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Reynald's number

Old 01-15-2003, 10:58 PM
  #76  
Ollie
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Default Reynald's number

Bill,

There are many reynolds numbers associated with your B25. At a particular air speed the tip of the wing has a different reynolds number than the root of the wing. The engine nacelles have have a different reynolds number than the fuselage and the tail. As the air speed changes so do all those reynolds numbers. In a standard atmosphere reynolds number is equal to 68459 times the length in meters (of the flow over the surface), times the airspeed in meters per second.

In english units, reynolds number is 780 times the length in inches (of the flow over the surface), times the airspeed in miles per hour (at sea level). At 5,000 feet of altitude the constant is only 690 because of the less dense atmosphere.

The boundary layer thickness increase slowly with reynolds number decrease.
Therefore, your B25 fuselage will have a somewhat thinner boundary layer than your B25's wing tip at the same airspeed. Your B25's collection of boundary layers will be much thicker than a full scale B25 because of differences in both size and speed.
Old 01-15-2003, 11:56 PM
  #77  
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Default Reynald's number

He asked and you answered in a clear, concise manner -
I do have a question tho --
How relevant is boundry layer at the speeds this scale B25 will be seeing-wich I am guessing will not exceed 100 MPH.
On a model of this size -at what speed does the boundry layer start to make a significant change in how the model flys.?
At sea level. (STP)
Did I ask the question correctly?
Old 01-16-2003, 01:33 AM
  #78  
Ollie
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Default Reynald's number

Originally posted by dick Hanson
He asked and you answered in a clear, concise manner -
I do have a question tho --
How relevant is boundry layer at the speeds this scale B25 will be seeing-wich I am guessing will not exceed 100 MPH.

Revelant to what? If you mean relevant to your set of values about scale models, I would guess that reynolds number is not all that relevant. If Bill has a problem with too high a landing speed then maybe it is of some small relevance. If Bill wonders why he can't get the B25 model to fly at a scale speed, the high wing loading and a poor airfoil selection are at fault. The boundary layer has a significant affect on airfoil performance. So much so that it is best to select different airfoils for the tip and root of a wing with a lot of taper. Some airfoils for tails have a jog in the coefficient of lift versus angle of attack curve that results in symptoms of a one or two degree dead band in the control response which is caused by laminar seperation bubbles of the boundary layer shifting from one side to the other when the airfoil pases through zero angle of attack. Some people might think this significant, others not.

On a model of this size -at what speed does the boundry layer start to make a significant change in how the model flys.?

Its hard to say because various people will have various ideas of what is significant. Generally speaking, reynolds number effects change slowly with changes in reynolds number but, at model sizes and speeds they change faster than at full scale and the smaller and slower the model the more pronounced the changes with changes in reynolds number.

At sea level. (STP)
Did I ask the question correctly?
Old 01-16-2003, 02:42 AM
  #79  
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Default Reynolds #'s......

Ollie-Dick-BtoTurn..et-al.......I know Nick Ziroli Sr. was an Aernautical Engnr for Grumman for many years.....I have 3 of his planes in my inventory(F4U, P-47 and B-25) and they are excellent flyers.....His past experience enabled him to come up with the flt characteristics that his models have......and I have no idea what the airfoils are on his models,but they work...I'll ask him next time I see him......I know there are literally hundreds of airfoils to choose from but can you look at one and know which one it is?...I can tell that on these models the Reynolds # can change quickly just by the stall characteristics...the bottom falls out just by one or two knots of airspeed, where as on a full scale an "imminent" stall is easily identified and corrections can be made to avoid the "full" stall......Ollie...just on several simple calculations as a "rough" estimate looks like the B-25 @ 100mph(close to top speed) might have a reynolds # of 1,248,000...or 780x100(MPH)x16(inches ave chord length) ?As far as flaps are concerned...How much does the Kowanda Effect change as the degrees of flap are selected towards full deployment say 40*?...........Bill.....
Old 01-16-2003, 03:24 AM
  #80  
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Default Reynald's number

The flow over the top separates at the flap hinge line somewhere around five degrees of deflection, give or take a degree or two. By 40 degrees the flap is producing a lot of drag and the air flow directly behind the flap is detached and quite turbulent.

I didn't mean any criticism of Zeroli's designs. Boundary layer effects are mostly about drag and maximum lift coefficient. Most powered models solve any drag problems with more thrust and any coefficient of lift limitations by flying faster. With enough thrust and enough control response, most guys like the way a model flies. Drag reduction is only a high priority for pylon racers, endurance models, long range models or, sailplanes. High lift coefficients are high priorities for low landing and take off speeds, competition scale models that get points for scale flight speed, load lifters, thermal duration sailplanes and pylon racers in the turns.
Old 01-16-2003, 03:33 AM
  #81  
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Default Reynald's number

Kowanda- I think that is celebrated in December
Coanda however - is a sticky subject---
Old 01-16-2003, 03:40 AM
  #82  
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Default Flaps....

Dick....thanks for the correction...had a brain f.... and couldn't remember correct spelling......I guess its getting late....Bill.......
Old 01-16-2003, 04:38 AM
  #83  
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Default Reynald's number

Dick,

I would say there are two primary ways that the boundary layer becomes significant. When it transitions from laminar to turbulent, and when it separates. The transition to turbulent is important, because skin friction is higher for a turbulent boundary layer than a laminar one. Separation is important because, in the context of a wing, lift goes down and drag goes up if the boundary layer separates before the trailing edge. For low Reynold's numbers, transition to turbulence can happen further back on the airfoil, which is the good news. The bad news is that separation is more likely to happen before the leading edge. This is the bad part of the 'scale effect'.

banktoturn
Old 01-16-2003, 01:46 PM
  #84  
rmh
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Default Reynald's number

The small , slow stuff still appears to be in an area of darkness - as all the explanations - which work for big stuff seem to deteriorate into " maybe and I don't know" tag lines.
I also don't know but so far -I see that most of the rules just don't apply to very slow - low wing loading airframe.
OR no one can explain them.
I am playing with some stuff that flys an 10-15 MPH has wing loadings of only a few (2-4) oz per sq ft.
As far as I can see - the only full scale comparison -may be a parachute.
I don't see that airfoils are of any use on these - simple flat plates work just fine - C/G location has to somewhere close not very tho -
Old 01-16-2003, 06:06 PM
  #85  
Ben Lanterman
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Default Reynald's number

Dick, the thing with out aero knowledge data base is that money drives the results for the most part. We needed big airplanes for commerce so the work was done on big airplane aero. We needed fighter airplanes so a lot of military money was spent. We needed good model airplane glider airfoils but the military doesn't so luckily a few die-hard modelers have done some amazing work. We need little spy airplanes so people are studying the mechanism of the clap-fling lift. The thing is that we tend to look at these things as four separate areas of aero. In reality they are all just parts of aero as a whole. There are areas that we would consider transitional from one to the other but they are not that interesting so don't get the money to study.

An integrated aero set of equations would handle all of them at the expense of being very hard to work with. Certainly real life doesn't have definite cut off points but rather has smooth graduations from one thing to another.

--------------- I am playing with some stuff that flys an 10-15 MPH has wing loadings of only a few (2-4) oz per sq ft.
As far as I can see - the only full scale comparison -may be a parachute.
I don't see that airfoils are of any use on these - simple flat plates work just fine - C/G location has to somewhere close not very tho --------------

I started with the slow-flight stuff myself a couple of years ago. I have a modified Steubenflege that flies nicely. I did add some airfloil sections across the otherwise flat wing. I think it flies better but don't know for sure. It would take a lot of testing with the techniques we have available to determine the differences. The same batteries giving flight times that have to be repeated many times to get a reasonablely good average. Usually the increase in wing efficiency is not good enough to make the work worth it.

I look to nature for the necissity of having an airfoil at these speeds/wingloadings. Albatross have great airfoil sections. A maple leaf has a nice curve while being relatively flat. The finches at my feeders have nice airfoils. The little flies that bother me have flat wings. A hummingbird's wing although working both directions is nicely flexible and has a round leading edge and curve to it. Butterflies bigger than hummingbirds can have flat wings. It would indicate that that size and weight is in the transistion area.

It suggests that anything bigger than your hand would benefit from an airfoil section. It is just that with rubber power or battery power variations that we now have the diifferences between airfoil and flat are masked.

I would also look to indoor microfilm airplanes for guidance. The guys with the patience and that win usually have an airfoiled single surface wing. I am sure they have tried flat and gone away from it.

It is pretty interesting isn't it. I would have loved to work in the insect flight studies but making a living always seemed to get in the way.
Old 01-16-2003, 06:16 PM
  #86  
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Default Reynald's number

Originally posted by Ben Lanterman

It suggests that anything bigger than your hand would benefit from an airfoil section. It is just that with rubber power or battery power variations that we now have the diifferences between airfoil and flat are masked.
[/B]
Ben,

I don't know if I quite agree. It seems to me that the data points you mention suggest that a thin, flat wing is well suited for the lift/thrust mechanism used by insects ( clap/fling, or something like that ), but not for the mechanisms used by birds and human-build aircraft. I don't think that size is the most important factor.

banktoturn
Old 01-17-2003, 08:14 PM
  #87  
Ikaros
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Default Reynald's number

For what you guys might think it could be worth, 2c perhaps

Re =Rho * v * l/ Ny

With units : Re [1]= Density [kg/m3] * speed [m/s] * length [m] / Kinematic viscosity [m/s]

The formula reveals the secrets you are discussing.

Like already said, it expresses the relationship between the momentum-forces and the viscous forces.

However, it also expresses the difference in having different 'l' ; the charecteristic length ( or diameter in case of flow in a tube ).

Like someone already said, it works like a scale-factor for compairing different sizes of objects sorrounded by a moving fluidum.

It also works as a meter in other ways. (like it also explains why flow charecteristics vary depending on air pressure (altitude))

Now, Mr. Dick, flame me please but don't expect me to be nice in return.

Cheers,
Ikaros
Old 01-17-2003, 08:36 PM
  #88  
rmh
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Flame you ?
Why?
You like formula
Ok by me-
Some prefer formula-
I was breast fed--so always prefer the hands on approach.
Old 01-17-2003, 09:52 PM
  #89  
Ikaros
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Default Reynald's number

Reynold's numbers makes no sense at all without using abstractions like formulas.

The Reynold's number is an abstract thing. It is not like a lawn-mower.

Cheers,
Ikaros
Old 01-17-2003, 10:19 PM
  #90  
rmh
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Default Reynald's number

Gosh -It made perfect sense to me the first time I read thru it
I saw the relationships but was hard pressed to find how it could be of a lot of help down at very low speeds on tiny chords etc..
So far I have not seen anyone answer that conundrum .
You answer sounds like the ol "God moves in a mysterious way" answer I once heard
I am familiar with lawn mowers tho - They make sense.--as do answers which address the question
Old 01-17-2003, 10:25 PM
  #91  
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Default Reynald's number

Originally posted by dick Hanson
Gosh -It made perfect sense to me the first time I read thru it
I saw the relationships but was hard pressed to find how it could be of a lot of help down at very low speeds on tiny chords etc..
Dick,

I am not sure what you mean. You don't see how what could be of a lot of help?

banktoturn
Old 01-17-2003, 11:01 PM
  #92  
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Default Reynald's number

When it comes to hydro-dynamics and aero-dynamics, it is in fact so, that the inner-workings are the mysterious ways of God.

That is because of the chaotic nature of flow.

Reynold, Froude, Nusselt and others, made a great work for us all, trying to explain it in a useful way.

It is possible to make quite accurate mathematic models, describing the nature of different kinds of flow, however, it will always be approximations only.

If this was not true, the weather forecasters would be able to forecast the wheater for every day in the infinite future, if they had computers fast enough.

Computers makes it possible to make new approaches in solving practical problems but the old theories still applies.

The inner workings of a chaotic system will always look mysterious to a human.

Cheers,
Ikaros
Old 01-17-2003, 11:32 PM
  #93  
Ikaros
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Default Reynald's number

I can read a clock, however, I know little of the inner workings of a mechanichal clock. ( I know how a pendulum and a oro works but not the rest).

That is not a problem to me because the time is linear. (to me it is anyway).

Aerodynamics is about a lot of interacting mechanisms that cannot be fully explained by known mathematics.

If you seek a layman's explanation of why the Reynolds number does not reflect the nature of flow like K or C explains temperature related things, you could look until the sun burns out all of its hydrogen and expands to a red giant, making a furnance of earth, still, you will find no layman's explanation.

Being sarcastic about people trying to explain such a thing will not help you understand.

I suggest you to forget Reynolds number altogether, Dick. Alternately, read a book.

Cheers,
Ikaros
Old 01-17-2003, 11:50 PM
  #94  
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Default Reynald's number

Dick Hansson,

Reason I am a bit sarcastic is that I read about your conversation with Ollie and I think you are trying to be a smart ass.

If you do not understand things, then you should not project your frustration at people trying to explain.

That is childish.

Cheers,
Ikaros
Old 01-18-2003, 12:23 AM
  #95  
rmh
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Default Reynald's number

You are being foolish.
If you read things a bit more carefully you would se that rather than denying the benifits of a "formula -in this case Reynold's Law , I have been looking for some applicable results for small models - using that law or any law.
Abstract is not an unfamiliar term to me -I also understand the term vague.
Old 01-18-2003, 12:48 AM
  #96  
David Cutler
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Originally posted by Ikaros
It is not like a lawn-mower.


It isn't? Damn! There goes another of my life-foundations.

David C.
Old 01-18-2003, 12:51 AM
  #97  
Ikaros
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Dick Hanson,

Reynold's number is not a law. And there is no law for what you are looking for, sorry.

Cheers,
Ikaros
Old 01-18-2003, 12:55 AM
  #98  
Ikaros
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Default Reynald's number

Another thing,

Could the forum administrator please change "Raynalds" and similair attempts to type hes name to "Reynold's"

You don't want this guy rotate like a propeller in hes grave, do you ?

Cheers,
Ikaros
Old 01-18-2003, 01:06 AM
  #99  
Ikaros
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Dick,

Shure i'm foolish, I'm just a human. There is no answer to your question that you would accept.

You can fly your models without those answers, just like a bird can fly.

Cheers,
Ikaros
Old 01-18-2003, 01:41 AM
  #100  
rmh
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Default Reynald's number

Chee- I did say Reynold's Law!
You got one right!
Yes- I can fly without knowing any law -but it took a while to come up with the answers I needed.
The most important being, most of the stuff I learned in textbooks about flight and what was necessary - was directed to and developed by those who were writing to themselves.
The classic " what makes a wing lift?
the answer typically dwelt on th classic Clark Y airfoil.
Replete with the old laws about (laws?) about low pressure above th wing. blah blah blah.
The more I tested the book - the more contradictions I found -
I felt like I had been wasting my time actually reading this stuff.
I was reminded of the man who searched for the secret of life - and having found the old Guru asked him - what is the secret?
The Guru answerd - "Life is a flower"
The man swore "You tell me it is only a fflower?"
The Guru said "it ain't?"
Way back when - I asked if anyone would/could explain RN (not registered nurse) as it applied to our little chicken sh-- models.
I got the formula layed out - -OK
fine -- but I had and still have not seen how it applies at very low speeds and sizes.
A couple of honest folk said - they were not sure -understandable as they probably never had to do a graph--for example. of 10 mph on a 5 " chord.
Now I am chastised for suggesting that the profound secrets of flight (don't fly to close to the sun, son), are of such an abstract nature that I could never fathom them.

I never claimed to have the facts - I just asked for a clear explanation of them .
If you can't explain something - just say so - - this isn't a test -

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