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Reynald's number

Old 10-30-2002, 12:33 AM
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lennyk
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Default Reynald's number

Can anyone give a layman's explaination of Reynald's number
especially with respect to the relation with bigger planes
flying better than smaller planes of equivalent wing loading.

Thanks,

L
Old 10-30-2002, 01:23 AM
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JohnW
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Default Reynolds #s

A Reynolds # describes the relations between interial forces and viscous forces. Thes # are used to descrbe any flow with relation to an object. With airfoils we are concerned with the size of an airfoil the velocity of the airfoil and viscosity of the medium in which it travels.

Think of a reynolds # as an efficiency factor. Since we really can't change the medium we fly in, we really only need to be concerned with airfoil size and velocity. This is not exactly correct in that our medium (air) does change with temp, humidity, altitude, etc.

As to velocity and size, fast speeds and big wings are more efficient. Because the medium (air) is still the same viscosity for both, the size and speed matter. Depending upon how fast something moves and how big it is the air "feels" different to the airfoil. Again not entirely exact, but layman's terms.
Old 10-30-2002, 03:30 AM
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Default Reynald's number

MonkeyBoy summed it up; I'll add this stuff:

A simple rule-of-thumb equation for Reynolds Number (Re) which includes a factor to eliminate viscosity and get the units compatible is Re = C x V x 780 where:
C = wing chord in inches
V = airspeed in miles per hour

To see the effect of Re on airfoil efficiency, consider the chart below of the SD7037 airfoil, a popular sailplane airfoil, at various Re. Note how the drag (Cd) increases as Re decreases at any given lift coefficient (Cl). This is especially apparent at Re below 100,000 due to formation of a thin "separation bubble(s)." Drag due to separation (as opposed to "skin friction" drag) is the type of high drag visually evident as vortices in the wake of a boat having a stubby back end.

At higher Re, flow transitions from a laminar to a turbulent state, and is less prone to separation, thus some models employ turbulators in the form of trip strips or wires strategically placed on the wing to induce turbulent flow, but at the (lesser) penalty of some increased skin friction drag. Some airfoils are amenable to turbulation, e.g. Eppler 214 and 387, some aren't (like many Selig airfoils, the SD7037 was specifically designed for low-Re flight, so it isn't responsive to turbulation). This type of surface "disruption" to reduce drag might go against one's intuition, thinking that a glossy-smooth surface will necessarily be better - Not so. Case in point are the dimples on a golf ball specifically intended to turbulate the flow around it, resulting in delayed separation, reduced wake size, bottom-line reduced drag.

Hope that gives some practical feel for Re.
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Old 10-30-2002, 03:48 AM
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Default Reynald's number

lennyk,

I'll just add a couple comments. As MonkeyBoy said, the Reynold's number can be thought of as the ratio of inertial forces to viscous forces. Rather than thinking of the Reynold's number as a real precise measure, though, it is useful to think of it as a rough indicator of what kind of flow you have. I like to think of the Reynold's number, intuitively, as a measure of how 'energetic' the flow is. This is not at all precise, and it is not an analogy that works in all circumstances, but for aircraft, it has some value. One of the big reasons we consider Reynold's number is that the lower Reynold's number that applies to our models implies that the boundary layer ( the air that is right next to the surface ) is not as energetic, and is more likely to separate ( the flow phenomenon that causes stall, speaking roughly ) than the energetic flow that we have at higher Reynold's numbers. The news is not all bad though. The less energetic flow also is more likely to be laminar, rather than turbulent, which means that we have a little better chance than the full-scale guys to get the advantage of lower drag due to laminar flow. We don't take advantage of it very often, but it's always nice to know we could.

Maybe this helps, or maybe it sounds as incoherent to you as it does to me now that I read it.

banktoturn
Old 10-30-2002, 04:10 PM
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Default Reynald's number

lennyk,

I've thought a bit more about this, and I'm not too happy with my explanation of Reynold's number. The problem is that, unlike some of the results from fluid mechanics and aerodynamics, the Reynold's number is kind of a complicated concept. The bottom line is that the Reynold's number tells you whether you can expect similar behavior from aircraft ( or other objects affected by airflow ) of different sizes. If I want to compare my model airplane wing to a full scale airplane wing, and it turns out the the Reynold's numbers at which they operate are very different, then the comparison is suspect. Most of the references to Reynold's number are extremely simplified, usually as generalized design rules. For example, "laminar separation is more likely at low Reynold's number". You can kind of explain this in terms of the "energetic flow" or "inertial forces vs. viscous forces" ideas, but what it comes down to in most cases is trusting these design rules to be correct, and using them without necessarily having a complete understanding. I have quite a bit of background in fluid mechanics and aerodynamics, but I still find myself looking up "design rules" and other factoids in my old books, and just trusting them, even when I can't necessarily come up with a theoretical derivation of them.

To get back to your original question, I full scale wings fly "better" than small scale ones largely because, at high Reynold's number, the flow on the upper surface has a stronger tendency to stay "attached" to the surface, so that the wing has less tendency to stall. Now I am going to get a little bit wordy, so you can skip to the end if that bothers you. The reason has to do with what an aerodynamicist would call the "adverse pressure gradient". When a wing generates lift, it is because the pressure on the top is lower than on the bottom. If you look at the pressure on the upper surface of the wing ( or airfoil ), you will see that it reaches its lowest value at a point fairly near the leading edge. That means that from that point back, the pressure is increasing. This is kind of unnatural, since air wants to flow from high pressure to low pressure, and we are asking it to go from low pressure to high pressure ( hence, the name "adverse pressure gradient" ). Well, the air can do this, to a limited extent. If you make the low pressure peak at the front of the wing too extreme, then it can't make it, and you have stall. If the air is more "energetic", or if it is turbulent, which is related, then it is better able to make it through the adverse pressure gradient.

I personally don't like the often heard assertion that larger wings "fly better", because I think it oversimplifies the issue. It is definitely true that there is much more high quality data for higher Reynolds numbers though, so it is easier to design for.

banktoturn
Old 10-30-2002, 08:13 PM
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Default Reynald's number

Banktoturn makes good points re Reynolds Number. It's a concept that's tempting to trivialize because the equation for Re itself is so simple, however Re is simply a rough predictor of various flow phenomena (transition, separation, laminar vs. turbulent, etc.) that are difficult to accurately assess.

Performance comparison between aircraft (or boats, rockets, etc.) of differing scale is based on a general concept that goes by several names: Dynamic Similitude, Dimensional Analysis, Similarity, etc. A specific method to accurately compare things of differing scale is the Buckingham Pi method, which is a step-by-step procedure involving simple math but, because it's been too long since my school daze, I can't remember how to do it

Here's a good plain-language non-mathematical article on Reynolds Number; gets into some pressure distribution concepts:
http://members.ozemail.com.au/~rcmn/...s/modeldyn.htm
Old 11-01-2002, 05:22 AM
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Default Yikes...

I think what we have discovered is that it is impossible to explain quantum physics in layman terms... i.e., complex issues lose specific data critical to their explanation when described in laymans terms. There are some very good and correct definitions and such for Reynolds but some started to get complex... not laymans terms IMO.

I'll throw this out as a laymans analogy of the "size factor", i.e. larger Reynolds number. This is just an analogy and like any analogy it has flaws. It really has nothing to do with flow. It merely simplifies how scale is not a linear function. This really is not what LennyK asked for but I think it does help partly explain why Bigger is Better.

Consider a helium balloon that has a 1 foot diameter. Lets say it can lift 1 lbs. This would be a balloon loading (wing loading) of 1 pound per foot. How much then could an identical shaped balloon lift if it had a 2 foot diameter. 2 pounds? Nope. The volume did not increase in a linear fashion. The new volume is 4/3*pi*r^3 which is actually eight (8) times larger. The 2 foot balloon could lift 8 pounds, or a balloon loading of 4 pounds per foot. While this has nothing to do with inertia vs. viscous forces (reynolds #), and my math isn't exactly correct, it is a good "laymans" example that shows how scale effects are not linear.
Old 11-04-2002, 01:02 PM
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Default Reynald's number

Lenny was after a "layman" description of why larger models fly better than small models if the wing loading of each is the same.
Years ago, whilst teaching an adult class , I found that "layman" information must start from the layman's base of information -then work UP to the information being presented.
In this case - formulas and charts are a considerable step UP from a layman approach to grasping a concept.
My own answer to this question about Reynolds Numbers goes like this:
Air is a sea of unconnected , heavy molecules -all spaced evenly and of the same size.
When you move something thru it, the molecules must move out of the way.
If you try to move rapidly, some molecules get suddenly pressed more closely together, as they are pushed out of the way.
If you move slowly OR the moving object is very small, the molecules move, away or around , easily.
Coversely, big objects ,moving thru the air , trap the molecules more easily , as they can't quickly move around the object .
That creates uneven distribution of the molecules -which IS pressure difference - which is ( ta-da) LIFT.
The SIZE of the moving object , compared to the SIZE of the molecules of air, is the important point here.
So- size really matters-in this case.
Now add the speed of the moving object to it's size and you get part of a "formula" for figuring how that moving object will compress or disturb the air ( also part of the formula you call Reynolds Number ).
In our applications - we find that as the model gets smaller - it gets less efficient -at an alarming rate.
All because of it slips thru the sea of molecules more easily.
Old 11-04-2002, 03:02 PM
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Default Reynald's number

Here is a simple but partial explaination that I prefer. The energy of the airflow is proportional to the square of the air speed. If the air speed doubles the energy is four times greater. If the airspeed tripples, the energy is nine times greater, etc. Therefore the energy in the flow is much, much smaller at model speeds. In addition, the energy is proportional to the volume of air involved at standard conditions. Model flight involves a smaller volume of air than full scale. Air clings to the surface of the object passing through it. Therefore, a boundary layer is formed between the air clinging to the surface and the air passing by the surface. The more energy there is in the flow the easer the air is brought up to speed as one traverses the boundary layer and therefore the thinner the boundary layer. So large fast objects have thin boundary layers and small, slow objects have thicker boundary layers. The boundary layer adds a virtual thickness to the object. If the boundary layer is laminar then the boundary layer is thinnest. When the boundary layer becomes turbulent it thickens and when the boundary layer seperates it becomes thickest. The drag is related to the object dimensions plus the boundary layer thickness. The lift is related to the angle of attack. Since the energy available at model sizes and speeds is less than at full scale, the energy available to keep the flow from seperating is lower for an airfoil operating at model sizes and speeds. This means that an airfoil operating at full scale will stall at a higher angle of attack than the same airfoil operating as a model. Because the airfoil operating as a model stalls sooner, it can not generate as high a lift as it could have under full scale scale conditions.

Reynolds number is just a convenient way of comparing flow conditions. When the reynolds numbers are the same, the flow conditions will be similar. The larger the difference in reynolds number between two flow situations, the greater the difference will be in the flow. Full scale is associated with high reynolds numbers and models are associated with small reynolds numbers. Insects are associated with smaller reynolds numbers than models. The low reynolds numbers associated with insects result in such poor airfoil performance that insects couldn't fly if they relied on the same flow mechanisms as models, most birds and full scale. So, insects use a clap-twist- snap mode of flapping that produces a stronger partial vacuum above their wings and a more powerful down wash under their wings. This mode is only possible in small sizes because of structural advantages at small sizes.
Old 11-04-2002, 03:55 PM
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Default Reynald's number

Or -Ollie - differences in pressure is the name of the game -right?
I agree with your explanation -of course but - I find that it is - for the layman - information which does not link easily to "common knowledge."
How airfoils work -in small models is typically pi-- poor. Most may as well be simple small stiff slabs of whatever.
We are a hell of a lot closer to the bumble bee approach- .
(I liked that explanation - even a child can grasp that easily)
Old 11-04-2002, 04:52 PM
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Default Reynald's number

As I mentioned before, I think that Reynold's number is truly difficult to explain in layman's terms. I think the closest we have been is probably MonkeyBoy's explanation that it is the ratio of inertial forces to viscous forces. When air is in motion, there are two "forces" that influence it's behavior: the inertial force ( in casual conversation, we say 'momentum' ) makes it want to keep going, and the viscous force ( friction ) makes it want to stop. When the Reynold's number is higher, the inertial force is more likely to win ( the flow is more energetic ), when the Reynold's number is lower, the viscous force is more likely to win ( friction will stop the fluid, stalling the wing ). To me, that is about as intuitive as it can be without inaccuracies, and even this explanation is not complete. I don't really think that using molecules and pressure give the right picture, and size of the body relative to the size of the molecules is not really relevant for Reynold's number ( but it is for another number, the Knudsen number ).

I really don't think it's the case that our small wings are ****-poor. The lower Reynold's number gives somewhat lower CLmax, but good airfoils work much better than flat plates, and in the same manner as full scale wings, apart from minor differences. We are seldom limited by CLmax, and if chose to make lemonade from the lemons we've been given, we might even be able to get some laminar drag reductions that are unavailable to full scale aircraft.

banktoturn
Old 11-04-2002, 05:03 PM
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Default Reynald's number

Dick,

We have a real, unresolved dilemma on our hands.

How much over simplification is appropriate for the layman?

For years, elementary texts explained lift by just refering to Bernouli's principal and the pressure differences it produces. This was such an oversimplification that it borders on a lie. See:
http://www.monmouth.com/~jsd/how/htm...ther-fallacies
and paragraph 3.14 in particular.

This oversimplification resulted in lenthy arguments about which of the four or five explainations of lift was correct when, in reality, there is one mechanism for aircraft that requires four consistent but different theories to fully explain lift.

Sometimes I think it is better to challenge the understanding of the layman with a relatively full explaination. In this way the people who are motivated to learn will inquire further and get the tools necessary to understanding and the rest will fall by the wayside when they run out of motivation to learn more.

I remember clearly asking my father how airplanes could fly when I was about seven years old (65 years ago). My quest is life long and I am still learning from people like Dr. Mark Drela.
Old 11-04-2002, 05:21 PM
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Default Reynald's number

There is no such thing as oversimplification .
That is the simple answer.
The important part is to make sure the path from an unknown to a known , has no holes to be lept over.
The textbook approachs to "why things fly" was always interesting to me - but it became apparant that the jargon used tended to isolate the information- much like trying to figure out what a heart doctor means when he says a "probable myocardial infarction"--
Once I figured out the the whole ball game was about the same as sticking your hand in a fast moving stream -then trying different finger positions to see what happened - I caught on to what the "theory" stuf -tho correct in most cases but poorly related, --was all about.
Old 11-04-2002, 05:25 PM
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Default Reynald's number

PS- I have been playing with models since the late 40's so -like you , I also have had time to see the various "fallacies" in explaining why things fly.
Tho it is a poor analogy- try learning to play a violin from just reading a book on it .
Old 11-04-2002, 05:45 PM
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Default Reynald's number

OK, Dick. You simplify to your hearts content and I will try to fill in for those who want to know more. Between the two of us we should address a wider audience.
Old 11-04-2002, 05:49 PM
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Default Reynald's number

Sounds good -
I will toss the questions about conservation of angular momentum, your way -
Old 11-04-2002, 05:49 PM
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Default Reynald's number

You know guys i just took over moderating this part of Rcu and you guys are really over my head....

but i love reading your stuff....hopefully i will learn some things from you guys....

I always figured if it didn't fly well just add A BIGGER MOTOR!!...lol

but i guess there are other variables....
Old 11-04-2002, 06:17 PM
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Default Reynald's number

Dick & Ollie,

I think there is a different answer. Trying to reduce every topic to a bunch of small enough bites for the layman is assuming that the layman wants to, and is willing to, become an expert. In many cases, the layman wants enough information, and/or enough intuition, to make a decision. My own belief is that you will have more luck giving a useful answer by finding out what decision the asker wants to make. Is he looking for a way to reduce drag? Get more lift? Determine CG placement? Decide whether to build a bigger plane? Whatever it is, it would probably be enough to allow an "expert" to give a concise, helpful answer. Unless you have a ton of time on your hands, you are not going to be able to provide the hole-free path that you mention, but I agree on that philosophy if you are in a bona fide teaching situation, with enough time available for the task.

In the situation at hand, I think it is reasonable to expect to give lennyk some intuition about why planes fly better at higher Reynold's number, but not to give him a very complete characterization of all the implications of Reynold's number.

lennyk,

Has any of this been helpful to you?

My 2 cents, no offense intended,

banktoturn
Old 11-05-2002, 12:43 AM
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Default Reynald's number

Just an interesting fact...

The sweep of the wings of the F-117 was based on a paper airplane. The prototype, have blue, had a sweep angle of about 72 degrees. It was decided to decrease the sweep and the designers looked towards the great paper airplane. When paper is folded twice, the resulting angle is 67.5 degrees. It was then rounded to 67 degrees. So, I guess there is a place for simplification....

But then getting it to fly and maintain it's stealthiness wasn't quite that simple. I don't think a paper airplane could do much here.

There is another thread on the Spitfire wing I've run into. The funny thing here is that the wing was mainly designed that way to fit the guns inside of the wing outside of the prop arc.

Angular momentum......now that's dynamics, not aerodynamics. I think we might have to start a new forum.

T28, dude you nailed it. More power. A barn door will fly (and loop) if it has enough power. Who cares about efficient lifting surfaces. I've seen it done. It just didn't fly all that well.

I'm bored, just having fun, no offense intended. I'm a fence sitter I guess.
Old 11-05-2002, 12:59 AM
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Default Reynald's number

You noticed the angular momentum bit-
actually -in a controlled spin- (spins can be controlled ) you can change the AOA of the spinning plane and increase the -OR decrease the speed of rotation--
just like a ice skater -
Just a passing thought --
but it is aerodynamics --
Old 11-05-2002, 02:08 AM
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Default Angular momentum

Think some good points were touched on about explaining anything in "layman's" terms. I always feel something is lost with any analogy, but analogies and "simplification" make things easier to understand to those that that don't have the proper background to understand the full equation. There is nothing wrong with simplification in my opinion as long as what is lost isn't significant, for example Newtons Laws are just fine for non-relativistic speeds. The problems occur when analogies or incomplete formulas are used to make new conclusions.

As for Reynolds numbers, the inertial vs viscous forces IS the actual definition of what a Reynolds number describes... it just isn't intuitively obvious to most what the heck that means. I think Banktoturn did a good job of explaining that better.

As for angular momentum, it must be conserved just like linear momentum. For a point mass, the area of an arc that a mass sweeps across in a interval of time must always be equal. Same basic rules apply to non-point mass but a little harder to explain. Easiest way to experience this is sit in a swivel chair and spin yourself up. Move your arms and legs in and out. If you ignore friction your rotational speed changes... that is conversation of angular momentum.
Old 11-05-2002, 02:42 AM
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'but it is aerodynamics --'

I stand corrected if you consider flight dynamics part of aerodynamics. Some people combine the two and others don't.
Old 11-05-2002, 09:30 PM
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Default Reynald's number

A comment:
I've been reading most if not all of the threads within "aerodynamics" since I found this forum...
My point is: Many read these threads. Not just the initial author. Limiting the discussion means limiting the audience, doesn't it?
I would find it wonderful to always have at least two opinions and at least two perpectives: the simpler and the more elaborated ones, in this case... It's great to be able to tell my wife what I have learned and not seem a weirdo (a psychologist, you know, NACA and bananas are no different to her...), but personally, the more the better -- I'm still motivated!
Thanks. Souza.
Old 11-05-2002, 11:23 PM
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Default Reynald's number

I know my explanations grate on those who like the text book language and the use of charts and graphs and the use of engineering jargon.
But - I have found out - the hard way - that explaining technical points can be a real exercize. The proof in an explanation is that the person to whom the explaination is proffered -- does understand .
At least well enough to feel their question was answered.
To clarify:
If you can't explain it in layman terms - y-you really do not thoroughly understand it.
Old 11-06-2002, 01:11 AM
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Default Reynald's number

i have followed your thread. do not understand most it. pretty new to this kind of stuff. question does the numbers of a airfoil tell you the renalds no. if so what can you tell me about airfoil nacan63. it is for a 80 in. aerocommander that i will be building this winter. thank you fxd.

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