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Old Nov 14, 2011, 09:39 PM
B for Bruce
BMatthews's Avatar
The 'Wack, BC, Canada
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Quote:
Originally Posted by Tim Green View Post
Wrong, and the experiment shows it to be wrong. Look at the chopper with the plate not folded (no ground), and the plate folded (has ground). One way the chopper lifts, the other way it doesn't.

This clearly proves that the plate is different from the ground.

Until you admit what the experiment shows, you are blowing in the wind.
Tim, the folded vs non folded test results is exactly what I've been saying.

With the plate flat and taped to the legs it is like you just tied the heli down to the ground. With it folded and slotted down between the boxes the boxes are now like the ground and it lifts normally. WHich is exactly what I've typed out all along.

If instead of taping the legs to the plate if he used a little fishing scale to hold the heli to the flat plate you'd see all the lift registering on the scale between the heli and the plate. There is still lift from the rotor. It's just being counteracted by the downwash building up pressure against the plate. The tension measured by a fishing scale between the two would show this.

THe only reason he did the folded plate between the two boxes was to show that the heli could lift the weight of the plate. Otherwise someone would have just said it was too heavy to lift. As it is the taking off with the folded plate is the same as taking off from solid ground.


At this point I give up. Either you're stubborn to the extreme or you are so focused on the trees that you can't see the forest. You also reply with phrases like the bolded part which clearly avoids any discussion whenever you seem to see something that you can't deal with. Yet when others do the same you cry "foul". Goodbye and good luck.
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Old Nov 14, 2011, 10:13 PM
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As an airfoil starts from rest, fluid particles adhere to the surface to create what is known as the no-slip condition. Because of the no slip condition and the viscosity of air, momentum (kinetic energy) is transferred from the airfoil to the surrounding flow field. As the airfoil begins to move at an angle of attack, fluid particles in the immediate vicinity of the airfoil move along with it. The particles on the bottom surface are essentially “pushed” forward into other air molecules causing an increase in pressure, while the molecules on the upper surface are “pulled” along creating a void where they once were and a decrease in pressure.

It is helpful to think in terms of the Ideal Gas law, PV=nRT. On the lower surface, molecules are pushed along so that more molecules (increasing n) occupy a fixed volume (fixed V) and accordingly Pressure must increase (increase P). Likewise, on the upper surface molecules are pulled away, hence less molecules (decreasing n) occupy a fixed volume (fixed V) and the pressure must decrease (decrease P).

Now here is where it get’s interesting: Molecules from both the upper and lower surfaces rush along and attempt to fill the void that was created in the wake of the upper surface. As the idiom states Nature Abhors a Vacuum. Molecules from the lower surface will attempt to move from the high pressure of the bottom surface to the low pressure of the upper surface by accelerating around the SHARP trailing edge. It would require an infinite velocity to make the U-turn around the trailing edge and as a result a vortex is created as the fluid attempts to go around the trailing edge. This is known as the starting vortex and it separates from the trailing edge as the airfoil moves away and eventually dissipates due to viscosity.

Kelvin’s circulation theorem states that the circulation around a closed curve moving with the fluid remains constant with time. In other words the circulation created by the starting vortex must be accompanied by an equal and opposite circulation around the airfoil itself. The system will reach equilibrium when the magnitude of the circulation around the airfoil is sufficient to move the rear stagnation point to the SHARP trailing edge. This is what is known as the KUTTA CONDITION, read it over thousands of times if you must, it is the TRUE foundation of modern aerodynamics.

An important property of Vortices is that they form lines called vortex lines, that must start at fluid boundaries or form closed loops. In the case of the finite wing, the vortex line creates a closed-loop comprised of the circulation around the finite wing connected through the tip vortices that span all the way back to the starting vortex. This is essentially just a large vortex ring. The Kutta-Joukowski lift theorem states that the lift created is directly proportional to the circulation. So say an airfoil increases it’s angle of attack, then more circulation is needed to move the rear stagnation point to the trailing edge, more circulation = more lift. An interesting side note here is that with a change in lift a starting vortex would be shed from the rear trailing edge that was equal in magnitude to the circulation required for the change in lift.

Bernoulli’s equation is a neat way of accounting for the conservation of energy. The drop in pressure requires an increase in velocity in order for energy to be conserved, nothing more, nothing less. A common technique for calculating lift is integrating pressure measurements over the upper and lower surfaces of the wing. This technique would not work if Bernoulli’s principle did not apply.

In terms of your helicopter plate experiment, aircraft fly because the wings possess surface area for the pressure to interact with. In your case the upper surface of the rotor creates a low pressure and the lower surface of the rotor creates a high pressure. By attaching the cardboard, the upper surface of the cardboard now interacts with the increased pressure generated on the lower surface of the rotor and produces a net downward force equal and opposite of the thrust being created by the rotor. As BMathews stated previously, if you were to measure the force in the landing gear it would be equal to the lift being created by the rotor.

I saw a similar situation in a cartoon where a character in a boat blew directly into his own sails to generate thrust, when in reality he would go nowhere, much like your helicopter with the cardboard.

Also Google Vortex Ring State in regards to downwash generated by a helicopter.

This link is also great reading.
http://www.df.uba.ar/users/sgil/phys...ewton_lift.pdf
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Old Nov 15, 2011, 06:51 AM
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Incidentally, for each blade of an helicopter rotor, the amount of upwash is the same as the downwash. The flow through the rotor is not the airflow the single blade sees. The rotor generates lift, that is used to push the column of air downwards. But you can't consider the rotor system as equivalent to a wing.
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Old Nov 15, 2011, 08:33 AM
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WOLFGANG LANGEWIESCHE had something to say about this, too...

Dear Tim Green:

The PIPE Here...and, from an era nearly seventy YEARS ago, someone named Wolfgang Langewiesche had something to say about this EXACT issue...

...as in his famous 1944-published volume, "Stick and Rudder, An Explanation of the Art of Flying", what should be the first subheading in the first chapter entitled "How a Wing is Flown"...but the words...

"FORGET BERNOULLI'S THEOREM"...

on page seven of the book.

That section is summed up by the simple words I've "lived my aeromodeling life by" ever since I got my own copy of it (open to those very words as I write this) waay back at the start of the 1980s, at the top of page 9 in that section...

"The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down."

This book even has its own Wikipedia entry at http://en.wikipedia.org/wiki/Stick_and_Rudder , and is still WIDELY available at airport bookshops all over North America.

Many libraries have this book too, as I understand it, so if you don't intend to purchase it, you can simply ask your local library if they've got a copy, if you don't already have an aeromodeling friend who has a copy. Also, a search here at RCGroups, specifically at http://www.rcgroups.com/forums/searc...y=Langewiesche using the author's family name, reveals SIXTY-SIX previous posts at RCGroups regarding one aspect or another of that book.

Give it a look, once you can get your hands on one...as my late (1925-1996) RC flight instructor Sammy Frey used to say, "it's the best single book to read on WHY your RC model behaves in the air in the ways that it does"...!

Yours Sincerely,

The PIPE....!!
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Old Nov 15, 2011, 10:05 AM
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the fact that it's writen by someone in a book does not make it true, I can publish a book that says the moon is made out of cheese, but that doesn't make it true either. As an example, I have seen this specific explanation for the dihedral effect in a few books:

but it is pretty obvious, if you apply a little math, that it's absolutely wrong.
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Old Nov 15, 2011, 11:16 AM
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Here's a streaming video (RealPlayer unfortunately) that illustrates the vorticity and circulation that Sefco so ably described. From about 3 minutes in.

In fact the page that this video comes from has a few topics that may be of general interest to our armchair proponents:

National Committee for Fluid Mechanics Films

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Old Nov 15, 2011, 11:19 AM
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Great example Brandano. Books do not have to be peer reviewed like journal articles.

One thing I have learned from this discussion is that the larger, bolder, and more colorful the text, the less I care about what I'm reading.

Sefco, thanks for the great explanation of a starting vortex, I agree that helps give understanding to how the steady state solution becomes the steady state solution.

Tim, the reason you will not be able to understand our explanations of downwash is because you say "stand behind a prop and feel the downwash". Let me make this clear. You are feeling "prop wash". This is not directly equal to the downwash produced by the props in any way. If you want to use the propeller for an example of downwash, you need to draw a diagram and show the local flow over a propeller section, because that is how you examine the downwash produced by it.

Bruce, I think you have clearly stated the flaw that I (and I think most of us) see in Tim's reasoning. Until Tim agrees that the flow leaving the helicopter is the same in the two cases (folded vs unfolded white plate), there is no way this discussion can proceed.
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Old Nov 15, 2011, 11:49 AM
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Quote:
Originally Posted by Brandano View Post
Incidentally, for each blade of an helicopter rotor, the amount of upwash is the same as the downwash. The flow through the rotor is not the airflow the single blade sees. The rotor generates lift, that is used to push the column of air downwards. But you can't consider the rotor system as equivalent to a wing.
Yes, that is what I was trying to explain on the other thread http://www.rcgroups.com/forums/showp...&postcount=297
Quote:
You can't do an analogy between the outflow of a prop and the downwash of a wing.

If you consider the reference volume as a cilinder whose horizontal section is the rotor, of course there is a downwash, but again, it's been said many many times, if we take an enough large reference volume there is obviously no net downwash (if you fly your model heli inside the room, the air wouldn't get floored so that you'd suffocate LOL).


As for "The main fact of all heavier-than-air flight is this: the wing keep the airplane up by pushing the air down." , I perfectly agree with and I highly doubt this will ever be proved wrong.
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Old Nov 15, 2011, 12:51 PM
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The fact that so many 3D model airplanes fly quite well with absolutely flat plate wings (and with unrounded leading and trailing edges) amply demonstrates that carefully- (or even casually) designed airfoils are not absolutely necessary for adequate performance. "Real" airfoils give different advantages, depending on flight profile mission and desired efficiencies.

Emotional energy expended on explanations of how and why are of little use.

Jim R.
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Old Nov 15, 2011, 04:18 PM
greg
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what's the difference between lift and thrust?
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Old Nov 15, 2011, 06:54 PM
B for Bruce
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The 'Wack, BC, Canada
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Quote:
Originally Posted by JRuggiero View Post
The fact that so many 3D model airplanes fly quite well with absolutely flat plate wings (and with unrounded leading and trailing edges) amply demonstrates that carefully- (or even casually) designed airfoils are not absolutely necessary for adequate performance. "Real" airfoils give different advantages, depending on flight profile mission and desired efficiencies.

Emotional energy expended on explanations of how and why are of little use.

Jim R.
Jim, while I agree with you on the face of it one does not need to stray far to see examples where a proper airfoil design optimized for model size use does pay off. Look at competition RC sailplanes. Then look at what competition FAI class free flight gliders use. Both being highly tuned to work the best with the flight requirements. And two prime examples where a flat plate would easily lose one the competition.

Even the case of your flat plate is more of a specialty airfoil than you're giving it credit for. In this case it uses the high drag produced with moderate angle of attack changes to stop the plane in mid air in a manner desireable to this peculiar style of flying. Yet that same high drag for modest angle of attack changes would be seen as a harsh disadvantage in many other styles of flying.

So perhaps there actually is something to be said for choosing the shapes in at least a broader sense than you're giving it credit.
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Old Nov 15, 2011, 09:20 PM
Grad student in aeronautics
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Quote:
Originally Posted by ciurpita View Post
what's the difference between lift and thrust?
Lift is exactly as I defined a post or two ago.

Thrust is the net force produced by a propulsion system. Rockets, propellers, turbo jets, rotors,.... all produce thrust in different (some are similar) ways and with different (some are similar) effects on the aircraft they are placed. Thrust should never be confused with lift. It is completely true that a propeller works the same way as a wing -- modern props are swept the same way modern wings are. However, thrust and lift are both vectors that are defined in completely different ways and are NOT interchangeable.
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Old Nov 15, 2011, 10:46 PM
t=1007606
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Quote:
Originally Posted by Tim Green View Post
Here's my contradictions you speak of ...

An object cannot have lift without throwing air down.
The direction of the thrust vector (which can be lift, if the thrust is down) follows (opposes) the final output direction of the air being ejected by an object.


Now - try to prove either statement wrong.

NOTE: Here's the link to the helicopter experiment, showing Bernoulli cannot lift a copter alone ...

http://rcuvideos.com/video/BladeCPBootStrap-wmv
interesting Video.
I wonder if the blades flex just the same amount, or not, in both experiments ?
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Old Nov 16, 2011, 01:24 PM
An itch?. Scratch build.
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Quote:
Originally Posted by ciurpita View Post
what's the difference between lift and thrust?
Isn't there an old saying, I think it may have been Mark Twain's 2nd cousin twice removed, Splitin Twain -

"In Lift We Thrust"
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Old Nov 16, 2011, 10:54 PM
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Here is a question for the OP if he comes back. If lift is due to air pushed down, how does a biplane work? If a plane weighs too much for it's wing area and you add a second wind and it flys, how is the bottom wing not blocking the downward air from the top wing for a net lift gain of zero?

Note: lift gain over having just one wing.

Oh, and don't try to try to tell me it depends on where the two wings are in relation to each other. Look at examples of the Beach Stagger wing to the many WW1 and pre WW2 biplanes. The wings are all over the place with basicly the same results.
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