To Vortex Generate, or Not?

[Steve_B]

My uneducated guess would be that they're trying to make the airflow stay attached to the wing as it passes over the aileron to improve aileron response.

Dead right, that's where VG's are usually located and for exactly this reason.

The previous point you made was also spot on. VG's delay separation of the boundary layer and so delay stall, that is their only purpose. We would have to ask if is this really what we want to achieve if we are trying to fly 'post stall'? Personally I don't think it sounds logical.
The other thing to bear in mind is that full size airplanes fly in a totally different Reynolds Number regime than even quite large scale models, the the aerodynamics are very different., VG's don't necessarily work in the same way at the scale of most RC models.. The size and location of VG's is also highly critical, most RC'ers just seem to stick them on in fairly random locations and make then arbitrary sizes and shapes which is likely to provide erratic and inconsistent results at best.

I think it was Scott stoops who pioneered the very sharp leading edge airfoils most 3D model use these days. That as I understand it was to give a relatively early stall and to make the wing, once stalled, stay stalled.. This seems to be diametrically opposite to what VG's might achieve.. no?

I have to admit though to never having tried VG's on my RC models. Most of the 'testing' I've read about seems to be highly subjective.

Steve

 

[RCAddict16]

From what I've heard its mainly to improve the control surface response. I think that VG's create little swirls of air that further enhance control surface authority.

 

[njswede]

To me, the elephant in the room is this: How stalled is really what we call post stall? Compared to 1:1, we're spinning huge props and have insane T/W, so the lift generated from prop wash can't be ignored. Also, the airflow vector generated by the prop is going to lower the true alpha. It's pretty easy to calculate:

Let's say I'm harriering at 5mph over ground on a calm day and I'm flying at a 45 degree angle to the horizon maintaining steady altitude. I'm spinning a 7" pitch prop at 8500 RPM, yielding a theoretical pitch speed of 56mph. Let's say the prop wash is down to 30mph when it hits the wing. We can calculate the true angle of attack (relative to the airflow) as:

atan((30 * sin(0) + 5* sin(45)) / (30 * cos(0) + 5* cos(45))) = 6 degrees.

So our true alpha is really only about 6 degrees and I'm not sure we have a full stall at that alpha.

(Somewhere in the back of my mind I remember Steve shooting this down, but I don't remember where the flaw is...)

 

[njswede]

From what I've heard its mainly to improve the control surface response. I think that VG's create little swirls of air that further enhance control surface authority.

Yes, that's certainly the explanation that makes the most sense to me.

 

[njswede]

I'm sure someone has already done this, but it would be a fun experiment that I think I will try to do next time I fly: Pieces of yarn and a key chain camera to try to find out what REALLY happens!

 

[Steve_B]

I'm sure someone has already done this, but it would be a fun experiment that I think I will try to do next time I fly: Pieces of yarn and a key chain camera to try to find out what REALLY happens!

'tufting' is something i've seen done on RC planes before but not 3D planes and not with VG's.. it's an excellent idea and wouldn't be much work to do.

 

[njswede]

'tufting' is something i've seen done on RC planes before but not 3D planes and not with VG's.. it's an excellent idea and wouldn't be much work to do.

I'll do it without VGs first. I have to fix my Sabre X first, since it has the nice quality of being able to be flown at a gradually increasing alpha without any bad tendencies. My idea was to go from straight an level flight through increasing angles of attack until I reach a hover. Having looked at the math a bit, I think we're in for a surprise...

 

[Steve_B]

(Somewhere in the back of my mind I remember Steve shooting this down, but I don't remember where the flaw is...)

Not sure about 'shooting down' the math looks pretty solid.. Only caveat is that the prop wash only covers a small proportion of the wing, also in high alpha you would have only moderate throttle so probably much lower prop wash velocity.

Tufting would indicate just how big the wash effect was.

 

[njswede]

Not sure about 'shooting down' the math looks pretty solid.. Only caveat is that the prop wash only covers a small proportion of the wing, also in high alpha you would have only moderate throttle so probably much lower prop wash velocity.

Tufting would indicate just how big the wash effect was.

I think you'll find that the prop wash is somewhat "funnel shaped", so you'll have a wider area exposed (at the expense of airflow speed). The bottom line is that you're going to have a substantial part of the wing that's never stalled.

And where am I going with this? I'm not sure. I guess that I'm just arguing that "post stall" is a relative term and that we need to take this into consideration when we think of how VGs work/don't work...

 

[Steve_B]

Having looked at the math a bit, I think we're in for a surprise...

My bet would be that the wing is fully stalled in a full high alpha harrier, except possibly for a few inches near the root that in the prop wash. You would need to use really flexible 'tufts'. Wool (what they often use in full size planes) is probably too stiff, strips of tissue or crepe paper might work quite well providing you kept it slow.