Fully Symetrical Airfoils?

[MicroRC Beginer]

Ok, beginer question, but how does a fully symetrical airfoil work, if it symetrical wont it produce just enough lift as is it does downforce?

[Bill Glover]

The wing needs to be at a positive angle of incidence to produce lift. Normally on an aerobatic model it's rigged at zero degrees relative to the fus. centre line & thrustline ... so the plane has to fly in a very slight nose-up attitude. Roll inverted and with a tiny amount of down elevator to lift the nose it flies exactly the same.

[faust9]

Wing cross sections are a lot more complex than we are initially led to believe (http://www.physlink.com/Education/AskExperts/ae25.cfm). Bernoulli's law (http://scienceworld.wolfram.com/physics/BernoullisLaw.html) is what most people are taught (air going over the upper surface travels faster...) but as you noticed there are symetrical foils (Cap 21 for instance) as well as flat wings. The lift does not come from the curve of the wing (http://www.eskimo.com/~billb/miscon/miscon4.html#wing). Also there is another theory, the Newtonian explanation (http://travel.howstuffworks.com/airplane2.htm), floating around; however neither theory is 100 % right or wrong. The fact of the matter is flight is derived from a multitude of fluid dynamics such as turbulence and angle of attach as well as the above theories. The analysis of flight is pretty complex, but if you do a goolge searve for "How airplanes fly turbulence", you'll find a wealth of information which delves deeper into the other effects present.

edit: look here (http://www.eng-tips.com) for a lot of information concerning the dynamics of flight.

[Ollie]

When the symmetrical airfoil is at zero degrees angle of attack, the flow is symmetrical, top and bottom and no lift is produced. As the angle of attack becomes positive, with the leading edge higher than the trailing edge, the flow no longer divides in a symmetrical maner at the leading edge. The stagnation point, where the flow divides, moves down and back a little along the lower surface. The flow over the top first has to change direction and flow back around the leading edge and then over the top surface. The acceleration of the flow around the leading edge and over the top produces a low pressure reagion that pulls down air above the wing. The flow over the bottom of the airfoil creates a high pressure region that pushes down the air below the wing. The flow behind the wing is angled down too. This down wash of air behind the wing has a vertical component to its velocity. The air some distance ahead of the wing has no vertical velocity. The lift force produced by the wing is the mass of the air accelerated downward by the wing times the acceleration of that air. F=MA. The fluid dynamics are complex enough to require several mutually consistent theories to describe the various aspects of the flow around a wing with the pressures, velocities, accelerations and forces that result.

[Sail 'n Soar]

[QUOTE]Originally posted by faust9
[B]Wing cross sections are a lot more complex than we are initially led to believe (http://www.physlink.com/Education/AskExperts/ae25.cfm). Bernoulli's law (http://scienceworld.wolfram.com/physics/BernoullisLaw.html) is what most people are taught (air going over the upper surface travels faster...) but as you noticed there are symetrical foils (Cap 21 for instance) as well as flat wings. The lift does not come from the curve of the wing (http://www.eskimo.com/~billb/miscon/miscon4.html#wing). Also there is another theory, the Newtonian explanation (http://travel.howstuffworks.com/airplane2.htm), floating around; however neither theory is 100 % right or wrong. The fact of the matter is flight is derived from a multitude of fluid dynamics such as turbulence and angle of attach as well as the above theories. The analysis of flight is pretty complex, but if you do a goolge searve for "How airplanes fly turbulence", you'll find a wealth of information which delves deeper into the other effects present.

MicroRC, in my humble opinion, many of these explanations become more misleading than informing. I'd guess Bill Glover's explanation is more what you were looking for. That is, a symmetrical foil works by being flown at an angle of attack. If you are wanting the 1st year engineering explanation, then Olie's explanation is right on.

Minor clarifications to the discussions of applicable subsonic flow theories, unless you are flying at near-space conditions:

A. Bernoulli's law DOES work - it's100% right. The flow over the upper wing IS faster, even for a symmetrical foil or flat plate, if it is producing lift, which means it is at a positive angle of attack. Of course, they way you calculate the velocities aroung a two-dimensional foil or three-dimensional wing is what's more complex than the simple statements imply.

B. The curve of the wing DOES contribute to lift. A curved cord line foil produces lift at zero angle of attack.

C. Newtonian theory really doesn't apply to our model applications at all.

Do you want to know how to build a watch?

[faust9]

A. Bernoulli's law DOES work - it's100% right. The flow over the upper wing IS faster, even for a symmetrical foil or flat plate, if it is producing lift, which means it is at a positive angle of attack. Of course, they way you calculate the velocities aroung a two-dimensional foil or three-dimensional wing is what's more complex than the simple statements imply.

This is a shakey position to hold when your talking about science (the study of flight is a science, engineering is the application of the knowledge gleened from said science). As an example, F=MA is, as most know, Newtons second LAW. A law being a theory that stands up under the test of time for a general situation(EG all situations covered under the context of the theory)... That LAW was disproven in 1914--I do believe that's the correct year-- and is now F=(gamma)^3MA. Newton's law needs to be amended for high speed conditions (approaching C). There is nothing that is 100% certain in the eyes of science.

Bernoulli's principle, though a decent explanation which gives a close approximate answer, doesn't hold true over all situations namely at the boundry layer.

B. The curve of the wing DOES contribute to lift. A curved cord line foil produces lift at zero angle of attack.

This is true, but the curve of the wing isn't what develops the lift. Take the flat wing for instance. We've all seen the nifty flying street sign models made from flat foam board, and these fly pretty well. The shape isn't what generates the lift, the AOA is (and all of the phenomena associated)... So, I assert that the shape does have an effect but not due to the Bernoulli principle as most of us have been taught at one point or another.

I agree that a curved camber line (the cord line being a straight line drawn from LE to TE) will produce lift at zero AOA while a zero chamber wing (symetrical wing) requires a positive AOA to generate lift; However this is not evidence that the Bernoulli effect is the dominate factor. I simply stated the the curve of the wing is not what develops the lift, but rather lift is a product of a very complex process. This complex process is easily beyond the scope of Bernoulli's law.

C. Newtonian theory really doesn't apply to our model applications at all.

I'd like to address this with an alagorical question. You're in a semi driving along loaded with 2000 lbm of parakeets in the trailer. All of the budgees are pearched and the trailer is airtight. the weight of the truck, trailer, and birds is 5000 lbm (just bear with me). How much would the truck weigh if it hit a bump scarring all 2000lbm of parakeets into flight?

Answer: 5000lbm. The mass of the birds in the trailer is still in the trailer. The trailer is a closed system so there wouldn't be a delta mass without a transfer into or out of the system. The lift generated by the birds would cause a newtonian equal, and opposite force against the trailed...

This same transfer if force downward is seen in small models as well as super guppies.

In closing I'd like to reference See How It Flies (http://www.av8n.com/how/) by John S Denker:

"3.14 Consistent (Not Cumulative) Laws of Physics

We have seen that several physical principles are involved in producing lift. Each of the following statements is correct as far as it goes:

* The wing produces lift ``because'' it is flying at an angle of attack.
* The wing produces lift ``because'' of circulation.
* The wing produces lift ``because'' of Bernoulli's principle.
* The wing produces lift ``because'' of Newton's law of action and reaction.

We now examine the relationship between these physical principles. Do we get a little bit of lift because of Bernoulli, and a little bit more because of Newton? No, the laws of physics are not cumulative in this way.

There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli's principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton's laws."

Thank you

[vintage1]

Actually, frm a strictly accurate point of view. it goes more like this.

Wings work.

They produce lift at forward airspeed and various positive angles of attack.

Bernoulli' therrem, Newtons laws of physics, and other models, do a pretty good job of predictng how well they work - at least until they stall.

When they stall, the simple explanations are no longer adequate, and even the best computer simulations find it very hard to predict how the wing will behave, since the mathematics needed to analyse every little rapidly changing vortex and the computer needed to do it exceeds the cost of a wind tunnel :)

Which is why we don't fly planes into stalls, and have test pilots. :D

Its a very convenient thing to regard the world as something that is 'governed' by natural 'laws'. Indeed science *assumes* that it is, and proceeds from there.

The reality of the situation is however, that the world is what it is, and 'Natural laws' are just something humans have invented in an attempt to describe how it does what it does.

As to WHY the world is there at all... well we can safely leave that in the capable hands of religion, can't we? :D

[HarryC]

You can read the answers from the specialists. NASA has kindly put a lot of info on the web about how lift is really made, (the reaction to downwash) and put up whole pages to explain how the different path length and half venturi explanations are wrong. They point out that it is a misuse of Bernoulli's Law to use it to explain lift. Take a look at these pages form NASA's Glenn Research Centre, look at the LIFT section and follow the links for the three Incorrect Theory of Lift pages. http://www.grc.nasa.gov/WWW/K-12/airplane/short.html

Please also note that speeding up of the air is not the cause of low pressures around the wing. Acceleration requires a force and that comes from the pressure difference - the low pressure already exists first and it is that which causes the air to speed up towards it and the slow down after it has passed it. See www.rz.uni-frankfurt.de/~weltner/Mis6/mis6.html
and
www.rz.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm

Harry

[HarryC]

Originally posted by Sail 'n Soar
C. Newtonian theory really doesn't apply to our model applications at all.
[/B]

Ignoring relativistic and quantum effects, neither of which are measurable at this level, there is simply no other form of mechanics to apply to an aircraft. Newtonian mechanics is the only theory available. Some people may have misapplied Newtonian theory by looking only at the lower surface but it is they who are wrong, not Newtonian theory. Also note that Bernoulli's Law is a Newtonian Law! Bernoulli simply described one particular instance of the Newtonian general rule of conservation of energy. Physicists can derive Bernoulli's law from Newtonian mechanics without ever seeing a venturi, pressure gauge and flow speed gauge.

Harry

[Sail 'n Soar]

Ok, Ok. What a lively discussion we are in now (and remember, the original question was analagous to asking a watch maker the time of day.)

First, addressing Harry C's point. We are not discussing Newtonian mechanics, but newtonian flow, which really only does a reasonable job with thigs like very rarified gases and billiard balls - it also works reasonably well for flying kites, which may have been where Sir Isaac came up with the theory to begin with.

Second, addressing faust9 et. al., relative to our good friend Bernoulli, was his principal really disproven in 1914? Isn't it still his principal if rho = rho(p)? Anyway, since we're talking about low subsonic, let alone low beam-me-up-Scotty speeds, doesn't conservation of energy still apply - isn't that how you derive Bernoulli's principal?

Relative to the offered web sites, the only thing that I note has improved since I was in school is what's been enabled through computational fluid dynamics. In any case, I'm pulling out of the debate before it really gets deep.

Gerry

[steve lewin]

This just keeps coming round and will go on and on and on, even though the initial question was answered in the first few posts. A fully symmetrical (zero-camber) airfoil only generates lift when it is flying at some angle of attack (when it no longer looks symmetrical with respect to the airflow).

Bernoulli's theorems are perfectly correct in their defined context. Trying to use them to fully explain how a wing creates lift is the error.

It is impossible to talk about anything involving motion without some of Newton's Law coming into the expanation. It is equally impossible to fully explain the creation of lift using just the basic laws.

The trick is either to put in the considerable intellectual effort required to fully understand the complexities of aerodynamics, fluid dynamics etc, or if like me you're too lazy, just accept that what you have are partial explanations that give you a reasonable idea of how things work but not the full story.

Fortunately I find that even though the pilot is pretty ignorant about these things my planes somehow still manage to fly ;).

Steve

[Ollie]

As Vintage I has pointed out, the whole of science is a human construct to help us understand and predict the world around us (nature). Turkey vultures know absolutely nothing about the human explanation of lift, aerodynamics. However, I've been out flown by them on many an occasion. They do very nicely without a knowledge of Bernoulli, Newton, circulation, Coanda effect boundary layer theory, etc. Observing the birds motivated humans to want to fly and the birds have taught us much. The rest is history.

Nature is the reality. Aerodynamics is meerly our cumulitive attempt at understanding part of it.

[vintage1]

Phew. And I thought I was the only one here who susbcribes to the old dictum 'the map is not the territory'

:D

Did you see that report a year or two ago where they calculated the size of a system that would be needed to predict the entire pattern of existence within the universe?

Turns out it's a big as - the entire universe :D :D