Vortex generators
 

Hi all.

I´m considering building a BUSA 1:3 Super Cub, and my plan is to incorporate some of the mods used on the full size STOL versions. Maybe extended squared wing, with bigger flaps and ailerons, and other cosmetic mods.
As far as I know the full size community is impressed by the performance when adding vortex generators, and I´m thinking that this may also work on model sizes?

At first I considered buying a "generic" VG set, made for ultralights, but they are going to be rather large and clumsy for a 1:3 scale, so I may have them 3D printed in nylon by a friend with access to all the fancy toys, and made in a more appropiate size.

But I´m not even sure if they will work as advertised when scaled down. Anyone with relevant experiences?
Can I use full size formulas for placement, or do I need to make changes due to the smaller wing chord?

Thanks in advance.

RE: Vortex generators
 

Vortex generators belong to a set of "bandaids" used to produce a turbulent boundry layer. A TBL is thin and quite "sticky" so it aids in delaying the formation of an upper surface separation bubble and eventual stall.

The thing is that this is not an automatic sort of thing. If you pick the right airfoil for a model then the airfoil itself can produce the same results without requireing that you resort to these fixes.

The tubulation also needs to take place ahead of where the air generally separates from the airfoil as the angle of attack climbs. If it behind that point then it won't do any good as it's inside the turbulence of the bubble.

Possible options for forming this turbulence are numerous. The little vertical angled bits are one option. Another is a simple raised strip such as you'd get from using two or three layers of 1/8 inch wide trim stripe tape added over each other to build up enough thickness to be suitable for the size of the airfoil chord. Others are cutting a saw tooth patthern in thick tape and adding it to the airfoil.

But all the results are the same. The early laminar flow is churned into a thin TBL in order to delay separation and the final stall event.

What works and where it should be located all depends not only on the airfoil used but the building method, chord dimensions and wing loading. All these things have an effect on where a possible separation can take place. But generally it's a safe bet to locate a trip trip, or other boundry layer trip at around 15% of the chord.

RE: Vortex generators
 

What he said!  :)

VG's are used as original equipment on full-size, like some Falcons and Learjets, not so much as 'bandaids' but as elements that do several things to enhance performance with minimum penalty. I can attest to their effectiveness on Cessna twins, the VGs totally transform them and tame some bad characteristics. For a real bandaid design approach, look at the tail of a Beech 1900.  [X(] 

As BM said, 15-20% of wing chord will be right. Height and shape will be up to your skills, as scale VGs will probably not be very effective due to scale effect, Reynolds numbers, etc.  Believe it or not, the aftermarket VGs use double-sided tape exactly like our servo tape for attachment.  You might start out with plastic angle stock from the home improvement store and trim it to your dimensions. You may have noticed that some VGs alternate evenly (facing left of center, then right), and others point toward the center. Just a difference in who designed them, and for which aircraft.  They do work, as many bush pilots will attest.

RE: Vortex generators
 

For the vortex generator to work we need scale air, not full size air and scale aircraft.
Darryl Usher

RE: Vortex generators
 

fluid velocity at the surface of the wing is 0 hence creating what is known as the boundary layer. This as well as an adverse pressure gradient along the top surface of the wing deplete the air of its energy and can cause air to separate from the wing which is known as a laminar separation bubble. When this happens you lose lift and end up creating drag. The air then becomes turbulent and is "re-energized by the surrounding air and will often reattach further along the wing. This is often seen at low reynolds numbers

This can all be very technical but simply put sometimes the air flowing above the wing doesn't have enough energy to "stick" to the wing and if it doesn't stick it becomes turbulent which creates little lift and a lot of drag. By incorporating Vortex Gens we can make the air turbulent before it separates which may be confusing because i just told you turbulent air is bad but turbulent air has more "energy" so a little bit of turbulence is good and we can do this by this using a VG. We can create little turbulent packages of air that stay attached to a wing for longer instead of having the air separate and become a big bad eddie. A lot of details are missing but hopefully I was able to convey to you the basic idea behind VGs

To answer your question, you can't just scale them and without doing some computational fluid dynamics or experimental research as previously mentioned you won't know where to place them, how many to place, how big to make them etc. As long as you make them small enough they may not be helpful and you may even lose a bit of lift but I think it'd still be cool if you incorporated them onto your wings.

I hope this was helpful Good luck:D

RE: Vortex generators
 

There is a new little aerobatic model - designed by a long time aerobatic flyer - which uses a full span set of bumps at the high point of the airfoil top n bottom - th airfoil is quite thick with the high point well forward.
The model is 3 D thing sold by Horizon and utilizes the new AS3X stability system.
Videos show excellent stability at high AOA -
I have never tried this approach-but have used air dams which stop spanwise flow works well-
Maybe the super thick "ice cream cone" airfoil contributes the most to the good stability but the little generators may really seal the deal -
what do ya think - there are videos of this on HH site .

RE: Vortex generators
 

WWII. Thanks for the explanation.
I have actually managed to find custom made VG´s for 1:3 scale models, being produced by a full size aircraft manufacturer. I´m about to order a set, and I will post some details and pictures if the order comes through as expected.

RE: Vortex generators
 

Can you share a website and costs?

RE: Vortex generators
 

Hope you make them temporary so you can see how they work on and off. That would also allow you to test their foward-aft positioning, yet another unknown.

RE: Vortex generators
 

hi guys, just had to bounce in here.. i fabricated and installed Vortex generators on my Bud Nosen citabria and they work great.. i made them out of Lexan left over from my new windows,, and i can tell you the difference is very noticeable. she will just hang in the air now with out stalling, and when i force a stall, i still have aileron control though the stall. i looked up Vortex generators and found several sites with the formula for spacing and placement and multiplied by .25 to get what i needed for 1/4 scale.. to give you an idea, i crashed this plane because it had a very nasty habit of snapping into a spin if she stalled ( i mean she would flip onto her back right now, and do at least 2 uncontrolled spins) now, with the vortex generators there is no tendency at all to stall. i can land her every time on the tail wheel.. i will try to attach some pics ( by the way, i only put them in front of the ailerons, so the root would stall first)

RE: Vortex generators
 

Bandit357, is that fabric, and how did you attach them?<div>Sweet bird, what power?</div>

RE: Vortex generators
 

yes its fabric, i attached them with silicone ( clear) i made them to have the curve of the wing using a covering iron, that way they would have a strong bond.. also as you can see.. they are barley visible being clear..

RE: Vortex generators
 

Put the VG's on horizontal tail and rudder as well and watch as your stick throws at higher angles become far more smooth.

RE: Vortex generators
 

BFoote, that's true. That's done on full-scale with great effect. Horizontal stab gets them on the bottom.

RE: Vortex generators
 

I've always wondered if it was applicable in scale, now you guys have proven it..<div>
</div><div>Will it work on my 1270mm wingspan Dynam T-28?</div><div>
</div><div>Thanks</div><div>
</div><div>Ed</div>

RE: Vortex generators
 

Some of the competition fun-fly planes of the late-80's/early 90's had diamond-shaped airfoils with the widest part well forward on the airfoil. Diamond-shaped stabs have been around since the early free-flight days.

Edgardo, give it a try!

RE: Vortex generators
 

Sorry for not updating this thread for so long time.

I did receive the VG´s today. I bought them from Zlin Aero (http://www.zlinaero.com/eng/index.php) where they are supplied with their 1:3 scale Bobber. The price shipped to my door was about 125€, which is quite a lot, but since they are sold by a company that normally sells full size aircraft, I guess that would be expected.

They are made from thin aluminum plates, cut and bent to shape. So if I ever need VG´s for another project I guess it would not be too difficult to fabricate them myself.
I bought them anyway since I have no experience with VG´s, and I wanted my first try to have a fair chance of success.

The set contains VG´s for the main wings along with the bottom of the stabilizer. 16 pcs. for each wing half, and 7 pcs. for the bottom of each stabilizer half.

It´s supposed to be taped to the structure and testflown until the best placement has been determined, whereupon they are to be fixed with glue.

Tinus , I hope you have had good success with the vortex gens ! If so please share the outcome , placement , perceived flight enhancement ,etc.
kind regards , Jim

Unfortunately I have not yet gotten around to mounting them.
I have spent the season dialing in the throws and trimming so I have something to compare to when they are mounted.
For some reason the BUSA SC needs quite a lot of down elevator for level flight, and when dumping the flaps I need full down trim, as well as a bit of stick pressure, to prevent a loop. But with adjustment of throws and pushrods I´m close to something useful. Without elevator authority in the flare there is no need for VG´s. :-)
But I will update this thread when they are mounted.

Tinus,
It sounds to me like your wing incidence is too positive ! Not saying you don't have it as suggested in build manual , just saying that its probably too much positive incidence . Had same thing happen to me . Thanks for your reply!

Tinus, Have you used the vortex generators yet? Thanks!

Nope, they are not installed yet. Winter is almost here i Denmark, so it´s not very likely I will get out flying much until spring, but I´m hoping for a few good days.