model in wind tunnel - RC Groups
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Feb 20, 2012, 11:07 PM
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model in wind tunnel

If you didn't see this it might spark some interest and kill some preconceived ideas...:

Wind tunnel cession with Jibe 2 (2 min 6 sec)

It has the links to the more theoretical side of the work as well. Enjoy!
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Feb 21, 2012, 12:01 PM
Registered User
I would have preferred an intelligent commentary on what's happening in the tunnel, rather than the cheesy pop music.

Jim R.
Feb 21, 2012, 12:39 PM
B for Bruce
BMatthews's Avatar
The left side wing oil patterns look pretty normal. What's with the right side having those three little steps starting out from the center?
Feb 21, 2012, 02:59 PM
Registered User
I guess they are turbulator strips?
Feb 21, 2012, 08:26 PM
Needs brain lubrication
biber's Avatar
'They' often place some small strips of zig zag tape as a reference in flow visualisation.

Feb 22, 2012, 08:12 PM
Cognitive dissonance
kcaldwel's Avatar
Thanks Francois! Nice link. There are some great technical papers there too. I've read the extended stability for models one many times, but I hadn't seen the others.

I did some screen captures to be able to look at the flow patterns in a bit more detail. This is what I think I see, and found interesting. I'd appreciate any other thoughts.

The flow is laminar back to about 70% at Alpha=0.1. The turbulators definitely trip the flow further forward, which would increase drag (Edit: On further inspection, the aft most turbulator does not seem to be causing the transition pint to move forward, so it is ineffective). The tail is about 100% laminar, except in the fuselage wake. The fuselage wake effects a pretty big chunk of the wing, with turbulent flow right to the Leading edge, and likely a big hole in the nice wing-only elliptical lift distribution as well.

At Cl=0.6, Alpha = 5.1, the transition is at forward of 10%. The turbulators are doing nothing except collecting fluid, as they are all aft of the transition. The swept back tip has separation and will be higher drag than a straight trailing edge. The transition is over a very small percentage of the wing chord, with little evidence of a transition bubble.

There are a couple of spots on the left wing where the flow is tripping right at the leading edge, causing a turbulent triangle back across the wing. This is good evidence of how critical the leading edge is.

Any thoughts on how much the hole in the lift distribution from the fuselage might decrease the span efficiency? I've gotten interested in flying wings again, but I am again disappointed by their span efficiencies. I wonder how close the fuselage hole brings the span efficiencies? Of course, if you have a pod on the flying wing you have the same issue.

Last edited by kcaldwel; Feb 22, 2012 at 10:07 PM.
Feb 22, 2012, 10:29 PM
Registered User
Yes Kevin, very interesting indeed how just how much we can learn qualitatively from a short video!!!
Mind you, you need the resources and the time to put together something like this.
All your comments are very valid and it shows how designing the "perfect wing" might not lead to a highly performant aircraft. Too many "other things" to consider.
For me, this visualization is very interesting for one point: the flow over the wing is parallel to the wind tunnel flow except at the very tip. Not in accordance with some "theories" developed sometimes around here.
But: this test is done at the proper Re and this is why it makes such a strong statement. We are not speaking of low to near critical Re, but this is not full scale Re in the 1 to 5 millions values either. For this reason a lot of "assumptions" need to be revisited. The wing/fuselage interference and the fuselage/tail interference being an other area that would benefit from some more thinking. After all for how many years professor Drela has designed wing mounts with a pod and a full flying horizontal stab on a yoke above the boom? Must be a reason???
Feb 23, 2012, 12:49 AM
Cognitive dissonance
kcaldwel's Avatar
I missed the theories about the flow over the wing not being parallel with the free stream. Sounds strange.

There is some interesting stuff in this paper (towards the end) about the flow direction on swept wings at Re from 22k to 60k:

If you align the airfoil with the 1/4c on a swept wing, then the airfoil gets thinner if the flow is closer to the free stream direction. You might be better off aligning the airfoil with the free stream direction on swept wings?

It looks like 3D effects of a wing change the pressure coefficient and skin friction coefficient quite a bit over a strictly 2D analysis. 2D airfoil modelling certainly won't tell the whole story of how a real wing works.

Feb 23, 2012, 02:29 PM
Registered User
I wish we all had more access to this sort of testing!

It won't be immediately, but I'm planning some tests with a DLG with a specially made wing in real world flying. Essentially fly it to the ground under different flight conditions and snap a picture of the flow pattern. It will likely take a little work to get ready for the tests and more work to get good data from them. We'll see. I'd rather use an oil or fluid that didn't need UV fluorescence for good visibility.

One thing I hope to answer is what Ncrit range (if any) gives the best correspondence to real world (near) 2D flow behavior (ie, near the middle of the wing half where flow is likely closest to 2D that we're going to get on a real plane). That one piece of information would be worth quite a bit in the design process.

I plan to use the feedback to assist in further refinement of designs.

Much to do before we get there I suppose.

Mar 04, 2012, 08:54 AM
Team Hillbilly (UK)
Space Bat's Avatar
Originally Posted by G_T
I wish we all had more access to this sort of testing!

Much to do before we get there I suppose.

You CAN do most of this at home

Have a large fan?? Able to position plane at various angles?? Have a camera with high shutter speeds that can take multiple exposures??

Then you can do it....

All you need is to attach small cotton/wool strips at various locations on your plane and turn the fan on and take pictures...adjust the planes AOA and pitch/bank and see what happens to these strips throughout different types of angles...

Similar if you want to observe prop wash ....etc just do the same and run the prop...

You can find out a great deal of how a plane works - where dead spots are on wings and can play around with vortex generators...etc - only imagination and patience is required - not fancy equipment

But yeah fancy equipment is good
Mar 05, 2012, 03:38 AM
Registered User
Yes Space Bat, but your analysis is qualitative, not quantitative. Furthermore you don't have a control over the flow and it is not a constant and/or not turbulent flow. Therefore a lot of "findings" might be completely off...
Mar 05, 2012, 03:42 AM
Registered User
The best poor man wind tunnel is in fact on a roof rack on a car. The only big drawback is: how to control the flow to get rid of the turbulences created by the car itself. Actually with a simple flat sheet of plywood and the right positioning of the whole contraption results can be amazingly good, but limited for the obvious reasons...
Mar 05, 2012, 04:20 AM
Registered User
Just use a pillar mount? Finding a long straight private road with steady air is going to be more troublesome, as well as actually making the observations. Also, you are limited to the car's speed, which might not be enough for those models where an in depth analysis would be beneficial.

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