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Reynald's number

Old 01-13-2003, 08:17 PM
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rmh
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Default Reynald's number

OK Ben-I better explain my "logic joke" on flaps.
"given a section of wing panel, let's assume it has an airfoil maximized for lift at a given speed and load".
So if we bend the airfoil -(add flap) do we gain or loose lift at the same speed load?
The AOA must surely change-but if the foil was alreay optomized -how do we add lift?
Lousy joke ---but I always get replies that flaps add lift -not that flaps allow the AOA to change -or NOT change as needed -to provide desired lift and drag desired.
My other lousy logic joke is "lift is drag and drag is lift"
It just changes in the eye of the beholder.
I always looked at the airframe as a whole thing concept - that is - how do we do the job with the least differences in air pressure on any opposing points of referrence.
Old 01-13-2003, 09:02 PM
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Default Flaps.....

Ben....Dick.....Bank-to-Turn....Are you gentlemen familiar with Hobby-Lobby hinges?.....They make great flap mounting hinges....I use them on all my "Big" warbirds....they make the flaps operate more closely to "Fowler" type of flap which during flap extension go "BacK" as well as down... this gives the wing a slightly larger surface area for lift as well as some drag @ lower flap settings.......There is a definite difference on T/O for example with flaps up and say flaps 15* ....T/O roll is shorter with ability for a higher rate of climb....not to mention nice stable approaches and landings.....Bill......
Old 01-13-2003, 10:33 PM
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Default Reynald's number

I have not used em -but saw some for "Fowler "setups - I agree- changing the AOA with flaps is a good idea for warbirds -
Anything to get a manageable "wheel landing."
Old 01-13-2003, 11:43 PM
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Default Reynald's number

-----------------OK Ben-I better explain my "logic joke" on flaps.
"given a section of wing panel, let's assume it has an airfoil maximized for lift at a given speed and load".
So if we bend the airfoil -(add flap) do we gain or loose lift at the same speed load?
The AOA must surely change-but if the foil was alreay optomized -how do we add lift?
Lousy joke ---but I always get replies that flaps add lift -not that flaps allow the AOA to change -or NOT change as needed -to provide desired lift and drag desired.
My other lousy logic joke is "lift is drag and drag is lift"
It just changes in the eye of the beholder. ------------

First of all a round of boos and hisses are in order. I'll never trust a design by you again:-)

Flaps can be added to a perfect airfoil and gain lift but at the expense of a lot of drag. The AOA does not need to change. It might but doesn't have to. Since there is a moment balance that must be satisfied and a tail trim required to trim out that moment it may indeed bring about a AOA change.

Warbirdz1, keep in mind that the flap does not gain primary lift as a function of increased projected wing area. When the area does increase as the flap moves rearward the increase in lift can't be more than the % increase in area which isn't much. The majority of the lift is from the bend down of the flap thing. I have seen a 5 segmeted flap to try to do this better. The flap produces a real lift due to yanking the air from the top of the wing downward and deflecting the air under the wing downward. It takes a lot of power and does cause drag (my understatement of the day).

-------------- I always looked at the airframe as a whole thing concept - that is - how do we do the job with the least differences in air pressure on any opposing points of referrence. ---------------

Absolutely, I tend to always want to see if the old modelers tales can withstand the light of math and logic. However since my short term memory is pathetic (at least compared to a bright flea) so sometimes the math and logic gets whoopsey. I do try to be accurate though, but don't write as precisely as Ollie.
Old 01-14-2003, 03:46 PM
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Default Reynald's number

Flaps can be added to a perfect airfoil and gain lift but at the expense of a lot of drag. The AOA does not need to change. It might but doesn't have to. Since there is a moment balance that must be satisfied and a tail trim required to trim out that moment it may indeed bring about a AOA change.

Ben,

Whether flaps increase drag depends on what you are comparing to what. Deploying flaps to increase lift can easily have a smaller drag penalty than increasing the AoA to increase lift.

Dick,

-------------- I always looked at the airframe as a whole thing concept - that is - how do we do the job with the least differences in air pressure on any opposing points of referrence. ---------------

I'm not quite sure what you mean by this.

banktoturn
Old 01-14-2003, 06:29 PM
  #56  
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Default Reynald's number

Bank to Turn, agreeded, you would need to look at the actual data to determine the real numbers. I have always been a fan of getting data first and coming up with the theory later, then I say I knew it all the time. As I have said before whoever says it first ends up being boss.
Old 01-14-2003, 06:41 PM
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Default Flaps......

Ben....B-T-Turn.....Dick....Being an Aircraft Mech and Pilot for years but not as Technically proficient as you guys are, I see things more from a "hands-on" experience ....I fully comprehend your info but find that sometimes its just plain easier to try something and go to the field and fly to see what the results turn out to be.....like the the afore-mentioned hinges for flaps......I look for results from a performance perspective....I can't fully explain aerodynamically why those hinges work but the Ziroli P-47 that I used them on first sure took a liking to them.....both for T/O's on shorter runways and landings with having to use higher sink rates......smooth and stable.........Bill....
Old 01-14-2003, 08:15 PM
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Default Reynald's number

Bee's wing don't produce lift by the same mechanism as aircraft wings. At the extremely low reynolds numbers of bee's wings the boundary layer is about the same thickness as the bee's wing is wide. If a Bee tried to glide it would glide worse than the space shuttle. Even so, bees can generate a higher lift coefficient than all but the best flap equipped aircraft wing. Here is how they do it.

On the upstroke the wings are in a plane roughly perpendicular to the way the bee is pointed. The leading edges come together and then the wings twist to bring the trailing edges together too. The air between the wings is expelled backwards, propelling the bee forward. Beginning the down stroke, the wing top sides are almost touching across the whole area of the wings. On the down stroke, the wings do not twist but travel through the air like a paddle. A partial vacuum is created before the air can rush in to the space between the wings and the bottom of the wings push against the air below them. As the wings reach the bottom of the stroke they rotate again so that they will be in the plane of the upstroke when it starts. The cycle repeats itself several hundred times per second. Variations in the stroke allow the bee to maneuver, hover, etc. To apply airfoil theory to a bee's lift generating mechanism would be a gross violation of the assumptions underlying airfoil theory. It is only since strobe photography has been applied to bee flight that we have been able to see in detail what is happening.

As Ben said. First get the data. Then find the right theory to fit the data. Then you are in a position to apply the theory to questions without having to go out and collect all the data for a similar case all over again. The engineering/scientific method is difficult but really quite efficient in the long run. When you finally get it right, it keeps you from going down some of the dead ends that have been previously explored. It's not the only way to get where you want to go but it is a great time saver. Biological evolution designs by trial and error. Engineering design is many orders of magnitude faster. The reason evolution is sometime ahead of engineering design is that it started hundreds of millions of years sooner. You can design an aircraft in minutes using REAL Flight or another simulation program and try it out on the computer compared to having to build a model first to try it out. We have X Foil to design and test airfoils on the computer compared to having to build a test model and test it in a wind tunnel. It's worth the effort to learn how to use such marvelous tools.
Old 01-14-2003, 09:14 PM
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Default Reynald's number

Sure - But IF one does not have a wind tunnel and a Cray computer- is it still OK to use the cut and try?
Please loan me one and I guarantee I will learn to use it --
Old 01-14-2003, 09:41 PM
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Default Reynald's number

I'm taking two pages out of your excuse book, Dick. X Foil is not only an airfoil design program but a virtual wind tunnel that runs on your home PC. Its available as a free download too. No need any more for a wind tunnel or a super computer to solve many model design problems. Now your only excuse is an aversion to technical knowledge. If I could I would do you a favor and hold you to your promise.
Old 01-14-2003, 09:42 PM
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Default Reynald's number

Ben,

Good, relevant data is nice to have. So is a valid theory. We seldom have the luxury of getting all of either that we need before we get some of the other. The partial, imperfect data we start with leads to a hypothesis (theory), and that leads to a new experiment to test the hypothesis. We go back and forth between better & more relevant data and more refined theory. For us in the modeling world, most of the theory is in place, and our main question is how well it applies at our Reynold's numbers. Without wind tunnels & Cray computers ( easier to use than a Windows/Intel PC, in my opinion, Dick ), cut and try is the method we have to test our hypotheses. Having said that, we modelers often do cut and try to answer questions that have already been answered, even for our Reynold's numbers. This might be a bit of a waste from a scientist's persptective, but it's a great hobby!

banktoturn
Old 01-14-2003, 10:53 PM
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Default Reynald's number

I do understand hands on learning. I have been a modeler for over 50 years having started at the age of 8. I am still surprised that my parents let me use double edge razor blades and amazed that I didn't cut off something that I would use later. I did however bathe many formers with a bit of red before the model was finished. I hate those old printed on brittle balsa wood kits but loved the airplanes.

It was neat when the things I learned in college seemed to make sense and agreeded with the things I had learned by the hands on method. I still get excited with new knowledge, the first photos of the clap-fling--(known by several names) of insect flight helped put the whole thing together.

The thing about hands on learning is that it can lead to so many dead ends. Though those dead ends may indeed be as interesting as where you were going sometimes they are a little expensive. The interesting thing is that the amount of information that a person needs to absorb to understand airplane flight is pretty small.

No calculus is needed (otherwise I would be up a creek most of the time, I have a son with a Dr. in EE to make me feel reasonablely humble), just a willingness to delete "old modelers tales" from the memory and repalce them with new good data. In my case getting a little senile has helped with deleting old brain files but makes poking in new data a little harder each year.

For myself I am still trying to understand the logic of the modern trim setup of aerobatic airplanes.

First adjust for level flight. Then do knife edge or dive power off and check for pull to canopy and then change incidence to reduce the tendency. But you need the incidence or equivalent uptrim for level flight. What is the difference? The incidence gives 1 g acceleration to the canopy in level flight, of course it will pull to the canopy when gravity is no longer making a balance of forces! Then they say go inverted and shift the CG aft to get good hands off lnverted flight. But all that does is make the airplane less stable so you need a smaller incidence angle (or less tail deflection) to trim. You could just start with a less stable airplane to start with. Of course it would still pull to the canopy no matter what you do if it flies level hands off ......... maybe the initial trim suggestions are a lot of old molelers tales?

Oh well, I will think about it some more................
Old 01-14-2003, 11:05 PM
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I get the idea that Ollie thinks I don't understand tech stuff-

I don't have a degree in aero stuff-as a matter of fact NO Engineering degree--but I do have a number of patents on very technical -and successful -- mechanical/ pneumatic and high temperature machinery-
My earlier comments on breaking things down into layman's "talk" -was based on having to write up explanations , suitable for use in civil court.
You may thing your background is too sophisticated to be reduced in this manner .
I assure you --it is not.
I use cut and try for very good reasons.
It is expedient
It has proven successful
Ultimately -all of the wind tunnel and paperwork has to be flown to prove it.
In the case of modeling - these two expensive, time consuming and often unsuccessful steps are eliminated.
And why not - the objective of modeling is to have fun .
Many of my designs have been flown by others at World, National and local (inc TOC)- very successfully - so Ollie - I am not completely devoid of technical ability -
Actually my advanced education was thru a full music scholarship - but that stuff requires no real skills --
right?
Old 01-14-2003, 11:15 PM
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Ben -many of the trim charts are a lot of hooey-
In too many cases - they are based on the scientific principle of a miracle.
The idea that the trim can be setup to work hands off -up/down -in a dive - abviously ignores gravity.
But the closer the trim guys get to neutral - the more they assume it can actually be done -
Sort of like weaning a horse from food - -just as you get him weaned - he dies.
I trim for a reasonable knife edge - and set inverted flight response with changes in expo /throw and/or servo to elevator geometry -
But if the model still has lousy snap recovery character - I shift CG ahead and start over .
Old 01-15-2003, 03:32 AM
  #65  
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I love the "scientific principle of a miracle". I have often prayed in the course of doing a wind tunnel or flight test but never put it into words quite as well.

Thanks for the insight!

My son got a degree in music. Actually he got one in music, EE and computer science in the 4 years of undergrad work. As I mentioned he keeps me humble but I brag on him anyway. I do think the same things that make a good musician can make a good computer science type.

Terrible handicap for understanding aerodynamics though:-)

I can't play a note, have tried, can't play, manage to hum and whistle sometimes but usually off key, don't know what a key is actually but have been told it was off.
Old 01-15-2003, 04:54 PM
  #66  
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Default Aerodynamics.......

Ben-Ollie-Dick.....B-to-T.....Its interesting to get different perspectives on Aerodynamic Theory and the like.....but think about the "Wright Bros'....They had a wind tunnel and had some forsight on aerodynamics but it was their persistant ambition and curiosity that truely got them to fly as they did......from bicycle shop to airplanes...and in a relatively short time....now, gentlemen are you familiar with "Charlie Taylor?"....not talked about nearly as much as the "Wrights"......he with his own expertise "hand built" the first engine for Wrights.....with a lathe and a drill press....only part that he didn't make was the eng block...it was cast at a local foundry....that means he made a crank,pistons,rods, bearings,cam ,valves..etc all by himself........Wrights "computed" they needed 8 hp for flight.......they ended up with 11 hp thanks to Charlie.....Charlie out-lived the Wrights by a long shot.....lived to see Jet-Age in full swing...died in 1954 at age 86.....my point being that all the Aerodynamic Theory really means little unless put to use and tested...thats where the real learning takes place...thru experience whether its something thats works or fails.....its still useful info..........Bill....
Old 01-15-2003, 05:28 PM
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Yeh -- Textbook expertise is very important in the present world -
First -it is a quick method of sorting out those who are not team players
It also spreads know info to a wider group - quickly.
The BUT--is that it is just another tool which has to be used correctly.
It is not an end all source of info-
I don't believe in black magic and miracle engineering but having worked with a LOT of graduate engineers -I found that really good engineers were as commmon as my good engineer prof pointed out - about 10%
The Russians recruit their aircraft designeers from (gasp) model aero clubs --people who actually like and apply hands on approaches.
Too bad many of our engineers are guys who liked working with numbers and just watch the clock and wait for payday -
This is not a putdown -just a long term observation.
Thank goodness I don't have to deal with em anymore.
signed
Oscar the Grouch
Old 01-15-2003, 05:46 PM
  #68  
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warbirdz1 & Dick,

I agree that the Wright Brothers had persistent ambition and curiosity. A lot of the people trying to build flying machines had those things. In point of fact, what the Wright Brothers brought to the table was an analytic view that allowed them to have the objectivity to accept that the currently available experimental data was very poor. From there, they proceeded to generate far superior data, and methodically use it to design effective machines. They didn't read the textbook, they wrote it. They were excellent theoreticians, and also very good hands-on guys.

Preference/aptitude for analysis and preference/aptitude for practice (doing it) are simply two characteristics of people. They occur separately in some people, and together in some people. They aren't exclusive by any means. Same with being a team player. At the same time, these qualities are useful for different things; they aren't interchangeable.

Aerodynamic theory means exactly what it means. We ( humans ) couldn't build the airplanes we do without it, and we couldn't build them if the theory were all we have either. There is no point in valuing analysis ( a term I prefer over theory ) over practice, or vice versa.

It is important, though, if one wants to use 'theory', not to mess it up in an attempt to simplify it. That happens here a lot. This is also not a putdown, just an observation.

banktoturn
Old 01-15-2003, 06:50 PM
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OK- but I never saw anything messed up by clarifying it.
The classic response is " I know how it works -I just can't explain it".
Once upon a time I bought into that -
not now -

If you can't explain it in layman's terms -you don't really understand it.
You may use it -but when you can explain it to a child - then you really have full grasp of it.
If making fun of something bothers you -you also don't understand it.
With full comprehension comes the absurd as well as the factual parts.
Shakyspear
Old 01-15-2003, 07:25 PM
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Dick,

The ability to explain a concept to a layman depends not only on the explainer's understanding, but on the background of the layman, and the amount of time & effort the layman will put into learning. To say that you don't understand an idea unless you can explain it to a layman is frankly ridiculous. To say that you don't understand an idea unless you can explain it clearly would be a little more reasonable. No offense intended, I will use an example from the beginning of this thread. Your analogy of molecules being pressed together, while fairly accessible to a layman, is fundamentally not related to the concept of Reynold's number. It is the beginning of a pretty good explanation of pressure, but the notion of Reynold's number is completely independent of particle size, or even whether the fluid is composed of particles at all. This simplification, while appealing, and capable of conjuring a satisfying image in the mind of a layman, does not clarify, because it is not valid.

If you want to truly explain the notion of Reynold's number, as understood by an accomplished Fluid Dynamicist ( if that's a word ), to a layman, you must first give them quite a bit of background information ( what are viscous forces, what are inertial forces, ... ). If said layman, or the explainer, doesn't have time, or won't spend the time, to do that, then the best you can do is explain how this concept affects them in a fairly narrowly defined situation ( for example, wings operating at low Re stall more easily ). This is true of a lot of ideas. Unfortunate, but true.

banktoturn
Old 01-15-2003, 08:27 PM
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My explanation was not complete--it was only a simple overview-
As for an accomplished aero person being unable to relate to a layman- I don't agree -
An instant explanation is of course - highly unlikely!
Why do you add time constraints to an explanation?
My point once again -is that on can relate the info -if it is properly done and time is not a fixed factor.
Do you understand how fluidic valves operate?
I do - yet I am a layman to you -
Is air compressible?
Is water compressible?
Why is it called fluid dynamics -if fluids are not involved?
if one is working with flying under water- does the idea of Reynold's numbers apply?
Did you suppose I thought Reynold's numbers were made up form molecules?
Why is it necessary for a model airplane guy -who just wants to know why his 1/2 A flys far differently -as compared against his 40% model, to understand boundry layer stuff - or stagnant points or much of the other stuff necessary for an airliner?
My point is that one CAN simplify and pluck out the necessary points- IF one understands-thoroughly.
When a guy asks what time it is - you don't need to explain how to make a watch.
perhaps that is a bit trite- oh well -
If I do get into a bind on the fine points of why my models do what they do - I know that there are those with greater knowledge, ready to lhelp.
I will keep my kindersprecht off the list.
Old 01-15-2003, 09:25 PM
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Default Reynald's number

Dick,

I add the time constraint because it is there. The time constraint is why we need to "pluck out the necessary points". I'm glad you mention the 'plucking', because it is on this point where you and I agree, however reluctantly. So, in this case, I would 'pluck' by telling lennyk that Reynold's number can be thought of as a resistance to separation, which is the phenomenon that causes stall, and because Reynold's number is much larger for full scale aircraft, its wing is more resistant to stall. This is not an explanation of the Reynold's number, but there is a good chance that it would tell him what he needs to know, or wants to know.

I'm not sure what you are getting at with the questions, but I'll bite:

Yes, I understand how fluidic valves operate. Yes, air is compressible. Yes, water is compressible, but not very. It's called fluid dynamics because it DOES involve fluids. Yes, the idea of Reynold's number applies to water, and all other fluids. I don't know why you mentioned particles in reference to Reynold's number, but to be honest, it's bugging me a little bit.

banktoturn
Old 01-15-2003, 09:32 PM
  #73  
warbirdz1
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Default Reynolds #'s....et al........

Ollie....B-to-T....Dick.......When I first learned to fly RC ,I didn't want to know about Reynolds ,nor did I care ....then.....I just wanted to get out and fly...wanted to make progress and ,hopefully soon, fly on my own.............What I did need to know were some basics....for building that first trainer......I think the more prevailing mentality for our hobby is to keep things on a simpler plane and go out and fly...... that doesn't mean thats all there is.........I enjoy reading and responding because I feel I become a better builder/pilot when aided with more facts and figures.....Another observation I have is depending on the style/type of plane being discussed, different people each have their own objectives.......I.E....I fly big Warbirds...they inherently have higher wing loadings and flight characteristics of their own.......I get "Bashed" from time to time by others about the wing loadings of my planes( which is roughly the same as anyone elses warbird).......but the reality is they fly very good .....and after-all its the end result that really matters....so Ollie, just out of curiosity what would be the Reynolds # for my Ziroli B-25 wing @ 102" WS ....Bill....
Old 01-15-2003, 09:47 PM
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Default Reynald's number

warbirdz,

Don't take any guff about high wingloading. It's that way for a reason. Hauling around a huge wing is very wasteful, which is why full scale planes use flaps when they need more lift. We get away with huge wings because no one cares how much fuel we burn, and some of us like to do outrageous maneuvers that require them. It always amuses me when I see someone put flaps on a plane that already has a very low wing loading.

banktoturn
Old 01-15-2003, 10:37 PM
  #75  
rmh
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Default Reynald's number

Maybe the guy wants to reduce lift by increasing drag?
Amusing?
I did it on a couple of models and it worked very well
I could have used speed brakes (ala Douglas Dauntless) but he flaps had a manifold use
I could couple em with the elevator and change lift without changing angle of fuselage and I could short field land by really dropping them.
Once upon a time -I made split rudders to slow vertical down lines - Itwas a lousy choice-
The amount of coupled up stuff required to hold the line as the rudder split , was just too much .
The pitch up with application of splitting rudder was quite profound.

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