Does a symmetrical airfoil produce any lift?

Hogster · Jul 22, 2004, 06:40 PM

Title says it all! I'm a bit

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 · Jul 22, 2004, 06:55 PM

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 · Jul 22, 2004, 07:31 PM

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!

Zeroaltitude · Jul 22, 2004, 07:32 PM

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 · Jul 22, 2004, 07:34 PM

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 · Jul 22, 2004, 09:20 PM

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 · Jul 22, 2004, 09:39 PM

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 · Jul 22, 2004, 09:46 PM

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 · Jul 22, 2004, 09:50 PM

Quote:

Originally Posted by LTChip

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 · Jul 22, 2004, 09:53 PM

Quote:

Originally Posted by pfm

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 · Jul 22, 2004, 10:14 PM

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 · Jul 22, 2004, 10:24 PM

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 · Jul 22, 2004, 10:30 PM

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 · Jul 22, 2004, 10:38 PM

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 · Jul 22, 2004, 10:48 PM

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.

Jul 22, 2004, 06:40 PM	#1
Hogster HeliAP'er! Join Date: Aug 2003 Location: Surrey, UK Posts: 7,212	Does a symmetrical airfoil produce any lift? Title says it all! I'm a bit 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.

Jul 22, 2004, 06:55 PM	#2
sukhoi26mx Scott Stoops Join Date: Mar 2002 Location: Mead, Colorado Posts: 7,672	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 Last edited by sukhoi26mx; Jul 22, 2004 at 06:58 PM.

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Jul 22, 2004, 07:31 PM	#3
Hogster HeliAP'er! Join Date: Aug 2003 Location: Surrey, UK Posts: 7,212	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!

Jul 22, 2004, 07:32 PM	#4
Zeroaltitude Cheapskate freeloader! Join Date: Oct 2002 Location: Orebro, Sweden Posts: 3,155	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

Jul 22, 2004, 07:34 PM	#5
Zeroaltitude Cheapskate freeloader! Join Date: Oct 2002 Location: Orebro, Sweden Posts: 3,155	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

Jul 22, 2004, 09:20 PM	#6
LTChip Registered User Join Date: Feb 2003 Location: Helena, MT Posts: 2,887	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.

Jul 22, 2004, 09:39 PM	#7
Zeroaltitude Cheapskate freeloader! Join Date: Oct 2002 Location: Orebro, Sweden Posts: 3,155	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

Jul 22, 2004, 09:46 PM	#8
seftonde Registered User Join Date: Apr 2004 Location: East Lyme, CT Posts: 62	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

Jul 22, 2004, 10:14 PM	#11
TooTall victim of the system Join Date: Jul 2003 Location: Home of the Toledo RC show Posts: 2,495	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?...

Jul 22, 2004, 10:24 PM	#12
Konrad Measure twice cut once Join Date: Jun 2001 Location: Everett Wa. Posts: 4,564	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

Jul 22, 2004, 10:30 PM	#13
LTChip Registered User Join Date: Feb 2003 Location: Helena, MT Posts: 2,887	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.

Jul 22, 2004, 10:38 PM	#14
Brutus Glue can fix that. Join Date: May 2004 Location: Sunny Miami Posts: 1,183	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.

Jul 22, 2004, 10:48 PM	#15
LTChip Registered User Join Date: Feb 2003 Location: Helena, MT Posts: 2,887	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.