What is this?

[jswayze]

I've seen this feature on several different aircraft. What aerodynamic effect does it have on the wing? Would it help to have this on a model, or is it only effective at full scale?

Thanks!

[Ollie]

It is a wing fence. It keeps the wild and domesticated air molecules from faternizing with each other.

[vintage1]

So true. I believe the idea is that, lemming like, the molecules all try to rush to the end of the wing and cast themslves over the edge, thus wasting their own potential to prop up the aeroplane.

I mean, someone had to stop it?

[jswayze]

OK, ok... everyone's a comedian... ;)

Though I've extracted the beginnings of an answer, it's still not clear. You're forward velocity is 100 mph and air wants to move sideways on the wing, so you put up a fence to keept it in line?

Still not sure if I asked a stupid question :confused:

[steve lewin]

You've got it, though bear in mind that planes are not always flying straight and level. Wing fences (also called boundary layer fence and flow fences) are most commonly fitted to swept wing aircraft and are most effect in bank and/or sideslip.

If you're interested in the aerodynamics a quick Google search "aerodynamic wing fence" found plenty of information. E.g. http://www.b2streamlines.com/WingFences.pdf is a reasonable start.

Steve

[Ollie]

Only the vector component of air flow perpendicular to the 25% chord line produces lift. Another component of the flow in the same direction as the 25% chord line produces no lift but only drag. Therefore, straightening the flow so that more of it goes perpendicular to the 25% chord line increases lift. The fence has both a drag increase affect of one type and a drag reduction effect of another type. The net effect on drag may be either positive or negative depending on the particular circumstances.

Your question wasn't stupid at all. It just resulted in a couple of jokes which gave me time to think of an answer.

The fence shown may be located at the inboard end of the aileron, in which case, it may also reduce the drag producing vortex and cross flow that results when the aileron is deflected. A similar observation could apply to the outboard end of a flap, if there was one.

[RSCherry]

So what about part 2 of jswayze's question- is there any value in a fence on a model plane?

Observation would tell me no, since I don't recall ever seeing one on a model that wasn't a scale ship.

[Ollie]

The value of a wing fence would depend on the purpose of the model and how well the fence harmonized with the rest of the wing design. I think that the wing fence, independent of other considerations, is not a good idea. I think wing fences are, in many cases, an after thought that is added in the later stages of the design process to correct a previously unanticippated problem without having to start all over again.

As Steve points out, wing fences on unswept wings are rare. I think that the reason is that there are usually better ways to design a wing.

[vintage1]

The classic way to NOT use a wing fence is high aspect ratio wing as in a sailplane. The idea is to not have tip vortices.

[JRuggiero]

Isn't the performance of a wing fence much like that of an end plate or winglet?

Jim R

[Ollie]

If end plates resulted in a net drag reduction you would have seen them on commercial airliners long ago. The effect of end plates and fences are similar in that they reduce the spanwise component of the flow some but at the cost of additional skin friction drag and possibly also drag due to turbulence. Winglets have a small improvement in net drag over a limited range of coefficients of lift associated with the aircraft's crusing speed. At air speeds outside the narrow design window, the winglets have a net increase in drag. Better performance would result from putting the weight and size of the winglets into an increase in span. Increasing the aspect ratio is better than tiplets unless the span is limited for some reason. Higher aspect ratio would improve the lift to drag ratio, reduce thrust requirements and improve range while reducing fuel consumption. It is no coincidence that the Rutan Voyager that flew around the world without refueling did it with very high aspect ratio wings and without tiplets.

The vast majority of models do not have range and fuel economy as high priorities. End plates at the ends of control surfaces will improve control effectiveness and that is often a priority with models but a full chord wing fence or end plate is not required for this purpose.

[Sparky Paul]

Ollie to be correct, it -is= a coincidence the Voyager had no tiplets.
One was scraped off on the takeoff run due to the extreme wing droop with all the fuel and the other was damaged enough Dick manuvered the plane to complete its removal.

[DeaninMilwaukee]

A couple of years ago a buddy and me had an ongoing speed competition with similar sized sp500-sp600 powered self designed planes, roughly pylon style looking.

Each week we'de show up with some modification to try to out against the other guy, and I'd finally taken the lead by going geared and inputting LOTS more watts.

The next week he showed up with these huge wing tiplets, tall,and swept way back, probally 1/2 the area as the vertical stab each. Looked funky, like something batman would fly. :)

Despite no other changes, he went from 2-3 mph deficeit compared to my plane to a 2-3 mph surplus.

In this particular case, there is no doubt he significantly reduced drag.

He did abandon them however as they made the plane really twitchy in gusty conditions. In hindsight, perhaps he just had too much effective vert. stab area? :confused:


Dean in Milwaukee

[Sparky Paul]

Well, was it pure straight line speed, or tighter turns, or less speed loss in turns? :)
This kind of stuff is difficult to quantify with two seperate pieces of equipment.
That's why I found the baseline trim with everything on that airplane, and changed only one thing at a time to check for changes.
The pilot goes out of the performance loop that way.
The straight line speed is relatively simple to check.. fly without and with. Same motor/battery etc.
The turning stuff.. good luck! :)
About the only to test that would be using a control-line model where speed and radius of turn could be determined.

[DeaninMilwaukee]

Well, was it pure straight line speed, or tighter turns, or less speed loss in turns?

Basically straight line. We would do large sweeping racetrack circuits in part to make it easier for us to tail each other, ( expert pilots we were not), accurately.
About turning ect, dunno. Much too hard to quantify small changes.

In the straights though, no question, he was faster with tiplets than without.


Dean in Milwaukee

[Sparky Paul]

Dean, your buddies plane must have been truly awful for a 10% or so increase in speed with winglets! :)
Were such a change doable in the real world, all planes would have winglets!
Our Director of Flight Test, Sam Wyrick said he's sell his grandmother for a single drag count improvement on the Tristar.
And that's less than 1%, an improvement the airlines would dearly love to get on their fuel bills.

[aeropuppy]

Originally posted by Sparky Paul
Dean, your buddies plane must have been truly awful for a 10% or so increase in speed with winglets! :)
Were such a change doable in the real world, all planes would have winglets!
Our Director of Flight Test, Sam Wyrick said he's sell his grandmother for a single drag count improvement on the Tristar.
And that's less than 1%, an improvement the airlines would dearly love to get on their fuel bills.

Perhaps your flight test director will be unpopular with his grandfather sooner than he thinks?

A friend of mine works for Avaition Partners in Seattle producing blended winglets. Here's a couple of quotes from thier website and a press release:

"Aviation Partners of Seattle has begun flight-testing a winglet retrofit for "classic" 747-200 and -300 transports. A prototype set, extending 14 feet over the planar part of the wing, has accumulated four hours of flight time and "cleared all flutter issues," according to Aviation Partners CEO Joe Clark. Initial indications show a 6%-7% reduction in drag,"

"Highly blended winglets have demonstrated more than 60% greater effectiveness over the same size conventional winglets with an angular transition."
"By reducing drag, blended winglets increase fuel efficiency 7.3% and boost range 200 N.M. for cruise between mach .74 and mach .80."

http://www.aviationpartners.com/intro.html
http://www.awgnet.com/shownews/00farn1/intell02.htm

[Ollie]

I'll bet that very powerful computational fluid dynamics software was used to develop those highly blended winglets. That sort of software is out of the reach of most modelers in both the dollar cost and the technical knowledge to use it. Furthermore, unless a model is intended to operate for long periods of time at a crusing speed, such blended winglets would be of little value. Modelers that aren't trying for an Atlantic crossing or a distance record would have little practical use for such winglets.

A 28 foot span increase on a 196 foot span Boeing 747 would have about the same or greater increase in lift to drag ratio as the blended winglets. The advantage of the winglets is that they wouldn't be a problem with existing ground handing facilities at airport arrival and departure gates that the 28 foot increase in span would be.

[Sparky Paul]

We tested two sizes of extended tiips on the Tristar in the middle '70s.. the second version I seem to recall was used on the -500 model.
I don't know what the benefits were, but any measureable improvement would have been a sales item.
Photo shows the standard wing on the left, -500 wing on the right..
But I bet any improvement at Mach .074 to .08 would be very difficult to quantify. :)

[DeaninMilwaukee]

"Dean, your buddies plane must have been truly awful for a 10% or so increase in speed with winglets! "


Perhaps, but in the end he did indeed go faster with a same sized plane and maybe 3/4 the watts.

Thats on of the probs with model airplane design. Its difficult to meaningfully test measuring of aerodynamics, and what looks fast is'nt always fast.
Good example is my jk aerotech 1/12 Zero. Does NOT look fast with the gaping cowling up front, and yet in real life it was very fast. Go figure.


Dean in Milwaukee

[Grejen]

Hmm... On straight wings those fences seem to show up on STOL aircraft such as the DHC-6 Twin otter in the pic. I'm gonna guess it's a lift for drag trade off. Increases both resulting in a better climb, shorter takeoff, payload increase, and trading off cruise speed and fuel economy.

Design limitations such as materials cost, structural and even air field/airport parking space limitations would necessitate such devices rather than higher aspect ratio wings. The DHC-6 has a pretty high aspect ratio already.

Also I've noticed more and more comercial liners with those blended winglets so they must be worth it. Not all design considerations are aerodynamic or even structural. I've heard that some aircraft have T tails for no better reason than to clear the top of the fuel truck!