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Reynald's number

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Reynald's number

Old 11-06-2002, 01:56 AM
  #26  
banktoturn
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Default Reynald's number

Dick,

Use of everyday language does not grate on me, and I agree very much that an inability to explain an idea in plain language often means that the explainer does not really understand. Unfortunately, some ideas are difficult to put into everyday language, without taking a very long time, and a lot of words. While jargon may be a problem, it is also a problem to put an idea into non-technical terms, and lose the essential meaning.

FXD,

A wing or airfoil does not have a Reynold's number. To calculate the Reynold's number, you need to know the properties of the air, which don't change much, the size of the wing, and the airspeed. The size we need is the chord of the wing ( distance from leading edge to trailing edge ). If you tell us what that is, we can assume the speed, and approximate the Reynold's number. More than likely, it will end up being a low Reynold's number, in the sense that the wing won't behave quite the same as full-size wings.

banktoturn
Old 11-06-2002, 02:30 AM
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rmh
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Default Reynald's number

bank to turn- fair enough -
Now then - here is an example of how a your good explanation of RN can go wrong.

Suppose FXD reads your reply and asks " why is a low number different than a high number " and whatthe blazes IS a "full size " wing.
You know and I know that low numbers mean that the LOW RN NUMBER means that the wing will not lift as much weight but the fact that the scale is not a straight line from 1-10 is not obvious to a newcomer.
As for" full scale " as a comparison.
This one is still a mystery to me that it is a common reference. .
The full scale chord of "what" comes to mind.
a 3 ft chord? a 12 ft chord?
The ability of model wings to work in the sizes commonly used -is the question I typically get -in a face to face situation.
Typical example - why does a model wing with a avg chord of 15" work well for aerobatics if it has a wing loading of 30 oz ft--yet a slightly smaller -yet absolutely perfectly scaled down model having an avg chord of 12" and the same wing loading -flys noticably differently
The question is -what is the scale effect curve like if these seemingly tiny differences are so easy to see in actual practice?
How would you answer that?
Old 11-06-2002, 03:50 PM
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Default Reynald's number

Dick,

Well, I guess my first comment would be that FXD could post a couple questions asking for clarification. In that exchange, I, or someone else, could ask him exactly what decision he wants to make, so that we could offer more helpful information, if possible.

Your example is an instructive one. If someone observes that a 12" chord wing with the same wing loading doesn't fly as well as a 15" chord wing, no simple, watered down explanation of Reynold's number is going to tell him exactly why. I am a little dubious that the observed difference in performance is a Reynold's number effect, myself. If such a big change is observed between those two wings, it probably means that we are operating in a critical range of Reynold's number ( or some other parameter ), and much more specific data would be required, likely more data that we could get our hands on. If someone wanted to gain some intuition about why the chord length can have such a big effect, that's pretty reasonable, and the posts in this thread so far may offer that. If he is a serious competitor, he will either use airfoils and dimensions that are supported by high quality data, or will simply avoid the dimension which experience showed to perform poorly. If he is not a serious competitor, he will pretty likely not notice, or just keep flying his plane and having fun.

As I said, I am a huge fan of accessible, clear explanations in everyday terms. I attempt to do that, as best I can.

banktoturn
Old 11-06-2002, 05:10 PM
  #29  
Ollie
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Default Reynald's number

I think it is instructive to look at reynolds number from the point of view of its impact on historical events. The Wright brothers built a wind tunnel to test various airfoils to see which was best. The wind tunnel airfoil models were small to fit the small wind tunnel. The speed of the air in the wind tunnel wasn't very high either. As a result of these test conditions the Wright brothers selected a thin highly cambered airfoil for their early gliders and their Wright Flyer. Their selection did not take into account all the effects of differences in scale and velocity (reynolds number related effects) between the wind tunnel and their full scale application. This resulted in wings that were overkill from a lift capability point of view and which had severe high speed limitations. It also resulted in the need for a wire braced biplane to meet the structural requirements. It wasn't until late in WWI that German aeronautical research revealed the superiority, at full scale (high reynolds numbers), of monoplane wings with thick airfoils that allowed strong enough spars to do away with wire bracing. Airfoils similar to the Wright brothers airfoil are still used in wire braced indoor freeflight wings where their high camber and thinness give superior performance at low reynolds numbers.
Old 11-06-2002, 05:16 PM
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Default Reynald's number

Imperical results are that wing loading is quite simply of more importance in these smaller sizes than any change in airfoils .
For example - the case mentioned is 12/15 (4/5) the wing chord -or about a 20% shift in chord.
Having built a number of a design - then increasing or decreasing them - we got a lot of hands on exposure to what happens .
Typically - in this size range - --if the model had a 30 oz ft loading at 15" chord-- the same power off characteristics for the 12" chord - required that the wing loading drop below 25 oz ft-
Power off was used as the comparison basis as power on comparison is simply not predictable
stalls and landing approachs show when the model starts to loose control- snaps show how easily the model regains control - this testactually shows the most striking differences in wing loading .
comparisons
Dropping sizs even further to small models with chords of avg 9" show that wing loading now must be in the mid teens - to "equal" the character seen at the larger sizes.
If these numbers sound strange to you - try some comparisons -if possible ,with models .
I really don't know what the shape of the curve would be on a graph- in this size - and really there is probably not a lot of referrence stuff.
For working this up- -Imagine all speeds stay at 60 mph and chord changes from 15-12 -9" avg on a wing with a 5-1 aspect ratio- your choice of airfoils
Old 11-06-2002, 05:17 PM
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Default Reynald's number

Emperical
Old 11-06-2002, 06:04 PM
  #32  
Ollie
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Default Reynald's number

Every aircraft wing operates over a range of speeds from stall to top speed. Because reynolds number is proportional to air speed the wing is associated with a range of reynolds numbers. Also, a tapered wing has a variety of wing chords along its span so, the reynolds number which is proportional to chord varies along the span of a tapered wing. Aircraft that operate over a wide range of altitudes experience air with a range of densities. This further extends the range of reynolds numbers associated with the aircraft because reynolds number is also proportional to air density. This is a practical consideration for models that are flown from fields at sea level and also from fields at 5000 feet like Denver where the performance is quite different.

Simple reynolds number explainations may satisfy curiosity but they aren't very useful to apply to aircraft design, selection and operation unless they have a lot of detailed technical and mathematical content.
Old 11-06-2002, 08:40 PM
  #33  
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Default Reynald's number

lennyk – Did you ever think that your original post would turn into this?

I'll chime in again for my nickel's worth.

Returning to the Reynolds Number (Re) and scale effect issue, the problem in trying to convey a real understanding in lay terms is that much of the underlying principles get glossed over, to the point where a lay explanation loses too much, and is akin to no explanation.

To grasp the significance of Re requires first getting comfortable with the concepts of viscosity, shear stress, momentum, boundary layers, work and energy, separation, wake, laminar vs. turbulent, etc. Without understanding these concepts, which are themselves based on further sub-concepts, a discussion of Re in the context of scale effect will generate little real understanding. Like trying to explain any other highly conceptual topic on a lay level - Yes, it can be done, but there will be holes in the explanation, and large ones at that.

Consider an everyday concept - Friction. Seems simple enough. We all intuitively "understand" friction and its effects, but could a deeper understanding involving the surface science involved, like the source and dynamics of the intermolecular attractive forces, really be conveyed in lay terms? How about the everyday concepts of light and color perception? We all relate to them in physical terms simply enough, but would a lay discussion attempting to delve deep really shed meaningful insight on spectral absorption, electron transitions, or the neurophysiology of the eye?

Problem is, every physical phenomena ultimately requires, at some level of understanding, a discussion transcending a lay level in order to accurately describe it. The complexity of the phenomena that Reynolds Number relates to is often underestimated, I believe, simply because the equation for Re is so simple - It gets thrown around and used as a rule of thumb so much that its conceptual foundations aren't fully appreciated.

The level of understanding that one chooses to aspire to, as Ollie stated earlier, is based on that person's motivation to get there - whatever his interests are and his "need to know." As I see it, advanced discussion of a lay topic may create misunderstandings, but a lay discussion of an advanced topic is bound to create misunderstandings.
Old 11-06-2002, 09:22 PM
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rmh
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Default Reynald's number

Interesting comments all.
Now put yourselves in the current application of RN:
Model aircraft and how the understanding of RN may help those in the hobby.
Simply stating that it is extremely technical and requires knowledge of sub contexts etc., is of little help to the modeler who just wants to know why his model flies differently, as compared to larger models.
As long as the idea of why is coveyed, it matters little how the idea is explained.
Empirical data ( my spelling!), can be used to show what the textbook tries to explain.
This isn't quantum mechanics-
Old 11-06-2002, 09:49 PM
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banktoturn
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Default Reynald's number

Dick,

I guess I'm not sure what you are taking issue with. If you go back through the posts, I think you'll find several that give pretty good intuition about why "big planes fly better" ( an assertion that I don't care for or agree with, in this simple form ), some of them in mostly non-technical terms. If anyone asking a question found that the answers weren't helpful, we sure didn't hear from them.

I think I will leave this discussion with a quote from Albert Einstein:

"Everything should be made as simple as possible, but not simpler"

banktoturn
Old 11-06-2002, 11:05 PM
  #36  
rmh
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Default Reynald's number

Interesting inputs.
I really had hoped to hear from others who use a hands on approach but it was not to be -As Al may also have said--- Solch ist das leben.
Old 01-05-2003, 10:40 PM
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Bruce Thompson
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Default Reynald's number

Since you all seem to be WAY over my head let me ask a question and see what comes of it. I am trying to design a wing for a racecar. from what I read above the RE# are approx 1.4mill to 2.4 mill. How do I choose an airfoil that will have a high Cl and low Cd?
Old 01-05-2003, 11:11 PM
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Default Reynald's number

Originally posted by T28pilot
You know guys i just took over moderating this part of Rcu and you guys are really over my head........
So this means you won't know if we're flaming or talking naughty as long as we keep it all covered up with technical stuff?.......

Tee hee hee.....

Edit;

After reading more of this thread I'd like to offer a couple of thoughts.

Dick, I agree 100% with the thought that if a person can't explain an idea in layman's terms then they may not totally understand it. BUT...... I think it's also expecting too much for the recipient of that basic fundemental concept to be able to take it and apply it in a meaningful way. I think it should be enough that they go away with an appreciation that they need to look into the subject in more detail before they can use it in their own designs. There's only so much you can do with a 30 second Reader's Digest version. We have all that math and stuff for a reason after all.

But I can also see where some will find Ollie's responses, accurate and thorough though they are, a pretty thick wade through wet sand.

But in the end the instructions for building a sand castle can't be used to build a high rise apartment tower. If you want to move to the next level the audience needs to get more involved.
Old 01-05-2003, 11:12 PM
  #39  
Ollie
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Default Reynald's number

You choose the airfoil by comparing the polar diagrams of the airfoils. The polar diagrams are plots of the coefficent of lift versus the coefficient of drag. They usually show curves for several reynolds numbers. Draw a line from the origin (zero for both coefficients) that is tangent to the curve for the reynolds number in question. Do this for every airfoil you are considering. Pick the airfoil with the steepest slope of the tangent lines you have drawn. That is the airfoil with the best lift to drag ratio. You can find many airfoil polar diagrams at:
http://www.nasg.com/afdb/list-polar-e.phtml

Then the issue becomes one of setting the angle of attack so that the airfoil works at the coefficient of lift that you want it to. This is very difficult to predetermine because you must take into account the direction of air flow around the car body where the airfoil will be located. You also must take into account the induced angle of attack which depends on the aspect ratio of the wing. Rather than try to account for all this beforehand, mount the wing so that you can adjust the angle of incidence. Then run a series of tests to see which angle gives the strongest down force for the highest speed. If the down force is too great for the car's setup, then reduce the chord of the wing while maintaining the angle of incidence until the download fits the car setup. The results won't be perfect but they will be very close. You can come even closer by reducing the angle of incidence a little to allow for any increases in aspect ratio. The induced drag of the wing is half the total drag at the best lift to drag ratio. The induced drag is inversely proportional to the aspect ratio. Therefore, you will get the best results with the highest aspect ratio you can acomodate.

The above proceedure assumes that you don't know beforehand the down load desired. If you do know the download desired at a given speed, then you can calculate the wing area required and determine the aspect ratio beforehand.
Old 01-06-2003, 01:25 AM
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rmh
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Default Reynald's number

As someone once said - - experience helps .
On race cars - the use of an "airfoil" is really a setup where cut and try works
As for aspect ratio on a car -- the aspect ratio on a car wing is very low - and must be so ---unless you consider a 3-1 aspect ratio to be high--
Are formulas of ANY value here?
Of course -of course--but
One of the best aerodynamic guys in the car business was a guy named Kamm -- who came up with a very interesting bit of logic.
"If you can't make the airfoil work for you - get rid of it"
(Ever wonder why all of those high speed road cars had that squared off rear end?)
Old 01-06-2003, 02:06 AM
  #41  
Ben Lanterman
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Default Perhaps this will help get a feel of Re

If memory serves me right (and it is known to be suspect) a full scale F-15 at some flight conditions operates at a Reyond's number of 30-40-50,000,000.

Reduced to a 5% model for wind tunnel testing we have found that using a wind tunnel capable of 1,000,000 Rn will give barely acceptable results in drag. We would chose to avoid using this type of tunnel if we could.

In a wind tunnel operating at 6,000,000 the drag results are nicely applicable to full scale.

The wind tunnel model is very carefully duplicated out of steel and aluminum. Exact airfoil copied, etc. They did great work. It is necessary though to add "transition strips" on the wing and tail surfaces. This is some form of fine grit or similar small objects laid in a strip along roughly the 5% chord point of all surfaces.

We have models in RC that are working at such a low Rn that it no longer duplicates flight conditions. In a greater sense it becomes something that unless you are working in the exotic FAI glider class or models it is pretty useless. I am impressed by the work of Eppler and the others in the low Rn airfoil work though. The other writers in this forum have covered the other aspects of the number pretty nicely.
Old 01-06-2003, 02:23 AM
  #42  
David Cutler
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Default Re: Reynald's number

Originally posted by lennyk
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
There have been some excellent answers to this already in this thread. However, here's my little bit!...

Reynolds number is the degree of 'stickiness' of a fluid. It becomes important when considering scale factors; hence its pertinence to model aircraft.

Scale factors happen because halving the size of something doesn't result in a halving of the other factors. For instance, halving the size results in a reduction of the weight by 8 (2x2x2). This has effects like, the legs of an elephant are proportionally bigger than a mouse due to the fact that the weight difference is 8 times bigger than the height difference.

This effect happens to surface tension as well, which is why an insect can walk on water, when a man can't.

Air has the same surface tension, density etc etc whether it's passing over a full scale aircraft wing or a model's wing, so the scale factors make it react differently from each other.

The Reynolds number is one of the ratios that can be used to predict this.

There's more info, related to scale factors in aerofoils here:-

http://www.desktopaero.com/appliedae...esections.html

David C.
Old 01-06-2003, 02:36 AM
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Ollie
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Default Reynald's number

Ben is right about reynolds number effects not being a vital part of the design of most powered R/C models because high lift and drag reduction efforts are very low priorities compared to higher thrust, lower weight and greater control authority. Only where speed, range, endurance, load carrying ability and fuel economy are high priorities, will drag reduction or high lift coefficients rear their ugly heads and reynolds number becomes much of a consideration.
Old 01-13-2003, 12:24 AM
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Bruce Thompson
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Default Reynald's number

Thanks Guys for your responses. Thanks also Ollie for the Delta CofG answer. Now for my next question. When talking about CLmax. It seems I have never seen anything much over 1.5. What does this number reference. Also what would been considered high for a no flap wing and a simple flapped wing.
Old 01-13-2003, 01:45 AM
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Ollie
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Default Reynald's number

The coefficient of lift is a dimensionless number that summarizes the effect of the airfoil on the lift of the wing. The lift force is equal to one half times the air density times the air velocity squared times the wing area times the coefficient of lift. the maximum lift coefficient is the lift coefficient just before stall. The maximum lift coefficient determines how slowly the plane can fly in level flight. The maximum lift coefficient determines how tightly an aircraft can turn at a given airspeed. Model airfoils can't usually achieve as high lift coefficients as full scale airfoils because there is less energy in the airflow and the flow seperates from the model airfoil at lower angles of attack which correspond to lower coefficients of lift.

A highly cambered airfoil like the S1223 can achieve a lift coefficient of 2.2. Its practical application is limited to load lifter type model aircraft.

There is not a lot of reliable wind tunnel test data of model airfoils at large flap deflections. Therefore it is hard to say with assurance much about the associated maximum lift coefficients. I think it is safe to say that flaps on models could produce lift coefficients above 2.0 and possibly as high as 2.5 or so in some cases.
Old 01-13-2003, 01:48 PM
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rmh
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Default Reynald's number

Here is another little conundrum about wings -
The wing that has has the best CL --may be a lousy setup for a model -
EXAMPLE:
To get best CL- you need a wing with a fairly long span (relative to chord)
N/Y?
Let's go with Yes
IF- you attempt very tight -skidding turns with this wing -at low speeds -- the inboard panel will see less air flow than the outboard panel.
N/Y?
Again, assume Yes (turning radius , difference effect.)
A very low aspect ratio wing , tho really not as good for max CL number ,
will tolerate this tight turn much better
N/Y?
I say yes -
I tested this theory - by flying stuff ranging from the Klemm trainer design (old German design) to some goofy stuff with more chord than span.
Also in playing with model flaps - I found that flaps do not increase lift - just change AOA.
If the flaps can actually increase usable area -then lift will increase - - OK how badly did I mangle that one ?
Old 01-13-2003, 03:41 PM
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Default Reynald's number

Most of this message text was accidently deleted.
Old 01-13-2003, 03:57 PM
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Default Reynald's number

Read my post again - I said inboard LESS
Anyway If you doubt this look at the speed difference in panels on tight skidding turn- Taken to it's illogical extreme - the inboard panel goes backward if the turn is tight enough -
And flaps - I stand by my comment - a flap just reshapes the existing area - unless it extends outward - so how would it add lift?
Change AOA required? - yes -
Old 01-13-2003, 04:15 PM
  #49  
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Default Reynald's number

Dick,

My mistake, I meant to type 'less'. I was under the impression that you meant inboard panels on each wing, rather than the inboard half of the wing. This makes it more clear to me why you specified a 'skidding' turn, and my comments were not really relevant.

On the flaps, I respectfully disagree. Flaps, unless they are just plain poorly designed, give higher lift at each angle of attack, and also cause stall to occur at a lower angle of attack. The maximum CL with flaps deployed should be significantly higher than the maximum CL without flaps deployed. This is true whether the wing area increases with flap deployment or not.

banktoturn
Old 01-13-2003, 06:46 PM
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Ben Lanterman
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Default Reynald's number

Good reasoning on span effects, pretty bad on flaps.

--------------- Also in playing with model flaps - I found that flaps do not increase lift - just change AOA.
If the flaps can actually increase usable area -then lift will increase -------------

Flaps are best evaluated in a wind tunnel. The problem is that deploying flaps also usually causes a big pitching moment and the airplane has to be retrimed to fly level. A simple test is using RealFlight. Some models pitch nose down with flaps and some pitch nose up. (not too scientific but an indicator).

In the wind tunnel looking at a Lift Coefficient vs angle of attack (CL vs alpha) the basic curve goes through the 0-0 axis intersection point. Increasing alpha causes the angle of attack to go up. Deploying flaps will offset the curve upward by the increase in lift of the flap. The pitching moment curve will show a big shift. If you look at the curves that I added to the end of the airfloiled stab discussion it will indicate a trend. A deployed flap will look like the +tail deflection on the CL vs alpha curve. Note though there are no nonlinear effects given. Too hard.


----------The wing that has has the best CL --may be a lousy setup for a model - ------------

Depends on what you want the model to do. Not a good general statement


------------------- EXAMPLE:
To get best CL- you need a wing with a fairly long span (relative to chord)
N/Y?
Let's go with Yes
IF- you attempt very tight -skidding turns with this wing -at low speeds -- the inboard panel will see less air flow than the outboard panel.
N/Y?
Again, assume Yes (turning radius , difference effect.)
A very low aspect ratio wing , tho really not as good for max CL number ,
will tolerate this tight turn much better
N/Y?
I say yes -
I tested this theory - by flying stuff ranging from the Klemm trainer design (old German design) to some goofy stuff with more chord than span. ---------------

Well look at the equations and save a lot of test flying :-) AC is the aero center of each panel.

The distance the ACs travel in a turn .........
Distance Inboard AC travels = RIac = 2 * Radiusin * pi
Distance Outboard AC travels = ROac = 2 * Radiusout * pi

Where Radiusin is the radius from the center of the circle to the inboard AC
and Radiusout is the radius from the center of the circle to the outboard AC

Velocity of ACs (rough approximation) in a turn .........
VInAC = RIac / circletime
VOutAC = ROac / circletime

The lift on the Wing ACs in a turn .......
LiftInAC = CLIn * 1/2 * rho * VInAC * VInAC
LiftOutAC = CLOut * 1/2 * rho * VOutAC * VOutAC

If CLOut = CLIn = CL then the difference in lift between the inboard and outboard wing is

LiftInAC = CL * 1/2 * rho * VInAC * VInAC
LiftOutAC = CL * 1/2 * rho * VOutAC * VOutAC

or another was of lookiing at it is

LiftInAC ~ VInAC * VInAC
LiftOutAC ~ VOutAC * VOutAC
DeltaLift = LiftOutAC - LiftInAC
DeltaLift = 1/2 * rho * CL * ( VOutAC * VOutAC * - VInAC * VInAC )
=( rho * CL / ( 2 * circletime * circletime ) ) *
( ROac * ROac - RIac * RIac )

or

DeltaLift ~ CL/ circletime * circletime * ( ROac * ROac - RIac * RIac )

So you have the differences in lift between the outboard and inboard panels proportional to the differences in squares of the distances from the center of the circle to the aero centers of the wings, proportional to the Lift coefficient of the wing and inversely proportional to the square of the time to make one circle.

What does it mean if I haven't goofed the math. The greater the span the greater the differences in the wing lift from side to side. The faster the turn the greater the differences. The greater the CL the greater the differences.

For the same airplane speed and radius level turn your low aspect ratio wing will show less response in roll (where the difference in CL will show up first) than a higher aspect ratio.

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