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Author Topic: Vortex Generators on a Twister  (Read 2440 times)

Online Ken Culbertson

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Vortex Generators on a Twister
« on: August 16, 2019, 06:02:31 PM »
Has anybody tried vortex generators on a Twister?  It may be to light to have any affect and I can't find anything that helps in a search.

Ken
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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #1 on: August 16, 2019, 06:07:54 PM »
There may be more direct ways to attain high performance.

AFAIK vortex generators only make sense when the wingloading is high.  How heavy is your Twister?  I'll admit that I never tried putting vortex generators on my 54 ounce Twister (see the graphical aid), but even so, I'm pretty sure that it wouldn't have overcome the airplane's inherent Twisterness.
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #2 on: August 16, 2019, 11:55:28 PM »
There may be more direct ways to attain high performance.

AFAIK vortex generators only make sense when the wingloading is high.  How heavy is your Twister?  I'll admit that I never tried putting vortex generators on my 54 ounce Twister (see the graphical aid), but even so, I'm pretty sure that it wouldn't have overcome the airplane's inherent Twisterness.
48oz electric & Fancherized with 4" added to span so it has a pretty low loading.  It has a nasty habit of turning early after a couple of corners slow it down.  Other than that I would put it up against a PA ship in good air.  Thought the generators might let me use a bit less elevator and stop some of it's Twisterness.  They really helped my PA ship which wobbles in at 70 ounces but is massively over powered so it flies quite nice and is great aerobic exercise.

Ken
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Online Brett Buck

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Re: Vortex Generators on a Twister
« Reply #3 on: August 17, 2019, 12:24:27 AM »
Has anybody tried vortex generators on a Twister?  It may be to light to have any affect and I can't find anything that helps in a search.

  So far, I haven't seen anyone that found a case that worked worse with VGs than not, I don't see why the Twister would be any different. Given that the theory of operation is, at best, "vague", I would suggest just trying it and seeing what happens. You can always take them off again.

    Brett

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Re: Vortex Generators on a Twister
« Reply #4 on: August 17, 2019, 09:22:32 AM »
I've discovered that with a top coat of 2-part automotive clear coat, almost nothing will stick to the stuff.  So how does one attach VG's?  (Not that I would want to try them, but just curious).
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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #5 on: August 17, 2019, 10:54:18 AM »
... Given that the theory of operation is, at best, "vague", ...

Whadinhell do you mean by that???

Igor uses them.  Paul uses them.  Any theory that starts with "well, the top guys use them" has to be solid, right?
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Offline Matt Colan

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Re: Vortex Generators on a Twister
« Reply #6 on: August 17, 2019, 11:22:14 AM »
I've discovered that with a top coat of 2-part automotive clear coat, almost nothing will stick to the stuff.  So how does one attach VG's?  (Not that I would want to try them, but just curious).

Iíve been putting a little piece of tape on the airplane and then CAed the VGís onto the tape. Havenít had any issues with them flying off or not sticking
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #7 on: August 17, 2019, 11:38:14 AM »
Whadinhell do you mean by that???

Igor uses them.  Paul uses them.  Any theory that starts with "well, the top guys use them" has to be solid, right?
The "Top Guys" once used Fox 35's
The "Top Guys" once used fixed leadouts.
The "Top Guys" once had 15% airfoils with pointed leading edges.

Ken
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #8 on: August 17, 2019, 12:23:24 PM »
Iíve been putting a little piece of tape on the airplane and then CAed the VGís onto the tape. Havenít had any issues with them flying off or not sticking
They CA to MonoKote, put them on my Sandpiper profile.  Making mine from the ends of Popsicle sticks cut in half.  Get 4 per stick.  I am going to use the tape idea on the Twister because I don't have high hopes for them.  There is a limit to how well you can make one of them fly and I may have already reached it.

Ken
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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #9 on: August 17, 2019, 01:03:47 PM »
The "Top Guys" once used Fox 35's
The "Top Guys" once used fixed leadouts.
The "Top Guys" once had 15% airfoils with pointed leading edges.

Ken

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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #10 on: August 17, 2019, 01:08:00 PM »
They CA to MonoKote, put them on my Sandpiper profile.  Making mine from the ends of Popsicle sticks cut in half.  Get 4 per stick.  I am going to use the tape idea on the Twister because I don't have high hopes for them.  There is a limit to how well you can make one of them fly and I may have already reached it.

Ken

Unless you can peel the CA off of the 'coat, use something (like the tape idea) that'll let you relocate them.

Not only because they may not work, but because when folks do get improvement out of them, they often then report even more improvement by moving them around, adding more, and/or by thinning them out.

I do know that the people who say they work tend to report that they help the most when the plane is heavy.  The vague description that fits best with my admittedly limited understanding of aeronautics is that they keep the turbulent flow stuck to the airfoil better, as the thing is tending toward a stall.

Come to think of it, they may be of more benefit in your pointy-airfoiled full-fuselage plane, possibly located closer to the LE that folks do on the current generation of blunt-airfoiled planes.
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Online Brett Buck

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Re: Vortex Generators on a Twister
« Reply #11 on: August 17, 2019, 01:19:16 PM »
Whadinhell do you mean by that???

Igor uses them.  Paul uses them.  Any theory that starts with "well, the top guys use them" has to be solid, right?

   Calm down there, Ace.

   I mean that no one has a particularly good theory of the mechanism by which they operate, and how they help, and the proposed mechanisms I have heard have ranged from complete nonsense, to (charitably)  hand-wavy.

    That doesn't mean they *don't work*, just that no one has a good theory of how they work, so you can't easily tell whether or how much they might help in any particular case,  unless you try it.

   Which is, I note *exactly what I recommended*.

     Brett

   

Online Brett Buck

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Re: Vortex Generators on a Twister
« Reply #12 on: August 17, 2019, 01:26:20 PM »
I've discovered that with a top coat of 2-part automotive clear coat, almost nothing will stick to the stuff.  So how does one attach VG's?  (Not that I would want to try them, but just curious).

      Paul uses rubber cement, that seems to work OK. I think Uncle Jimby (whose airplanes are apparently so shiny on such a regular basis he has convinced some people that stunt is too easy) uses double-sided tape. There's not a lot of load on them, and it's not the end of the world if you have to reinstall them after they get knocked off going in and out of the car, so that seems to be plenty adequate.

     In my limited experimentation, I used Ambroid or Duco cement, which stuck well enough, and could be popped off pretty easily when necessary. Clean up with acetone, it won't hurt the clear.

     Brett

     

Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #13 on: August 17, 2019, 01:32:04 PM »
   Calm down there, Ace.

   I mean that no one has a particularly good theory of the mechanism by which they operate, and how they help, and the proposed mechanisms I have heard have ranged from complete nonsense, to (charitably)  hand-wavy.

    That doesn't mean they *don't work*, just that no one has a good theory of how they work, so you can't easily tell whether or how much they might help in any particular case,  unless you try it.

   Which is, I note *exactly what I recommended*.

     Brett

   

Yes, whoever that Emperical guy was, he sure invented a sound method for testing stuff.
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Offline Howard Rush

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Re: Vortex Generators on a Twister
« Reply #14 on: August 17, 2019, 08:06:07 PM »
I mean that no one has a particularly good theory of the mechanism by which they operate, and how they help, and the proposed mechanisms I have heard have ranged from complete nonsense, to (charitably)  hand-wavy.

That doesn't mean they *don't work*, just that no one has a good theory of how they work, so you can't easily tell whether or how much they might help in any particular case,  unless you try it.

The mechanism is well-known: they make vortices that bring high-speed air down to the surface of the wing to reenergize the boundary layer. Here's a paper: Pauley, W. R. and Eaton, J. K., Experimental study of the development of longitudinal vortex pairs embedded in a turbulent boundary layer, AIAA Journal, 26 (7), 816 (1988).  I recently stumbled on some wind tunnel results that indicate that they can reduce lift hysteresis in an airfoil that increases in angle of attack until it's near stall, then decreases in angle of attack.  That sounds useful for stunt.  I'd like to do (or better yet, have others do) some more experimenting on where to put them.  That's why I asked where Ken put them that helped so much.   

Vortex generators were once an issue in a French national election.  The party in power backed development of an Airbus airplane, the wing of which the opposition claimed was so poorly designed that it required vortex generators to fix. 

Igor uses them.  Paul uses them. 

Igor doesn't use them, maybe because his airfoil doesn't have lift hysteresis.  Paul uses them, but in different amounts on different airplanes, as determined by experiment.  I first used them in 1975 on a Jr. Satan, which they didn't help.  PJ was the first person I saw using them on a stunt plane. 

Here's a piece I wrote on how to stick them on a wing:
I have used two methods to attach them:

1. first putting down a strip of Scotch Multitask Tape (the stuff we use to seal control surface hinge gaps), then gluing the VGs to the tape with cyanoacrylate glue.. 

2. gluing the VGs directly to the airplane.  I have tried various glues.  I recommend the following, both of which allow nondestructive VG removal:

Elmerís Rubber Cement
This works a treat.  VGs stay on in flight, come off easily on demand, and leave no residue on either acrylic lacquer or catalyzed polyurethane.  The brush in the bottle isnít much use for VG work, though.  Starbucks stirring sticks work well for applying the glue.  Q-Tips work well for removing excess glue around the VGs easily without solvent.

UHU Twist & Glue
This holds the VGs on a little better than rubber cement, and it dries faster.  VGs still come off when you want them to.  They leave some glue on the surface, but itís easily removed (from catalyzed polyurethane clearcoat) with isopropyl alcohol.  I have only used the version of this glue commonly available in Poland, but I presume that the same glue is available elsewhere. 

Mind you, this is for my ABS VGs with wide bases that provide some area for glue.

I should mention that there are some substandard VGs coming out of Canada, a foreign country, made out of cheap polystyrene by immigrant labor.  They are devoid of color ("colour", as they put it), and have deformed sickle-cell-looking bases. I would avoid these.  Some authentic American-made artisanal VGs may be available at the Goyet and team trials if the wind blows in the wrong direction this week at JCT manor.  Otherwise, I'll be practicing stunt.
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #15 on: August 17, 2019, 10:08:37 PM »
That's why I asked where Ken put them that helped so much.
Not sure I answered that so I will do it again.  It is a 64" span and I started them 7" from the fuselage 4 on each wing 4" apart in two "towed in" pairs set at 20 degrees with the back end about 1/8" from the high point. (See Picture)  It is a classic with a 40% high point so they look further back than they would on a modern airfoil.

What I got for my effort was a small increase in the locking out of corners it already did that well so it was hard to judge improvement) and less tendency to hunt.  The major improvement was the one I was looking for.  The stall was pushed out enough that it no longer has difficulty with the 6th triangle corner, the 4th hourglass corner and the last loop of the clover.

Finally have a plane that flies better than the pilot!

Ken
« Last Edit: August 18, 2019, 10:44:20 AM by Ken Culbertson »
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Offline Russell Bond

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Re: Vortex Generators on a Twister
« Reply #16 on: August 18, 2019, 04:09:13 AM »
Igor uses multiple zigzag turbulators under the paint, that's why they are hard to see.
They are on the wing and stabilizer.
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #17 on: August 18, 2019, 07:10:40 AM »
There ya go Ken. A 4 % plus is better than 0. Most mods today are just tiny increments of advance over the past. D>K
Tru dat
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #18 on: August 18, 2019, 11:25:50 AM »
End of speculation.  They worked.  Tested them in a 10mph wind so it was really easy to see the change in the amount of turn it took to make it stall.  No stall even with a legal 4th turn in the hourglass and the last turn of the clover from 42 which I simply could not do in any significant wind before.  Less bouncing around and clearly more steady in inverted flight.  I use the word "steady" because it did not hunt before the generators were added.  It just stopped much of that bouncing around that profiles due and made keeping it level much easier.  Basically the same improvement areas that I experienced on my other ships in differing degrees.  The change in the stall was dramatic.  I had trimmed the Twister a bit nose heavy so that it turned more like my full size ships. 

I think that the biggest benefit may be psychological.  If you are worried that you might stall on one of those "gut check" corners you are more likely to soften it and not even give it a second thought.  The infamous "&" Hourglass (reversed of course).  If I am confidant the plane will make those corners, I am more likely to do them right.

For those curious as to where I put them, I attached a snapshot.

Ken
« Last Edit: August 19, 2019, 11:32:02 PM by Ken Culbertson »
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Offline Dave_Trible

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Re: Vortex Generators on a Twister
« Reply #19 on: August 18, 2019, 02:05:09 PM »
I didnít get enough flying time or weather before the Nats to really get the bugs worked out on my new airplane.  Once I got to Muncie and started working it hard the airplane kept making obscene gestures at the bottom of the triangle and a couple other places.  These aerodynamics are identical to my Desperados and hadnít seen this before.  The only difference was I tried the very sharp stab leading edge instead of the rounded one I had always used.  These new ship weigh plenty but no more than any of the Desperados.  I was convinced the stab was stalling.  Zig zag tape help quite a bit but didnít totally solve the problem.  We limped on through the Nats.  Before I left I hit Howard up for a small batch of VGs.  Once I got home I built a new stab with the standard rounded LE and I also applied the VGs.  The difference is night and day.  Now the first airplane flies quite nicely.  Now, though I donít know what fixed the problem.  Iíll make just one change at a time on the second and know. 
From all the stuff I found to read about the VGs and a video on Utube with a guy testing them on an RC airplane with a camera and tastles on board,  they say 7-9% back from the LE and canted no more then 15 degrees.  Frank Williams told me about 10 % back.  Thatís what I did and they obviously work.  If nothing else the glide stretched out a mile with the VGs.  From what Iíve seen I think guys are mounting them waaayy too far back.  BTW Frank seems to think the sharp stab drives my wing at a higher alpha causing the wing to stall at times....

Dave
« Last Edit: August 18, 2019, 05:58:07 PM by Dave_Trible »
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #20 on: August 18, 2019, 03:38:15 PM »
I didnít get enough flying time or weather before the Nats to really get the bugs worked out on my new airplane.  Once I got to Muncie and started working it hard the airplane kept making obscene gestures at the bottom of the triangle and a couple other places.  These aerodynamics are identical to my Desperados and hadnít seen this before.  The only difference was I tried the very sharp stab leading edge instead of the rounded one I had always used.  These new ship weigh plenty but no more than any of the Desperados.  I was convinced the stab was stalling.  Zig zag tape help quite a bit but didnít totally solve the problem.  We limped on through the Nats.  Before I left I hit Howard up for a small batch of VGs.  Once I got home I built a new stab with the standard rounded LE and I also applied the VGs.  The difference is night and day.  Now the first airplane flies quite nicely.  Now, though I donít know what fixed the problem.  Iíll make just one change at a time on the second and know. 
From all the stuff I found to read about the VGs and a video on Utube with a guy testing them on an RC airplane with a camera and tastles on board,  they say 7-9% back from the LE and canted no more then 15 degrees.  Frank Williams told me about 10 % back.  Thatís what I did and they obviously work.  If nothing else the glide stretched out a mile with the VGs.  From what Iíve scene I think guys are mounting them waaayy too far back.  BTW Frank seems to think the sharp stab drives my wing at a higher alpha causing the wing to stall at times....

Dave
This sounds familiar.  I had never considered that the stab may be stalling.  I really liked what the sharp LE did for corners but come to think of it the problems at the end of the triangles and it's unwillingness to turn in the 4th loop of the clover could very well be the stab.  When I added VG's to the wing both problems went away.  Perhaps with the added wing lift I never reached the point the stab stalled but as I replay the what the plane was doing before it sure looked like both stalled, wing first but the way the tail rose instead of falling...interesting.  Thank God for brick lifting engines/motors to pull us out.

At some point I will have time to play more with location.  I just went with what others had uses to claim success.  VG's on real aircraft seem to be more forward and ahead of the high point yet extending just above it.  Some have a second row ahead of the flaps.  Worth a try.  I think delaying the stall as little as 1/10th of a second is a huge improvement.

In my experience, turbulators work a little different and I am not convinced that they do as much to help on a symmetrical airfoil to delay a stall.  I think they more increase lift but maybe both roads lead to Rome.  Anyway, who is to argue with Igor?  Not me.

ken
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Offline Dave_Trible

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Re: Vortex Generators on a Twister
« Reply #21 on: August 18, 2019, 06:53:09 PM »
Here is how I placed mine.  I need to use more rubber cement though.  Iím losing them too often.  The last two on the top of the outboard wing seem to pop off the most- likely the hardest worked and most important on those triangle bottoms.  Hope Howard has that printer up to speed.......

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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #22 on: August 18, 2019, 07:26:22 PM »
I placed mine much closer together, as I had seen on a L4 Bird Dog that landed at the El Dorado, Ark  RC runway. It's about 300 feet long, if that. It had VG all along the entire LE on the upper part of the wing.

I beg to differ!  An L-4 is only about 22 1/2 feet long!
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Offline PJ Rowland

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Re: Vortex Generators on a Twister
« Reply #23 on: August 18, 2019, 07:39:59 PM »
Its great to see more people giving VG tech a shot


I first used them back in 1995 thats a long time to test.
Ive used them on a 45oz Ares and they worked as expected. Weight isnt really an issue with getting them to work. Obviously a heavy plane will see improvement over a plabe that already works..
VGs are more about getting consistency out of the wing than anything else. These planes wobble and bobble about its eas to wash airspeed off in weird ways. Vgs just clean it up.

Ive tested them everywhere from 5% le to just infront of the fflap.. and the best location I find is on the highpoint.

Vgs are good... but thats 25 years old tech now.. im working on the new stuff.. and its WAY more exciting.

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Re: Vortex Generators on a Twister
« Reply #24 on: August 19, 2019, 09:05:03 PM »


These things ( Below ) were a all weather interceptor , After fitting the V. G. Theyed only do a deep stall , T tail blanketed . So still could fall out of the sky .
Problem worse at night . No Horizon . Intresting second row is at Ailerons , but designs almost a flying wing delta .



The Harrier Flys Real Slow . Seems the benefits are generally at the stall aproach / Comparitivly low airspeed / airflow ? ? .

Positionings again intresting . Starting around the Dog Tooth . Whats the Dog tooth FOR . WHY does no one use Dog Tooth L.E. !  S?P



Could look at a Avro Vulan Mk II L.E. too . . . VD~


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Re: Vortex Generators on a Twister
« Reply #25 on: August 19, 2019, 11:18:07 PM »
Frank Williams told me about 10 % back.  Thatís what I did and they obviously work. 

Thanks. I added a pair forward of the others today. It certainly didnít hurt.
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Offline Ted Fancher

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Re: Vortex Generators on a Twister
« Reply #26 on: August 31, 2019, 04:56:53 PM »
Whadinhell do you mean by that???

Igor uses them.  Paul uses them.  Any theory that starts with "well, the top guys use them" has to be solid, right?

Well, yeah...but so does Howard!!!!

Ted




post mortem salvation mode invoked!  (i.e. juuuuust kidding, Howie!)

Offline Ted Fancher

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Re: Vortex Generators on a Twister
« Reply #27 on: August 31, 2019, 05:01:34 PM »
48oz electric & Fancherized with 4" added to span so it has a pretty low loading.  It has a nasty habit of turning early after a couple of corners slow it down.  Other than that I would put it up against a PA ship in good air.  Thought the generators might let me use a bit less elevator and stop some of it's Twisterness.  They really helped my PA ship which wobbles in at 70 ounces but is massively over powered so it flies quite nice and is great aerobic exercise.

Ken

Hi Ken,

Just curious if you enlarged the stab/elevator commensurate with the increased wing span/area? 

If not, I'm thinking maybe the CG might have had to be a little far forward which could be bogging the ship down after the first couple of corners.  If you're one to one on the flap/elevator deflection you might consider less flap movement as, with that high aspect ratio wing, you almost certainly aren't at a loss for lift.  Reducing the flap movement would reduce the wing's negative pitching moment plus reduce the elevator deflection required for the brisk corners you're after.  Ergo, you would reduce the loads causing the ship to slow in corners.  Could be sort of a win/win???

Just a thought.

Ted
« Last Edit: August 31, 2019, 05:32:03 PM by Ted Fancher »

Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #28 on: August 31, 2019, 05:52:59 PM »
Hi Ken,

Just curious if you enlarged the stab/elevator commensurate with the increased wing span/area? 

If not, I'm thinking maybe the CG might have had to be a little far forward which could be bogging the ship down after the first couple of corners.  If you're one to one on the flap/elevator deflection you might consider less flap movement as, with that high aspect ratio wing, you almost certainly aren't at a loss for lift.  Reducing the flap movement would reduce the wing's negative pitching moment plus reduce the elevator deflection required for the brisk corners you're after.  Ergo, you would reduce the loads causing the ship to slow in corners.  Could be sort of a win/win???

Just a thought.

Ted
I did add to the stab.  The wing got a 4" span increase and the stab 2"  Before I added the vortex generators I had changed the flap ratio from 3:4 to 1:1.  It helped smooth out the corners.  I may change it back now that I have them.
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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #29 on: August 31, 2019, 06:07:14 PM »
Well, yeah...but so does Howard!!!!

Hey!  My personal rule is that I'm not good enough to just copy the likes of Paul Walker or Igor Burger, so I do what Howard does.

Of course, Howard just does what Paul tells him to do, but that's different...
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Offline FLOYD CARTER

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Re: Vortex Generators on a Twister
« Reply #30 on: September 10, 2019, 05:01:44 PM »
All my designs are relatively light, and they don't stall.

Therefore, I can get along without.
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Offline Howard Rush

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Re: Vortex Generators on a Twister
« Reply #31 on: September 11, 2019, 02:24:21 AM »
Any theory that starts with "well, the top guys use them" has to be solid, right?

Well, yeah...but so does Howard!!!!

That's a good point, Ted.  I made a spreadsheet and entered the top guys.  That initially told me to use VGs.  Then I entered myself and got a warning about circular reference.  I found out that I could get it to converge if I used fewer VGs than the top guys. 
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #32 on: September 11, 2019, 07:01:15 AM »
That's a good point, Ted.  I made a spreadsheet and entered the top guys.  That initially told me to use VGs.  Then I entered myself and got a warning about circular reference.  I found out that I could get it to converge if I used fewer VGs than the top guys.
Now I am getting confused.  Should I be using the average number the "Top Guys" use?  Would that be arithmetic or mode?  If I use the median, will someone's feelings be hurt?   LL~ LL~

All I know is that the Twister bounced around a lot and would stall if I "popped" a corner.  Now it bounces around a little and I have to "Bang" a corner to get it to stall.  To me that is success, or at least progress.  I will leave it to those of you who make a living "determining the difference between an orange" to find the optimal number and position.

Thanks for all the help! 

Ken
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Offline Tim Wescott

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Re: Vortex Generators on a Twister
« Reply #33 on: September 11, 2019, 03:49:25 PM »
... Whats the Dog tooth FOR . ...

Lots of LE sweep is good for low drag at supersonic speeds ('cuz of the area rule, I think, or distributing the shock wave, or something aerodynamic-y).  But it messes up low-speed handling because the air wants to flow along the span of the wing instead of across the chord.  Dog legs (and LE fences, like on the MIG-15) sorta-kinda make it stop.

I don't think you see them on later planes -- engineers must have figured out how to get by without 'em.
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Offline PJ Rowland

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Re: Vortex Generators on a Twister
« Reply #34 on: September 28, 2019, 12:21:48 AM »
It does ammuse me to read that the old axiom of stunt lore " if a little helps then alot must help alot"

I see it all the time..
A. Reluctance to use them
B.TestIng
C.Night and day improvement lots of improvment everywhere
D. So why dont we add more or alter the location to get MORE..

Well.. you can if you like.  I wont ever discourage someone from their own experiments. However... all the testing that brought the product to the point where "the top guys use them" has already been done..

If there were more improvement to be had THAT would be the suggestion. The facts are that only 1 PAIR located 1 foot from the root chord is sufficient. This will get you to 95% improvment. This exact setup has also won the Nats. So that version as initially described is pretty darn solid.

What happened originally was after Dave Fitzgerald did some of his own testing he wanted to gain 100% out of the airframe and thus added only 1 more set just to clean up the middle part of the wing in wind. He also did some extensive stab testing of his own and found the same results as I had found. The "turn amd lock" improved.

But these extra improvments are only subtle over the original 1 pair and move on philosophy.

No disrespect to Brett, but there IS indeed a very clear understanding as to the how and why they work. The reasons WE use it arent going to be found in any textbook of research paper because CL stunt is a very domain specific disipline.
Only through testing did I find they needed to be the current size and the current locations. I trialled them as large as 1 inch.. at different canted angles.. its all been done for you.

There really is no additional improvements to be had by doimg additional testing.. if ALOT more would make alot more improvments ,thats what we would be using.

Having them further forward of the highpoint POSSIBLY may work better for electrics as the CG is generally more forward anyway. One thing I didnt do 25 years ago was test them in an electric at 10%.. mainly because Igor had not invented it yet..

The basic answer you need to solve is where will maximum lift be located.. we locate everything for max lift around the CG. Most intelligent design follows this predication. The vgs are no different.
They clean up the wing design and allow it to perform in better air. There are no negative side effects that ive ever found to using them.
Even if you build as flawlessly and as light as Paul walker there is STILL the issue of flow seperation. There is a reason he uses them. He has done the testing also.. and found exactly the same results as I did.

Its no a fluke.. its not a gimmick.. its just a small addition you can add to any design to make it perform better.. this was acheived through comprehensive testing.. Now its more mainstream as has gone through its own independant testing via the hands of some of the most elite stunt fliers of the last 50 years.. the VG concept time and time again provides improvement that you simply cannot ignore.

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Offline Chuck_Smith

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Re: Vortex Generators on a Twister
« Reply #35 on: September 28, 2019, 06:10:47 AM »
You can buy really nice turbulator tape from full-size sailplane distributors.

It's one of the things that make you go "hmmmm." If I need a turbulator, why did I just spend half an hour cleaning the bugs off the leading edge?

Stunt wings are weird. Since the chord is small and velocity is low we'd expect the flow to remain laminar pretty far back on the wing. But - the curve is so high it adds a non-trivial rotation to the flow (not to be confused with circulation).

I suspect also, that the high mgnatude of airframe vibration on a stunt ship trips the flow earlier than would be otherwise expected. Seems to me that moving to electric power would allow the flow to remain attached longer and then yes, there just might be something to using vortices to manage the flow.

It may be that they help on electric but not on glow, but who knows? We have no real hard data at conditions that are dynamically similar to CLPA. Until we put a CLPA section in the flow with and without vortex generators or rent a supercomputer for CFD all we can do is experiment.
 
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #36 on: September 28, 2019, 07:25:24 AM »
You can buy really nice turbulator tape from full-size sailplane distributors.

It's one of the things that make you go "hmmmm."

It may be that they help on electric but not on glow, but who knows?
You may be on to something here with the vibration.  I put them on both my electrics and IC at the same time.  Both were improved but the electrics were improved more.  The largest improvement was with a Classic that has a very thin (by today's standards) airfoil with a high point at about 45% and a curved aft section.  It worked well on the 1963 original at 42oz and 680sq but the 2018 rebuild came out at 70.  Difference was original had a puppy dog engine and the new one had wild horses up front.  Had a real stall problem with tight turns (triangles, hourglass, clover).  VG's totally fixed that.  Each plane was improved and each improvement was different which tells me that VG's act differently on different airfoils and you have to experiment a bit to find out where your best placement is.

Ken
« Last Edit: September 28, 2019, 07:50:10 AM by Ken Culbertson »
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Re: Vortex Generators on a Twister
« Reply #37 on: September 29, 2019, 02:50:51 PM »
I've discovered that with a top coat of 2-part automotive clear coat, almost nothing will stick to the stuff.  So how does one attach VG's?  (Not that I would want to try them, but just curious).

Use  rubber cement on the  surface,  or  place a strip of  scotch invisible tape on the area of the wing you are going to glue them to, then use  CA  or  whatever  glue you  want.

Randy

Offline Larry Wong

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Re: Vortex Generators on a Twister
« Reply #38 on: August 06, 2020, 06:20:41 PM »
This is what I found
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Offline Ted Fancher

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Re: Vortex Generators on a Twister
« Reply #39 on: August 15, 2020, 03:19:18 PM »
That's a good point, Ted.  I made a spreadsheet and entered the top guys.  That initially told me to use VGs.  Then I entered myself and got a warning about circular reference.  I found out that I could get it to converge if I used fewer VGs than the top guys.

I rest my case!

Ted

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Re: Vortex Generators on a Twister
« Reply #40 on: September 10, 2020, 04:45:07 AM »
Chuck.

You dont nees a supercomputer to do a CFD analysis. I was playing around with CFD systems 10 years ago. Now I can gst an app on my phone that will predict flow..


CFD is nice but it wont tell you in flight how its going to feel.
One thing is for certain, IC vibration helps with flow seperation and this is mitigated with electrics.


If you always put limit on everything you do, physical or anything else. It will spread into your work and into your life. There are no limits. There are only plateaus, and you must not stay there, you must go beyond them.Ē - Bruce Lee.

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Offline Rick Huff

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Re: Vortex Generators on a Twister
« Reply #41 on: September 10, 2020, 05:45:25 AM »
Where does one obtain vortex generators?
I have an older plane that's stalling at the bottom of the triangle and hourglass and was hoping the VGs might help.
Thanks,
Rick

Offline Jim Hoffman

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Re: Vortex Generators on a Twister
« Reply #42 on: September 10, 2020, 11:59:37 AM »
I find carpet tape to be the best adhesive.  Much better bond than rubber cement

Offline Fred Underwood

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Re: Vortex Generators on a Twister
« Reply #43 on: September 10, 2020, 12:10:33 PM »
Where does one obtain vortex generators?
I have an older plane that's stalling at the bottom of the triangle and hourglass and was hoping the VGs might help.
Thanks,
Rick

They were here, but currently out.

https://gator-rc.com/micro-vortex-generator

Some make their own.
Fred
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Online Ken Culbertson

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Re: Vortex Generators on a Twister
« Reply #44 on: September 10, 2020, 12:34:13 PM »
Where does one obtain vortex generators?
I have an older plane that's stalling at the bottom of the triangle and hourglass and was hoping the VGs might help.
Thanks,
Rick
They will help some but increasing the area of the outboard flap may help as well.  I had added tip weight once and I started getting that same stall.  I added the tab and it cured it.  I had done that prior to adding vortex generators.  With VG's the flaps seemed more effective and I felt less control pressure to make those turns so I may not have needed the tab.  I had another ship, an overweight Classic that the VG's totally cured the stall problem but it was more wide spread and the by-product of a bad airfoil for the weight I was carrying. Will never know, both planes are gone.

VG's aren't  going to fix a bad CG but they may point it out.  My take on VG's after having put them on every ship I had is that they make good things a little bit better and more consistent but they don't *fix* bad things although they may mask them somewhat.

Ken
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Offline Rick Huff

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Re: Vortex Generators on a Twister
« Reply #45 on: September 11, 2020, 05:33:23 AM »
Thanks for the link, Fred.  I guess I'll just have to wait. 
Ken, I've already increased the area of the flaps by 10%, but haven't had a chance to fly it with the larger flaps.
Thanks,
Rick


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