Vortex generators?

On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square hollow corner where the wing meets the fuselage. Due to the pressures in that area, sooner or later the air can't follow the contour, especially at high angles of attack in slow flight. The "standard" cure for this is a fillet to provide a contour the air can follow. Putting a turbulator/vortex generator ahead of the separation can entrain more air into the boundary layer there and possibly reduce the effect somewhat. I once put some fillets on the root of a 1-26 and reduced the separation enough to lower the stall speed about 4 mph if I remember correctly. It is worth noting that this is a touchy area on certificated aircraft.
UH

I figured the first thing I'd do is tape some yarn to the wing and film it, but doesn't having the tape and yarn on the airfoil effect what's going on too? I am NOT pouring dirty oil on my wing! (unless I REALLY have to) Yuck.

Boggs

On Monday, September 5, 2016 at 1:18:21 PM UTC-4, wrote:
On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square
hollow corner where the wing meets the fuselage. Due to the pressures
in that area, sooner or later the air can't follow the contour, especially
at high angles of attack in slow flight. The "standard" cure for this is
a fillet to provide a contour the air can follow.

Its the gradient that causes the separation. The fuselage is angled away
from the wing, the top of the wing is angled downwards. Combination reduces
the pressure too much too fast, hence separation. It may take a pretty big
fillet to fix, for example on Genesis.

Putting a turbulator/vortex generator ahead of the separation can entrain
more air into the boundary layer there...

Different animals though, no? A turbulator takes laminar flow and turbulates
it to make it stickier. But it doesn't really change the boundary layer,
thickness does it? Any aero experts out there? A vortex generator sticks
up to mix air outside the boundary layer, reducing the boundary layer
thickness in the resulting vortex area, right?

I once put some fillets on the root of a 1-26 and reduced the separation
enough to lower the stall speed about 4 mph if I remember correctly.

Cool! Of course you need a pitot-static and ASI better than factory
to really find out the speeds down at that end.

Anyway, all these solutions improve the situation at high CL but
add just add drag at low CL, right? Probably don't improve max LD
unless something is drastically wrong with the original design...

On Monday, September 5, 2016 at 2:50:02 PM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 1:18:21 PM UTC-4, wrote:
On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square
hollow corner where the wing meets the fuselage. Due to the pressures
in that area, sooner or later the air can't follow the contour, especially
at high angles of attack in slow flight. The "standard" cure for this is
a fillet to provide a contour the air can follow.

Its the gradient that causes the separation. The fuselage is angled away
from the wing, the top of the wing is angled downwards. Combination reduces
the pressure too much too fast, hence separation. It may take a pretty big
fillet to fix, for example on Genesis.

Putting a turbulator/vortex generator ahead of the separation can entrain
more air into the boundary layer there...

Different animals though, no? A turbulator takes laminar flow and turbulates
it to make it stickier. But it doesn't really change the boundary layer,
thickness does it? Any aero experts out there? A vortex generator sticks
up to mix air outside the boundary layer, reducing the boundary layer
thickness in the resulting vortex area, right?

I once put some fillets on the root of a 1-26 and reduced the separation
enough to lower the stall speed about 4 mph if I remember correctly.

Cool! Of course you need a pitot-static and ASI better than factory
to really find out the speeds down at that end.

Anyway, all these solutions improve the situation at high CL but
add just add drag at low CL, right? Probably don't improve max LD
unless something is drastically wrong with the original design...

The contrast is more about degree than action, I think. Vortex generators are commonly larger and many are placed at angles to the local flow to disturb much more air than the smaller turbulators used to transition attached laminar flow into attached turbulent flow. Both stick up through the local layer into higher energy air and cause mixing. It's mostly about degree.
On big jets it is probably almost all intended to add energy to the boundary layer so that controls work better and air stays attached to high lift devices at very high C/L.
One interesting application I heard from Mike Opitz was the adding of turbulators forward of the hinge line of the rudder on a Grob Twin. He reported a remarkable improvement in feel and handling.
FWIW
UH

As both Dave and Hank have insinuated, turbulators and vortex generators generally have different functions. Turbulators are used to force transition from laminar to turbulent flow. This is done to eliminate (drag-producing) laminar separation bubbles that form for Reynolds numbers typical of sailplanes. Turbulators have heights less than the boundary-layer thickness. Conversely, vortex generators are usually used to alleviate turbulent separation. They act like tiny wings, generating a vortex that causes higher velocity flow farther from the surface to be directed closer to the surface, thus energizing the boundary layer. This reduces the tendency of the boundary layer to separate in the adverse pressure gradient toward the trailing edge of the wing, particularly at the high lift coefficients corresponding to low airspeeds.

To get to Boggs's original question, vortex generators may alleviate a separation in the wing-fuselage juncture. Tufting the region in question is certainly a good idea. The (yarn) tufts will have negligible effect provided they are attached downstream of transition. As Hank noted, the modification may run afoul of the law for a certificated aircraft. Alleviating a wing-root separation may also have unintended consequences, for example, by changing the separation progression along the span and, therefore, the stall behavior. There is, of course, a drag penalty due to the vortex generators that generally increases with increasing airspeed. As one aerodynamicist put it: "There's no free lunch."

I agree oil-flow visualization of a separated region should be avoided. While it may be enlightening for an aerodynamicist, the almost explosive scattering of the oil downstream will create quite a mess.

Wing-root fillets are a significant undertaking, often involving as much art as science. Be prepared for the inevitable research project.

Finally, to clarify a few misconceptions:
No transport aircraft employ laminar-flow airfoils---yet; some business jets do, fewer actually achieve laminar flow (e.g., the HondaJet).
Turbulators increase the boundary-layer thickness because they force the thin, laminar boundary layer to transition into the thicker, turbulent boundary layer. Turbulators do not make the boundary layer "stickier."
Vortex generators do not make the boundary layer thinner; they alter the boundary-layer velocity profile, making it "fuller."

Dan Somers/ID
Port Matilda, Pennsylvania

On Monday, September 5, 2016 at 4:15:09 PM UTC-4, wrote:
On Monday, September 5, 2016 at 2:50:02 PM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 1:18:21 PM UTC-4, wrote:
On Monday, September 5, 2016 at 11:05:05 AM UTC-4, Dave Nadler wrote:
On Monday, September 5, 2016 at 10:22:02 AM UTC-4, wrote:
Possibly some of these aircraft have laminar separation bubbles
that are made better by tripping the flow directly to turbulent.

As I think I understand it...

A turbulator is used to transition laminar to turbulent flow to
avoid laminar separation; the turbulent flow causing less drag
than a big bubble. Turbulators can be a relatively thin tape, or
blow holes...

A vortex generator is a lot taller, and is used to pull
higher-speed flow down to the much slower boundary layer,
and create a vortex that inhibits span-wise flow in the
slower boundary layer. That can be used to prevent separation
in some cases; either in laminar or turbulent flow.
It also be used to aid in cooling - there was a great article
recently (KitPlanes ? Sport Aviation?) about a rear-engined
canard that needs tall turbulators to get air into the
aft cooling scoops.

Gary - start with some thin tell-tales and a GoPro to look
at the flow around the top wing surface at the root, aft
of the spar.

Hank - Could Gary be seeing the same issue as the 20's
experience - double-bad-adverse-gradient at fuselage
junction? You install turbulators on the 20 top surface
at the root for this, no?

Interesting stuff....
Best Regards, Dave

Dave- In a word- yes. The simple construction geometry leaves a square
hollow corner where the wing meets the fuselage. Due to the pressures
in that area, sooner or later the air can't follow the contour, especially
at high angles of attack in slow flight. The "standard" cure for this is
a fillet to provide a contour the air can follow.

Its the gradient that causes the separation. The fuselage is angled away
from the wing, the top of the wing is angled downwards. Combination reduces
the pressure too much too fast, hence separation. It may take a pretty big
fillet to fix, for example on Genesis.

Putting a turbulator/vortex generator ahead of the separation can entrain
more air into the boundary layer there...

Different animals though, no? A turbulator takes laminar flow and turbulates
it to make it stickier. But it doesn't really change the boundary layer,
thickness does it? Any aero experts out there? A vortex generator sticks
up to mix air outside the boundary layer, reducing the boundary layer
thickness in the resulting vortex area, right?

I once put some fillets on the root of a 1-26 and reduced the separation
enough to lower the stall speed about 4 mph if I remember correctly.

Cool! Of course you need a pitot-static and ASI better than factory
to really find out the speeds down at that end.

Anyway, all these solutions improve the situation at high CL but
add just add drag at low CL, right? Probably don't improve max LD
unless something is drastically wrong with the original design...

The contrast is more about degree than action, I think. Vortex generators are commonly larger and many are placed at angles to the local flow to disturb much more air than the smaller turbulators used to transition attached laminar flow into attached turbulent flow. Both stick up through the local layer into higher energy air and cause mixing. It's mostly about degree.
On big jets it is probably almost all intended to add energy to the boundary layer so that controls work better and air stays attached to high lift devices at very high C/L.
One interesting application I heard from Mike Opitz was the adding of turbulators forward of the hinge line of the rudder on a Grob Twin. He reported a remarkable improvement in feel and handling.
FWIW
UH

I wish this forum had a like button. Thank you all for this input. Now all I need is the double sticky tape...

Boggs

On Monday, September 5, 2016 at 5:20:34 PM UTC-4, wrote:
...Turbulators increase the boundary-layer thickness because they
force the thin, laminar boundary layer to transition into the thicker,
turbulent boundary layer.
Turbulators do not make the boundary layer "stickier."

Vortex generators do not make the boundary layer thinner; they alter
the boundary-layer velocity profile, making it "fuller."

Thanks Dan!
Best Regards, Dave

One interesting application I heard from Mike Opitz was the
adding of
turbulators forward of the hinge line of the rudder on a Grob
Twin. He reported
a remarkable improvement in feel and handling.
FWIW
UH

Since my name was brought up, I just want to mention that the
Grob airfoils were of the generation before turbulated airfoils came
out. Therefore, the Grob airfoils have tended to incur the large
laminar separation bubbles that are so typical of that earlier
generation. Some glider repair shops in Europe started
experimenting with retrofitting the Z-tape turbulators to gliders with
the older airfoils in hopes of minimizing how far forward the laminar
separation bubbles would creep, thus improving the performance.

In the case of the Grob gliders, it was discovered that putting the
Z-tape in front of the control surfaces can noticeably increase their
efficacy on some of the gliders, thus improving handling
characteristics.

Although Grob went bankrupt, the German glider repair center LTB
Lindner has picked up all of the rights to the Grob glider line, to
include molds, spares, and just about anything else that one would
need for true factory support. If they don't have a part, they can
manufacture one for you as well.

This brings me to UH's point about modifications to certified
aircraft. LTB Lindner has Service Letter #SL-12 published on their
web site which covers the LEGAL application of mylar seals and Z-
tape to all Grob gliders. Just go into the service letter, and it will
show you how and where one can LEGALLY apply the newer seals
and Z-tape.

http://www.ltb-lindner.com/service-letter.html

From personal experience, I can say that the Z-tape makes a very
noticeable difference in the handling of our club's Twin Astirs (Twin
1's). For anyone looking through previous old internet posts, you
will find that some people had gripes with the Twin 1's handling. I
would hazard a guess that those complainers had been flying Twin
Astirs that were not Z-tape equipped.

Last year at the Elmira Nationals, Erik Mann (P3) approached me
about the handling of his ACA (Blairstown, NJ) Twin Astir. I just
referred him to SL-12, and told him to talk to me after he
accomplished it. They did it over the winter. This year at the
Sports Class Nationals, I believe the quote from him was something
like "It's like it's a whole different glider. I actually have people that

like flying it now." Then, he told me that they hadn't even done the
rudder yet. I told him that the rudder would show a bigger
handling improvement than the ailerons if he were to do it...

This is a case where you don't have to experiment because someone
has already done it, and they got it certificated as well - a simple
solution to an issue that once had many pilots aggravated...

RO