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Jun 28, 2011, 07:39 PM
Build straight - Fly twisty
Whiskers's Avatar
Quote:
having the wing a zero incidence to the flow would be things that would be closer to the reality of how they are most used too.
You may rig tour plane at zero, but I'd bet my boots that it flies with some measure of Angle of Attack.
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Jun 28, 2011, 07:39 PM
Dreamer

the cfd run


i guess i should explain my backround a bit, i just graduated with a bachelors in Aerospace Engineering. this was the result of our project for the course Viscous flow. we got an A+ Our whole idea behind doing the project was that KF airfoils are easy to make, but have more drag than proper airfoils, but this can be overcome by the powerful brushless motors available for scratch builders.

Im not highly versed in the math behind it all but i have the experience in doing basic CFD and interpreting the results. the software is Ansys CFX. i believe it is used in the industry for this sort of stuff. i have access to our labs in our university, so my group mate and I were able to do a range of AoA for this.

So im gona try to use the resources available to me to do more acurate models. im using Catia V5 another industry standard software for the modelling. Just have to figure our how do do the the cad shape as it was made of 6 mm depron. I'm a pretty good modeler so I'm confident i can model it. just takes a bit o time.

My goal is to get good coefficients so I can use it on a plane we are building to host the ArdupilotMega. I'm still learning to fly. haven't flown any R/c planes for longer than 40 secs . So i can't do field tests of KFs.

Ill post images and results and i get them. it takes quite a bit of computing power and time.
Jun 28, 2011, 07:43 PM
Dreamer
Quote:
Originally Posted by JetPlaneFlyer View Post
What software produced that picture?.. Has the software been wind tunnel verified on similar stepped airfoils?

the software is ANSYS CFX. I think its one of those aerospace industry standard softwares, as for wind tunnel testing... i do have sort of access to a wind tunnel at my university... but ill have to talk to some people..
Jun 28, 2011, 07:56 PM
Jack
jackerbes's Avatar
Quote:
Originally Posted by Whiskers View Post
You may rig tour plane at zero, but I'd bet my boots that it flies with some measure of Angle of Attack.
Yes, exactly right. My planes KF winged planes are all built without any incidence. And they appear to fly level. So to me they don't seem to have much or any incidence.

And I wasn't sure how the testing is done. Do they start with some incidence? Or at zero and vary it?

Jack
Jun 28, 2011, 08:01 PM
Jack
jackerbes's Avatar
"..i guess i should explain my backround a bit..."

Welcome to the forums if you're pretty new here, you are a welcome addition.

Most of us had to do more plebeian things for a living and then we could and play with airplanes in our spare time. Maybe you'll be lucky enough to be able to come home from work and apply your work to your recreational efforts.

If you want to, you can edit you profile and people can see where you are at. Some times that opens doors to more people and thiing...

Good luck with it!

Jack
Jun 28, 2011, 08:05 PM
Dreamer
i did zero to 15.. heres an image of 15 .. makes a cool picture.. i dont know how accurate the results were, we were on a rush to hand in the report, so my friend and i were up late doing quick iterations.. so the quality of the work is poor..

now that i have time i can spend a bit more time into it

these are velocity streamlines the lines represent the speed of the airflow around the KF...
Jun 28, 2011, 08:39 PM
Build straight - Fly twisty
Whiskers's Avatar
Quote:
My planes KF winged planes are all built without any incidence. And they appear to fly level. So to me they don't seem to have much or any incidence.
Yes, my conventional symmetrical section and KFm4 planes rigged at zero also seem to fly 'flat' but they must have some positive AOT in flight because wind-tunnel tests and logic show they produce no lift at zero AOT.
Jun 29, 2011, 12:49 AM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Quote:
Originally Posted by sketchillus View Post
the software is ANSYS CFX. I think its one of those aerospace industry standard softwares, as for wind tunnel testing... i do have sort of access to a wind tunnel at my university... but ill have to talk to some people..
I thought it might be an Ansys product. There was a discussion on the use of the similar Ansys Fluent CFD software over on the Modeling Science forum recently: https://www.rcgroups.com/forums/show...ghlight=fluent

The consensus was that Ansys Fluent did not (in the tests they did) accurately model flow where transition and separation was present... I suspect the same might be true on your analysis because what actual wind tunnel testing that has been done on 'trapped vortex' airfoils has shown that a stable vortex cant be maintained in the way your image indicates.. Or at least it cant be done without forced blowing and sucking of the vortex cavity.

Taking nothing away from the work you have done, it's very interesting and if you could get access to a wind tunnel and verify the Cl and Cd analysis results that would be amazing...

Steve
Jun 29, 2011, 07:35 AM
Dreamer
Steve,

you are most likely right, CFX and Fluent does have its pitfalls. but im sure one can analyze it better, but that will require a more knowledgeable approach, probably changing some default settings and using diffrerent solvers. ( i mean people use this software for more complicated CFD analysis)

when i animated the streamlines, there vortex seems to be loosing air flow and tends to move towards the tip of the air foil, at the time i didnt know how acurate that was and even now im still not sure how to interpret that. I can maybe talk to one of our amazing profs who's specialty is CFD, and he likes RC stuff.

hopefully i can use our wind tunnel to get some data and we can compare them.

Madhuran
Jun 29, 2011, 05:58 PM
Build straight - Fly twisty
Whiskers's Avatar
Those flow diagrams are interesting, but I am sure they lack detail.
Like, you can draw a face with an outline, two dots and 2 curved lines but it is not a good representation of the real thing.
However they do show why (I think) this type of KF works, and that is by using air flow to create a virtual, and variable, wing section.
Jun 29, 2011, 08:16 PM
treefinder
springer's Avatar
Following on me Older Bro's thoughts, here's a hypothesis: The air coming over the tips that flows inboard through most of the wing span (or maybe all the way to centerline - that needs to be tested with more tufts) feeds the vortex that "plumps" (new high tech term???) the flow over the wing stabilizing the vortex, and giving the variable/virtual wing section.

Another thought that came to me on way back to MI from OH was based on the comments that the over the tip flow only runs for a small distance on the top of the wing, and in order to achieve a stable vortex in earlier testing, it had to be "fed" with more air dynamically. Perhaps there is a scale factor involved, in that the mass flow will only go so far for the amount of "push" or "pull" involved, and for wings above a certain size, there's not enough available to make it work, but down in our sizes there is proportionately enough.
Jun 29, 2011, 10:53 PM
internet gadfly
nmasters's Avatar
Quote:
Originally Posted by springer View Post
Following on me Older Bro's thoughts, here's a hypothesis: The air coming over the tips that flows inboard through most of the wing span (or maybe all the way to centerline - that needs to be tested with more tufts) feeds the vortex that "plumps"
A vortex is fed by the shear layer that separates from the step. Air can only flow out at the ends; that's why they are either closed loops like smoke rings or one end is attached to a surface. The air flowing around the tip probably just causes turbulence in that area


Quote:
Another thought that came to me on way back to MI from OH was based on the comments that the over the tip flow only runs for a small distance on the top of the wing, and in order to achieve a stable vortex in earlier testing, it had to be "fed" with more air dynamically. Perhaps there is a scale factor involved, in that the mass flow will only go so far for the amount of "push" or "pull" involved, and for wings above a certain size, there's not enough available to make it work, but down in our sizes there is proportionately enough.
Well there certainly isn't a size limit on vortices in general. Tornadoes, hurricanes and Jupiter's great red spot are vortices. The largest example of a step induced vortex I can think of right now is what sailplane pilots call a rotor which forms downwind of a mountain. On a somewhat smaller scale back in the '40s and '50s a guy named Edward Lanier built several airplanes with doors on the upper skin that opened inward to reveal steps

Jun 30, 2011, 10:14 AM
treefinder
springer's Avatar
[QUOTE=nmasters;18634531]A vortex is fed by the shear layer that separates from the step. Air can only flow out at the ends; that's why they are either closed loops like smoke rings or one end is attached to a surface. The air flowing around the tip probably just causes turbulence in that area

Ok, how do we explain the consistent flow of air inboard along the step almost all the way to the centerline of the wing? Almost 30 inches of turbulence? Maybe just flow inboard without a vortex? Got more vids today with tip plates, but haven't uploaded to vimeo yet. I'll post when they are done.
Jun 30, 2011, 11:05 AM
treefinder
springer's Avatar

KFM2 wing with tufts


More Fodder for the discussion and your viewing enjoyment....two new vids with KFM2 wing on the OSG. First is pretty much the same as first vid posted earlier. The winds are nearly calm this morning, and the tufts overhanging the step get sucked into it pretty much same as before. Dew was pretty heavy this morning, and the tufts seemed to get stuck from time to time. But changes in attitude and turns seem to dislodge them and they return to the inboard pointing mode.

(2 min 0 sec)


The second vid is a comparison between no tip plate, a small tip plate set 90 degrees to the wing tip, and a larger tip plate angled outboard about 15 degrees. The flight is the final approach, which seemed the most consistently the same between all vids. It looks to me like the tips do reduce the inboard flow but not stop it. I only added them to one side, and with the small one didn't notice any different flight characteristics, but with the larger one, noticed a tendency to want to turn toward it, so I'd guess it creates more drag then no tips. (as one might expect)

(1 min 18 sec)
Jun 30, 2011, 12:30 PM
Grumpy old git.. Who me?
JetPlaneFlyer's Avatar
Springer,

I'm not really seeing this consistent inboard pointing 'thing'. The only time I see the tufts clearly all pointing inboard is when the glider is circling with the wing that's in view low.. In that scenario inboard flow would be expected on the low wing due to sideslip.

The tufts do clearly show a stall a few times in the first video.

Steve


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