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-   -   wingtip vorticies (http://www.rcuniverse.com/forum/aerodynamics-76/1138176-wingtip-vorticies.html)

fainjon 09-15-2003 12:05 PM

wingtip vorticies
 
i was wondering if a flat wingtip has less drag than a round, and say they both have the same area. does the vortex created by the round tip induce more drag than the flat tip.

KenLitko 09-15-2003 02:27 PM

RE: wingtip vorticies
 
Yes, the flat tip (or cutoff tip) has less induced drag because it is more difficult for air to swirl around the cutoff tip.

A better solution than the cutoff tip is actually a sharp tip, but it is more difficult to build... or you could use a simple Hoerner tip (used on a lot of model aircraft out there). Again, the Hoerner tip is more difficult to build than a cutoff, but it is a better solution.

fainjon 09-15-2003 04:57 PM

RE: wingtip vorticies
 
I talked to a fluids professor and she said that the rounded smooth tip would be more efficient than a cutoff tip with sharp edges. Also better when you increase angle of attack. Think about that and let me know any reasons why that would not be the case.

KenLitko 09-15-2003 05:52 PM

RE: wingtip vorticies
 
"I talked to a fluids professor and she said that the rounded smooth tip would be more efficient than a cutoff tip with sharp edges."

That's not correct, for the reason that I stated. With a sharp tip (that is, looking at the tip of the wing nose-on), there is less induced drag because high-pressure air on the bottom of the wing doesn't escape to the top as easilily as with a round tip. Same thing with a simple cutoff tip (though it's not as efficient as with a sharp tip).

Increasing the angle of attack won't alter that phenomenon that much.

RCaillouet3 09-15-2003 06:23 PM

RE: wingtip vorticies
 
Ok guys, this is a fun one for me! As for the question about wingtip vorticies, the answer is that for the type of flying that we do, it doesn't really matter!

I have been flying full scale airplanes commercially for about 10 years now, and have flown model aircraft for about 10 years longer!!! Also, I completed 3 years of Aerospace Engineering before deciding to fly them instead of design them for a living. The only reason I give this info is to show you how I have come to the above conclusion.

Wingtip Vorticies are cause by the relative high pressure of air on the lower surface of the wing meeting the relative low pressure of air on the top of the wing. One of the laws of lift is Bernollis' Principle. His theroy roughly states that as speed increases pressure decreases. As to how it works with a wing, imagine a flat bottom aerofoil. Now picture that 2 air molecules hit the leading edge of the wing at the same momment. Now picture them travelling across the top and bottm of the wing and reaching the trailing edge of the wing at the same exact time. Which one went faster? The answer is the one going across the top of the wing because it had to travel a longer distance in the same amount of time! Insert Mr. Bernollis' set of math, and VOILA, the upper surface moving air is travelling faster, and thus must have lower pressure than that air passing under the wing.
NOW, what exactly does that tell us??? Easy!! One thing about nature is that pressure ALWAYS wants to equalize in the atmosphere! The realitive HIGH pressure under the wings is wanting to meet and equalize the realative LOW pressure ABOVE the wing. This action is what gives a wing 90% of its lift!!!
Now that I have throughly bored you, wingtip vorticies is just that area of air where the pressures can meet! At the wingtip, the HIGH Pressure air below the wing is able to rise and meet the LOW Pressure air on top. This cause a swirling motion of air behind the wing.
The only time that this type of situation can be dangerous is when another airplane flies into your wingtip vorticies!!! The worst scenario that can take place is when the proceeding airplane is Heavy (at max gross weight,) Clean configuration (landing gear and flaps up,) and Slow speed. When the following airplane flies through the vorticies of an airplane operating as above, the results can be DEADLY!!!
Now as for flying models, you should not eer have to worry, unless you are flying at a real airport with real airplanes operating in the near area! The amount of vorticies created by a model are not very signigant considering how most airplane are flown with greater thrust to weight ratios as the full size bretheren! If you get in a situation where the airplane rolls without warning, just apply max power and do what it takes to get the airplane to climb! That's it!

With all the boredom of that stuff out of the way I can answer your question better. Because the nature of Wingtip Vorticies is to descend behind the airplane, the total amount of drag is less than 2% of the total drag on an airplane in flight!!! As someone stated before, a tapered wingtip design has slightly less drag may be true but only to a small amount. The actual reason that a tapered wingtip cause less drag is due to the fact that the smaller edge area is less for that swirling wind to conteract with! This means it just hits the wing with less area! The down side to a tapered wingtip is that it gives you LESS lift than a flat wingtip because of the lower speed of the air passing over that area on a tapered wingtip.

As for the differences in wingtip design for models, there are advantages either way. Want speed, go with tapered. Want Lift go with flat.

Any questions about this, let me know.

Thanks,

Reg

LouW 09-15-2003 07:12 PM

RE: wingtip vorticies
 
RCaillout3 has my vote. For the type of flying model pilots do the difference in induced drag between flat , rounded, or sharp tips is just plain insignificant. Full scale designers are concerned with range which is where small drag reductions become important. There have been many special tip shapes tried, the most prevalent at the moment seem to be the Hoerner used on some production light aircraft, and the "winglets" used on commercial airliners. If I remember correctly, the Rutan round the world airplane had the winglets. On takeoff for the record flight, one tip was damaged and the winglet came off. It made it around the world anyway.

If the question is just for sake of argument, though the squared off tip may restrict the flow of air bottom to top thus reducing "induced" drag it causes turbulent flow that represents energy loss adding to "form"drag, so the overall drag is likely the same or maybe a little higher. The only way to know for sure would be to run comparative tests under controlled conditions (in a wind tunnel maybe) and measure the difference.

On the typical R/C model if you are serious about drag reduction the first move would be get rid of the nose gear.

probligo 09-15-2003 07:27 PM

RE: wingtip vorticies
 
By comparison, I am only a fairly average accountant.

But the main area of my interest (F1A glider) drew me into the area of induced drag many years back.

My argument must be simple, and comparatively brief - so here goes...

What Reg says, I generally agree with except;
[ul][*]The formation of the vortex, the fact that the air is "moved" from lower surface to upper as he describes, requires energy.[*]To the best of my knowledge, that energy does not come out of the surrounding air, it comes from the aircraft itself.[*]The amount of energy "absorbed" by the vortex depends upon the lift coefficient of the wing, its velocity, and the shape of the wing (planform) itself.[*]The planform of the wing relates the "local wing loading" to the "local lift generation". To illustrate - an elliptical wing generates no vortex because the "element loading" always equals the "local lift". A square wing generates a very large vortex because the distribution of loading differs considerably from the distribution of lift across the span.
[/ul]
(I am doing my best here...)
So, why my interest in induced drag? Because in F1A gliders, it constitutes some 60+% of the total drag budget. Very slow, high lift wings making very large slow vortices.

So I disagree with Reg to this extent - in some model aircraft, and in some 1:1 aircraft as well, induced drag is an important part of the design. HOW important is the real question.

For the general sport power model, I do agree with Reg - it is a relatively small part of the equation.

To the original question, the simplest form of "vortex control" -
[ul][*]Square edge is fine for most sport models[*]Rounded edge adds a little bit of aesthetics perhaps.[*]The Hoerner tip is the first really practical move. [*]Then you can get really fancy with winglets if you want...
[/ul]
If you are designing your own model, especially for competition purposes, you will get far more result from moving toward an elliptical planform, than from adding any winglets or tips or anything else.

Then you get into the compromises...

RCaillouet3 09-15-2003 07:46 PM

RE: wingtip vorticies
 
Probligo is right to some extent. The aerofoil does "create" some of the vorticies itself, but only by design. A thicker aerofoil generally will creat greater vorticies due to the fact that the upper air has to travel that much of a longer distance in a given time fram making it travel faster with even lower pressure. The lower pressure, as compared to a wing 1/2 as thick, will make a greater vorticies potential.
The fact of the matter is that the vorticies actually do NOT form on the wing. The vorticies starts at a small distance off to the side of the wing depending on aerofoil design. A longer thinner wing as used in Probs example will create the same amount of vorticies, but at a reduced rate of swirling effect at the tip.

ANYTIME a wing is creating lift, it IS creating Vorticies.

Now as to why airliners are using raised wing tips is simple. Remember that Wingtip Vorticies swirl and that motion has a speed. So what they are doing is diverting the vorticies from coming off to the immediate side and put this air motion OVER the wing! This gives the wing anywhere from 2 - 5 % more lift!!! The greater the lift adds the benefit of letting the aircraft fly with less energy imparted to the aircraft by the engines. What this means is that the engines don't have to work as hard to make the same amount of lift. In a sense, they are doing to the wing what modelers have been tring to do with weight on airplanes! If you have to have the weight, you might as well make it USEFUL weight!! That's the only reason that they are going to the raised winglets! it allows the airplane to operate the same, but with slightly less cost (gas / trip.)

Just my 2 cents worth.

Thanks,

Reg

KenLitko 09-15-2003 08:48 PM

RE: wingtip vorticies
 
WOW... you guys really know how to make a simple question complicated. :eek:

Ed_Moorman 09-15-2003 09:39 PM

RE: wingtip vorticies
 
Theory is good, but have you cut and pasted any? I have flown all the above wing tips except the winglet and let's face it, for RC work, except for maybe sailplanes, they don't work upside down and they'd get broken off the first time out of my garage in into my truck.

Also a buddy of mine made a rudimentary wind tunned and a wing covered n yellow Monokote that could have tips bolted on. We CA'ed red thread along the leading edge and tried different tips at various algles of attack.

End plates worked the best-they had no outward flow of the threads.
Hoerner tips worked well, but only in one direction, upright.
Square ends were next best with only a little outward flow.
Rounded blocks were next with noticeably more outward thread movement.
Stick type tips were next to worst.
Large, round tps like on 30's and most WW II planes except the Mustang and Grumman designs was absolutely the worst. The outward flow was extreme.

Next, we flew a plane with bolt on tips. End plates gave a lower landing speed and more resistance to stalls and spins.

I mentioned this in my column and several people duplicated the bolt-on tips. They all reported that end plates gave the same results on their planes, including a biplane.

KenLitko 09-15-2003 10:19 PM

RE: wingtip vorticies
 
End plates are great. Their disadvantage is that they add a lot of wetted area. The cutoff tip or Hoerner tip add no or little wetted area, respectively.

End plates generally add more drag than they save lift. An engineered tip can be looked at as adding to the effective span of the wing. End plates add about half the lift that would be added by simply extending the wing out their height.

Winglets are designed for a single flight condition... away from that and they lose efficiency fast. The nice thing is that winglets can actually save double the lift that would be had by simply extending the wing out their height.... but again... that's for a single flight condition.

Weight is another factor. Hoerner tips and simple cutoff tips are lighter than your typical plastic end plate.

But again... simple question... simple answer... cutoff tips (or better, Hoerner tips) have less induced drag than rounded ones.

LouW 09-16-2003 09:59 AM

RE: wingtip vorticies
 
Ken has answered a simple question with a simple answer. I like that.

This is to clarify a point in an earlier reply. In a glider (sailplane) there are two flight conditions of primary interest, minimum sink rate (for riding thermals) and minimum glide angle (for moving between thermals with minimum loss of altitude. These two conditions are specific points on the drag curve.

The drag curve for an airplane consists of the combination of the induced drag, which is a maximum just before the stall, and approaches zero as speed is increased, and the form drag which starts at zero at rest, and rises as the square of the speed. The resulting drag curve has a minimum at the point where induced drag and form drag are equal. This it the speed for mimimum sink rate and at that point induced drag is 50% of the total. The minimum glide angle occurs at a little faster speed (where induced drag is a little less) is indicated by a tangent to the drag curve intersecting the origin (this is the overall L/D maximum).

A infinite wing produces induced drag which is the toll for producing lift. The additional increment of induced drag due to a finite span (tip vortices) varies inversely with aspect ratio so that for A.R. greater than 10 is a pretty small percent of total induced drag. This increment can be affected a little by tip design but reducing an already small number by a small amount doesn't really gain much. This small amount may be worth pursuing if chasing a world record but simply increasing the aspect ratio, even with simple tips will probably give a better return on the effort.

KenLitko 09-16-2003 10:37 AM

RE: wingtip vorticies
 
Quote:

A infinite wing produces induced drag which is the toll for producing lift. The additional increment of induced drag due to a finite span (tip vortices) varies inversely with aspect ratio so that for A.R. greater than 10 is a pretty small percent of total induced drag. This increment can be affected a little by tip design but reducing an already small number by a small amount doesn't really gain much. This small amount may be worth pursuing if chasing a world record but simply increasing the aspect ratio, even with simple tips will probably give a better return on the effort.
Actually, an infinite wing produces no induced drag. Induced drag is due to vortex rollup, and an infinite wing has no vortex rollup. It does have, however, both pressure (form) drag and skin friction drag (collectively a.k.a. parasite drag).

Using an engineered tip is good if you are limited by span, but not necessarily by tip design.

By the way... I know nothing of sailplane design... so you can roll with it LouW! :D

Johng 09-16-2003 10:42 AM

RE: wingtip vorticies
 
I'm with Ken, way to much verbage for a short, simple question.

Much misinformation included as well. A few things need correcting:

Quote:

As to how it works with a wing, imagine a flat bottom aerofoil. Now picture that 2 air molecules hit the leading edge of the wing at the same momment. Now picture them travelling across the top and bottm of the wing and reaching the trailing edge of the wing at the same exact time. Which one went faster? The answer is the one going across the top of the wing because it had to travel a longer distance in the same amount of time!
This is an example which has been repeated so often it is now an urban legend. Like most urban legends, it is false. This is not a good example to use because it is not true. For more information do an RCU search on Bernoulli and you'll find some long pi$$ing contests and some good explanation of the real cases.

AND

Quote:

an elliptical wing generates no vortex because the "element loading" always equals the "local lift".
Also, not true. The elliptical wing is the most efficient - but that does not mean it generates no vortex. It a wing has tips, there's a vortex close by.

banktoturn 09-16-2003 10:55 AM

RE: wingtip vorticies
 
fainjon asks two separate questions. The first is whether a square tip would give less drag than a rounded one. The second is whether the rounded tip causes the tip vortex to induce more drag. This second question seems to be referring specifically to induced drag, rather than overall drag, as the first question was. As has been pointed out, induced drag is usually a small component of overall drag. Induced drag becomes significant when the wing is operating at a high lift coefficient ( landing, slow speed flight, pylon turns, etc. ).

I think there is a widely held misconception about the cause of induced drag. The wingtip vortex itself is not the cause of induced drag, per se. The cause of induced drag is the distribution of downwash along the span of the wing. Downwash is the slight downward component of velocity of the air approaching the wing, caused by the very act of generating lift. You simply cannot prevent induced drag by making it more difficult for air to spill from the bottom of the wing to the top. To say that a square tip has lower drag for this reason is to misunderstand the phenomenon. It is possible that some wingtip designs could reduce induced drag by shifting the distribution of downwash ( usually this means that the tip design pushes the center of the vortex out from the end of the wing, emulating a longer wing ). Some winglet designs also take advantage of the tip vortex by using it to generate some thrust, which partially compensates for induced drag, but doesn't necessarily reduce it.

Since most of the time, induced drag is very small, it makes sense to consider other components of drag when choosing a wingtip. A square tip will certainly be more likely to have a region of local separation, causing additional drag ( profile drag ). Streamlining the wingtip is beneficial for the same reason that streamlining the rest of the airframe is beneficial. A rounded tip would certainly be closer to an ideal, streamlined shape than a square one.

It would be hard to quantitatively predict the relative overall drag between a square tip and a rounded one. My educated guess would be that the one which reduces the more important component of drag is better. This is particularly true since the other one cannot be expected to reduce any component of drag.

My advice is to not worry about induced drag or wingtip vortices, and round your wingtips. Experiments of the kind which have been suggested could shed additional light, if you care to run them.

banktoturn

KenLitko 09-16-2003 11:34 AM

RE: wingtip vorticies
 
Quote:

I think there is a widely held misconception about the cause of induced drag. The wingtip vortex itself is not the cause of induced drag, per se. The cause of induced drag is the distribution of downwash along the span of the wing. Downwash is the slight downward component of velocity of the air approaching the wing, caused by the very act of generating lift. You simply cannot prevent induced drag by making it more difficult for air to spill from the bottom of the wing to the top. To say that a square tip has lower drag for this reason is to misunderstand the phenomenon. It is possible that some wingtip designs could reduce induced drag by shifting the distribution of downwash ( usually this means that the tip design pushes the center of the vortex out from the end of the wing, emulating a longer wing ).
Induced drag is caused by spanwise flow, inboard on the top of the wing, and outboard on the bottom. This spanwise flow is caused by the pressure differential between the top and bottom of the wing. So, you could say that induced drag is a byproduct of having to use finite wings... as an infinite wing has no induced drag.

Induced drag, especially at low speeds and/or high angles of attack is definitely the largest component of drag on an aircraft (neglecting difficult-to-quantify drags like gear, flaps and other protuberances or high lift devices).

banktoturn 09-16-2003 11:46 AM

RE: wingtip vorticies
 
[quote]ORIGINAL: KenLitko

Quote:


Induced drag is caused by spanwise flow, inboard on the top of the wing, and outboard on the bottom. This spanwise flow is caused by the pressure differential between the top and bottom of the wing. So, you could say that induced drag is a byproduct of having to use finite wings... as an infinite wing has no induced drag.

Induced drag, especially at low speeds and/or high angles of attack is definitely the largest component of drag on an aircraft (neglecting difficult-to-quantify drags like gear, flaps and other protuberances or high lift devices).
Induced drag is not caused by spanwise flow. It is indeed a byproduct of finite span, but not for that reason.

Induced drag is sometimes the largest component of drag, but ONLY under high CL conditions, including low speed, etc.

banktoturn

KenLitko 09-16-2003 12:08 PM

RE: wingtip vorticies
 
Banktoturn,

We're saying the same thing in two different ways. I'm saying that spanwise flow causes vortices, vortices cause downwash, downwash results in (effectively) a backward-tilted lift vector. The upward component is the actual lift, the backward component is the induced drag.

I'm going to the root cause of induced drag, your saying that it strictly a product of downwash.

The fact is that by modifying the spanwise flow on the wing (with engineered wingtips), you can modify this downwash field. In so doing, you can modify the induced drag (hopefully lowering it, and not getting more parasite drag in echange due to the size/wetted-area of the engineered tip).

fainjon 09-16-2003 12:10 PM

RE: wingtip vorticies
 
I understand vorticies and why they happen and how a wing work. i was wodering if i was flying 100mph and pulled up hard would the flat tip slow me down more than a rounded tip. But you guys are right for a model airplane it probably wont matter much. i just now am getting into aeronatical engieering and just try to analyze everything.. good practice.

banktoturn 09-16-2003 01:11 PM

RE: wingtip vorticies
 
Quote:

ORIGINAL: KenLitko

Banktoturn,

We're saying the same thing in two different ways. I'm saying that spanwise flow causes vortices, vortices cause downwash, downwash results in (effectively) a backward-tilted lift vector. The upward component is the actual lift, the backward component is the induced drag.

I'm going to the root cause of induced drag, your saying that it strictly a product of downwash.

The fact is that by modifying the spanwise flow on the wing (with engineered wingtips), you can modify this downwash field. In so doing, you can modify the induced drag (hopefully lowering it, and not getting more parasite drag in echange due to the size/wetted-area of the engineered tip).
KenLitko,

I think it is useful to go to the root cause, but I don't think we are saying the same thing. Spanwise flow is not the root cause of induced drag. Lift is the root cause of induced drag (the term induced drag is short for lift induced drag). If you could build a wing with no spanwise flow, it would still suffer from induced drag whenever generating lift. One reason that it is useful to go to the root cause of a phenomenon is so that one can rationally determine how to design around it. In particular, it is not valid to choose a wing tip with the idea of making it harder for flow to spill from the bottom surface to the top, at least not for the purpose of reducing induced drag. If you could show, or reason, that modifying the spanwise flow near the tip could favorably modify the downwash distribution, that would be a valid approach, but it is not the same thing as trying to impede the spill from the bottom to the top. This rationale seems to stem from the notion that the tip vortex causes drag, so if you can prevent the tip vortex, you can prevent the drag. This notion is invalid, much like the commonly heard explanation that entropy is disorder. Unfortunately, it seems to be almost as widely believed.

banktoturn

KenLitko 09-16-2003 01:30 PM

RE: wingtip vorticies
 
Quote:

If you could build a wing with no spanwise flow, it would still suffer from induced drag whenever generating lift.
Actually, no it wouldn't. The only way you're going to get no spanwise flow is from a 2D wing (a.k.a an infinite wing, or some approximation thereof). The only drag that a 2D wing suffers from is parasite drag.

A 2D wing, has no induced drag because it has no induced downwash. In a 2D wing, upwash always equals downwash... this is not the case in a 3D wing. In a 3D wing, energy is lost (from lift) to the trailing vortex sheet. This vortex sheet induces downwash.

The tip vortex is simply the "rolled-up" vortex sheet caused by a 3D wing moving through a fluid. You CANNOT prevent the tip vortex, but you can modify it. Modifying it by effectively spreading it out further from the other tip (which is what a cutoff, Hoerner, or end-plate does) can reduce drag.

LouW 09-16-2003 02:24 PM

RE: wingtip vorticies
 
I think the difference here is one of definition. If you look at section data (plotted from wind tunnel data where the wing extends from wall to wall simulating an infinite span wing) you see that as the angle of attack is increased from the zero lift angle, there is a drag component that increases as lift increases. The actual force on a wing section developing lift is always tilted rearward resulting in "induced drag". This is the penalty nature extracts for producing lift. The total drag of a wing section is the sum of "form" drag (the parting of the air and friction as the body passes through it), and the "induced" drag (caused by the rearward tilting of the resultant force on the wing). This induced drag is always present whenever the wing is generating lift.

There is another factor to be considered for a wing of finite span that is the result of spanwise flow. This is also frequently referred to (even in some texts) as "induced" drag. This factor would more properly be called "additional induced" drag. This is the factor that appears in the equation:

Cdi=CL (squared)/(pi)(aspect ratio)

As A.R. becomes infinite, this additional induced drag (Cdi) becomes zero.

Total drag = form drag+induced drag(due to lift)+Cdi(due to finite span)

Cdi is affected by planform and tip design but even in high angle of attack situations is seldom the major source of drag.

I believe if this confusion of terms is cleared up we will find that the several opinions are indeed saying the same thing.

banktoturn 09-16-2003 02:31 PM

RE: wingtip vorticies
 
Quote:

ORIGINAL: KenLitko

Quote:

If you could build a wing with no spanwise flow, it would still suffer from induced drag whenever generating lift.
Actually, no it wouldn't. The only way you're going to get no spanwise flow is from a 2D wing (a.k.a an infinite wing, or some approximation thereof). The only drag that a 2D wing suffers from is parasite drag.

A 2D wing, has no induced drag because it has no induced downwash. In a 2D wing, upwash always equals downwash... this is not the case in a 3D wing. In a 3D wing, energy is lost (from lift) to the trailing vortex sheet. This vortex sheet induces downwash.

The tip vortex is simply the "rolled-up" vortex sheet caused by a 3D wing moving through a fluid. You CANNOT prevent the tip vortex, but you can modify it. Modifying it by effectively spreading it out further from the other tip (which is what a cutoff, Hoerner, or end-plate does) can reduce drag.
Well, I was speaking hypothetically. It seems to me that you're saying that a wing without spanwise flow would not have induced drag because it would have to be 2D, and a 2D wing couldn't be built. If that wasn't your meaning, please excuse me. My contention is that the downwash experienced by a 3D wing is not dependent on spanwise flow. It would almost certainly be impossible to build a wing with no spanwise flow at all, but try this thought experiment. Imagine a lifting, 3D wing, with some kind of super-duper fence system which eliminates spanwise flow at the surface. If you are a CFD guy, you can imagine instead a flow simulation of a lifting, 3D wing with a boundary condition on the wing which precludes spanwise flow. There is nothing I can see about the absence of spanwise flow in this situation which prevents the lift of the wing from causing downwash. If you can articulate how spanwise flow is necessary for downwash, please help me here. This was your claim before.

Please compare your third paragraph to your first post, in which you stated: "it is more difficult for air to swirl around the cutoff tip" as the reason that a square tip would result in lower induced drag. In my first post, I took issue with this explanation, stating: "some wingtip designs could reduce induced drag by shifting the distribution of downwash ( usually this means that the tip design pushes the center of the vortex out from the end of the wing, emulating a longer wing )". We seem to have come back to that issue, and you seem to have changed your explanation to something fairly similar to mine. In any case, it is not obvious to me how a square tip would have a beneficial effect on the downwash distribution, but I would be interested to hear it, if there is some rationale. This discussion of induced drag ignores the issue of overall drag, which is where the square tip would seem to have a disadvantage, and of which induced drag is a small component for most flight regimes.

banktoturn

RCaillouet3 09-16-2003 04:55 PM

RE: wingtip vorticies
 
One thing I see in all of this is a slight misue or miunderstanding of some terms.

DRAG- This funny named thing comes in 2 forms!

1. Surface Drag, aka Form Drag is the drag caused by the resistance of the airplane by the air while in flight. Think of it as trying to push a stop sign in heavy wind. If you stand it up with the STOP logo to the wind the air pushes on a larger surface and tries to push it backwards. If you turn the sign on its side, the drag decreases signifigantly due to less surface area. A simple and rough formula is that as speed doubles, surface drag quadruples.

2 INDUCED DRAG - This type of drag is a BYPRODUCT of lift!! Imagine an aerofoil in level flight. Now most wings in LEVEL flight will have a small degree of what is known as an ANGLE OF ATTACK. This roughly means the difference between the chord line of the wing and the actual flight of the aircraft. If you have ever seen an f-16 do a REALLY slow flyby you see an airplane traveling FORWARD but with its wings pointed in a VERY HIGH Angle! That angle is called the angle of attack!
NOW, let us use that F16 example. The Actual LIFT on the wing is NOT Directly Vertical!! The wing is creating enough lift in an upward moment to keep the airplane airborne. BUT, LIFT IS ALWAYS MADE PERPINDICULAR TO THE CHORD OF THE WING! With this in mind the High Angle of Attack the F16 is using has a LARGE amount of the Lift Actually going BACKWARDS to the Direction of flight. That amount of lift that is causing the wing to generate a motion OPPOSITE to the forward motion of flight is called INDUCED DRAG!

If you don't believe me try this, take your airplane up to altitude and SLOW the airplane down. You will reach a point to where if you PITCH the airplane to a higher angle, all you will do is to slow the airplane more! In this area POWER is needed to maintain altitude!
Ask ANY full scale pilot if this true and EVERY one will tell you YES!!! This is trained to ALL full scale pilots from hour 1!!!

Lift and drag, and the relation of the 2 can be a VERY complicated matter indeed! Lift can create drag, just as speed can create drag. Confusing, yes. Should it be, No! Because with models these two or important, but not life threatening for the most part.
So relax, and enjoy building and flying models. THERE IS NO WRONG WAY TO DO IT!!! Just have fun and enjoy it!! Even, a TERRIBLE designed airplane should give you an idea of how to make your next one better!!!

Thanks,

Reg

KenLitko 09-16-2003 05:15 PM

RE: wingtip vorticies
 
Quote:

Well, I was speaking hypothetically.
Ahhh... mind games! :D

Quote:

It seems to me that you're saying that a wing without spanwise flow would not have induced drag because it would have to be 2D, and a 2D wing couldn't be built. If that wasn't your meaning, please excuse me.
That's accurate. Just so you know... I'm not trying to bicker here... this forum is fun... Ben L. put me in my place when I first got here. I wouldn't be suprised if you would do the same.

This is my understanding of the phenomena we're writing about.

Quote:

My contention is that the downwash experienced by a 3D wing is not dependent on spanwise flow. It would almost certainly be impossible to build a wing with no spanwise flow at all, but try this thought experiment. Imagine a lifting, 3D wing, with some kind of super-duper fence system which eliminates spanwise flow at the surface. If you are a CFD guy, you can imagine instead a flow simulation of a lifting, 3D wing with a boundary condition on the wing which precludes spanwise flow. There is nothing I can see about the absence of spanwise flow in this situation which prevents the lift of the wing from causing downwash.
You just described a 2D wing. Downwash is created, but not at the expense of lift (in other words, it is not induced). Downwash in this case is equalled by upwash. Induced drag is a differential between these two... and none exists in a 2D wing.

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If you can articulate how spanwise flow is necessary for downwash, please help me here. This was your claim before.
This differential is what I am talking about... but we we're talking about induced drag, so I wasn't getting into the 2D stuff that I mentioned above because it doesn't contribute to 3D drag.

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Please compare your third paragraph to your first post, in which you stated: "it is more difficult for air to swirl around the cutoff tip" as the reason that a square tip would result in lower induced drag. In my first post, I took issue with this explanation, stating: "some wingtip designs could reduce induced drag by shifting the distribution of downwash ( usually this means that the tip design pushes the center of the vortex out from the end of the wing, emulating a longer wing )". We seem to have come back to that issue, and you seem to have changed your explanation to something fairly similar to mine. In any case, it is not obvious to me how a square tip would have a beneficial effect on the downwash distribution, but I would be interested to hear it, if there is some rationale. This discussion of induced drag ignores the issue of overall drag, which is where the square tip would seem to have a disadvantage, and of which induced drag is a small component for most flight regimes.
I don't think I changed my explanation... maybe the verbage. Think of it this way... the cutoff tip is like a sharp bend in a pipe. It takes more energy for a fluid to get around the sharp bend than a round bend. In other words... it's more difficult for the air to swirl around that corner at the tip (remember, this is spanwise flow we're talking about). What does this do... it effectively pushes the span and those vortices out further from each other. There will be a local separated flow bubble... but that bubble serves a purpose. Is this the best solution? No. That's why I suggested the other tips.

So, like I said, I think that we are saying the same thing... just the difference with the relationship of spanwise flow to induced drag. I contend, based on the above statements, that without spanwise flow, there is NO induced drag. Without spanwise flow, the only thing that you have is parasite drag.

Let me know if I wrote anything stupid. :D


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