Ken,

Wrong. I suspect that there is no way to get the relevant idea across. Time to stop trying.

banktoturn

I am amazed.

There is a principle called diminishing returns which says that when pursuing a goal, address first the factors that can most significantly effect the results, then the lessor factors until you finally come to factors that have such little potential payout, that they are hardly worth the effort.

Generally R/C model airplanes are so grossly overpowered that drag is not a major deterent to performance. However if your goal is to reduce drag, some factors offering the greatest payout are, geting rid of the nosewheel, cowling the engine, retracting the main lg, etc. Some lessor factors are, useing thinner wheels, adding wheel pants, fairing the fuselage-wing interface, etc.

The small differences in induced drag between the several wingtip designs (which are only apparent at high angles of attack anyway) are not really very significant.

A simple question asked and answered, yet the thread has grown to two long pages of impassioned posts about something in the category of "how many angels can dance on the head of a pin?".

Go figure.

Lou has finally hit the nail on the head!!!

After discussing a little bit of my knowledge, I have been sitting back and keeping up with things that have been posted here. What I have said doesn't seem to be contested, and it seems that for one reason are another, people have been getting more into theory than actual practice of designing an efficient high lift/ low drag wing.
First and formost, we should look at the big picture wing. A 2D wing is ONLY for theory! This is a statment that was taught to me the first time I started in Aerospace Engineering. The only thing a 2D wing is used for is discussing the effects of all the different types of things that happen to a wing while in flight. As for practical uses, a wing must have 3 things to make it useful as a wing! LENGTH, WIDTH, AND THICKNESS. This make ANY practical wing design 3D!!!

Lou has said a PERFECT statment that I have stated in another way! Most RC airplanes are OVERPOWERED!! If you don't believe me, look at it this way. As an RC designeer and pilot, what is the SINGLE thing that we CANNOT scale down from the full size planes??? Answer is THE ATMOSPHERE!!!! So, for the most part RC engines are by nature greatly more efficient than the full size bretheren for the size of the aircraft to be used on. A close way to get it to the right power to weight ratio is to take a 1.80 size aircraft and place a .40 on it! An example of this is the Piper Cub! Originally had a gross weight of close to 2000 lbs, with a 65 HP engine! This gives roughly 30 lbs/ 1HP as compared to most .40 size aircraft of 5lbs/ 1HP!!!!!

This being said! There are to many factors that go into the design of a wing for different types of operations. A more specific question should have been asked I agree! But again, for RC purposes, unless you are trying to design something for a VERY SPECIFIC purpose, IT DOESN'T REALLY MATTER!!!

JUST HAVE FUN AND EXPERIMENT, EVEN A BAD DESIGN WILL GIVE YOU INFO FOR YOUR NEXT ONE!!!

Relax, you don't have 138 people sitting in this airplane with their lives in your hands that you have to design a GREAT, RELIABLE, AND TOTALLY EFFICIENT wing! You are only going to have to worry about the ribbing your fellow pilots will give you when you bury the thing into the ground and throw balsa all over the runway!

Relax, have Fun, and the rest will take care of itself!

Thanks,

Reg

LOL!

I thought that bank' and myself had a good ol' discussion about induced drag! The original poster didn't even say whether or not this was for an RC model. S/He just asked a question about induced drag due to tip effect. That's all I was trying to discuss. Besides, I'm an aero engineer and love talking about theory.

And whether you guys like theory or not... I think all this negativity about theory is funny. Without theory we'd still be building hand tossed gliders and not truly understanding why they fly (or don't, for that matter).

Every wing starts with an airfoil, and there have been plenty of threads in this forum about airfoil selection. Whether you know it or not, an airfoil is a section of a 2D wing. The characteristics of a 3D wing start with those 2D characteristics. If they didn't, we wouldn't be able to predict how a wing will perform without actually testing it... and that gets expensive fast... paper and electrons are cheaper.

Besides... talk is cheap too, right? LOL!

Good weekend everyone!

How does an annular wing (ring wing) effect induced drag and what does the vortex look like on one of these wings? Does it shed the vortex? Is the vortex bound? Is a box wing a close cousin to the ring wing? Why don't we see any of these designs in practice?

I can not answer for the box wing, but I can tell you that the ring (annular) wing is nowhere near as efficient as you might believe.

I don't know of any scientific research that might have been done, so I can not quote you any lengthy and worthy links on the topic.

But, having built one (Lee Richards III) I can tell you it is a real B*T*H to get to fly. When it does, it is not very good at it either.

Personally, I think that the aerodynamics at the "tip" of an annular wing would be almost impossible to "model" (as in mathematical model). Perhaps the nearest example in recent times would be the "Flying Pancake" series of prototypes.

wow, i hope nobody else replys to my thread cause you guys are just beating a dead horse.

I don't mean to be rude, here, but I find it worrisome that someone who has completed 3 years of training in Aerospace Engineering could make this statement:

I wonder if RCaillouet3 learned this concept in Aerospace Engineering School or Highschool (I am serious, now), and would like to refer him (and anyone interested) you to a pretty good aerodynamics reference:

http://www.av8n.com/how/htm/airfoils...sec-flow-intro

The bottom line is: THERE IS NO GOOD REASON WHY AIR TRAVELLING OVER THE WING MUST REACH THE TRAILING EDGE AT THE SAME TIME AS AIR TRAVELLING UNDER THE WING

Respectfully

Luis

Ken,

I just visited a web page, compliments of Irumd, which contains the following interesting text:

* Winglets, etc.
It is a common misconception that the wingtip vortices are somehow associated with unnecessary spanwise flow, and that they can be eliminated using fences, winglets, et cetera. The reality is that the vortices are completely necessary; you cannot produce lift without producing vortices. By fiddling with the shape of the wing the designer can control where along the span the vortices are shed, but there is no way to get rid of the vorticity without getting rid of the lift.


The URL is: [link]http://www.av8n.com/how/htm/airfoils.html#sec-flow-intro[/link]

banktoturn

When testing with NASA in 1980, we mounted 8 smoke generators on our #1 Tristar. The test was for wake turbulence, a result of vortices... Here's a typical image.. We found that a Tweet couldn't get closer than 5 miles behind the L-1011 in this configuration, before it would be thrown out of the smoke trail.
Howsomever, by cycling the control wheel, which raised and lowered the spoilers on the wings, the Tweet could get as close as a mile behind the Tristar.
Passengers of course would probably object to the rolling of the plane on approach
From inside it looked this....

banktoturn,

Try this for a more scientific explanation:

http://www.centennialofflight.gov/es...ortex/TH15.htm

Everything that I have been discussing is all there. You do NOT need tip vortices for lift, they are a result of having to engineer airplanes to work in the real world. Please read.

Ken,

1) That write-up is not 'more scientific'.

2) It didn't say that tip vortices are unnecessary for lift.

3) If something is necessary in the real world, it is necessary.

banktoturn

Luis, I understand where you are coming from, but I have to tell you this. I mde that statement with an idea of making it easier for people to understand lift and some of the ways it is created. If you disbelieve me, then I must say that all the experiments that I have conducted must not worth a thing.

You see, I have been part of experiments where a colored drop of fluid was imparted into the steam stream we were using to test a wing in a wind tunnel. The tunnel used, the half dozen computers, with 4 dozen digital video cameras, all watching this an measuring the effects, WITH ALL BEING THE ABSOLUTE STATE OF THE ART IN DESIGN AND FUNCTION, would say that the molecules do actually seperate and meet again at the trailing edge of a wing at the same time. The only time this not true is when you pitch the wing to extreme angles for its design, and start to make the air seperate from the top, or bottom, of the wings surface. At this time, they still meet up again, and still they are at meeting again, just this time at the point of equal pressure.
I don't mean to be rude, but if you don't believe that, then was does this happen with the full scale airplanes. Another experiment was to see if by seperating the flow of air from over the wing will it cause the wing to stall and not create lift. The FAA, NASA, and the USAF have done MULTIPLE studies and experiments showing that the moment the airflow does not conform to one side of a wing, the wing is in a stalled condition and will NOT generate lift until the flow of air is restored! I have been part of this type of experiment as a pilot!!! I have seen it MANY times!!!

I am not trying to be rude. Please if anyone takes me as that, I humbly apologize! But I try to state things in a way that makes it easy for people to understand what is goin on with airplanes while in flight, and on the ground. I agree that under the most techincal of terms, I may be slightly wrong. But we are talking about something that is very simple to understand if you knock it down to its most basic of forms. If you are an engineer, then it needs to be more specific (which leads to the types of disagreements we have here.) But for most of us, we just want to make an airplane that works with a .40 or .60 sized engine!

Thanks,

Reg

A couple of things do not make sense to me (an this coming from someone who is NOT an engineer or even a full scale pilot; I am a Pulmonary Disease specialist how knows JUST a little about airflow, loves aviation, and like most of us here, crashes his airplanes more than we should):

Can lift not be created with a simmetrical airfoil with a positive angle of attack (AOA)?
If you think of it, a simmetrical airfoil does not generate lift if perfectly horizontal, does it? Then, if the only thing that we change is the AOA and we generate lift, could it not be that the kinetic energy of the air molecules hitting the bottom of the wing generate a vector of vertical force (ie lift)?

Does lift not increase as the AOA increases until a point where turbulence occurs and the wing stalls? Doesn't this "critical AOA ( the AOA where you stall the wing)" become smaller with a wing that has rectangular cross section? If so, could it not be that the flat leading edge and trailing edge generate turbulence which alters airflow over and under the wing, therefor decreasing the effective kinetic energy of the air hitting the bottom of the wing?

Forget the idea that the air molecules "have to meet" at the trailing edge. If you take that for a fact and you base all your concepts on a wrong principle, many of your conclusions may be wrong. You say you have participated in many experiments and tha NASA, FAA, etc, have made numerous experiments showing that the air molecules do meet at the trailing edge. With all due respect, do you have any reference we can look at? I happen to be one of those persons who prefers to read the source directly and make my own conclusions, rather than take tour experience or anyone else's for granted. If I didn't look at my patients' XRays myself, I could not practice Medicine. I am serious. Radiologists are wrong so often, it is scary. And I am not saying I am not wrong, don't be mistaken, but I try to minimize the risk of being wrong by looking at the data carefully. (Sorry, rambling here).

Respectfully,

Luis

Okay Luis no problem. As for the symmetrical aerfoil issue. That is an easy one. The answer is yes, even with a high angle of attack, a symmetrical aerfoil can stall! Easy way to test this one. Take a RC model with this aerofoil and slow it down, sooner or later it will stall and the airplane will fall. I guess a little more explination is in order.

Lift is made perpindicular to the chord line. Can you have a wing flying with a high pitch angle and still be tracking forward, anyone that has seen an F16 doing a slow flyby will attest to that. Okay, the forward motion of this plane is the relative wind, and since the chord line in this example is pointed very high, this causes lift to go rearward and resist motion forward. At this point if the pilot pitches the airplane up, they will notice that the airplane will slow down and a loss of lift occurs. This is do to the fact that the wings are producing as much lift as possible. At this point power becomes the factor governing lift, and pitch governs airspeed. If the pilot then continues to pitch up, the "critical angle of attack" will be reached. The critical angle of attack is the point at which a wing can no longer create lift.

The experiments that I have participated in were mostly for school projects; therfore I don't have much of personal information other than what I have seen. Now as for as NASA goes, that should be rather easy to find, and I will do my best to get this info for you in the very near future.

Another place that is a wealth of knowledge on this matter can be found with the Aircraft Owners and Pilots Association. This is a group that supports the needs of new pilots, and to Certified Flight Instructors of the full scale aircraft. I have seen videos that show some of tests done by NASA for AOPA. One such video shows the effects of wind over the wing as it relates to wing stall. They tied hundreds of little streamers on the top of a wing. While in flight the streamers stay attached to the wing by the realitive wind. As the airplane was slowed down, AND pitched up, you could see the air starting to seperate from flowing along the wing by the streamers starting to blow in every which way, including forward. At the instant the streamers on the leading edge of the wing started to sway, the wing stall and lift was lost!
I din't participate in the actual test that AOPA video taped, but I have replicated this experiment in college.

Again, I'll get something to look at soon (sorry don't have the time any time to dig into it right now.)

Reg

Banktoturn,

In your first post to this thread, you point out that the cause of induced drag is the distribution of downwash along the span of the wing. I think that there is a definite connection between downwash and spanwise flow. One of the simplest induced drag models, Prandtl's Lifting Line Theory, relates the downwash distribution along the wing to the shedding of spanwise circulation to the wake. In addition to downwash, circulation in the wake also induces a discontinuity in the spanwise velocity component (in the inviscid case). This is analogous to the discontinuity in the magnetic field across a current sheet. There are many ways to approach this phenomenon with increasing complexity/fidelity (Helmholtz Vortex Laws, Kelvin's Circulation Theorem), but I think they will all lead to the conclusion that there is an inescapable connection between circulation/vorticity in the wake and spanwise flow.

I think that a "super-duper fence system which eliminates spanwise flow at the surface", would be equivalent to carrying all of the bound circulation out to the wingtips. Certainly possible (such as with a constant-section wing terminating at the walls of a wind tunnel), but it is a very special case that doesn't shed much light on the problem of induced drag.

I am a true newcomer to this forum, and I found this discussion interesting. Along the way, this thread has touched on just about every misconception about induced drag. I think your contributions have been very sound and have done far more that any others to help clear the waters. Even with respect to the spanwise flow issue, I think you were pushing the discussion in the right direction (while spanwise flow is inevitable on a 3D wing, it doesn't really help us understand induced drag)

To Fainjon, who got the discussion started, I think the answer to your question lies in the many responses... It is hard to say definitively which wingtip has lower drag. The answer depends on Aspect Ratio, Planform, Reynolds Number, Wing Section near the tips, Surface Roughness, and the list goes on and on. One of the things that keeps aerodynamics interesting is its complexity and the lack of simple answers. Good Luck! Fly Navy!

Hi Ya'll,
I have been away for a while so don't know how long this has been going on.
Good seems to be it has created action!
Topic is lageled Wing Tip Vorticies but that seems to have drifted into how an
airfoil (wing) creates lift which awakens me.
First know I have been a student of Aerodynamics for 70 odd years, that is
with miniature flying aircraft. In all that time I have never seen a understandable,
reasonable explaination of how lift is created.
Some of you people seem to have explainations but to me they are complex
and a bit confusing so hard to accept.
An old adage says solutions to mysteries usually are symplixtic and quite often
just plain common sense. Have found that adage suits much in aerodynamics
Experience has led me to a common sense way that lift is created that I have
never seen broadcast.
Would be interesting to see your thoughts regarding it.

Fore notes on which this therory is based>

Aerodynamics is the science of objects moving through air
which implies that the air is stationary and the wing moves throuh it.
\
An object moving through a liquid displaces it and such movement creates a force

Air is composed of seperated molecules which can be compressed closer together
The molecules tend to return to their origional spacing when the compression
device moves on.

Compressed air can have considerable force. A tire inflator has much force at the
nozzel and at a short distance from it the compresion has disipated, molecules have returned to origional spacing

With those thoughts consider>

A stationary wing does nothing but create weight.

A moving wing displaces air, in the process compreses the molecules.


With its movement the wing compresses the air before it, creating a force.

The wing is flying at an angle of attack

At the airfoil leading edge air is seperated to pass above and below it
This is compresed air containing force

The lower airfoil side is at a positive angle so the airstream passing it exerts its
stored energy against the airfoil bottom creating a positive upward ptessure on
the foil.

The upper curve of the airfoil, being a curve, further compresses the air until the
curve moves in the opposite direction.

Stored energy from the compression has inertia.

When the upper airflow reachs the foil "high point" the inertia tends to cause
the air to follow it's previous path, thus the airstream leaves the airfoil.

When the inertia force disapates the air molecules return to there origional spacing.

The area between where the air leaves the foil and where it returns is a low
pressure zone.

Sherlock to Watson.> high pressure always moves to low.

Common sense would believe this therory, your comments and observations
would be most interesting

Note> this thinking was brought about by the stucy of an excellent photo of
an airfoil in a smoke tunnel.
Smoke was pure white so discoloration was eazy to observe
What was noted was the white smoke entering the photo gained a greyish tint
as it approached the airfoil. The grey held in some amount until the smoke took
the downward turn on the upper side.

The smoke molecules intersperce with the air molecules, reside between the air
molecules, The color change would suggest the compression also effects the smoke, closer together more color?

Ain't this fun?


Hal [email protected]

Bravo Hal, simplistic yet quite convincing. Now, do the air molecules that got separated at the leading edge and flowed OVER the wing meet the molecules that travelled UNDER under the wing, at the trailing edge?

Why should they?

My question for RCaillouet3 was "did you learn that in highschool or is this something you learned in Aerospace Engineering School"? Because I understand the concept is wrong, and I would like to know if this is what erospace Engineering students are taught. There is no law of physics that says that MUST happen. I certainly MAY happen but it doesn't HAVE to. And if it DOESN'T HAVE TO happen, we cannot use that explanation as to why a wing creates lift, you follow me?

Yes, this is very interesting

Luis

Aero Engineers are taught circulation theory. What your talking about is the "equal transit time" theory, which is bunk.

Hal,

Some thoughts...

For low Mach number flow, there is very little air/fluid compression. Bodies moving through water (which is nearly incompressible) can generate plenty of lift. I think your explanation comes closer to explaining how supersonic airfoils generate lift.

I will bore you with my best shot at a simple explanation for lift generation...
Few people have trouble understanding what makes a helicopter fly. Like a fan turned on its side, a helicopter rotor pushes down on the air, and the air pushes back up. In technical terms, a helicopter in 1 g flight imparts momentum to the air at a rate proportional to its weight (assuming it is well out of ground effect). An airplane is really no different. The wings push down on the air and the air pushes back up. I think it's really that simple. Magical qualities are often attributed to airfoil shapes, but anyone who has ever stuck their hand out of a car window knows that shape really isn't all that important in creating lift (ultimately, the shape of an airfoil does become very important, but for only issues of efficiency and lifting capacity).

Understanding the details generating lift in order to make accurate predictions often becomes very involved. My opinion is that the complex details often complicate an otherwise intuitive phenomenon. My two cents.

Hal,

I would say that your explanation, reduced to its essence, captures the 'bottom line' of lift generation, which is that the flow of the air over the wing causes a pressure difference between the top and bottom surfaces of the wing, which gives net lift. This is accurate, but is not necessarily complete, in that it doesn't answer all of the other questions that people seem to have about the secret of lift generation. If someone interested in R/C planes asks how a wing generates lift, I think the best answer would be "The flow of air over the wing generates lower pressure on the top than on the bottom, and the details can get kind of involved". If that same person wants to know more so that he/she can design a better plane, then is the time to start learning more about the characteristics of airfoils/wings that would help them out. For example, for high lift, a highly cambered airfoil section is desireable, or for low induced drag a high aspect ratio is desireable, etc. Even at that level, the all-encompassing explanation of every related phenomenon is not really necessary. That's not to say that some physical insight is worthless, in fact, I think it is extremely valuable.

Irumd,

I doubt that the 'equal transit time' nonsense was ever taught in any engineering curriculum. I think it was developed to explain the idea to laymen. This happens all the time. For instance, we have a population that generally equates entropy with 'disorder', which is almost completely worthless, and results in faulty conclusions about the consequences of entropy. Generally, what is taught to engineers is a set of methods to design airfoils/wings/aircraft. A thorough, detailed, consistent explanation of the minute detail of lift generation and all its consequences is not necessarily taught. For airfoils in particular, pressure difference is what its all about. Tools are available that reliably allow engineers to predict the upper and lower surface pressures for any proposed airfoil, at least for design conditions. To a great extent, tools are even available to produce an airfoil which will give a desired pressure distribution. Users of these tools do not need to possess the end-all explanation of all the details of lift generation, and probably don't. Tools for designing wings are similar in nature, but somewhat less accurate and less 'black box' in capability.

banktoturn

Guys, you all bring up some interesting points. And to answer the question, yes I was taught this in aeprspace engineering, BUT it was taught to us for a way to explain in simple terms the things that occur to a wing in flight. You see if we were to go into depth what is going on, we could spend YEARS writing about all of these factors. I was just trying to make things simple. You see, if anyone read my posts, I was saying that for RC purposes, it really doesn't matter. HAVE FUN!!!

Sorry, but this may be the last post I can make for a feew months, I am being detachment is being sent overseas for a few months. I amafraid that I may be without computer access for a while. Until then, remember, this is not life or death for most of the world! Just have fun! As I have said before, even a bad design will give you good ideas for your next one!

Reg

I can't believe you Engineers of sorts think of yourselves as superior to the rest of us that have to come up with "simple explanations" for the non-initiated. We may not hold a title in Aerospace Engineering but do not be condescending, please.
Luis

Irumd,

I'm assuming that your reply was aimed at me as well. I certainly did not mean to insult you, and I'm very sorry if it seemed I did. I don't consider myself superior. I was taking issue with "simplified" explanations that are wrong, not with any people. Having said that, I don't think it is an insult, or condescending, to refer to someone as a layman. I am a layman on most topics, and hence I appreciate useful, accurate information on those topics, from people who are not laymen.

banktoturn

Hi y'all,
I have not read al;l that is posted but the subject is wing tips?
Are you aware of NASA's engineer Witcomb? He added modern efforts to an
acient finding and the report says jt created a break throuh
Back in the 20s NACA investigated "tip plates" and found a positive advantage
at low speeds but that was negated by drag at higher speeds.
Witcomb apparently took that report in recent times and further developed it.
The result was positive in all respects and at all speeds.
The new finding was labeled "Winglets".
In short they increased aircraft efficency by 3% or more.
What a Winglet does>
Reduces tip vortex and thus drag
Shares performance with an increased aspect ratio without the increase.
Creates forward thrust by providing lift in a forward vector.
Examples of useage>
When Rutan uses a factor on ALL his designs he is confident they are an advantage? All Rutan designs include Winglets
Many other modern aircraft are seen with them. Most modern Airliners have them.
Some years ago I evaluated them on miniature aircraft.
Findings were very positive. Improvement was seen and was obvious.
Details are a bit extensive but if desired I will provide them via Email.
How about that?

Hal [email protected]