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Old Sep 17, 2012, 11:02 AM
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How does the shape of the wing affect its performance?

When I say 'shape of the wing' I don't mean the cross sectional profile of the airofoil. I'm referring to the overall design of the wing like delta, elliptical, rectangular, tapered, swept back/forward, etc.

Also, if 2 wings have the same wing area and have the same wing profile, but differ in the overall shape, would their lift coefficients be the same? If not, why and how?
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Old Sep 17, 2012, 11:24 AM
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Google:
wing design parameters

The first link will be exactly what you need. Also, lift coefficient is a function of both the state of the aircraft and the geometry. There is no one lift coefficient for a wing.
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Old Sep 17, 2012, 11:47 AM
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It gets more exciting -- depending on the size of the wing--- it all changes
that is - a wing for a plane the size of a 747 -will have much different lift characteristis than one for a one pound model
When it gets really small the differences can become vague.
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Old Sep 17, 2012, 01:33 PM
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Add into the fun that a by-product of the aerodynamics of a wing is that different planforms have different areas of weakness and strength.

Fox example, a forward swept wing is likely to need to be made more torsionally stiff than a swept back wing.

Also, there are some other natty little issues, such as a swept wing having a dihedral effect.

On the subject of lift coefficient, that only tells you part of the story, you need the lift/drag coefficients, at the expected Re. numbers (basically how big and how fast) and the expected range of angle of attack, to be able to make any meaningful comparisons.

What works at a large size, as Richard so rightly says, doesn't always work in small sizes. As an example, very high aspect ratio wings are employed on full size sailplanes, due to the higher efficiency and lower drag. However, if you scaled those wings down, you'd find that as the model got smaller, the less appropriate (from an aerodynamic and structural point of view) a high aspect ratio wing would be. Hence a lot of smaller scale sailplanes "cheat" with broader chord, especially at the tips.

Swept wings were a boon for transonic/supersonic flight, but in the sort of speed range we encounter in model flying, a swept wing is no "faster" than any other, not to any significant degree.

As with everything, there are not really any hard and fast rules, there are some generalities, but you'll find they quite quickly run out of validity once you start digging deeper.

Its all about designing to a purpose. That might be aerobatics, efficiency, strength, speed, glide ratio, scale fidelity or just looking pretty, but start with what you want to acheive, and hone in on that.
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Old Sep 17, 2012, 02:39 PM
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The others are right in that it gets complicated... However one of the main factors in wing shape is 'aspect ratio' aspect ratio is the slenderness of the wing, defined as wing span divided by average wing chord.

As a general rule high aspect ratio wings have the following advantages over low aspect ratio (assuming wing area is the same):
  • More efficient making more lift against drag (l/d).
  • Steeper lift slope, that is per degree of angle of attack lift will increase more quickly.
  • Maximum lift will be higher.

And disadvantages:
  • More difficult to make structurally strong enough, wings might be 'flexy' and/or heavy.
  • Low rate of roll.
  • Lower stall angle
  • Sensitive to small changes in pitch.
  • Higher drag at transonic/supersonic speeds.
  • At RC model scales and low speeds narrow chord becomes inefficient due to Reynolds Number considerations, which puts a limit on aspect ratio.
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Old Sep 17, 2012, 05:00 PM
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One big PLUS of the small size is that the critical shapes (entry, thickness percentages etc., ) all be come very NON critical.

There are limitations but compared to shapes which are four times as large , the performance differences from slight undercamber or slightly different entry points, simply fade to nothing
Those who suppose this simply is not so , should actually build a few (or a few hundred or thousands as many of us have).

When one gets to the micro sized stuf
about the only critical thing left is weight.

This whole business can be summed up pretty easily
As size decreases weight importance increases and becomes harder to achieve.
As size increases , power importance increases and becomes harder to provide

The necessary "wing" requirements along these paths changes dramatically .
It is all a compromise.
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Old Sep 17, 2012, 06:06 PM
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Richard, do you have any data to back up your claims?
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Old Sep 17, 2012, 06:24 PM
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Originally Posted by Montag DP View Post
Richard, do you have any data to back up your claims?
What would you prefer?
I have no math models nor any textbook referrences .
I simply do not prefer to use em
What I claim, is easily demonstrated with flying models in various sizes
My experience in years of flying stuff from 45 pounds down to 1/2 ounce has shown that what I noted is quite factual.
Do you fly various models ?
what have you found which may show otherwise .?
I would like to know if I missed something here .
If you thumb thru your data on Reynolds Numbers and the physics of power required to increase/provide speed -you will likely note some of the rusults I am commenting on.
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Old Sep 17, 2012, 07:29 PM
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I was talking about your reference to things that become non-critical, and that differences fade to nothing. Under what conditions is this true? Is there a certain size of aircraft where it doesn't matter whether I use a wing with an aspect ratio of 5 or one with an aspect ratio of 1/5? If so, how does one determine at which size this occurs? Do airfoils not matter either?

I'm sure you have plenty of experience flying models. So do most of us on this board. I would wager that a fair number of other pilots don't agree with your conclusion. So whose experience is correct?
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Old Sep 17, 2012, 07:46 PM
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Originally Posted by Montag DP View Post
I was talking about your reference to things that become non-critical, and that differences fade to nothing. Under what conditions is this true? Is there a certain size of aircraft where it doesn't matter whether I use a wing with an aspect ratio of 5 or one with an aspect ratio of 1/5? If so, how does one determine at which size this occurs? Do airfoils not matter either?

I'm sure you have plenty of experience flying models. So do most of us on this board. I would wager that a fair number of other pilots don't agree with your conclusion. So whose experience is correct?
With a little hands on experience -these points could be obvious to you .
You appear to most comfortable with absolute numbers

I prefer basic concepts - Are mine too abstract?
Reading other comments -I see quite a bit of agreement
Read Jet Plane flyer's comments -for example
airfoils really loose critical nature when th RN gets very low
You know this --
flat wings on stuf with wing loadings in the very few oz to the ft are as good as anything . Do you have any experience with these setups
aspect ratios of 1-1 works very well on these sizes- 3-1bis also good - NONcritical
better use of the area.
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Old Sep 17, 2012, 09:28 PM
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I do not believe JPF or anyone else said things like aspect ratio and airfoil cease to matter at low Re. They said that the optimum design changes. In your example, one of the two wings (either AR=3 or AR=1) will perform better for a given task. They will not be equal. Sure, if you give it enough power either one will fly, but one of the two will have a better L/D ratio, one of the two will have a higher maximum lift coefficient, etc.
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Old Sep 17, 2012, 11:16 PM
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Originally Posted by Montag DP View Post
I do not believe JPF or anyone else said things like aspect ratio and airfoil cease to matter at low Re. They said that the optimum design changes. In your example, one of the two wings (either AR=3 or AR=1) will perform better for a given task. They will not be equal. Sure, if you give it enough power either one will fly, but one of the two will have a better L/D ratio, one of the two will have a higher maximum lift coefficient, etc.
Quite so but both very usable for the same general type performance.
I have made many of these and have done comparisons.
The single noteworthy part is not the aspect ratio but rather the wing loading.
Much like the Clark Y shape - any reasonable execution of this basic shape wil lwork -for all practical purposes the same - in model application
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Old Sep 18, 2012, 01:01 AM
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From a flying qualities perspective, it is generally true that the airfoil doesn't matter for the "smaller stuff". Although it often becomes less important at lower Reynolds numbers, airfoil shape does still affect performance (L/D). I'm quite sure I could compete with anyone in the world if they were forced to fly a DLG with a flate plate or rectangular wing section. I think a statement like: if it's small and light enough, AND you don't care about performance then airfoil shape doesn't matter is more accurate.
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Old Sep 18, 2012, 04:53 AM
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At low enough Re, a single strand of spiderweb (that's an AR of significantly smaller than 1 since it's half a hair's breadth in span but around 12 inches or more in chord) becomes efficient enough to take spiders up to 10000ft.
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Old Sep 18, 2012, 08:30 AM
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Originally Posted by ShoeDLG View Post
From a flying qualities perspective, it is generally true that the airfoil doesn't matter for the "smaller stuff". Although it often becomes less important at lower Reynolds numbers, airfoil shape does still affect performance (L/D). I'm quite sure I could compete with anyone in the world if they were forced to fly a DLG with a flate plate or rectangular wing section. I think a statement like: if it's small and light enough, AND you don't care about performance then airfoil shape doesn't matter is more accurate.
I might correct that statement to say that performance IS best using the flat plate - IF you are flying small aerobats

DLG's?
I don't fly em - flew hand chucks long ago
and some soaring stuff.
What is apparant is that there are few guys on this forum who fly or have flown small aerobatic stuff. especially indoor competition aerobatics.
IF we now shift to 40% scale aerobats - the picture changes and a odd thing happens.
A streamlined wing is used but non of the flyers adopt the airfoil so successful on the full size competition craft - the " icecream cone shape".

Being heavily interested in airfoils -I wonder if you could shed some light on why this is so.
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