Vortex generators on laminar flow airfoils

Homebuilt Aircraft & Kit Plane Forum

Help Support Homebuilt Aircraft & Kit Plane Forum:

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
I was browsing the web this morning, and chanced upon vortex generators. After following several links, I came across a technical paper which stated:
It’s better to mount farther forward than too far aft from the leading edge. The permissible range is considered to be 6-10% of wing chord back from the leading edge to the front of vortex generator
That seems awfully far forward, but I guess on a turbulent airfoil that makes sense. But what about a laminar airfoil? The NACA737A415 (for example) is laminar to 40% of the upper chord surface. Why would one destroy all that lovely laminar flow with vortex generators placed at 10% of the chord?

I can't find any web references to laminar airfoils and VG's, but if one WERE to use them on a laminar airfoil, surely one would place them much further aft - close to the transition region? But when they are actually needed (at takeoff or landing) the laminar region of the wing is much reduced.

Mmmm My premature and half-considered conclusion is that VG's are no good on laminar airfoils.

Anyone have any knowledge regarding VG's?

Duncan
 
Last edited:

Voidhawk9

Well-Known Member
HBA Supporter
Joined
Mar 26, 2012
Messages
555
Location
Timaru, NZ
What are you trying to achieve with the VGs? If it is high-AoA, low speed stuff, then placing them too far aft will probably not help a great deal.
 

Victor Bravo

Well-Known Member
HBA Supporter
Joined
Jul 30, 2014
Messages
8,964
Location
KWHP, Los Angeles CA, USA
VG's and laminar airfoils do not go together as a general statement.

In SOME rare cases, very very small VG's, called "turbulators" are used to trip a laminar flow airfoil to becoming a turbulent flow airfoil, at a certain chordwise location. Usually pretty far rearward on the bottom and just about in the middle on top.

But the "normal" kind of VG's from StolSpeed and Micro Aerodynamics are completely different. These bigger VG's destroy any laminar flow, and immediately render a laminar airfoil useless. They are only used on turbulent flow airfoils. They work very well, for a specific purpose or two, but you ALWAYS sacrifice all laminar flow behind where any VG's are located.
 

rtfm

Well-Known Member
Joined
Jan 3, 2008
Messages
3,532
Location
Brisbane, Australia
I wasn't thinking of a specific application, just wondering about laminar and VG. But I think I answered my own question anyway. Thanks guys.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
7,957
Location
Saline Michigan
Duncan,

VG's, stall strips, stall fences, vortillons, and other externally applied "fixes" are for undesirable characteristics after you have an airplane. Discussed elsewhere on HBA.com. They are a non-elegant solution where the elegant one is expensive and/or inconvenient and/or impractical. They can turn poorly behaved airplane into a well behaved one with modest added drag.

Fixes are not to be applied in initial design - a more appropriate combination of airfoil, planform, and control surface configuration is a much better solution that we then hope has need for only minor tweaking by addition of "fixes". If you apply "fixes" in initial design, and then find undesirable characteristics in flight, what do you do then? Your moves have already been made.

Yes, they spoil laminar flow. Fortunately we know of some nicely behaved airfoils and flap/aileron configurations that generally behave very nicely. So, if you are inclined to laminar foils, use the well behaved ones with the known good behaving control configurations, and then wreck laminar flow on the minimum of places you must.

Billski
 
Last edited:

allonsye

Member
Joined
May 28, 2013
Messages
21
Location
Winchester, VA
v/g's can be a very effective tool for some wings. I recently made a huge improvement to my Avid (undercambered) STOL wing with results that exceeded my expectations. I installed the smaller Airwaves under the leading edge. Interestingly, no decrease in stall speed, but an increase of 13 mph to cruise. I suspect if I rebuilt the flaperons to a laminar shape from the present flat bottom, it would gain even more efficiency. I got the idea from a write-up by Harry Riblett.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,721
Location
Rocky Mountains
I installed the smaller Airwaves under the leading edge. Interestingly, no decrease in stall speed, but an increase of 13 mph to cruise.
Interesting! My 'in the box' brain just never thought of that use for VGs.
A quick Google didn't turn up any relevant Riblett/VG paper. Can you give a link or name of the paper?
 

BBerson

Light Plane Philosopher
HBA Supporter
Joined
Dec 16, 2007
Messages
14,960
Location
Port Townsend WA
Sailplanes use "zig-zag" tape at the transition, not vg's.
The purpose is to trip the low drag laminar into medium drag turbulent instead of high drag separated that would occur without the tape.
 

allonsye

Member
Joined
May 28, 2013
Messages
21
Location
Winchester, VA
There's been lots of discussion in the AvidFoxFlyers forum on airfoil shapes which is where I pulled the attached. Sorry, they were bad images to begin with but there's some enormously interesting discussion within and worth gleaning through. The name for the undercambered airfoil escapes me but was atypical of the 1 - 3 KF's and the Avid STOL.
 

Attachments

allonsye

Member
Joined
May 28, 2013
Messages
21
Location
Winchester, VA
There's been lots of discussion in the AvidFoxFlyers forum on airfoil shapes which is where I pulled the attached. Sorry, they were bad images to begin with but there's some enormously interesting discussion within and worth gleaning through. The name for the undercambered airfoil escapes me but was atypical of the 1 - 3 KF's and the Avid STOL.
BB1E1963-61E7-4DD3-B6B8-B6DDA862C336.jpeg
 

Attachments

  • Like
Reactions: dog

speedracer

Well-Known Member
Joined
Feb 4, 2020
Messages
113
v/g's can be a very effective tool for some wings. I recently made a huge improvement to my Avid (undercambered) STOL wing with results that exceeded my expectations. I installed the smaller Airwaves under the leading edge. Interestingly, no decrease in stall speed, but an increase of 13 mph to cruise. I suspect if I rebuilt the flaperons to a laminar shape from the present flat bottom, it would gain even more efficiency. I got the idea from a write-up by Harry Riblett.
That's interesting. I've never heard of VG's making a plane go faster, only slower because of the added drag.
 

Norman

Well-Known Member
HBA Supporter
Joined
Nov 28, 2003
Messages
3,108
Location
Grand Junction, Colorado
That's interesting. I've never heard of VG's making a plane go faster, only slower because of the added drag.
It's explained in the letter from Harry Riblett attached to post #11 but it's worth saying again. VGs delay separation which usually only occurs at high AoA but on airfoils with a concave lower surface there is a big separation bubble on the lower surface at high speed. Since the pressure drag of a separation bubble is much higher than the friction and induced drag that VGs add there's a large net reduction associated with scrubbing the bubble off thus less drag at high speed.
 
Last edited:

speedracer

Well-Known Member
Joined
Feb 4, 2020
Messages
113
It's explaned in the letter from Harry Riblett attached to post #11 but it's worth saying again. VGs delay separation which usually only occurs at high AoA but on airfoils with a concave lower surface there is a big separation bubble on the lower surface at high speed. Since the pressure drag of a separation bubble is much higher than the friction and induced drag that VGs add there's a large net reduction associated with scrubbing the the bubble off thus less drag at high speed.
Thanks for that explanation. I learned something new today.
 

geosnooker2000

Well-Known Member
Joined
Mar 30, 2019
Messages
179
Location
Somerville, TN
Duncan,
........
........
Yes, they spoil laminar flow. Fortunately we know of some nicely behaved airfoils and flap/aileron configurations that generally behave very nicely. So, if you are inclined to laminar foils, use the well behaved ones with the known good behaving control configurations, and then wreck laminar flow on the minimum of places you must.

Billski
Which ones are those? To be more specific in my question, what database says "- this one has good stall characteristics"?
 

BJC

Well-Known Member
HBA Supporter
Joined
Oct 7, 2013
Messages
13,110
Location
97FL, Florida, USA
Which ones are those? To be more specific in my question, what database says "- this one has good stall characteristics"?
I'm not aware of a data base the lists stall characteristics, but there is more to the stall behavior than just the 2D airfoil selection. Reference all the discussion about the "deadly" NACA 23012, which behaves very nicely in the real world.

One example that I have flown is the LS(1)-0413 aka GA(W)-2 that is used on the Glasair series (likely meets the definition of laminar) and the GlaStar and Sportsman which do not, with riveted the metal wings, have laminar flow. The Sportsman and the Glasair both have benign stall characteristics IMO.


BJC
 
Top