Votex Generators..how do they work?
 

Hi guys. Pardon the silly question. I read flying magazines(fullscale) and was reading a while back about a guy who built an exp. float plane. He stuck a zillion vortex tabs on the wing near the leading edge and was able to take the stall from 43 knots or so to 28 knots. Can somebody explain to me how they work? I see them on a lot of Jets I fly on now too. Thanks guys. Keith

RE: Votex Generators..how do they work?
 

Adding ENERGY to the boundary layer will cause it to remain attached to the form longer. A turbulent boundary layer has more energy than a laminar boundary layer and will stay attached longer, whereas the laminar boundary layer will detach, taking the main flow with it. This is why golf balls have dimples, causing the WAKE area to be much smaller. The drag on a body is a combination of 'form' drag (having a lot to do with the size of the wake) and 'skin' drag (skin friction). Of course there is also 'induced drag' (from the wingtips) but that is beyond my feeble attempts at understanding or trying to explain.
Vortex generators will immediately trigger the boundary layer from laminar to turbulent and they will add a lot of energy to it. The flow will simply follow around the back side of the form farther before it detachs into a wake area. The smaller the wake area, the less the 'form' drag. Skin friction is increased but that is less important, in air at least.
Bottom line: an energetic boundary layer will increase the angle of attack that a wing can sustain before it 'stalls'.
This is true even at low speeds and with even with small models, regardless of wing loading or power. Whether it makes a difference on the model YOU fly depends on your specific case - the type of plane & the type of flying. In some cases, as a few folks will point out, it doesn't make any practical difference at all. In other cases it will make a lot.
Allan

RE: Votex Generators..how do they work?
 

A little bit confusing but I think I grasp the idea. Thanks for sharing your thoughts.

RE: Votex Generators..how do they work?
 

Also known as turbulators, trippers, and invigorators.;)

RE: Votex Generators..how do they work?
 

Turbulators is by far the most fun to say :)

RE: Votex Generators..how do they work?
 

Sometimes they will make things much worse, be particularly careful with the low Reynold's number laminar airfoil type wings popular on R/C aircraft.

It's not always the case but the higher camber designs, i.e. S1223, is not a good choice for this practice.

RE: Votex Generators..how do they work?
 

Actually, I was not thinking about putting them on an rc or any aircraft for that matter. I was merely curious. Thanks. Keith

RE: Votex Generators..how do they work?
 

Really interested?
Search Web for Mini Vortex Turbulators etc..
There is plenty of info on kits available to stick these on light aircraft of all types also figures on performance improvement etc..
For our small models -not really any point in adding them-other than as a conversation piece.

RE: Votex Generators..how do they work?
 

Turbulator strips (and even downstream invigorators) are used all the time in FAI Free Flight models. They definitely can be effective in preventing/delaying flow separation at model-size Reynolds numbers. This has been proven over the years by back to back tests by many modellers (myself included). That said, there is an aspect of "conversation piece" to their use, as there are several styles and various people feel strongly that one may be better than another... etc.

-David

RE: Votex Generators..how do they work?
 

The one issue that tends to be glanced over in the use of vortex generators is design. Simply applying randomly sized and placed bits to the wings of any given aircraft and expecting improved performance is craps shoot at best. Companies such as Boeing, spend millions of dollars in wind tunnel testing and computational fluid dynamics studies to get the vortex generation sizing and layout just right. Factors such as airfoil design, surface roughness, and control surface location come into play during the studies. More focused on the vortex generator itself, designers look at height, placement, orientation to the free stream, and more.

Like winglets the use of vortex generators is espoused as a major perfomance improvement, however as the addage goes - you never get something for nothing. Often times added weight or unintended control surface flutter degrade performance benefits gained from adding vortex generators, or winglets, or...

All that being said, the great thing about this hobby is that it never hurts to experiment, in fact that is half of the fun in my mind. Best case scenario you improve the performance of your plane and you learn something new. Worst case scenario you get to build a new plane:D