Eagler's Nest

Airframes => Single Seaters => Topic started by: Dan_ on October 22, 2013, 08:38:06 PM

Title: VGs and lift
Post by: Dan_ on October 22, 2013, 08:38:06 PM

The lift of a given wing varies with the AOA.  If the VGs allow the air to stay attached over a wider range of attack angle, then is it not true that they increase the lift of a wing..?

Title: Re: VGs and lift
Post by: rockiedog2 on October 23, 2013, 07:32:13 AM

the way ive always understood it is they delay separation and thus loss of lift rather than increase lift. understand that it would seem that if stall is delayed til a higher AOA then it could conceivably be said that the lift had increased due the higher AOA but the speed has decreased a like amount so the lift remains the same. the confirmation i think is that  we are supporting the same aircraft weight at one G thus lift is unchanged.
like i said...this is the way i understand it and that was from a long time ago. i will defer to a credible manual. www.stolspeed.com has a really good explanation

Title: Re: VGs and lift
Post by: Dan_ on October 23, 2013, 08:43:30 AM

Quote

the confirmation i think is that  we are supporting the same aircraft weight at one G thus lift is unchanged.

Joe,
I think you are thinking of landing, and I am thinking of climbing... 

This was in your link...  at the bottom I highlighted some of it...


How VGs Work
With the installation well underway, I asked Bob Desroche and Jay Falatko if they could explain to me the theory behind how vortex generators reduce stall speeds and Vmc. What ensued was a cram course in Aerodynamics 101 which I found illuminating and fascinating.
VGs are boundary layer control devices, so it isn't surprising that to understand how they work you first need to know something about the boundary layer. I'd certainly heard the term before, but never really understood its significance. Bob and Jay were glad to fill me in, and here's what I learned.
When an airplane is in flight, we usually think in terms of air passing over the top of the wing at the airspeed of the aircraft. But it turns out that the viscosity of the air and the friction of the wing surface cause the air molecules in contact with the wing to adhere to its surface and therefore have zero velocity. Air molecules slightly farther away from the wing surface will be slowed due to friction with the zero-velocity molecules but won't be completely stopped. As we move still farther away from the wing surface, the air molecules will be slowed less and less, until at some distance from the surface a point is reached where the air molecules are not slowed at all. The layer of air from the surface of the wing to the point where there is no measurable slowing of the air is known as the boundary layer.

(https://www.eaglersnest.com/forum/proxy.php?request=http%3A%2F%2Fwww.avweb.com%2Fnewspics%2Fvortex5.gif&hash=613a4c9865dd4cd85246707cc9fd135f08f74a6d)  Boundary layer changes from laminar to turbulent flow as it moves aft along the wing.

(https://www.eaglersnest.com/forum/proxy.php?request=http%3A%2F%2Fwww.avweb.com%2Fnewspics%2Fvortex6.gif&hash=89f4d7a790de85db2b0b12ca6648edab2fadf749)  Laminar vs turbulent.

Near the leading edge of the wing, the boundary layer is very thin, and the air molecules in it move smoothly and parallel to the wing surface. This is known as laminar flow. But as the airflow progresses aft from the leading edge, the boundary layer becomes progressively thicker and more unstable, and transitions to turbulent flow in which intermixing of faster and slower air molecules starts to take place. (Another easily-seen example of laminar and turbulent flow can be seen by watching the smoke rise from a lighted cigarette in a draft-free room.)
It turns out that laminar flow is a good-news/bad-news situation. The good news is that laminar flow provides greatly reduced drag compared to turbulent flow. The bad news is that laminar flow permits the boundary layer to separate easily from the wing surface at high angles of attack. That's why so-called "laminar flow airfoils" (which are designed to move the transition to turbulent flow further aft) tend to provide low drag at cruise but nasty stall characteristics.
Turbulent flow in the boundary layer produces more drag, but is much more resistant to separation (and therefore to stalling). However, even in areas of turbulent flow, there tends to be a thin sub-layer of laminar flow in the immediate vicinity of the wing surface which becomes increasingly slow-moving and stagnant toward the trailing edge of the wing. It is this "aerodynamically dead" sub-layer that allows airflow to separate and the wing to stall.

(https://www.eaglersnest.com/forum/proxy.php?request=http%3A%2F%2Fwww.avweb.com%2Fnewspics%2Fvortex7.gif&hash=da32d64590fd9b30e2b1368bdb605d52d5440b4b)   By energizing the boundary layer, VGs allow the airfoil to operate  at higher angles-of-attack without airflow separation. (Copyright Micro AeroDynamics)

If we could find a way to energize this sublayer, flow separation would be supressed and the onset of stall delayed. This is precisely what vortex generators do. Each VG creates a pencil-thin tornado-like cone of swirling air that stimulates and organizes the turbulent flow of the boundary layer on the aft portion of the wing. The swirl of the vortices pull fast-moving air down through the boundary layer into close proximity to the wing surface, energizing the previously-dead air there.  The result is a wing that can fly at significantly higher angles of attack before the onset of boundary layer separation, and can therefore achieve a significantly higher maximum lift coefficient.


When mounted on the wings, VGs reduce stall speed and increase climb capability. When mounted on the vertical tail, they increase rudder effectiveness and lower Vmc.

Title: Re: VGs and lift
Post by: rockiedog2 on October 23, 2013, 09:19:20 AM

no argument with any of that in the link
i'm speaking primarily of level one G flight near/at the stall speed
which also translates to the landing flare in a practical sense. as we hold it off, hold it off the AOA is increasing and the speed is decreasing and we are (in a practical sense anyway and ideally) holding altitude just a hair above the runway. yes in reality this rarely happens...we're descending and plunk it on short of the stall speed...but it illustrates the point. i hope.
as to the climb...no argument with the BL theory. but *in my experience* the benefit there is so secondary to the lowered stall speed that i rarely give it any thought so yes you are correct, i wasn't thinking of climb. the lower stall speed is a dramatic change and again *in my experience* the increase in climb is hardly noticable, if there at all.( the link didn't go into whether they were speaking of an increase in ROC due the *max rate speed being lower with the vgs*. if it is lower then that could be where the increased rate comes from. i don't know about that. ive never seen that addressed in any manual. I'm speaking  simply of the 45 mph speed that most consider the best ROC for a legal eagle. for instance). the exception might be if trying to climb very near the stall speed, which is neither max rate or angle so i don't see that as being much of a consideration. i have seen the ROC decrease measurably with the paired aluminum vgs due to increased drag. but never have seen a measurable increase in ROC at a given climb speed.
whew. i got smoke comin outa my ears..

Title: Re: VGs and lift
Post by: Justin on December 16, 2013, 02:11:07 PM

Quote from: rockiedog2 on October 23, 2013, 07:32:13 AM

Yes, VGs will increase lift to a low speed wing design by delaying separation, but at some point the reduction of air speed of the air that is laminar would result in the reduction of lift.

Utility and normal category aircraft typically limit their use to areas fwd of primary flight controls. Air flowing over primary control surfaces during stall is a safety feature to allow the flight controls to remain responsive in a stall.

I have seen mods and production runs with VGs all the way down the wing on restricted category aircraft, typically high power to weight ratio (a crop duster is the first type that comes to mind.) My opinion is that they need to bite the bullet and change the chord for their mission.

It would be interesting to see pros and cons for the different configurations in flight testing!

Title: Re: VGs and lift
Post by: Jess on November 23, 2014, 11:58:59 PM

Hello Eaglers,

I just received my DE plans and just found this community, so this is my first post; please forgive any faux pas.

I believe the issue with "increasing lift for climb out" would be in that although lift increases as a function of AOA (alpha), so does drag. I would assume that the best climb rate for an eagle, is not at max AOA, therefore, increasing the available AOA would not necessarily increase the max climb rate. 

I imagine that if one tried to climb at max AOA for any appreciable amount of time, the increased drag would slow the aircraft down below stall speed (even the reduced stall speed allowed by VGs). Clearly, more power is the answer :)

It's been a while since I looked at any of this stuff, so there may be holes in my logic.

Regards,

Title: Re: VGs and lift
Post by: Dan_ on November 24, 2014, 02:38:06 AM

Quote from: Jess on November 23, 2014, 11:58:59 PM

  Clearly, more power is the answer :)

Yep, I reckon if the power is the same and the angle of attack goes up there is a point the wing will quit.  That point will be later/slower with VGs...

Title: Re: VGs and lift
Post by: LSaupe on May 05, 2018, 07:05:49 AM

I know this is an old topic, but thought I would touch base to see if there has been any practical experience with these on an LE (i.e. a good before and after comparison).

Anyone using these?

Title: Re: VGs and lift
Post by: Tom XL-7 on May 05, 2018, 03:04:59 PM

Get a hold of Joe Spencer. Might find him under RockieDog. He used them and is a fan of stol aircraft. He can give details but he swore by them. Joe also worked with wing fences at tips. He actually recorded his data . not seat of pants