Varying wing section info?

[David Hall 9]

On quite a few gliders, I have seen the wing section quoted as, for example: "AG35-36-37".

I guess that this will mean that it varies linearly along the wing...is that what it probably is? is there a design convention for the way this changes?

I am considering having a go at glider wing design and wonder how to incorporate this feature.

 

Edited By David Hall 9 on 25/04/2020 12:11:25

[Dickw]

A linear change between sections along the wing is one possible option (and perhaps the most common), but that it really is up to the designer and that depends on what he hopes to gain by using the different sections at different locations. For example, you might use a thicker tip section to try to avoid tip stalling or a thicker root section to give added strength. Not saying either would work - just simple examples.

Sites like Airfoil Tools allow you to compare how different airfoils work.

Go for it - and lets us know how you get.

Dick

[David Hall 9]

Posted by Dickw on 25/04/2020 12:39:50:

A linear change between sections along the wing is one possible option (and perhaps the most common), but that it really is up to the designer and that depends on what he hopes to gain by using the different sections at different locations. For example, you might use a thicker tip section to try to avoid tip stalling or a thicker root section to give added strength. Not saying either would work - just simple examples.

Sites like Airfoil Tools allow you to compare how different airfoils work.

Go for it - and lets us know how you get.

Dick

Thanks Dick,

I am considering building a scaled-up version of a glider that I have at the moment. As you say, altering the thickness of the airfoil is something that I might expect. I will have to dig a little deeper. I guess that the different sections incorporate similar features and advantages when compared to simply altering the thickness of a single section.

[Simon Chaddock]

David H

In a structurally high performance wing the requirements of strength usually exceed the optimum wing section. i.e. they are thicker at the root and use root wing sections that promote this attribute. This is particularly noticeable in modern airliners where the aerodynamic performance to payload of the wing is critical.

Whilst a modern high performance glider may use a broadly similar section root to tip the advantage of a 'laid up' construction means the internal structure of the wing can be very different over the span.

At model sizes the reduced geometry improves the effective strength to weight of materials so 'optimum' wing sections root to tip are structurally easier to obtain. The problem is of course determining exactly what they are and the benefits that might be achieved.

I find the "Incomplete Guide to Airfoil Usage" a useful reference to the actual wing sections used in full size planes.

Edited By Simon Chaddock on 27/04/2020 09:35:11

[Peter Miller]

JUst a little thought on wing sections.

Some years ago there was a glider plan published in Model Flying in the USA.

This glider had been winning competitions all over the place and all the other competitors kept asking the designer what wing section he had been using as he kept beating models with their highly accurately built Seligs ad Eppler sections.

He had a very simple answer which upset an awful lot of people.

He was using good old ClarkY

Now there is food for thought!!!

[Martin Harris - Moderator]

Interesting observation Peter - I've often wondered just how important wing sections are at model sizes. Perhaps competition pilots may notice some differences but having flown models with flat plate wings on many occasions, the importance of accurate and carefully selected sections might be a little overstated for many purposes.

Perhaps we should simply follow the guidance of the great scientific authority, Anne Elk:

[PatMc]

Posted by Peter Miller on 27/04/2020 10:21:35:

JUst a little thought on wing sections.

Some years ago there was a glider plan published in Model Flying in the USA.

This glider had been winning competitions all over the place and all the other competitors kept asking the designer what wing section he had been using as he kept beating models with their highly accurately built Seligs ad Eppler sections.

He had a very simple answer which upset an awful lot of people.

He was using good old ClarkY

Now there is food for thought!!!

Who was it & what glider plan was published ?

[Martin Harris - Moderator]

Whoops - just watched the Anne Elk clip I posted and it isn't as per the original - for those not well versed in classic Monty Python, her original theory, expounded after great preamble and clearing of throat was that the brontosaurus started off thin at the front, was much, much thicker in the middle and thin again at the tail.  Sorry if anyone was wondering how women with moustaches could relate to aerofoils!

Edited By Martin Harris on 27/04/2020 12:05:51

[David Hall 9]

I found this in a thread elsewhere. It explains the reason for the changing section.

" Dr Drela designs airfoils for particular Re operation ranges, which is one of the reasons he uses different sections along the span of a wing. As the cord shrinks and the Re gets smaller, he uses an airfoil optimised for that range."

These are fine design changes indeed.

So, if I were to adhere to this idea, then my scaling up of a wing that uses a range of AG airfoils by ~25% might change the airfoil sections to a new set of AG airfoils.

[brokenenglish]

Posted by PatMc on 27/04/2020 11:39:17:

Posted by Peter Miller on 27/04/2020 10:21:35:

Who was it & what glider plan was published ?

Pat, it was probably down to the pilot's flying and air picking skills. There have been hundreds, if not thousands, of cases like that, over the years, in FF competition.

[fly boy3]

A similar analogy, years ago before mobiles etc had cameras, if you saw a particularly good picture, people would often ask what camera was used lol. It was all down to the photographer. Cheers

[Former Member]

[This posting has been removed]

[PatMc]

Posted by brokenenglish on 27/04/2020 14:28:39:

Posted by PatMc on 27/04/2020 11:39:17:

Posted by Peter Miller on 27/04/2020 10:21:35:

Who was it & what glider plan was published ?

Pat, it was probably down to the pilot's flying and air picking skills. There have been hundreds, if not thousands, of cases like that, over the years, in FF competition.

Add meticulous trimming, practicing & selecting which comps to enter. That's why some win or have won regularly despite the model designs they use often also being used by their opposition.

Still like to know the answer to my questions.

[Peter Miller]

Posted by PatMc on 27/04/2020 15:14:28:

Posted by brokenenglish on 27/04/2020 14:28:39:

Posted by PatMc on 27/04/2020 11:39:17:

Posted by Peter Miller on 27/04/2020 10:21:35:

Who was it & what glider plan was published ?

Pat, it was probably down to the pilot's flying and air picking skills. There have been hundreds, if not thousands, of cases like that, over the years, in FF competition.

Add meticulous trimming, practicing & selecting which comps to enter. That's why some win or have won regularly despite the model designs they use often also being used by their opposition.

Still like to know the answer to my questions.

Sorry, it was at least 10 or more years ago and I wasn't really in to Gliders.

Flying Models has since vanished along with many other magazines

[Peter Jenkins]

Posted by David Hall 9 on 27/04/2020 12:17:18:

I found this in a thread elsewhere. It explains the reason for the changing section.

" Dr Drela designs airfoils for particular Re operation ranges, which is one of the reasons he uses different sections along the span of a wing. As the cord shrinks and the Re gets smaller, he uses an airfoil optimised for that range."

These are fine design changes indeed.

So, if I were to adhere to this idea, then my scaling up of a wing that uses a range of AG airfoils by ~25% might change the airfoil sections to a new set of AG airfoils.

The key point here is Reynolds number or Re as quoted above. Re is defined as the relationship between viscous and inertia forces and is calculated by (air density x wing velocity x critical length (wing chord)) divided by air viscosity. You can Google air density and viscosity and plug them into the equation as a standard number - good enough for our purposes. You now need wing speed and wing chord to calculate the Re. Below a Re of around 750,000 a laminar boundary layer predominates while above that a turbulent boundary layer predominates.

Why does this mumbo jumbo matter? Well, most full size aircraft operate above the transition Re number while models, other than really large ones, operate in the laminar boundary layer area. In real terms, a wing operating in the laminar region only works for that portion of the wing where the laminar bl is attached and they only stay attached in a negative pressure gradient i.e the flow is speeding up and the pressure drop is what keeps it going. Once past the max thickness part of the wing, the pressure gradient rises as the airspeed reduces and at that point a laminar flow detaches and the wing aft of that point contributes no lift but lots of drag.

A turbulent boundary layer starts off as a laminar bl but then transitions to a turbulent boundary layer and remains attached even in an adverse pressure gradient so the whole wing works until you reach the stalling angle of attack.

That is why big models fly better than small models and why scale models have to have much larger tail surfaces to remain controllable.

When comparing aerofoil performances it is vital to know the Re at which their performance was obtained. For most models which operate in the laminar bl region, trying to use a full sized aerofoil in the hope of getting similar results is usually disappointing to say the least. The free flight guys used to use turbulators ( a thin string stuck onto the aerofoil spanwise just after the leading edge to trigger the laminar bl to transition to a turbulent bl and thus stay attached for more of the wing.

Beware of expecting the benefits of optimised wing sections from full size to model size unless you can get the Re up to above at least 750,000.

[David Hall 9]

Thank you for the very detailed explanation Peter Jenkins.

I guess then, that the value of a specific varying section is arguably overkill on a model, but especially on a model wing of film covered open ribbed structure. The ribs might be accurate, but sagging film section between the ribs must be difficult to design to a known section.

 

Edited By David Hall 9 on 02/05/2020 11:14:55

[Peter Jenkins]

David, that is exactly so. If the wing section testing was at a Re > 800,000 and your model is operating below or significantly below that Re then you would not expect to get the same performance. Incidentally, an airliner on approach (i.e. low speed) is operating at an Re of 1.5 million approx and at normal cruise at about 7 million.

You would need a low speed wind tunnel to run tests on model aerofoils at the correct Re in order to be able to test the differences in drag, L/D and coefficient of lift with angle of attack before you could be sure that your efforts would be suitably rewarded. That's why most aeromodellers will just try out wing sections to see which one performs better - a bit like used to be done for full size aviation before more advanced aerodynamic theory and testing provided something other than rules of thumb to achieve success.

Today, aerodynamacists use computer programmes to design advanced aerofoils and can determine all the main characteristics of the design before getting near a wind tunnel. However, there is still an extensive flight test programme to check that the design points are correct and whether there are any odd handling features.

[Simon Chaddock]

David

Your assumption is certainly correct in most cases.

Most smaller models have power to weight ratios that far exceed that found in the equivalent full size. They have to be like this for Reynolds number reasons as they are aerodynamically inefficient. With this sort of disadvantage other parameters like the frontal area of a wing rather than its section have a greater significance.

In other words a thinner wing may outperform a thicker one although it is likely to stall more readily and at higher speeds. The higher power to weight of models tend to mask such deficiencies but it does put more emphasis on having control surfaces (over size?) that remain effective over a wide speed range.

If power to weight is a key factor just don't forget about the other parameter - weight.