Tall Paul

I've been trying to eliminate the "head-shaking" typical of the ZAGIâ„¢ design. It's quite annoying in the sloper, somewhat less so in the electric powered version.
A suggestion made was to add winglets below the wing in addition to those above.
Previously I've flown several flying wings with different combinations of vertical fins; from none to 3, and noted most a interesting relaxation of concern by the model as to which is the front , with no vertical, while it would fly quite well... until the speed dropped.
Then any edge served as a leading edge.
I added below-the-wing winglets to a sloper and an e-powered ZAGIâ„¢ and yesterday got a chance to fly the e-plane. First launch was miserable, and the plane hit the runway, ripping off the lower winglet on the right side.
Tossed it again, and it had a persistent left-turn, which lessens as the speed goes up.
This is in accordance with Dr. Eppler's demonstration that a plane will turn toward a single downturned winglet with an upturned winglet on the opposite wing.
I was surprised at the amount of imbalance.
Glued another winglet on the right side.
Today's weather appears benign, so maybe later today I can toss it again.
As the effect appears to be speed related, when the wind comes back up the slope, I can more easily test both, either or no winglets at slower speeds than the e-power plane flies.
.
Note the tiplet on one-side only plane has MORE drag.. not what a racer wants!

BMatthews

Yet another example of truly Buddhist like humility..... LOL

Ken, yes induced drag DOES dominate the low speed end of the spectrum but it's all relative rather than absolute like you imply by trying to put a set speed to the definition. High Cl flight IS the low speed regime for any airplane. That's what the graph in the link page is all about. You can't put an absolute speed to this description.

Strykaas, proper wing tip winglets are carefully designed to control and limit (but not eliminate, that's impossible) the vortex formation by transitioning the lift around the wingtip and onto the winglet. The winglet is strongly twisted or otherwise shaped using a transitioning airfoil to taper off the lift towards the tip. At least that's the old NASA method. I notice that Airbus uses much smaller tip plates that go quite against the concept of the NASA types. I suspect these were derived through much wind tunnel time to be able to place them and shape them to reduce the size of the votices while not producing other side effects. The original NASA winglets were strickly a one speed solution and has some bad side effects at other speeds. Without a wind tunnel of your own and without knowing the specific flight speed you are setting the plates up for it's all a guess and the cut and try solutions can easily produce more interference drag due to bad corner shapes and other factors than the reduction in vortex drag then produce (if any). Also any tip plate solution is going to only be effective over a relatively narrow Cl range and perhaps not at all during very high Cl flight like in tight high speed turns. Only the tunnel would tell for sure.

This is why sailplanes have gotten away from tip plates or winglets in favour of the crescent or Schumman (sp?) planform. The raked back or crescent shape has been shown to strongly reduce (but not eliminate) vortex formation and help prevent and control inrushing spanwise airflow in all flight modes so that high Cl flight drag is reduced. And to some extent high speed high G tip stall susceptability is reduced as well. The raked or crescent tips are not as good at ONE speed as the NASA tiplets were shown to be but they are the best compromise for a wide range of operating speeds and Cl's.

Tall Paul

Flew the ZAGIâ„¢ yesterday with the downside tip reinstalled, to get a base line. As I kinda suspected the "turn towards the down side" was more a feature of mis-trim than more drag on the other side.
But now I have a baseline setting on the ZAGIâ„¢
The wind was too high today to fly during the day, but after sundown it died, so I took my YardStik down to the park, and flew a flight without a downside tip to set the trims. Taped on a downsider, and flew again.. definite pull to the left side where the tiplet was.
They do ADD drag. Not something a racer might desire.
ZAGIâ„¢ results later in the week, maybe. I can do a more extensive test when there's no one at the field.

Montague

Paul, can you post some pictures of what you're doing? I'm not totally sure I understand what the various configurations are.

KenLitko

Did you have to make any roll corrections Paul, or just yaw?

Tall Paul

Kirk, I'm using a standard ZAGIâ„¢ 400-X, as it already has the tiplets going up.
The downwards are merely some plastic sign material cut to fit and glued on.
As it went left with the right lower tip missing, I thought I'd cut off the lower left on the next flight.
I plan on using a transmitter with a digital display of the lateral trim position, and read what it says with any trim changes.
I expect the changes won't be large.
The next flight should be with the upper left tip removed, but I'm unsure as to how the electric will handle that, and I'd prefer to use the ZAGIâ„¢ 3C which I've modified the same, on the slope. More room to correct for an "oops" on launch.
But the electric also works as a sloper.... we'll see.
When I did the tip thing earlier to see how little vertical area was needed, (turned out be none, as long as the speed is kept up) I wasn't paying much attention to the lateral trim..

Tall Paul

Ken, it's lateral on the ZAGIâ„¢, elevons only.
The YardStik has RE, so the trim is yaw. Wasn't much at all.

Tall Paul

Things went well.
Changed the radio to a Flash-5, which has the trim position in perentage.
Flew another base-line flight to get a value, which was 12% right trim, 10% up trim.
Landed.
Took off the lower left tiplet.
Flew again, power on and off.
No observable trim change required.. changed anyway and it came back to the same point.
top photo
Landed.
Took up the upper left tiplet.
Again, no observable trim change required, power or off.
2nd photo.
Landed.
Took off both right side tiplets.
Again, no observable trim change, power on or off.
3rd photo.
Trim kept returning to the 12% figure.
A reasonable quantitative (but still anecootal, as alasdair would say) conclusion is the presence or non-presence of tip fins has no discernible effect for the length of the flight path involved being near or less than 1%.
As anything sticking out into the air must have an effect, the effect is too minute to detect with the mehods used, and would need a different set of circumstances for its evaluation.
But for the question, the presence of such devices would make zero difference in the aerodynamic results.
It's still up to the pilot..
And it was a beautiful day in the Antelope Valley!
4th photo.

FHHuber

Flying wings are often special cases... More testing needed.

I gotta get to building again and get some data to add to this "pot"

dump in data stir well... and you still got a mash of results because we just don't have the stuff to do it right.... yet.

For your Shrike race... just be sure you don't use flat plate endcaps on the wingtips... can't get a much more draggy tip than the style on a 4*40. (well you can if you TRY... but you really gotta WANT to make some drag. to even consider them.)

***********

Note... I am not trying to argue with the test results obtained by Paul. His tests are very valid for his model, and the wingtip designs used.

LouW

All of the various tip designs have as their goal fooling the air into thinking that the wing has a wider span than it does. Unless the actual span is limited by contest rules, hangar doors, etc, I suspect in most cases, the additional structure and weight could be more effectively used to simply extend the actual span a little instead of putting on a bunch of weird tip shapes.

MOJO65-9or10

http://www.mh-aerotools.de/airfoils/index.htm

Check out the info on wingtips - pretty cool[sm=RAINFRO.gif]

LouW

That is a very interesting site. However in reading his data on tiplets, in every case he compared the tiplets to a wing of the same span without tiplets. It would have been more informative if he had compared them with a wing of a span increased by the height of the tiplets. As small a difference as appears in his data, I still suspect that the wider span without tiplets would have at least as little drag if not less than the shorter span with them.

Tall Paul

Any drag change would be in the milli-ounces for a model. For this to have any -observable- effect the plane would have to fly trimmed for a long time.. much longer than in a race.
There's other potential benefits other than drag reduction, but these also wouldn't be observable over the short term.

LouW

I agree completely. Fooling around with tip design, while interesting academic discussion, is a complete waste of time for the highly overpowered aircraft that seldom fly straight for more than a few seconds like the typical R/C model.

Strykaas

To get back to the initial subject, I would like to mention something interesting. I have heard that B757s produce massive wake turbulence, nearly as important as B747's, although B757s are not that big/heavy.
Does anyone have any good explanation for this ? Is it simply related to airfoil ?

BMatthews

They may be refferring to takeoff and landing wakes where folks pay attention to this sort of stuff. All the work in reducing drag in an airliner goes into the cruise phase of flight in order to reduce the lbs of fuel used per km to the minimum. Meanwhile the wake of a plane such as this during takeoff and landing is probably set at least as much by the rotation angle as it is by the size of the plane. So it can't be blamed on any one item on the plane or even the size itself, within reason. However at cruise speed and altitude the smaller plane should make a smaller wake. But there's no one up there to test it....

Getting back to tip plates or winglets....

Adding fences such as these do CAN reduce the votex size and thus energy taken from the model but they produce their own form of parasitic drag due to airflow down in the inside corner or turbulating as it goes around the outer corners. The NACA style winglets used a fairly smooth curved transition from the wing to the tip to help avoid this. But we modelers often don't bother with stuff like that. As LouW has already suggested using a good flat tip shape much as sailplance models have these days will provide much of what a tip plate promises without the junction parasitic drag. And it'll do it at all speeds and angles of attack. So if you want cheap and easy efficiency use raked back tips like the fancy performance gliders have.

Tall Paul

Wing loading generates the downwash.
I wonder what the 777 downwash is like?