GliderIreland - Ireland RC Forum - Flying Model forum in Ireland
Techniques => Misc => Topic started by: joe on December 11, 2006, 11:19:22 AM
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Recently read an article in a mag about HQ35/12.The writer had used this profile on many sailplane to great effect.
It is aparently designed specifically for use with scale sailplanes.
It has a good level of efficiency and excellent low speed behaviour.
Where the root of many vintage gliders is quite thick.The thickness of the airfoil(thats the 12 part of 35/12) can be incressed at that point to say 16%and than tapered back to 12% at the tip.
All sound pretty good and apparently no need for washout!
Maybe worth looking into for a next project?
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Hi there,
On the Retroplane forum, there is a scale airfoil project going on.
We are lucky to have some professional Aerodynamist working with us !
The latest profile is the MS 535
(http://www.retroplane.net/forum/images/uploads/Matthieu_S/ProfilsMS535_206.gif)
Most of the vintage sailplanes are using the Go535. Mathieu (MS) have updated this profile for us with a bigger TE (easier to cut) and made some calculations for 3 thickness :
(http://www.retroplane.net/forum/images/uploads/Matthieu_S/PolaireMS535_192.gif)
The file can be found here : http://www.retroplane.net/forum/download.php?id=177
A second profile has been developped : The MS Retro. Flat profile, a bit like the ClarkY, but once again, developped for our scaled use. Easy to build also with a big TE too :
(http://www.retroplane.net/forum/images/uploads/Matthieu%20S/25032006085617_MSretro313.jpg)
And the file, with 3 thicknesses can be downloaded here : http://www.retroplane.net/forum/download.php?id=26
If interrested, I can give you the use on the glider for each thickness.
Fred
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Okay chaps,
If the lift from an aerofoil is suposed to come from the camber on the underside, how does a flat bottomed wing creat lift?
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There is more in it than the camber. :)
If not how a flat wing shockflyer would fly.
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Hi Happy Days. Welcome to the forum.
A wing can still have camber with out having a curve on the botton of the wing. I'll try to explane with the diagram at the bottom. The straight line distance (the blue line) from the leading edge of the wing to the traling edge is called the chord line. The line that is equally distant from the top surface and the bottom surface (the red line) is called the camber line. The maxium distance between the chord and camber line is the camber of the wing.
Also, hopefully this won't mess things up :D , a wing does not have to have camber to creat lift. It's the angle that the wing meets the air at (called the angle of attack) which creats lift.
Hope this helps.
(http://www.gliderireland.net/forum/images/uploads/Alan_Perse/Airfoil.jpg)
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Fernando got there before me :D . Doh!! :D . Yes there is more to lift generation than camber.
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Ahh!
I changed back mine because your explains way better! :)
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Yes,..........I'm fine with the concept of lift being derived from having pressure exerted on the underside of a wing. As I remember from my school days (circa 1897) about 25% of a wing's lift comes from air 'hitting' the underside of the wing.
However. I'm sure we were taught that 75% of the lift comes from air having to travel over the greater distance over the top camber of the wing and thereby ahving to travel faster than the air traveling under the wing This causes a reduction of pressure on that top camber due to what I think was called the "Venturie" effect. (The same effect that causes petrol to rise from the float chamber in a carburettor. But let's not get bogged down with internal combustion engines. Horrid smelly things.)
This then leaves the wing with relatively higher pressure on the botton, and relatively lower pressure on the top, hence LIFT.
Am I right so far???
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The lift is generated by the top surface (difference of speed between the top and the bottom of the traveling air), usually, on the first 1/3 of the root (then, the air layer simply don't stick to the surface).
So, basically, you don't need camber to generate lift (Clark Y and so on..) but... :D
...Adding camber to a wing will generate more lift ! Same story, the air travel further, generating more lift, increasing also the critical angle of attack before stalling (a cabered profile generate lift, even at 0 degrees of angle of attack)
All perfect in the good world ? Well, not really, as adding camber, generate more drag too... Question of compromise ! :D
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Yes,..........I'm fine with the concept of lift being derived from having pressure exerted on the underside of a wing. As I remember from my school days (circa 1897) about 25% of a wing's lift comes from air 'hitting' the underside of the wing.
However. I'm sure we were taught that 75% of the lift comes from air having to travel over the greater distance over the top camber of the wing and thereby ahving to travel faster than the air traveling under the wing This causes a reduction of pressure on that top camber due to what I think was called the "Venturie" effect. (The same effect that causes petrol to rise from the float chamber in a carburettor. But let's not get bogged down with internal combustion engines. Horrid smelly things.)
This then leaves the wing with relatively higher pressure on the botton, and relatively lower pressure on the top, hence LIFT.
Am I right so far???
Yep, you are right !
The lift is created by the difference of pressure between the top and the bottom.
Generaly, most of the lift is generated by the first 1/3 of the profile, on the upper surface.
This is the air speed difference between the top and bottom that create the lift.
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Okay,.............Thanks Fred, and you other guys, for replying. I must go away and think about this. It's all very interesting stuff. Hmmmmmmmmmmmmm.................!
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You're welcome ! :D
btw, forgot to say, Fernando, in a shockflyer, this is the incidence of the wing that create the lift (well, sort of lift, I will more call that a air deflection :P ), not the "profile". :D
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So Fred, you're saying that the shockflyer merely deflects air in one direction and the wing "reacts" in the opposite direction? like a rudder perhaps?
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Exactly ! :D
The wind speed at the top and bottom are the same (flat), no lift generated, only drag.
Change the incidence, and you will go up or down. :D As you say, abit like a rudder / elevator or your hand through the window of your car in good speed ! :D
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I'm thinking of a symmetrical wing. the camber on the top is equall to the camber on the bottom. Assuming zero angle of incidence and zero angle of attack where does the lift come from?
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Well, the symetric profiles generate no lift at 0 degrees and generate lift at incidence, but this thin type of profile stall rapidely, well, before starting to generate "lots" of lift.
That's why, on the "fun fly" type of plane, you do a really thick profile (stall later and can go to greater incidence) with a big LE (if that stall, that will stall "gently"). but those profiles are neutral, fly the same inverted and normal.
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Right...................... (I think :?: )
So now we come to the core of the matter, Fred.
A symmetric wing produces no lift at 0 degree of incidence, the same as a "flat" wing produces no lift at 0 degree of incidence. They both only produce lift by deflecting air downward. Neither wing produces any induced lift
A non-symmetric wing produces lift by both deflecting air downwards, via the underside of the wing, AND produces lift from the camber on the top of the wing. (What I call 'induced lift')
Assuming what I've just written to be true, Question; Why does any one bother producing a symmetricaly cambered wing when, according to the above, it flys no better than a plank of wood?
Why aren't all wings semi-symmetric or flat bottomed? They produce far more lift. (Induced and reactive lift)
I know there must be a reason for symmetric wings, but I just don't know what it is. :!:
So come on Fred, you aerodynamic guru,..........hit me with the answer!
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That's an easy one ! :D
As ymetric wing is far more efficient than a plank !
Less drag, and will stall later at far greater incidence !
Why aren't all wings semi-symmetric or flat bottomed? They produce far more lift. (Induced and reactive lift)
Because they don't produce more lift ! :D A cambered profile like the Go535 will produce far more lift than a ClarkY or maybe any flat bottomed profile.
All that without taking account of the speed of the plane of course ! A flat bottomed or semi-symetric profile (well, not all of them.) will be much more efficient at a speed range, before the air layer stop sticking to the profile.
And all that without even talking about the Reynolds number ! :D :P
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Hmmmmmmmmmmmm :?:
Firstly Fred I have to say I'm very impressed with you being awake at such an early hour of the day.............very good :clap:
Now I'm starting to get confussed, which is very easy at my age!
Could you tell me please where I might be able to find a diagram of a Go535 and a Clark Y profile?
You see, I'm thinking if simply having a later stall and offering less drag is the only advantage of a symmetric wing over a plank, why not have a semi symmetric wing with slots in the LE. Surley they reduce the stall yet offer less form drag than a symmetric wing.
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I've found a Clark Y airfoil diagram, still can't find a Go535 though. :x
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OK we need to make somthing clear first, this may be a long post :D :D . The camber of a wing is the maxium distance between the camber line and the chord line ( the red and blue lines on the diagram on the prevoius page). It is not the curvature of the top and bottom surfaces of the wing, although the curvature of the top and bottom surfaces of the wing effect the shape of the camber line.
And here's a spanner in the works :) , wings that have camber can creat lift at small negative angles of incidence. The reason a symetrical wing creats no thift at 0 degrees incidence is because it has no camber. The top and bottom surfaces of the wing have the same shape, thus the camber line is in the same position as the chord line. As a result there is no distance between them thus no camber.
If your wondering why i know so much its because I have two or three books on aerodynamics to look at :D . But I'm not an expert, its the books that are talking :D :D . One of them is called "Model Aircraft Aerodynamics" by Marin Simons, its very good book.
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Alan that seems a good reading!
Thanks for sharing. :)
I still prefer to read real books than be reading from the web...
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I still prefer to read real books than be reading from the web...
I agree
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Yes,..........I think I agree with Alan in that probably the best thing for me is to go back to basics. :roll:
At the moment as soon as I hear one answer, my brain, (what there is of it) pops up another question.
The problem with understanding lift is that there seems to be no black and white answers. The 'goal posts' are not fixed
Think I'll click on Amazon Books and order a copy of that "Model aircraft Aerodynamics"
As I remember it,..................... "Pi multiplied by R squared over 4', ................ take away the number you first thought of!! "
Hmmmm, this could take a bit of time :?:
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Well, compelled by the curiosity i went to Amazon... and yes i bought the book.
There is much info out there i still need to learn. :)
I dont wanna be around RTF's and ARTF's forever!!!
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Oh No don't tell me I've started something :) :) :P .
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Ahah!
You probably are getting some commission by making ppl buy that book aren't you? :D
From my side I've been trying to gather some more info regarding RC airplanes, when you mentioned that book and i went to amazon and saw the comments it seemed a good choice. Also the price for a used one was cheap. ;)
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Just to warn my book has arrived.
Pretty soon you may start asking questions! :D
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Whats a Book? :D :D :D :wink:
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I was reading an article today on the physics of sailing which mentioned a Nasa aerodynamics e-book site at http://www.grc.nasa.gov/WWW/K-12/airplane/short.html. Some quite interesting stuff here, including quite a bit on theories of lift (see earlier posts here). Check out the simulations of airflow and pressure over aerofoils and plates.
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Very interesting Peter, thanks for posting that link :clap:
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Yes thanks for that link Peter.
I instantly hit the 'bookmark' button.
Interesting bit on kites too.
I find sailing very like flying turned 90 degrees.
The foresail acts like slots when going to windward, allowing the mainsail to have a much greater angle of incidence without stalling.
And many people don't appreciate that you can stall a keel, when the side pressure becomes too great with little forward motion. This is especially so on these modern 'plastic bathtubs' with wing shaped keels, and is why the old sailing fishing boats had large flat keels to use the 'barn door' effect when hauling nets.
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Hi Ron,
Yes - the gist of the article was that sailing was just like flying. The keel is a symmetrical airfoil and will not work if the boat is pointing in exactly the same direction as the boat is going.
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HI Peter,
Yes that's it.
Leeway it's called.
If you're pointing, say, north, and the wind is from the west, there will be perhaps 5 to 7 degrees of leeway. So you're actual course will be 005 to 007.
So the leeway angle is the angle of incidence.
Mind you, there are lots of other considerations for keels, like righting moment due to ballast, performance when heeled, and even "will it fall over when dried out against the quay". :lol:
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I guess you can add me to your list of glider fliers who also like to sail! Although in my case all I have access to is a very old Heron dinghy that I resurrected and redecked a few years ago. Good to get out on the water though. Looks like you have a nice boat there. Can't beat wooden boats (and gliders!) - although both are a lot of work!
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Yes it's amazing just how many sailors also fly, either full size or models.
Like the owner of Rossbrin Boatyard where my boat is at the moment. He flies model gliders.
There must be something about Herons.
My boat is a Purbrook-Rossiter 'Heron'. Built 1960 of Mahogany on Oak. 22' 3" LOA. Needs a lot of work doing this year. Well it's needed doing for many years, but this summer she's staying ashore so I get some dry/warm weather for the work.
Can't say the keel is exactly 'airfoil' shaped though. More 'barn door' principle. :)