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All Forums >> RC Airplanes >> Aerodynamics >> RE: symmetrical wings
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RE: symmetrical wings - 3/27/2008 6:39:21 PM   
BMatthews



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quote:

ORIGINAL: combatpigg

Bruce, I'm talking about a 4 pound model with a 50"x8" wing. The target speed is 200 mph and it will have a V-tail plus whatever else is humanly possible to make those air molecules happy.

BTW, from now on when I fly up-side-down, I guess I'll have to call it out in advance so you guys won't think that I "lost it"



For finding out the lift needs of things like this I always love to turn to Foilsim. Further let's assume that your model will come in around 4 lbs and let's start with a 9% symettrical airfoil.

At 200 mph Foilsim tells me that your 4 lb plane will be flying at a rousing wing angle of attack of about .085 degrees. As in less than .1 degree to generate the 4 lbs of lift need. The lift coefficient being a staggering 0.009.

This is why symetrical and thin for fast models makes a lot of sense.

Now for sake of argument let's say this is a pylon racer that will pull 10G's in the turns. So it'll need to increase the Cl and angle of attack to generate 40 lbs of lift. At 200 mph 40 lbs of lift occurs with an AoA = 0.92 degrees and a Cl = 0.108

If we look at the low drag end of the Cl vs Cd curves of a 9% airfoil I think we would see that the airfoil will have no trouble at all staying in the lower end of the curves over this range. If you think you'll be generating higher peak G loads then there may well be some advantage to adding up to perhaps 0.3 to 0.5% camber to the airfoil to shift the low drag curves up slightly.

If you were flying at around 100 mph and wanted to do the same 10 G turns your 4 lb model would need to pull a 3.2 degree AoA and run at up to 0.43 Cl using a 9% thick 0.4% camber airfoil. Now things get more demanding and to stay within the lower drag portion of the curve you will want to run with a little more camber but carefully study the 1G end of the operating range to ensure you stay fully within the left side vertical portion of the curves as much as possible.


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Free Flighters go that extra mile........

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       Post #: 26

RE: symmetrical wings - 3/27/2008 6:51:25 PM   
Mike Connor



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quote:

ORIGINAL: victorzamora

...Problem is, it doesn't "know" of the lower density until the end of the airfoil..therefore creating no lift... The way lift "actually" works is through Newton's 3rd Law of Motion...

Back in the 80's we were taught in flight school that Bernoulli's effect was 75% and impact was 25% of wing lift. Things change but are you saying that Bernoulli's effect is a flawed concept and has nothing to do with lift? Please explain the positive lift of a cambered wing at a negative AOA.

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       Post #: 27

RE: symmetrical wings - 3/27/2008 8:26:17 PM   
Red B.


 

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From: Jonkoping, SWEDEN
Status: offline 		FWIW, from a physics point of view the whole Newton vs. Bernoulli debate is rather absurd. There is no conflict between Newtons's laws and Bernoulli's law. Newton's laws explain on a microscopic level how a wing produces lift by deflecting air molecules downward causing a reaction force that we on a macroscopic level observe as lift and drag. What Bernoulli's law does is simply to keep track of the pressure and velocity changes that will occur in the vicinity of the wing as a result of the airstream being deflected.

The confusion seem to arise because it is incorrectly argued (with some variations) that the hypothesis of path lengts applies: 'The profile of a wing is such that the air has further to travel over the upper surface and hence has to flow faster to maintain streamline flow and air which is adjacent before the separation by the airfoil has to meet again behind the airfoil.'. If this hypothesis is used to calculate air stream velocities above and below the airfoil and these velocities are used to calculate the amount of lift using Bernoullis law things doesn't add up. In reality the airfoil will produce more lift than predicted by the calculation.

This is not because there is something wrong with Bernoulli's law. The problem is in the hypothesis above. Air molecules that were adjacent before being separated by the wing, travelling above and below the wing, will not meet again at the trailing edge. In reality the air molecules above the wing will travel faster than the hypothesis of path lengths implies. This has been demonstrated experimentally over and over again in wind tunnels (see poor quality photo below) as well as theoretically using Computational Fluid Dynamics simulations based on the Navier-Stokes equations, i.e., a set of equations based on Newton's laws that are suitable for use with fluids rather than solid bodies.



< Message edited by Red B. -- 3/27/2008 10:20:44 PM >

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       Post #: 28

RE: symmetrical wings - 3/27/2008 9:41:30 PM   
victorzamora


 

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Status: offline 		I'm sorry about the confusing wording...I in no way meant to imply that Bernoulli's Law conflicted with Newton's. What I meant to say is that the idea that Red so eloquently mentioned, which is in fact based upon Bernoulli's Principle isn't the way lift is created and that THAT argument (the quote below) is flawed.
quote:

"The profile of a wing is such that the air has further to travel over the upper surface and hence has to flow faster to maintain streamline flow and air which is adjacent before the separation by the airfoil has to meet again behind the airfoil,"


I'm an 18 year old high school senior, I by no means know exactly what's going on...but I believe the Coanda effect aids Newton's 3rd law of motion to still be in effect. I'm not sure, I think that's what goes on at least .

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       Post #: 29

RE: symmetrical wings - 3/27/2008 10:40:11 PM   
vertical grimmace


 

Posts: 1143
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Status: offline 		Just keep your plane light. Symmetrical airfoils like light weight, otherwise you can have tip stall issues.

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       Post #: 30

RE: symmetrical wings - 3/27/2008 11:24:02 PM   
Mike Connor



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quote:

ORIGINAL: victorzamora

I'm an 18 year old high school senior,

With science knowledge well above most in your age group I suspect.


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       Post #: 31

RE: symmetrical wings - 3/27/2008 11:38:07 PM   
gboulton



Posts: 1862
Joined: 5/28/2005
From: La Vergne, TN, USA
Status: offline 		Couple of comments:

First, to those who provided links/etc to such info, and especially to Red B. for posting his info, well done and kudos.

As William Beaty pointed out in jlkonn's link, the whole "argument" is rather absurd, since BOTH "theories" are correct as far as they go.

I might, however, suggest that the "argument" is even MORE absurd for a much SIMPLER reason...and, admittedly, I'll paraphrase a FEW folks here, among them Langewiesche and...dare I suggest it...the Wrights.

There's no question that there are a few DOZEN ideas or theories surround how a wing flies. Some more or less accurate than others...however, with one exception, every single one of them addresses mathematics, engineering, structure, or dynamics that the pilot can not change. Further more, every one of them, again with one exception, has "loopholes" or "exceptions" for this condition, that attitude, or the other situation.

The exception? Angle of attack.

It is the ONLY thing the pilot can change about a wing, and understanding it will allow him to make the correct decision EVERY time, in EVERY situation.

Toss around all the math and fluid dynamics you want....and, certainly, much of that information is important and meaningful (that part of it which is accurate, anyway...an iffy proposition at best on $random_internet_forum). However, knowing Navier-Stokes Equations, or the flaws in the path-length hypothesis (or even the name of it) don't do the pilot a whole lot of good when his airplane is sinking faster than he wants it to. There's not a formula, wind tunnel, or equation in the world that will direct him to do the right thing.

Angle of Attack...what it means...the fact that the elevators are NOT up down controls, but ANGLE OF ATTACK controls (and thus, fast-slow controls) will save him. It is the only thing that will.

So...in that sense (or, as Obi Wan might have said, from that certain point of view) a wing flies because of its angle of attack.

Is that the only thing that explains it? Is that the only factor at work? Of course not. It is by no means the "be all and end all" of fixed wing flight.

It IS, however, the be all and end all of th PILOT'S CONTROL OF FIXED WING FLIGHT...and thus, from the pilot's point of view, the only theory that matters.

So to speak.

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RE: symmetrical wings - 3/27/2008 11:58:55 PM   
victorzamora


 

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Status: offline 		Well, Mike...I'd like to think so at least .
Really, I'm just passionate about airplanes and want to know everything about them!

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       Post #: 33

RE: symmetrical wings - 3/28/2008 2:42:02 AM   
Jim Thomerson



Posts: 2822
Joined: 10/9/2002
From: Austin, TX, USA
Status: offline 		Here is what Frank Zaic (Model Glider Design, 1943) has to say about it. " However, the point is to reduce the air pressure over the top so that the higher pressure under the wing will have an easier job to push the wing upward. The shape of the airfoil and the placing of the wing at an angle does this job for us. By combining the two actions we end up with lift."

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       Post #: 34

RE: symmetrical wings - 3/28/2008 3:05:10 AM   
victorzamora


 

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Status: offline 		but he's still talking about AoA referring to differential pressure.

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RE: symmetrical wings - 3/28/2008 5:01:14 AM   
BMatthews



Posts: 8943
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Let's not get into the whole "what is lift" issue. That was the topic in a VERY long thread a year and a bit ago. Search it up if you want to bring it back or read it. I know it was an eye opener for myself. But we don't need another 18 pages AGAIN! ! ! ! !


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       Post #: 36

RE: symmetrical wings - 3/29/2008 5:53:33 PM   
combatpigg



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From: arlington, WA, USA
Status: online 		Bruce, thanks for taking the time to illustrate that point with me. I never would have taken the time to educate myself, I just copy what I see works for others. I wonder what FoilSim would say about the same 9%, 400sq" wing that had to support 7 or 8 pounds for a plane that was having its' top speed measured right out of a dive?

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RE: symmetrical wings - 3/29/2008 8:24:10 PM   
BMatthews



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quote:

ORIGINAL: combatpigg

Bruce, thanks for taking the time to illustrate that point with me. I never would have taken the time to educate myself, I just copy what I see works for others. I wonder what FoilSim would say about the same 9%, 400sq" wing that had to support 7 or 8 pounds for a plane that was having its' top speed measured right out of a dive?



Well, here again you're pulling a G turn and that means you need more lift. It's a safe bet though that a transition from a high speed dive into level to go through some speed traps needs to be done in a fairly open manner to limit the G load and thus the lift generation required. But it'll be a tradeoff between pulling back to level too fast and bleeding off speed and taking a long gentle pullup where just the overall cross sectional area drag slows you down too much. Like so many things it'll be a happy medium solution that maximises the dive speed retention at the entry point to the measuring trap.

FWIW If I was doing a max speed trap pass I'd start high dive but use just a hair of up pressure so the angle goes from about 80 through to perhaps 30 degrees out just past the trap. Then just a little more pressure to level the model as it enters the trap gate.

Keep in mind what's been written in the past about how even on a speed model it is entirely possible to outrun the prop in a dive so at that point the engine and prop is actually a dive brake. So the trick is to find a pitch that only works from mid way through the dive and through most of or all of the trap but then starts lugging the engine on the pull up at the end of the trap. If you prop the model for maximum level flight speed that doesn't degrade and the model is sufficiently clean enough then you're going to have the prop slowing you down in a long dive. Of course such a model will dog out pretty quick without the speed from the dive to keep the prop and engine up to speed. But if you're ONLY after a record then I think this would be an avenue to persue. Much testing would be needed and this is one case where being a little bit heavier would help. The mass would help to retain the dive speed. Again much testing to find the top of the bell curve that balances the need to lift the weight against the speed retention through the course would be needed.

And obviously all this would need to be done in harmony with any rules governing the use of diving prior to the course that will enhance the speeds.


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Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

(in reply to combatpigg)
       Post #: 38

RE: symmetrical wings - 3/29/2008 9:27:07 PM   
Mike Connor



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quote:

ORIGINAL: combatpigg

Bruce, thanks for taking the time to illustrate that point with me. I never would have taken the time to educate myself, I just copy what I see works for others...

With the type of things you are doing you really should try Foilsim. Quick and easy to learn and a ton of information to help you make decisions.

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http://www.mikesrc.us/

(in reply to combatpigg)
       Post #: 39

RE: symmetrical wings - 3/29/2008 9:27:38 PM