Does a symmetrical airfoil produce any lift?

[Hogster]

Title says it all! I'm a bit :confused: on that one - I'd say no, as being symmetrical means there is no pressure difference any more, and hence no lift should be created. Have I just answered my own question?

Thoughts are welcomed. :)

[sukhoi26mx]

A symmetrical airfoil creates lift by flying at a positive (or negative) angle of attack. It performs identically upright vs. inverted. It does require an angle of attack, however, to create lift (i.e. zero angle of attack does not create any lift).

Scott

[Hogster]

Thanks! :)

Umm .... just on the side lines .... what's the performance difference between a symmetrical airfoil and a flat-plate wing (ie. one which has no airfoil)? Surely they both create lift with the presence of an angle of attack? What is the advantage of the symmetrical airfoil?

Learn something new everyday! :cool:

[Zeroaltitude]

The issue of different pressure (Bernoullis principle) being the main cause for lift on airfoils is an old but persistant myth. Bernoullis principle IS correct, but it can only explain a TINY amount of the lift we need to make an airplane fly, and it doesn´t explain ANY of the lift created by a symmetrical airfloil or a flatplate airfoil.

Instead it´s the AoA that creates the vast majority of lift on our planes, by pushing air downwards thus creating lift. If you look at a flatbottom airfoil (which is where Bernoullis principle would be strongest) lying flat on a table straight from the side you will be able to draw a straight line through the airfoil, dividing it´s AREA in the middle from the trailing edge (at the surface of the table) to the leading edge (higher than the surface of the table). This tells you that a flatbottom airfoil has an AoA built in, even when lying flat.

As Scott said, a symmetrical airfoil, or a flat plate airfoil (which is actually nothing but a very thin symmetrical airfoil) needs an AoA to create lift.

I just can´t belive that the myth of Bernoullis principle as the major contributor to lift is so hard to "kill". It is even still tought to schoolchildren! ;)

Anders O

[Zeroaltitude]

Hogster, a symmetrical airfoil is more aerodynamically efficient due to it´s teardrop-shape. When at an AoA it produces less turbulence, hence is more efficient.

Anders O

[LTChip]

All wings need AOA to produce lift (in still air). In the case of under cambered wings - the concavity of the underside of the wing effectively increases AOA of certain parts of the wing relative to the plane of the aircraft's motion. Wing shape has to do with aerodynamic efficiencies specific to the speed and type of flight in question. Slow-flyers can do well with more drag (or as ZeroAltitude says turbulence) because the lift from the shape is better at the slow speeds whereas a sport plane needs a sleeker shaped wing to reduce drag.

The grade school version of lift is only true in the case of ground effect flight. It amazes me that it continues as it is tougher to understand. I always knew intuitively that those particles of air could not have some innate desire to "catch-up to their buddies" as the grade-school version wants us to believe.

Someplace there was a link to a great explaination of flight debunking the differential pressure caused by airfoil shape theory.

[Zeroaltitude]

LTChip, when you´re talking about undercambered wings, isn´t it still true that the trailing edge needs to be lower than the leading edge to produce that lift? I allways felt that an undercambered airfoil placed at zero AoA would produce zero lift, just as a flat plate at zero AoA?
As I said, a flatbottom airfoil has AoA built in, but if you take away the bottom surface (thus producing an undercambered airfoil) wouldn´t the curvature from the leding edge to the highest point produce as much "sink" as the curvature from the highest point to the trailing edge would produce lift, thus resulting in no lift at all at zero AoA?

Anders O

[seftonde]

put your hand out the window of your car at 80 KPH. Increase angle of attack - feel the lift! No airfoil and not even a very even or symmetrical surface...

I've always been enamoured of this (and anything related to flying) and have done this as a kid as early as I can remember. When I started flying (full scale) - I flew a Grumman TR2, which has a sliding canopy that you can fly with half open. After I soloed I couldn't wait to try this trick - increased angle of attack and sprained my arm (airspeed over 120 kts).

FWIW
Daniel

[pfm]

Someplace there was a link to a great explaination of flight debunking the differential pressure caused by airfoil shape theory.
There was a sizeable article on this in Quiet Flyer a few issues ago.

[seftonde]

There was a sizeable article on this in Quiet Flyer a few issues ago.

...was an excellent one in, I think the most recent AMA mag. It's in the car - I'll go get it after the kids go to bed...

[TooTall]

somewhere in this equation you have to add power.....an undercambered kite design can fly with no power and some wind bellowing it .....such as sails on a ship....when you take a flat plate and release it , does it not tend to "flutter" to the surface...So with power and AoA a symetrical or flat foil will produce some kind of lift....My question to you David is ....are you looking for a slow flyer that produces lift , or something with power that provides it's on lift?...

[Konrad]

Of course it does. You have seen it for yourself. The benefit of a symmetrical airfoil is it offers zero pitch moment at zero lift. The benefit of cambered surface (same amount on both side but in opposite directions) vs. two parallel surfaces is less drag at all but the lowest Reynolds number.
It’s long past do to lynch Bernoulli. OK those that preach Bernoulli as the phenomenon that explains lift. Bernoulli was a 17 century mathematician that never saw flight other than birds
Konrad

[LTChip]

Zero - I am no aeronautical engineer and only have a semester of fluid dynamics but I think you are basically correct (except that attempting to classify under cambered wings in their infinite varieties of shapes might be hard to then generalize).

If you use the geometric plane created from the LE to the TE as the measuring point then it is hard for me to think of a way that any under cambered - or any wing at all - can generate lift without some AOA relative the airflow / direction of travel. I did not mean to infer that this rule does not apply to under cambered wings but rather was just pointing out some reasons why, depending on the type and speed of flight - an under cambered wing produces a net positive effect on lift even with its increased drag which is generally otherwise consider detrimental.

[Brutus]

if you really want to understand read this: http://www.aa.washington.edu/faculty/eberhardt/lift.htm

don't say I didn't warn you.
every answer leads to two questions.

[LTChip]

http://www.amasci.com/wing/airfoil.html#parts

Here is that excellent link I mentioned before - finally found it. Click through all it has to offer - very well done site.

[m4mm]

Birds in air and Fish in water all obey the same principle to produce lift. The only difference is the medium in which the wing or fin operates (air verses water). So regardless if Bernoulli never saw classical mechanical wing in flight does not mean the physics of the problem is changed. The principle of physics remains unchanged. Yes their are absolutes in this world!!!

Mike

[Konrad]

Hogster,
LTChip points to a very good site. The battle will rage as long as the question is not well defined. Bernoulli is alive and well in fluid dynamics: that a fluid traveling in a system at a higher velocity will have a lower pressure than a fluid traveling at a lower velocity in the same system. Perfume atomizers still work (but stink). In aerodynamics the question does a flat wing generate lift is answered by YES. No curves needed. Do curves help yes in all but the slowest fluids (small Reynolds numbers). So the answer to Hogster's above question is YES. If Hogster had asked at zero incidence the answer would be different.

We all need to keep thinking clearly and push our teachers when we see what is taught does not reflect what we see. We might not be seeing the whole picture or they may not be answering the right question.
Konrad

PS. M4mm, Lets agree to keep compressible vs. non compressible fluid systems out of this.

[Alex J]

If the air molecule is deflected downwards by the airfoil, then according to Newton's Conservation of Energy Law, to every action (air being deflected downwards), there is a reaction (airfoil being pushed upwards). That and the difference of pressure above and underneath the airfoil. Both create lift and make airplanes fly.

http://www.aviation-history.com/theory/lift.htm


...oops... same article as in post #14...

[Eclipse_7]

I like the part about a wing of infinite span does not require power for lift.

Where do I get one of those?

[Hogster]

Wow! Looks like I touched a nerve! :) Thanks for all the replies and the links - I've bookmarked both sites and I'll go back to them when I have the time! :) I guess the next important question is where should the CofG be on a plane with a flat-plate or symmetrical wing? On a non-symmetrical, it's usually above the highest point on the upper side ... well that's how it's always seemed on my models. Can anyone throw a light on that for me?

Thanks! A most interesting topic! :rolleyes:

[madrob]

CG location is usually from 30% to 20% MAC (mean aerodynamic cord). Most airfoils reach max thickness between 25% and 30% cord so that's a good place to start. I usually ballance the models I design at 30% MAC unless the tail volume is small (Gee Bee Z) and then I go around 20%.

Flight testibg will allow you to tune in the CG to give best performance but I would sart atr 25%.

Good Luck!

MadRob

[1965ford]

Just strap a rocket to it, anything will fly if given enough motivation.

[Brutus]

http://www.amasci.com/wing/airfoil.html#parts

Here is that excellent link I mentioned before - finally found it. Click through all it has to offer - very well done site.
excellent post LT

[Zeroaltitude]

Zero - I am no aeronautical engineer and only have a semester of fluid dynamics but I think you are basically correct (except that attempting to classify under cambered wings in their infinite varieties of shapes might be hard to then generalize).

If you use the geometric plane created from the LE to the TE as the measuring point then it is hard for me to think of a way that any under cambered - or any wing at all - can generate lift without some AOA relative the airflow / direction of travel. I did not mean to infer that this rule does not apply to under cambered wings but rather was just pointing out some reasons why, depending on the type and speed of flight - an under cambered wing produces a net positive effect on lift even with its increased drag which is generally otherwise consider detrimental.

LTChip, thanks, I wanted a clarification because I read it as if you knew something I didn´t (which you probably do anyway! ;) ). And I made a huge error in my reasoning, since when I referred to what you wrote I was exclusively thinking of an undercambered wing in the form of a bent flat plate (such as seen on many depron slowflyers).
Just as you say, there are indefinite variations to an undercambered wing, and those closer in shape to a flatbottom airfoil than a true "bent flatplate" should have some degree of "built in" AoA, just to a smaller degree than the flatbottom airfoil.

Anders O

[Zeroaltitude]

m4mm, while noone so far in this thread has suggested that Bernoullis principle is WRONG, what we are debating (IMHO) is the RELEVANCE of this principle when it comes to the flight of airplanes (fullsize of models), and whether it should be taught as the principle that makes flight possible or not.

Think of it like this, one of the "demonstrations" of Bernoullis principle as used in schools is to take a leaf of paper, and blow on the upper surface. The difference in airspeed creates a lowpressure on the upper surface of the paper thus sucking it upwards. Voilà, there you have flight!!! Hmm..., no you don´t! You have sufficient lift to make a paper bend upwards, but nothing more.

If it´s just a matter of different airspeed on the upper and lower surfaces of the wing, then it wouldn´t matter what the airspeeds where, just that there was a big enough difference between them. So what is the difference in airspeed between the upper and lower surface of your typical model? 15-20% (I doubt that it is that much, but whatever)? Then what if you take a model, and place it on a scale, tether the model by the prop with a line. Place a fan (or a row of fans) in front of it so that it blows air at 5mph over the upper surface, and next to nothing over the lower surface. This would roughly simulate a model traveling at 20-25mph (if we accept 20% as the airspeed difference for a "general" model). Now let´s assume that this particular model has a stallspeed lower than 20mph. Do you think it would achieve anything close to liftoff? I would be surprised if the "lift" (as represented by a lower wheight on the scale) would reach over 1% of the AUW of that model!

Yes, Bernoulli was right. What´s wrong is using his principle as the explanation for flight, he sure never did!

Anders O

[Konrad]

Hogster,
I think you are in the wrong form. Try to search or ask these fundamental questions in the Model Science form.
http://www.rcgroups.com/forums/forumdisplay.php?f=136
We are all willing to help answer your questions. I’m just trying to keep thing in order so that the next guy can research his answer,
Remember there are no bad questions just incomplete answers.
Konrad

[Zeroaltitude]

That´s a good thought Konrad. And although the issue has allready been discussed several times there, perhaps this thread should be moved?

Anders O

[Hogster]

It's funny you should mention that but I did actually look at the Modelling Science forum before posting, but it looked quite quiet there (a few days worth of postings on the first page) whereas the Parkflyer forum is extreeemely active (few HOURS worth of postings on the first page!). It pained me to post this slightly OT question in the Parkflyer forum, but the thing I'm building which stimulated me to ask this question IS a parkflyer so it's at least partially legitimate.! ;) The plane is a 'Tiny' by the way, and it's my first balsa-build - yipeee! :D

[Brutus]

It's funny you should mention that but I did actually look at the Modelling Science forum before posting, but it looked quite quiet there (a few days worth of postings on the first page) whereas the Parkflyer forum is extreeemely active (few HOURS worth of postings on the first page!). It pained me to post this slightly OT question in the Parkflyer forum, but the thing I'm building which stimulated me to ask this question IS a parkflyer so it's at least partially legitimate.! ;) The plane is a 'Tiny' by the way, and it's my first balsa-build - yipeee! :D
Good point. I appreciate that it was discussed here.

[Konrad]

Hogster,
Yes, but you will probably get a more qualified answer to your questions. As the emperor Claudius said in "I Claudius" when he was called a half wit "it’s not the quantity of one’s wit that counts but the quality of one’s wit’s that counts". I'm saying that the amount or frequency of posts has nothing to do with the validity of the form.

Your C of G is not an issue of airfoil but one of airframe stability. I have many airplanes that the C of G is totally off the airfoil at the mean aerodynamic cord. Some ahead of it some behind it. Look at a canard in Rutan's artwork or look at the old-timer sailplanes with their lifting stabilizers.

Konrad

[Luke303rd]

if you really want to understand read this: http://www.aa.washington.edu/faculty/eberhardt/lift.htm

don't say I didn't warn you.
every answer leads to two questions.

I used this website by Mr. Eberhart for my 7th grade science project on aerodynamics..