Wing Tips, Vorticies or a Fin?  
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All Forums >> RC Airplanes >> Aerodynamics >> Wing Tips, Vorticies or a Fin?
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Wing Tips, Vorticies or a Fin? - 2/7/2003 10:12:27 AM   
a088008



Posts: 404
Joined: 6/14/2002
From: San Diego, CA, USA
Status: offline 		What are the advantages of wing tips vs. a fin? I'm faced with this descision while finishing my latest design. It's an unconventional plane with a lifting body and a wing. I'm torn between putting on wing tips and having tip vorticies adding to the plane's drag, or smooting out the wing tips and adding a central fin.

Can you guys walk me through the pros and cons (get as technical as you like, I can handle it) of wing tips, tip vorticies and a fin. This will help me evaluate and come up with an optimal solution. Any information with a slany on flying wings would be most appreciated. I'm primarily interested in yaw stability and drag secondly.

P.S. The plane is already in balsa, but I have yet to complete the wing tips, so I've still got time to change things.

-Q.

< Message edited by a088008 -- Feb 7 2003 6:41AM >

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-Q
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Wing Tips, Vorticies or a Fin? - 2/7/2003 11:19:46 AM   
BMatthews



Posts: 8943
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		If it's a flying wing you may not get as much votex as you think...

With a swept flying wing the tips are usually washed out lots to obtain the stability that is needed. So the tips are doing very little lifting or even are lifting in a negative manner in extreme cases. And since vortex formation is related to pressure differences between the top and bottom surfaces it's likely that there are very little losses to what vortices that do form.

Having said that on a swept flying wing the obvious spot to put any fins is at the ends of the swept wings so they are as far rearward as possible for leverage. It's just a bonus that they act as tip plates or winglets.

Winglet design is quite a mouthful all at once. Basically they are small vertical wings that are severley washed out. The idea is that the lift coefficient of the root of the winglet matches the cruise lift coefficient of the wingtip. The winglet is then twisted to the point that the top of the winglet is flying at its no lift angle which is often a few degrees negative for lifting airfoils. The other option is to blend from the lifting airfoil that matches the wingtip to a symetrical section with a degree or two of toe out so the negative lift at the top of the tips fights the pressures from below for a much smaller vortex formation.

You'll still have SOME vortices but it can be made less.

BUT..... there's always a "but" isn't there. Winglets only work when the wingtip Cl matches the Cl of the winglet at it's base so there is a consistent pressure from the bottom of the wing to the outside of the base of the winglet. In other words it's a one speed wonder. Our models do anything BUT one speed.... as do full size gliders. Hence the crescent or Schumman raked tips that are so popular today. These help reduce the size and severity of the tip vortices. And those that do form are smooth and tight so they don't make as much drag as those from a square wingtip.

Just slapping on a tip plate can help some aspects but it can also produce MORE drag than just a regular smooth tip. The sharp corners can produce their own turbulence that fans outward across the wing forming a wedge of bad airflow. It's worse on the upper surface with it's critical low pressure area.

One option that you don't see very often is a tip "tank" that is shaped like a high zoot laminar flow airfoil. This "tank"needs to be large enough that the high point of the pod is located at the trailing edge of the wing and extends back from there. The idea here is that the vortices will still form but they will spin around the pod and the tail of the pod will make the spinning air stick to the pod much like water will flow down an inclined string or rod thanks to surface tension. The end result being that the vortices are smaller and less energetic. The article I read even went so far as to suggest that the energy that's drawn out of the vortices would act on the rear of the pod and produce a net pressure that pushed the pod ahead. If this was true then the energy that was going to be wasted by the vortices is converted to air pressure pushing the pods and the model ahead. So you get your power BACK instead of loosing it. It wasn't a full tech article but it seemed to have possibilites. But we don't see aircraft zooming around with big pods on their tips so perhaps it was all conjecture and fodle dodle...


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Bruce-
Proudly wasting balsa since 1965.

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

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Wing Tips, Vorticies or a Fin? - 2/7/2003 11:50:49 AM   
a088008



Posts: 404
Joined: 6/14/2002
From: San Diego, CA, USA
Status: offline 		Thanks for the info B! I appreciated the interesting read.

My design is basically a flying wing, but WITHOUT the swept wing. It has a symmetric airfoil and constant cord wing at 90 degrees to the fuselage, so there is not going to be the typical flying wing vorticies at the tips.

I had once read and article on tip vorticies and how tip plates could be used to create a stronger vortex to increase stability (I think it was the roll axis?) at the expense of increased drag and a resultant slower flying plane. I thought this might be something to consider, but I guess models don't fly fast enought to take advantage of this.

I think that I'm going to go with a central fin and smooth out the tips, unless someone can convince me otherwise.

-Q.

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-Q

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Wing Tips, Vorticies or a Fin? - 2/7/2003 12:29:14 PM   
Ben Lanterman



Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		Of course like anything flying wings of the "plank" style have been around for years. I am not sure what "smoothing" the wing tip implies but the vortex will still be there because tip vortex formation is basically a function of span and how hard the wing is working. It doesn't matter if the airplane is tailess or not and doesn't matter if the wing is swept or not (within reason).

If you are carrying 5 pounds of airplane then the wing has to provide 5 pounds of lift. That implies that the lift mechanism/vortex action/circulation of the wing to produce 5 pounds of lift will result in tip vortex formation that is appropriate.

Nicely rounded wing tips still produce vortex action though it may be modified in strength distribution a little from just a flat airfoil end. Tip plates or winglets modify it some more. Certainly the Schumman tip form helps to spread out the effects and is quite effective.


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Ben Lanterman

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Wing Tips, Vorticies or a Fin? - 2/7/2003 12:55:27 PM   
a088008



Posts: 404
Joined: 6/14/2002
From: San Diego, CA, USA
Status: offline 		Can you eleborate on the Schumman tip shape? I'm not sure if it'e the tear-drop shape or the one see on the "stick" type models.

[QUOTE]Originally posted by Ben Lanterman
Certainly the Schumman tip form helps to spread out the effects and is quite effective. [/QUOTE]

Should I even bother with smoothing out the tip? My plane's wingspan will only be 47". I do however plan to produce a scaled up version with a wingspan of around 70-80". Should I bother smoothing the wingtips only on the larger version?

-Q.

< Message edited by a088008 -- Feb 7 2003 8:03AM >

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-Q

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Wing Tips, Vorticies or a Fin? - 2/7/2003 9:09:09 PM   
banktoturn



Posts: 700
Joined: 8/22/2002
From: Bloomington, MN,
Status: offline 		a088008,

There are some things about this whole winglet/tip plate issue that I am not clear on, but I'll give you my 2 cents' worth.

The component of drag that winglets are intended to reduce is induced drag. We tend to talk about tip vortices causing induced drag, but that's not really a very good view. Downwash causes induced drag, and downwash is caused by lift. If the wing doesn't generate lift, it doesn't generate induced drag. I think that the phrase 'induced drag' probably started out as the phrase 'lift induced drag', and this is healthy view to take. Downwash can be thought of as a big vortex, centered a little bit inboard of the wing tip, which causes a component of downward flow on the wing. This downward flow means the the actual angle at which the wing encounters air is not the same as the overall angle of attack of the plane, and it is not the same at each point on the span. This perturbed angle of the flow is the cause of induced drag, which is actually a component of the lift vector which had been turned rearward. Something that is very important to realize is that no type of winglet or tip plate is going to make the downwash go away, or eliminate the induced drag that results. I have not learned enough about the benefits of winglets to be completely comfortable with the topic, but I have read in one article that the well-designed, effective winglets used on some full-scale aircraft actually use the vorticity at the wing tip to generate a small amount of thrust. This is very different from the notion of 'eliminating' the tip vortex, which is not possible in any case.

On a practical level, there is a difference between trying to absolutely minimize induced drag and deciding where to put fins, since they have to go somewhere, and could possibly reduce induced drag a little bit if placed at the wing tip. If you want to reduce induced drag, then changing the wing planform and increasing aspect ratio are more effective than any wing tip design. Minimum induced drag occurs for an elliptical spanwise lift distribution. This doesn't necessarily require an elliptical planform, but often an approximately elliptical planform is a good way to get an approximately elliptical lift distribution. Some taper in your wing would be one step to take, if reducing induced drag is your main goal. Increasing aspect ratio also reduces induced drag. If you consider adding winglets, you need to be aware that, for the same weight, you could extend the wing, thereby increasing the aspect ratio, and achieve the same goal. As I mentioned, if you need fins anyway, then it could make sense to put them at the wing tips, for whatever induced drag benefit they may give.

My final answer is: 1) no wing tip treatment makes a big difference in induced drag 2) induced drag is probably not a big deal for your plane 3) a single fin will probably result in less wetted area, and lower weight, unless it compels you to add an empennage that you wouldn't have needed otherwise, 4) a simple, smoothly rounded wing tip will probably minimize profile drag.

banktoturn

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Wing Tips, Vorticies or a Fin? - 2/7/2003 11:00:48 PM   
Ben Lanterman



Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		The wing tip shape can be seen here

http://www.sailplanes.com/mventwing.html

http://www.sailplanes.com/PhotoGallery/NewScale2/Ventus2c_Axel.htm

http://www.canterburysailplanes.co.nz/index.php/item/department/RC%20Gliders/item/Prima.html

http://www.multiplexrc.com/asw27b.htm

http://www.ies.si/

http://www.pinnacle-enviro-tech.org/side.jpg

It is basically a constant chord wing until you get to the wing tips. The trailing edge is straight. You have double taper in the leading edge. The exact dimensions are not terribly critical as long as it looks the same. The wing wing get thinner of course as the chord is smaller.

A standard wing has the tip vortex all at the tip. The double taper or even triple taper supposedly gives a little vortex action at each apex which is favorable. Then there is a general feel that by the time you get to the tip there isn't much left. Probably true, I imagine it has been verified in wind tunnel and flight tests. Certainly the best gliders are using them.

A better shape might be a straight trailing edge with an ellipitical leading edge. But you lose the little vortex action at the apex of the tapers.


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Ben Lanterman

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Wing Tips, Vorticies or a Fin? - 2/8/2003 11:08:04 AM   
a088008



Posts: 404
Joined: 6/14/2002
From: San Diego, CA, USA
Status: offline 		Thanx for the info and the links.

I can now make an informed decision on any design I attempt now, and in the future.

-Q.

_____________________________

-Q

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Wing Tips, Vorticies or a Fin? - 2/9/2003 5:03:24 AM   
Daniel Nelson


 

Posts: 87
Joined: 11/20/2002
From: Lancaster, CA,
Status: offline 		Another possible way to reduce tip vortices is this kind of tip shape:

One of the professors here at Davis (C.P. Van Dam) has been experimenting with it, and has had very favorable effects. Don't ask me how it works though, probably something to do with the highly swept LE keeping the vortex formation away from the main wing.

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