Vortex Generators
 

Hi Everybody;

I'm testing the effect of vortex generators on stall angle of a NACA 6313 aerofoil in low-speed wind tunnel.
Based on my understanding VGs are used to delay the separation and thus delay the stall.
however in my test they accelerate the stall.
my plain aerofoil stalled at around 16 degrees, however when placing the aerofoil on different chord positions and different spaces apart, in ALL situation my aerofoil stalled earlier.
I'm trying to find a reason behind it, or any test result from any journal/book that could explain my situation.

Any help is highly appreciated.

Thanks

RE: Vortex Generators
 

I did a lot of testing with turbulator strips on glider wings, and could not see a difference in
the stall behavior of the model.
Modifying the wing with washout achieved results.
I've seen turbulators on top of the wings of a Learjet, amazing.
You'd think it would ruin the lift.
I think turbulators only work on fullsize airplanes because of Reynolds number,
and air viscosity.
The freeflight Nordic gliders of the 50's used a piece of thread strung ahead of the lead edge,
the competitors swore by them, but I never understood the theory.
I'm tempted to try some sort of turbulator on my scratchbuilt AT-6, it has a bad tip stall

RE: Vortex Generators
 

For a start, without a wind tunnel and smoke, you are working in the blind. Every wing is unique. It has been discovered that if you can identify areas where the boundary layer has weakened, and is about to separate, that a turbulater can put energy back into the flow, delaying separation. Basically the turbulater is designed to create little vortices. Without identifying these areas, I think the idea is not worth pursuing.

RE: Vortex Generators
 

Vortex generators and turbulators don't always work as advertised. A lot depends on the scale of the airflow. I've never worked with vortex generators like the angled tabs you see on some airfoils but I have tried turbulators on a few different wings. In two cases I had good results and tow others I had one that made no difference and the other that made things worse.

We're talking about bandaid fixes here. A lot of things come into play. For example some inaccuracy or the level of finish on your test wing or even the residual turbulence in your airflow may have provided all the help you needed to extend the stall. And when you added your vortex generators you may have gone over the edge and it hurt more than it helped.

In his first report Michel Selig emphasized just how important the test wing finish and accuracy was and how important it was to reduce the turbulence level in his wind tunnel work on model sized airfoils.

Without knowing more about your own setup it's very hard to say if it's your tunnel, your test model or the size, placement and pattern of your vortex generators. But off the top of my head I'd suggest you try turbulator strips instead of the tab like vortex generators.

RE: Vortex Generators
 

Arent the little dents in golf balls turbulators?
The old Model Builder magazine had a humorous technical section
where some guy wrote in and said he took a ball peen hammer
and put hundreds of little dents in his RC model and it crashed

RE: Vortex Generators
 

That was because all the dents let out all the aerions that let our models fly... :D

Yes the dimples are turbulators. They aid in holding the air closer to the ball's surface and make it bend around the ball further before separating. This greatly reduces the wake the ball is pulling behind it and lets it travel further. It also incidentally makes any spin you impart on it more effective as well.

Some guy a while back wanted to put dimples on his props. But dimples are only one sort of tubulator. And the first thing you need to do is determine if you even need turbulators. Not every airfoil benifits from adding them. And they DO add drag. Like most things in life it's all about tradeoffs. Give up a little here to gain some there.

RE: Vortex Generators
 

To clarify the earlier statement, the dimples in a golf ball actually reduce drag due to increased pressure from behind the ball (this is the result of an attached positive pressure from the turbulent flow still attached behind the ball). The vortex generators on your wing will increase drag, however, because turbulent air is inherently more energetic than a laminar flow and therefore surface friction increases as well as induced drag from increased lift.