzie
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Old Mar 07, 2012, 07:57 PM
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regarding the "refinements" of my diagram in post 725 i might be in a position to actually test the hypothesis of stardust, aeronaut, et al that the second produces more lift. if this turns out to be true then i must rethink my model of flight.

i do own a wind tunnel as described in the kf science thread . however it would take some time to set up again so there might be an easier test. i also have a couple microgram scales left over from my attempts at sub-gram planes. this weekend when i go home i will set up the two "chiselfoils" next to each other and see how they behave outside in wind. it would be an excellent way to kill time and avoid family members if nothing else.

but remember, confuscious say: "man with open mind... sometimes end up with brain on the linoleum"
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Old Mar 07, 2012, 08:06 PM
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Dave, that would be neat if you did test it and let us know. I could be wrong with my hypothesis...I don't really know. I am just going with my gut feeling due to the fact that with it "right side up" in picture #2, it appears to have a really rough "chamber" like a normal airfoil. Of course that trailing edge could be the complete undoing of my theory I do suspect that with the bevel up, any sort of positive angle of attack, even very slight, will be when we really see the lift. With it upside down, I think it would require more AoA, but I do not know for sure.

Test it, lets find out
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Old Mar 08, 2012, 10:42 AM
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Quote:
Originally Posted by stardustertoo View Post
any sort of positive angle of attack, even very slight, will be when we really see the lift.
i agree. both would probably have more lift since more air would be deflected down by that bottom surface.
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